In this letter we report an investigation of the interfacial electronic structure formed by metals and conjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metal substrates for two five-ring phenylenevinylene oligomers: unsubstituted p-bis[(p-styryl)styryl]benzene (P5V4) and the analogous oligomer with 2-methoxy-5-(2’-ethyl-hexyloxy) substitution on the central ring (MEH-P5V4). We found for all interfaces a lowering of the energy levels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably due to interface dipole layers, was always such as to keep the hole injection barrier nearly constant and therefore at most weakly sensitive to the work function of the metal or the ionization potential of the oligomer.

Hadziioannou, G.,

Heeres, A.,

Jonkman, H.T.,

Krasnikov, V.V.,

Sawatzky, G.A.,

Stalmach, U.,

Veenstra, S.C.,

Veenstra, S. C.

Stalmach, U.

Krasnikov, V. V.

Hadziioannou, G.

Jonkman, H. T.

Heeres, A.

Sawatzky, G. A.

University of Groningen

   University of GroningenEnergy level alignment at the conjugated phenylenevinylene oligomer/metal interfaceVeenstra, S. C.; Stalmach, U.; Krasnikov, V. V.; Hadziioannou, G.; Jonkman, H. T.; Heeres,A.; Sawatzky, G. A.Published in:Applied Physics LettersDOI:10.1063/1.126312IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Veenstra, S. C., Stalmach, U., Krasnikov, V. V., Hadziioannou, G., Jonkman, H. T., Heeres, A., &Sawatzky, G. A. (2000). Energy level alignment at the conjugated phenylenevinylene oligomer/metalinterface. Applied Physics Letters, 76(16), 2253 - 2255. https://doi.org/10.1063/1.126312CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 16 17 APRIL 2000Energy level alignment at the conjugated phenylenevinyleneoligomerÕmetal interfaceS. C. Veenstra, U. Stalmach, V. V. Krasnikov, and G. Hadziioannoua)Department of Polymer Chemistry, Materials Science Centre, University of Groningen, Nijenborgh 4,9747 AG Groningen, The NetherlandsH. T. Jonkman, A. Heeres, and G. A. SawatzkyLaboratory of Solid State and Applied Physics, Materials Science Centre, University of Groningen,Nijenborgh 4, 9747 AG Groningen, The Netherlands~Received 19 November 1999; accepted for publication 17 February 2000!In this letter we report an investigation of the interfacial electronic structure formed by metals andconjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metalsubstrates for two five-ring phenylenevinylene oligomers: unsubstituted p-bis@(p-styryl)styryl#benzene ~P5V4! and the analogous oligomer with 2-methoxy-5-~2’-ethyl-hexyloxy!substitution on the central ring ~MEH-P5V4!. We found for all interfaces a lowering of the energylevels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably dueto interface dipole layers, was always such as to keep the hole injection barrier nearly constant andtherefore at most weakly sensitive to the work function of the metal or the ionization potential of theoligomer. © 2000 American Institute of Physics. @S0003-6951~00!01216-X#The charge transfer process at the interface between anelectrode and a polymeric semiconductor plays a key role inthe performance of optoelectric devices like polymer light-emitting diodes (pLEDs) and photovoltaic devices ~PVDs!.These devices typically consist of one or more polymer lay-ers sandwiched between two electrodes. The transfer processat the interface between the contact and the active layer is afunction of the barrier height between the Fermi energy levelof the electrode and the frontier orbital levels of the polymerlayer. In order to estimate the injection barrier one usuallyassumes a rigid band model and an alignment of the vacuumlevels at the interface. However, these assumptions ignorethe possible formation of dipole layers or covalent bonds atthe interface and doping of the interfacial region. Althoughmeasurements of the vacuum level alignment are importantfor understanding device behavior and optimizing device ef-ficiency, they have seldom been reported for the interfacebetween metals and phenylenevinylene oligomers.1–3Here we report ultraviolet photoelectron spectroscopy~UPS! measurements on the interfaces between metals ~Auand Ag! and two different five-ring phenylenevinylene oli-gomers ~P5V4’s!, unsubstituted ~P5V4! and 2-methoxy-5-~2’-ethyl-hexyloxy!-substituted P5V4 ~MEH-P5V4! ~see in-set Figs. 1 and 2, respectively!. These oligomers can be usedfor optoelectronic applications and as model systems for thetwo most widely used corresponding conjugated polymers.The advantage of using these oligomers instead of the poly-mers is that their high chemical purity and relatively lowmass enables us to prepare thin films in situ by deposition ofthe oligomer on a cleaned metal surface under ultrahighvacuum ~UHV! conditions.In all cases we measured a misalignment between thevacuum levels of the metal and the oligomer. The levels ofthe oligomers shift downward relative to the energy levels ofa!Electronic mail: hadzii@chem.rug.nl2250003-6951/2000/76(16)/2253/3/$17.00the metal ~a negative shift!. The magnitude of this shift isbetween 0.4 and 1.2 eV, depending on the metal as well ason the oligomer. The observed energy level alignment mayhave a strong effect on the charge injection process at themetal/oligomer interface as it is found in LEDs and PVDs.All experiments were performed in a single UHV cham-ber with a base pressure of 1029 mbar equipped with a Hedischarge lamp (He I,hn521.22 eV) and an effusion cell ofour own design. A hemispherical analyzer, in combinationwith a lens system, was used as electron energy analyzerwith an overall resolution of 150 meV. The metal substrateswere cleaned by polishing and washing in an ultrasonic bathwith toluene and acetone as solvents. Next, the substrateswere placed in a sampleholder on a x ,y ,z stage. Under UHVconditions, the substrates were Ar1-ion sputtered in order toobtain UPS spectra without features from impurities or ad-sorbed overlayers. During measurements, a bias voltage of24.00 V was applied to the sampleholder to clear the low-kinetic-energy cutoff of the spectrum. The oligomers weresynthesized and purified as described elsewhere.4,5 Theoligomer/metal interfaces were formed by slow molecularbeam deposition monitored by a calibrated thickness moni-tor, after degassing the effusion cell for several hours at120 °C. Typical sublimation temperatures range from 225 to275 °C and the pressure ranges from 0.531027 to 231027 mbar, giving a deposition rate of several monolayersper minute. The oligomer film thickness used in determiningthe energy level offset was typically 5–10 nm.Figure 1 illustrates how an interfacial energy diagram,including the presence of an electric field, is deduced fromtwo UPS spectra. The figure shows the He I UPS spectra ofAu ~left! and P5V4 on Au ~right!. Both spectra were re-corded while a bias voltage of 24.00 V between sample andanalyzer was applied. The energy diagram of the interface isshown in the middle of Fig. 1. The y axis represents thekinetic energy of the photoelectrons at the entrance slit of the3 © 2000 American Institute of Physics2254 Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.analyzer. The total spectrum consists of photoelectrons suf-fering no energy loss processes on their way out of the solidproviding information on the binding energies of the occu-pied states of the solid, and scattered electrons which havesuffered energy loss which contributes to a structured back-ground. The high-kinetic-energy onset of the spectra corre-sponds to emission of electrons from the Fermi level of themetal @Ekmax(Au)# or from the highest occupied molecularorbital ~HOMO! of the oligomer @Ekmax(P5V4)# . The low-kinetic-energy cutoff @Ekmin(Au),Ekmin(P5V4)# correspondsto electrons having suffered energy loss processes and endedup at the minimum kinetic energy in the solid required toescape into the vacuum.Thus the position of the vacuum level of a material inthe diagram is determined by adding 21.2 eV to the cutoffenergy to give Evac(Au) and Evac(P5V4). Now one can de-termine the work function of Au using:FAu5hn2@Ekmax~Au!2Ekmin~Au!# ~1!and the position of the HOMO level of P5V4 according to:Is5hn2@Ekmax~P5V4!2Ekmin~P5V4!# . ~2!By comparison of the two UPS spectra, we can construct anenergy diagram as shown in the middle of the plot and de-termine the positions of the energy levels at both sides of theinterface. We can determine the difference (D) between thevacuum levels of the metal and the oligomer layer using:D5Ekmin~P5V4!2Ekmin~Au!. ~3!FIG. 1. Method for determining the energy diagram of the metal/oligomerinterface using the ultraviolet photoelectron spectra of the metal and theoligomer. The spectra were recorded with a bias of 24.0 V between sampleand analyzer. The y axis represents the kinetic energy of the photoelectronsat the entrance slit of the analyzer. ~a! Photoemission of the Au substrate. ~c!photoemission of 5–10 nm of P5V4 on the Au substrate. ~b! Energy dia-gram of the Au/P5V4 interface. Ekmax(Au): maximum kinetic energy of aphotoelectron excited from Au with hn: photon energy ~21.22 eV!;Ekmax(P5V4): maximum kinetic energy of a photoelectron excited fromP5V4; Ekmin(Au): minimum kinetic energy of a scattered photoelectron ex-cited from Au; Evac(Au): vacuum level of Au; Ekmin(P5V4): minimumkinetic energy of a scattered photoelectron from P5V4; Evac(P5V4):vacuum level of P5V4; FAu : work function of Au; EHOMO : energy level ofhighest occupied molecular orbital of P5V4, evF : hole injection barrier or theenergy difference between the Fermi level of Au and the HOMO level; D:vacuum level shift. The structure of P5V4 is shown at the top, right.Since the vacuum level of the oligomer is lower than thevacuum level of Au, the electric field points from the oligo-mer (d1) to the metal (d2), making D,0 ~as defined inRef. 3!. In this case, the vacuum level shift leads to an in-crease of the barrier for hole injection (ev8F) according to:ev8F5Is2FAu2D . ~4!This hinders hole injection from Au to P5V4, but facilitateselectron injection from the fermi level of Au to the lowestunoccupied molecular orbital ~LUMO!.In Fig. 2 we show the interface formation of MEH-P5V4on a clean Ag substrate. In the center, the full spectra aregiven. The bottom spectrum shows the UP spectrum of theAg substrate with a thin layer of MEH-P5V4 evaporated ontop ~in the order of 1 nm!. The subsequent spectra show thephotoemission of the surface with increasing MEH-P5V4thickness ~0.5–1 nm increase per deposition step!. The topline, with the largest offset, shows the Ag spectrum with theFermi edge well resolved ~at hn2FAg1Vbias520.6 eV!. Atthe right-hand side, the gradual change in the HOMO regionis depicted in more detail. As the film grows thicker, thefeatures arising from the Fermi level of Ag give way to thepure MEH-P5V4 spectrum. The low-kinetic-energy cutoffregion is plotted at the left-hand side of Fig. 2. The shift inthe onset (D) is complete after depositing approximately 1nm on the substrate, it does not increase further with filmthickness. This result indicates that one or very few molecu-lar layers are involved in the process.The upper two spectra of MEH-P5V4 show a small shiftof 0.1 eV towards lower kinetic energy. A slight charging ofthe sample may cause this shift, which is within our mea-surement accuracy. The lowering of the vacuum level iscaused by an electric field at the interface due to ~partial!charge transfer in the interfacial region. The exact mecha-nism creating the electric field is so far unknown. Severalprocesses can cause an electric field at an interface, for ex-ample, electron transfer from a donor to an acceptor, imageeffects, tailing of the electron cloud of the metal towards theFIG. 2. UPS spectrum of MEH-P5V4 on Ag as function of the depositiontime ~thickness!. All spectra were recorded with a bias of 24.0 V betweensample and analyzer. The x axis represents the kinetic energy of the photo-electrons at the entrance slit of the analyzer. The dotted line is the spectrumfrom the polycrystalline Ag substrate. The plots on the right- and left-handside show an enlargement of the central plot. After each spectrum, thethickness of the MEH-P5V4 layer was increased by approximately 0.5–1nm, except for the last two spectra where the thickness was increased byseveral nanometers. The inset in the middle plot shows the structure ofMEH-P5V4.2255Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.adsorbent causing dipoles or metal induced gap states, orchemical interactions between metal and overlayer.6–8Charge transfer at the interface formed by similar PPVoligomers on a Ca substrate was observed by Park et al.1,2The charge transfer caused an overall energy level bendingof 0.5 eV in a space-charge region extending over 10 nm.The energy level bending in their experiment indicates elec-tron transfer from the metal to the organic overlayer makingD.0, which is opposite to the values reported here.Image potential effects lower the ionization energy ofinsulators and semiconductors adsorbed on metal surfaces.This stabilizing effect is linearly proportional to 1/d where dis the distance between the metal surface and the photoion-ized molecule.9,10 Such a distance dependence is not re-flected in a dependence on the film thickness in this study~see Fig. 2!, indicating that the image potential effect is ei-ther small for even the thinnest layer measured or compen-sated for by another electro-static effect. Chemical interac-tions at the interface formed by evaporating Al on asexithiophene layer have been reported3 with a significantcharge transfer from the metal to the oligomer, resulting inthe formation of a bond between the oligomer and the metal.Such a charge transfer causes an electric field pointing fromthe metal towards the organic layer (D.0). For mostmetal–organic interfaces the dipole is negative ~see for ex-ample Fig. 15 in Ref. 6!, similar to our measurements.Table I summarizes the results of the UPS measure-ments. The shift between vacuum levels at the interface isnegative in all cases, moving the energy levels of the oligo-mer downward relative to the levels of the metal. The mag-nitude of the shift is larger for Au than for Ag and alsodepends on the oligomer: the substituted oligomer causes alarger shift. The ionization energy seems to be independentof the metal substrate and compares well with previouslyreported values for similar P5V4s.11 The hole injection bar-rier can be obtained in two ways: by measuring the energydifference between the high kinetic energy onset of the metalTABLE I. UPS measurements on P5V4 and MEH-P5V4 (FAu55.160.1 eV and FAg54.460.1 eV!. D represents the vacuum level shift; Is ,ionization energy; evF, measured hole injection barrier; ev8F , calculated holeinjection barrier using Eq. ~4!.Interface D ~eV! Is ~eV! evF ~eV! ev8F ~eV!P5V4 / Au 21.060.1 5.660.1 1.460.1 1.560.3P5V4 / Ag 20.460.1 5.660.1 1.760.1 1.660.3MEH-P5V4 / Au 21.260.1 5.260.1 1.260.1 1.360.3MEH-P5V4 / Ag 20.560.1 5.360.2 1.360.2 1.460.4and the oligomer, as shown in Fig. 1 (evF), or by using Eq.~4! (ev8F). The differences between the values for the sameinjection barrier ~see column 4 and 5 in the table! give anindication of the systematic error. The values clearly showthat the misalignment of the vacuum levels strongly influ-ences the hole injection barrier and acts as to keep the barrieralmost independent of the work function of the metal sub-strate.In conclusion, we reported UPS measurements of theinterface formed by evaporating PPV-like oligomers ontometals ~Ag and Au!. We found for all these interfaces amisalignment between the vacuum levels of the metal andthe organic overlayer. This shift of levels, presumablycaused by an interfacial dipole layer, strongly influences thehole injection barrier in such a way as to keep this barriernearly constant and therefore at most weakly sensitive to thework function of the metal or the ionization potential of theoligomer. Knowledge of this interfacial dipole layer is there-fore crucial for understanding the electrical characteristics ofoligomer-based electronic devices.The authors are grateful to J. Wildeman and M. Serliefor providing P5V4, P. F. van Hutten for many discussionsand J. Kappenburg for his technical support. This researchwas financially supported by the Dutch Organization for Sci-entific Research ~NWO-CW! and the TMR program of theEC.1 Y. Park, V. Choong, E. Ettedgui, Y. Gao, B. R. Hsieh, T. Wehrmeister,and K. Mu¨llen, Appl. Phys. Lett. 69, 1080 ~1996!.2 Y. Park, V.-E. Choong, B. R. Hsieh, C. W. Tang, T. Wehrmeister, K.Mu¨llen, and Y. Gao, J. Vac. Sci. Technol. A 15, 2574 ~1997!.3 W. R. Salaneck, S. Stafstro¨m, and J.-L. Bre´das, in Conjugated PolymerSurfaces and Interfaces ~Cambridge University Press, New York, 1996!.4 P. F. van Hutten, J. Wildemann, A. Meetsma, and G. Hadziioannou, J.Am. Chem. Soc. 121, 5910 ~1999!.5 L. Ouali, V. V. Krasnikov, U. Stalmach, and G. Hadziiouannou, Adv.Mater. 11, 1515 ~1999!.6 H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater. 11, 605 ~1999! andreferences therein.7 W. Mo¨nch, in Semiconductor Surfaces and Interfaces, Springer series inSurface Sciences 26 ~Springer, Berlin 1993!.8 J. Ho¨lzel and F. K. Schulte, Solid Surface Physics, Work Function ofMetals in Springer Tracts in Modern Physics ~Springer, Berlin, 1979!,Vol. 85.9 R. Hesper, L. H. Tjeng, and G. A. Sawatzky, Europhys. Lett. 40, 177~1997!.10 G. Kaindl, T.-C. Chang, D. E. Eastman, and F. J. Himpsel, Phys. Rev.Lett. 45, 1808 ~1980!.11 A. Schmidt, M. L. Anderson, D. Dunphy, T. Wehrmeister, K. Mu¨llen, andN. R. Armstrong, Adv. Mater. 7, 722 ~1995!.

Energy level alignment at the conjugated phenylenevinylene oligomer/metal interface

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In this letter we report an investigation of the interfacial electronic structure formed by metals and conjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metal substrates for two five-ring phenylenevinylene oligomers: unsubstituted p-bis[(p-styryl)styryl]benzene (P5V4) and the analogous oligomer with 2-methoxy-5-(2’-ethyl-hexyloxy) substitution on the central ring (MEH-P5V4). We found for all interfaces a lowering of the energy levels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably due to interface dipole layers, was always such as to keep the hole injection barrier nearly constant and therefore at most weakly sensitive to the work function of the metal or the ionization potential of the oligomer

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   University of GroningenEnergy level alignment at the conjugated phenylenevinylene oligomer/metal interfaceVeenstra, S. C.; Stalmach, U.; Krasnikov, V. V.; Hadziioannou, G.; Jonkman, H. T.; Heeres,A.; Sawatzky, G. A.Published in:Applied Physics LettersDOI:10.1063/1.126312IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Veenstra, S. C., Stalmach, U., Krasnikov, V. V., Hadziioannou, G., Jonkman, H. T., Heeres, A., &Sawatzky, G. A. (2000). Energy level alignment at the conjugated phenylenevinylene oligomer/metalinterface. Applied Physics Letters, 76(16), 2253 - 2255. https://doi.org/10.1063/1.126312CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 01-02-2024APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 16 17 APRIL 2000Energy level alignment at the conjugated phenylenevinyleneoligomer Õmetal interfaceS. C. Veenstra, U. Stalmach, V. V. Krasnikov, and G. Hadziioannoua)Department of Polymer Chemistry, Materials Science Centre, University of Groningen, Nijenborgh 4,9747 AG Groningen, The NetherlandsH. T. Jonkman, A. Heeres, and G. A. SawatzkyLaboratory of Solid State and Applied Physics, Materials Science Centre, University of Groningen,Nijenborgh 4, 9747 AG Groningen, The Netherlands~Received 19 November 1999; accepted for publication 17 February 2000!In this letter we report an investigation of the interfacial electronic structure formed by metals andconjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metalsubstrates for two five-ring phenylenevinylene oligomers: unsubstitutedp-bis@(p-styryl)styryl#benzene~P5V4! and the analogous oligomer with 2-methoxy-5-~2’-ethyl-hexyloxy!substitution on the central ring~MEH-P5V4!. We found for all interfaces a lowering of the energylevels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably dueto interface dipole layers, was always such as to keep the hole injection barrier nearly constant andtherefore at most weakly sensitive to the work function of the metal or the ionization potential of theoligomer. © 2000 American Institute of Physics.@S0003-6951~00!01216-X#nehtaycisveelluuoaghtaeacpi-dthrolwigthooll asaythe.-fionyzertesathatesd-e ofw-relarni-attorsingm,oma of-de istheThe charge transfer process at the interface betweeelectrode and a polymeric semiconductor plays a key rolthe performance of optoelectric devices like polymer ligemitting diodes (pLEDs) and photovoltaic devices~PVDs!.These devices typically consist of one or more polymer lers sandwiched between two electrodes. The transfer proat the interface between the contact and the active layerfunction of the barrier height between the Fermi energy leof the electrode and the frontier orbital levels of the polymlayer. In order to estimate the injection barrier one usuaassumes a rigid band model and an alignment of the vaclevels at the interface. However, these assumptions ignthe possible formation of dipole layers or covalent bondsthe interface and doping of the interfacial region. Althoumeasurements of the vacuum level alignment are imporfor understanding device behavior and optimizing deviceficiency, they have seldom been reported for the interfbetween metals and phenylenevinylene oligomers.1–3Here we report ultraviolet photoelectron spectrosco~UPS! measurements on the interfaces between metals~Auand Ag! and two different five-ring phenylenevinylene olgomers ~P5V4’s!, unsubstituted~P5V4! and 2-methoxy-5-~2’-ethyl-hexyloxy!-substituted P5V4~MEH-P5V4! ~see in-set Figs. 1 and 2, respectively!. These oligomers can be usefor optoelectronic applications and as model systems fortwo most widely used corresponding conjugated polymeThe advantage of using these oligomers instead of the pmers is that their high chemical purity and relatively lomass enables us to prepare thin filmsin situ by deposition ofthe oligomer on a cleaned metal surface under ultrahvacuum~UHV! conditions.In all cases we measured a misalignment betweenvacuum levels of the metal and the oligomer. The levelsthe oligomers shift downward relative to the energy levelsa!Electronic mail: hadzii@chem.rug.nl2250003-6951/2000/76(16)/2253/3/$17.00anin--essalrymretntf-eyes.y-heffthe metal~a negative shift!. The magnitude of this shift isbetween 0.4 and 1.2 eV, depending on the metal as weon the oligomer. The observed energy level alignment mhave a strong effect on the charge injection process atmetal/oligomer interface as it is found in LEDs and PVDsAll experiments were performed in a single UHV chamber with a base pressure of 1029 mbar equipped with a Hedischarge lamp (He I,hn521.22 eV) and an effusion cell oour own design. A hemispherical analyzer, in combinatwith a lens system, was used as electron energy analwith an overall resolution of 150 meV. The metal substrawere cleaned by polishing and washing in an ultrasonic bwith toluene and acetone as solvents. Next, the substrwere placed in a sampleholder on ax,y,z stage. Under UHVconditions, the substrates were Ar1-ion sputtered in order toobtain UPS spectra without features from impurities or asorbed overlayers. During measurements, a bias voltag24.00 V was applied to the sampleholder to clear the lokinetic-energy cutoff of the spectrum. The oligomers wesynthesized and purified as described elsewhere.4,5 Theoligomer/metal interfaces were formed by slow molecubeam deposition monitored by a calibrated thickness motor, after degassing the effusion cell for several hours120 °C. Typical sublimation temperatures range from 225275 °C and the pressure ranges from 0.531027 to 231027 mbar, giving a deposition rate of several monolayeper minute. The oligomer film thickness used in determinthe energy level offset was typically 5–10 nm.Figure 1 illustrates how an interfacial energy diagraincluding the presence of an electric field, is deduced frtwo UPS spectra. The figure shows the He I UPS spectrAu ~left! and P5V4 on Au~right!. Both spectra were recorded while a bias voltage of24.00 V between sample ananalyzer was applied. The energy diagram of the interfacshown in the middle of Fig. 1. They axis represents thekinetic energy of the photoelectrons at the entrance slit of3 © 2000 American Institute of Physicssuoluacrrthrndtointo-o:t adeth:he-in-sst4aretheonheV4heioneeffin1lmcu-iftofa-isha-ralex-ageheethonmxiono-trumndthe–1d bye of2254 Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.analyzer. The total spectrum consists of photoelectronsfering no energy loss processes on their way out of the sproviding information on the binding energies of the occpied states of the solid, and scattered electrons which hsuffered energy loss which contributes to a structured baground. The high-kinetic-energy onset of the spectra cosponds to emission of electrons from the Fermi level ofmetal @Ekmax(Au)# or from the highest occupied moleculaorbital ~HOMO! of the oligomer@Ekmax(P5V4)#. The low-kinetic-energy cutoff@Ekmin(Au),Ekmin(P5V4)# correspondsto electrons having suffered energy loss processes and eup at the minimum kinetic energy in the solid requiredescape into the vacuum.Thus the position of the vacuum level of a materialthe diagram is determined by adding 21.2 eV to the cuenergy to giveEvac(Au) andEvac(P5V4). Now one can determine the work function of Au using:FAu5hn2@Ekmax~Au!2Ekmin~Au!# ~1!and the position of the HOMO level of P5V4 according tI s5hn2@Ekmax~P5V4!2Ekmin~P5V4!#. ~2!By comparison of the two UPS spectra, we can construcenergy diagram as shown in the middle of the plot andtermine the positions of the energy levels at both sides ofinterface. We can determine the difference (D) between thevacuum levels of the metal and the oligomer layer usingD5Ekmin~P5V4!2Ekmin~Au!. ~3!FIG. 1. Method for determining the energy diagram of the metal/oligominterface using the ultraviolet photoelectron spectra of the metal andoligomer. The spectra were recorded with a bias of24.0 V between sampleand analyzer. They axis represents the kinetic energy of the photoelectrat the entrance slit of the analyzer.~a! Photoemission of the Au substrate.~c!photoemission of 5–10 nm of P5V4 on the Au substrate.~b! Energy dia-gram of the Au/P5V4 interface.Ekmax(Au): maximum kinetic energy of aphotoelectron excited from Au withhn: photon energy~21.22 eV!;Ekmax(P5V4): maximum kinetic energy of a photoelectron excited froP5V4; Ekmin(Au): minimum kinetic energy of a scattered photoelectron ecited from Au; Evac(Au): vacuum level of Au;Ekmin(P5V4): minimumkinetic energy of a scattered photoelectron from P5V4;Evac(P5V4):vacuum level of P5V4;FAu : work function of Au;EHOMO : energy level ofhighest occupied molecular orbital of P5V4,evF : hole injection barrier or theenergy difference between the Fermi level of Au and the HOMO level;D:vacuum level shift. The structure of P5V4 is shown at the top, right.f-id-vek-e-eedffn-eSince the vacuum level of the oligomer is lower than tvacuum level of Au, the electric field points from the oligomer (d1) to the metal (d2), makingD,0 ~as defined inRef. 3!. In this case, the vacuum level shift leads to ancrease of the barrier for hole injection (ev8F) according to:ev8F5I s2FAu2D. ~4!This hinders hole injection from Au to P5V4, but facilitateelectron injection from the fermi level of Au to the loweunoccupied molecular orbital~LUMO!.In Fig. 2 we show the interface formation of MEH-P5Von a clean Ag substrate. In the center, the full spectragiven. The bottom spectrum shows the UP spectrum ofAg substrate with a thin layer of MEH-P5V4 evaporatedtop ~in the order of 1 nm!. The subsequent spectra show tphotoemission of the surface with increasing MEH-P5thickness~0.5–1 nm increase per deposition step!. The topline, with the largest offset, shows the Ag spectrum with tFermi edge well resolved~at hn2FAg1Vbias520.6 eV!. Atthe right-hand side, the gradual change in the HOMO regis depicted in more detail. As the film grows thicker, thfeatures arising from the Fermi level of Ag give way to thpure MEH-P5V4 spectrum. The low-kinetic-energy cutoregion is plotted at the left-hand side of Fig. 2. The shiftthe onset (D) is complete after depositing approximatelynm on the substrate, it does not increase further with fithickness. This result indicates that one or very few molelar layers are involved in the process.The upper two spectra of MEH-P5V4 show a small shof 0.1 eV towards lower kinetic energy. A slight chargingthe sample may cause this shift, which is within our mesurement accuracy. The lowering of the vacuum levelcaused by an electric field at the interface due to~partial!charge transfer in the interfacial region. The exact mecnism creating the electric field is so far unknown. Seveprocesses can cause an electric field at an interface, forample, electron transfer from a donor to an acceptor, imeffects, tailing of the electron cloud of the metal towards tres-FIG. 2. UPS spectrum of MEH-P5V4 on Ag as function of the deposittime ~thickness!. All spectra were recorded with a bias of24.0 V betweensample and analyzer. Thex axis represents the kinetic energy of the photelectrons at the entrance slit of the analyzer. The dotted line is the specfrom the polycrystalline Ag substrate. The plots on the right- and left-haside show an enlargement of the central plot. After each spectrum,thickness of the MEH-P5V4 layer was increased by approximately 0.5nm, except for the last two spectra where the thickness was increaseseveral nanometers. The inset in the middle plot shows the structurMEH-P5V4.,PVdinmlecinocioreudi-eaaitaroregoagoeseslrgetmeowu-rrierb-thetoaandlytheiertheee-ofrliensrchci-er,K., J.v.fev.2255Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.adsorbent causing dipoles or metal induced gap stateschemical interactions between metal and overlayer.6–8Charge transfer at the interface formed by similar Poligomers on a Ca substrate was observed by Parket al.1,2The charge transfer caused an overall energy level benof 0.5 eV in a space-charge region extending over 10The energy level bending in their experiment indicates etron transfer from the metal to the organic overlayer makD.0, which is opposite to the values reported here.Image potential effects lower the ionization energyinsulators and semiconductors adsorbed on metal surfaThis stabilizing effect is linearly proportional to 1/d wheredis the distance between the metal surface and the photoized molecule.9,10 Such a distance dependence is notflected in a dependence on the film thickness in this st~see Fig. 2!, indicating that the image potential effect is ether small for even the thinnest layer measured or compsated for by another electro-static effect. Chemical intertions at the interface formed by evaporating Al onsexithiophene layer have been reported3 with a significantcharge transfer from the metal to the oligomer, resultingthe formation of a bond between the oligomer and the meSuch a charge transfer causes an electric field pointing fthe metal towards the organic layer (D.0). For mostmetal–organic interfaces the dipole is negative~s e for ex-ample Fig. 15 in Ref. 6!, similar to our measurements.Table I summarizes the results of the UPS measuments. The shift between vacuum levels at the interfacnegative in all cases, moving the energy levels of the olimer downward relative to the levels of the metal. The mnitude of the shift is larger for Au than for Ag and alsdepends on the oligomer: the substituted oligomer causlarger shift. The ionization energy seems to be independof the metal substrate and compares well with previoureported values for similar P5V4s.11 The hole injection bar-rier can be obtained in two ways: by measuring the enedifference between the high kinetic energy onset of the mTABLE I. UPS measurements on P5V4 and MEH-P5V4 (FAu55.160.1 eV andFAg54.460.1 eV!. D represents the vacuum level shift;I s ,ionization energy;evF , measured hole injection barrier;ev8F , calculated holeinjection barrier using Eq.~4!.Interface D ~eV! I s ~eV! evF ~eV! ev8F ~eV!P5V4 / Au 21.060.1 5.6 0.1 1.460.1 1.560.3P5V4 / Ag 20.460.1 5.6 0.1 1.760.1 1.6 0.3MEH-P5V4 / Au 21.260.1 5.260.1 1.260.1 1.360.3MEH-P5V4 / Ag 20.560.1 5.360.2 1.360.2 1.460.4orng.-gfes.n--yn-c-nl.me-is--antyyaland the oligomer, as shown in Fig. 1 (evF), or by using Eq.~4! (ev8F). The differences between the values for the sainjection barrier~see column 4 and 5 in the table! give anindication of the systematic error. The values clearly shthat the misalignment of the vacuum levels strongly inflences the hole injection barrier and acts as to keep the baalmost independent of the work function of the metal sustrate.In conclusion, we reported UPS measurements ofinterface formed by evaporating PPV-like oligomers onmetals ~Ag and Au!. We found for all these interfacesmisalignment between the vacuum levels of the metalthe organic overlayer. This shift of levels, presumabcaused by an interfacial dipole layer, strongly influenceshole injection barrier in such a way as to keep this barrnearly constant and therefore at most weakly sensitive towork function of the metal or the ionization potential of tholigomer. Knowledge of this interfacial dipole layer is therfore crucial for understanding the electrical characteristicsoligomer-based electronic devices.The authors are grateful to J. Wildeman and M. Sefor providing P5V4, P. F. van Hutten for many discussioand J. Kappenburg for his technical support. This reseawas financially supported by the Dutch Organization for Sentific Research~NWO-CW! and the TMR program of theEC.1Y. Park, V. Choong, E. Ettedgui, Y. Gao, B. R. Hsieh, T. Wehrmeistand K. Müllen, Appl. Phys. Lett.69, 1080~1996!.2Y. Park, V.-E. Choong, B. R. Hsieh, C. W. Tang, T. Wehrmeister,Müllen, and Y. Gao, J. Vac. Sci. Technol. A15, 2574~1997!.3W. R. Salaneck, S. Stafstro¨m, and J.-L. Bre´das, inConjugated PolymerSurfaces and Interfaces~Cambridge University Press, New York, 1996!.4P. F. van Hutten, J. Wildemann, A. Meetsma, and G. HadziioannouAm. Chem. Soc.121, 5910~1999!.5L. Ouali, V. V. Krasnikov, U. Stalmach, and G. Hadziiouannou, AdMater.11, 1515~1999!.6H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater.11, 605~1999! andreferences therein.7W. Mönch, in Semiconductor Surfaces and Interfaces, Springer series inSurface Sciences 26~Springer, Berlin 1993!.8J. Hölzel and F. K. Schulte,Solid Surface Physics, Work Function oMetals in Springer Tracts in Modern Physics~Springer, Berlin, 1979!,Vol. 85.9R. Hesper, L. H. Tjeng, and G. A. Sawatzky, Europhys. Lett.40, 177~1997!.10G. Kaindl, T.-C. Chang, D. E. Eastman, and F. J. Himpsel, Phys. RLett. 45, 1808~1980!.11A. Schmidt, M. L. Anderson, D. Dunphy, T. Wehrmeister, K. Mu¨llen, andN. R. Armstrong, Adv. Mater.7, 722 ~1995!.

English

   University of GroningenEnergy level alignment at the conjugated phenylenevinylene oligomer/metal interfaceVeenstra, S. C.; Stalmach, U.; Krasnikov, V. V.; Hadziioannou, G.; Jonkman, H. T.; Heeres,A.; Sawatzky, G. A.Published in:Applied Physics LettersDOI:10.1063/1.126312IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Veenstra, S. C., Stalmach, U., Krasnikov, V. V., Hadziioannou, G., Jonkman, H. T., Heeres, A., &Sawatzky, G. A. (2000). Energy level alignment at the conjugated phenylenevinylene oligomer/metalinterface. Applied Physics Letters, 76(16), 2253 - 2255. https://doi.org/10.1063/1.126312CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 25-04-2023APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 16 17 APRIL 2000Energy level alignment at the conjugated phenylenevinyleneoligomer Õmetal interfaceS. C. Veenstra, U. Stalmach, V. V. Krasnikov, and G. Hadziioannoua)Department of Polymer Chemistry, Materials Science Centre, University of Groningen, Nijenborgh 4,9747 AG Groningen, The NetherlandsH. T. Jonkman, A. Heeres, and G. A. SawatzkyLaboratory of Solid State and Applied Physics, Materials Science Centre, University of Groningen,Nijenborgh 4, 9747 AG Groningen, The Netherlands~Received 19 November 1999; accepted for publication 17 February 2000!In this letter we report an investigation of the interfacial electronic structure formed by metals andconjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metalsubstrates for two five-ring phenylenevinylene oligomers: unsubstitutedp-bis@(p-styryl)styryl#benzene~P5V4! and the analogous oligomer with 2-methoxy-5-~2’-ethyl-hexyloxy!substitution on the central ring~MEH-P5V4!. We found for all interfaces a lowering of the energylevels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably dueto interface dipole layers, was always such as to keep the hole injection barrier nearly constant andtherefore at most weakly sensitive to the work function of the metal or the ionization potential of theoligomer. © 2000 American Institute of Physics.@S0003-6951~00!01216-X#nehtaycisveelluuoaghtaeacpi-dthrolwigthooll asaythe.-fionyzertesathatesd-e ofw-relarni-attorsingm,oma of-de istheThe charge transfer process at the interface betweeelectrode and a polymeric semiconductor plays a key rolthe performance of optoelectric devices like polymer ligemitting diodes (pLEDs) and photovoltaic devices~PVDs!.These devices typically consist of one or more polymer lers sandwiched between two electrodes. The transfer proat the interface between the contact and the active layerfunction of the barrier height between the Fermi energy leof the electrode and the frontier orbital levels of the polymlayer. In order to estimate the injection barrier one usuaassumes a rigid band model and an alignment of the vaclevels at the interface. However, these assumptions ignthe possible formation of dipole layers or covalent bondsthe interface and doping of the interfacial region. Althoumeasurements of the vacuum level alignment are imporfor understanding device behavior and optimizing deviceficiency, they have seldom been reported for the interfbetween metals and phenylenevinylene oligomers.1–3Here we report ultraviolet photoelectron spectrosco~UPS! measurements on the interfaces between metals~Auand Ag! and two different five-ring phenylenevinylene olgomers ~P5V4’s!, unsubstituted~P5V4! and 2-methoxy-5-~2’-ethyl-hexyloxy!-substituted P5V4~MEH-P5V4! ~see in-set Figs. 1 and 2, respectively!. These oligomers can be usefor optoelectronic applications and as model systems fortwo most widely used corresponding conjugated polymeThe advantage of using these oligomers instead of the pmers is that their high chemical purity and relatively lomass enables us to prepare thin filmsin situ by deposition ofthe oligomer on a cleaned metal surface under ultrahvacuum~UHV! conditions.In all cases we measured a misalignment betweenvacuum levels of the metal and the oligomer. The levelsthe oligomers shift downward relative to the energy levelsa!Electronic mail: hadzii@chem.rug.nl2250003-6951/2000/76(16)/2253/3/$17.00anin--essalrymretntf-eyes.y-heffthe metal~a negative shift!. The magnitude of this shift isbetween 0.4 and 1.2 eV, depending on the metal as weon the oligomer. The observed energy level alignment mhave a strong effect on the charge injection process atmetal/oligomer interface as it is found in LEDs and PVDsAll experiments were performed in a single UHV chamber with a base pressure of 1029 mbar equipped with a Hedischarge lamp (He I,hn521.22 eV) and an effusion cell oour own design. A hemispherical analyzer, in combinatwith a lens system, was used as electron energy analwith an overall resolution of 150 meV. The metal substrawere cleaned by polishing and washing in an ultrasonic bwith toluene and acetone as solvents. Next, the substrwere placed in a sampleholder on ax,y,z stage. Under UHVconditions, the substrates were Ar1-ion sputtered in order toobtain UPS spectra without features from impurities or asorbed overlayers. During measurements, a bias voltag24.00 V was applied to the sampleholder to clear the lokinetic-energy cutoff of the spectrum. The oligomers wesynthesized and purified as described elsewhere.4,5 Theoligomer/metal interfaces were formed by slow molecubeam deposition monitored by a calibrated thickness motor, after degassing the effusion cell for several hours120 °C. Typical sublimation temperatures range from 225275 °C and the pressure ranges from 0.531027 to 231027 mbar, giving a deposition rate of several monolayeper minute. The oligomer film thickness used in determinthe energy level offset was typically 5–10 nm.Figure 1 illustrates how an interfacial energy diagraincluding the presence of an electric field, is deduced frtwo UPS spectra. The figure shows the He I UPS spectrAu ~left! and P5V4 on Au~right!. Both spectra were recorded while a bias voltage of24.00 V between sample ananalyzer was applied. The energy diagram of the interfacshown in the middle of Fig. 1. They axis represents thekinetic energy of the photoelectrons at the entrance slit of3 © 2000 American Institute of Physicssuoluacrrthrndtointo-o:t adeth:he-in-sst4aretheonheV4heioneeffin1lmcu-iftofa-isha-ralex-ageheethonmxiono-trumndthe–1d bye of2254 Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.analyzer. The total spectrum consists of photoelectronsfering no energy loss processes on their way out of the sproviding information on the binding energies of the occpied states of the solid, and scattered electrons which hsuffered energy loss which contributes to a structured baground. The high-kinetic-energy onset of the spectra cosponds to emission of electrons from the Fermi level ofmetal @Ekmax(Au)# or from the highest occupied moleculaorbital ~HOMO! of the oligomer@Ekmax(P5V4)#. The low-kinetic-energy cutoff@Ekmin(Au),Ekmin(P5V4)# correspondsto electrons having suffered energy loss processes and eup at the minimum kinetic energy in the solid requiredescape into the vacuum.Thus the position of the vacuum level of a materialthe diagram is determined by adding 21.2 eV to the cuenergy to giveEvac(Au) andEvac(P5V4). Now one can determine the work function of Au using:FAu5hn2@Ekmax~Au!2Ekmin~Au!# ~1!and the position of the HOMO level of P5V4 according tI s5hn2@Ekmax~P5V4!2Ekmin~P5V4!#. ~2!By comparison of the two UPS spectra, we can construcenergy diagram as shown in the middle of the plot andtermine the positions of the energy levels at both sides ofinterface. We can determine the difference (D) between thevacuum levels of the metal and the oligomer layer usingD5Ekmin~P5V4!2Ekmin~Au!. ~3!FIG. 1. Method for determining the energy diagram of the metal/oligominterface using the ultraviolet photoelectron spectra of the metal andoligomer. The spectra were recorded with a bias of24.0 V between sampleand analyzer. They axis represents the kinetic energy of the photoelectrat the entrance slit of the analyzer.~a! Photoemission of the Au substrate.~c!photoemission of 5–10 nm of P5V4 on the Au substrate.~b! Energy dia-gram of the Au/P5V4 interface.Ekmax(Au): maximum kinetic energy of aphotoelectron excited from Au withhn: photon energy~21.22 eV!;Ekmax(P5V4): maximum kinetic energy of a photoelectron excited froP5V4; Ekmin(Au): minimum kinetic energy of a scattered photoelectron ecited from Au; Evac(Au): vacuum level of Au;Ekmin(P5V4): minimumkinetic energy of a scattered photoelectron from P5V4;Evac(P5V4):vacuum level of P5V4;FAu : work function of Au;EHOMO : energy level ofhighest occupied molecular orbital of P5V4,evF : hole injection barrier or theenergy difference between the Fermi level of Au and the HOMO level;D:vacuum level shift. The structure of P5V4 is shown at the top, right.f-id-vek-e-eedffn-eSince the vacuum level of the oligomer is lower than tvacuum level of Au, the electric field points from the oligomer (d1) to the metal (d2), makingD,0 ~as defined inRef. 3!. In this case, the vacuum level shift leads to ancrease of the barrier for hole injection (ev8F) according to:ev8F5I s2FAu2D. ~4!This hinders hole injection from Au to P5V4, but facilitateelectron injection from the fermi level of Au to the loweunoccupied molecular orbital~LUMO!.In Fig. 2 we show the interface formation of MEH-P5Von a clean Ag substrate. In the center, the full spectragiven. The bottom spectrum shows the UP spectrum ofAg substrate with a thin layer of MEH-P5V4 evaporatedtop ~in the order of 1 nm!. The subsequent spectra show tphotoemission of the surface with increasing MEH-P5thickness~0.5–1 nm increase per deposition step!. The topline, with the largest offset, shows the Ag spectrum with tFermi edge well resolved~at hn2FAg1Vbias520.6 eV!. Atthe right-hand side, the gradual change in the HOMO regis depicted in more detail. As the film grows thicker, thfeatures arising from the Fermi level of Ag give way to thpure MEH-P5V4 spectrum. The low-kinetic-energy cutoregion is plotted at the left-hand side of Fig. 2. The shiftthe onset (D) is complete after depositing approximatelynm on the substrate, it does not increase further with fithickness. This result indicates that one or very few molelar layers are involved in the process.The upper two spectra of MEH-P5V4 show a small shof 0.1 eV towards lower kinetic energy. A slight chargingthe sample may cause this shift, which is within our mesurement accuracy. The lowering of the vacuum levelcaused by an electric field at the interface due to~partial!charge transfer in the interfacial region. The exact mecnism creating the electric field is so far unknown. Seveprocesses can cause an electric field at an interface, forample, electron transfer from a donor to an acceptor, imeffects, tailing of the electron cloud of the metal towards tres-FIG. 2. UPS spectrum of MEH-P5V4 on Ag as function of the deposittime ~thickness!. All spectra were recorded with a bias of24.0 V betweensample and analyzer. Thex axis represents the kinetic energy of the photelectrons at the entrance slit of the analyzer. The dotted line is the specfrom the polycrystalline Ag substrate. The plots on the right- and left-haside show an enlargement of the central plot. After each spectrum,thickness of the MEH-P5V4 layer was increased by approximately 0.5nm, except for the last two spectra where the thickness was increaseseveral nanometers. The inset in the middle plot shows the structurMEH-P5V4.,PVdinmlecinocioreudi-eaaitaroregoagoeseslrgetmeowu-rrierb-thetoaandlytheiertheee-ofrliensrchci-er,K., J.v.fev.2255Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.adsorbent causing dipoles or metal induced gap stateschemical interactions between metal and overlayer.6–8Charge transfer at the interface formed by similar Poligomers on a Ca substrate was observed by Parket al.1,2The charge transfer caused an overall energy level benof 0.5 eV in a space-charge region extending over 10The energy level bending in their experiment indicates etron transfer from the metal to the organic overlayer makD.0, which is opposite to the values reported here.Image potential effects lower the ionization energyinsulators and semiconductors adsorbed on metal surfaThis stabilizing effect is linearly proportional to 1/d wheredis the distance between the metal surface and the photoized molecule.9,10 Such a distance dependence is notflected in a dependence on the film thickness in this st~see Fig. 2!, indicating that the image potential effect is ether small for even the thinnest layer measured or compsated for by another electro-static effect. Chemical intertions at the interface formed by evaporating Al onsexithiophene layer have been reported3 with a significantcharge transfer from the metal to the oligomer, resultingthe formation of a bond between the oligomer and the meSuch a charge transfer causes an electric field pointing fthe metal towards the organic layer (D.0). For mostmetal–organic interfaces the dipole is negative~s e for ex-ample Fig. 15 in Ref. 6!, similar to our measurements.Table I summarizes the results of the UPS measuments. The shift between vacuum levels at the interfacnegative in all cases, moving the energy levels of the olimer downward relative to the levels of the metal. The mnitude of the shift is larger for Au than for Ag and alsdepends on the oligomer: the substituted oligomer causlarger shift. The ionization energy seems to be independof the metal substrate and compares well with previoureported values for similar P5V4s.11 The hole injection bar-rier can be obtained in two ways: by measuring the enedifference between the high kinetic energy onset of the mTABLE I. UPS measurements on P5V4 and MEH-P5V4 (FAu55.160.1 eV andFAg54.460.1 eV!. D represents the vacuum level shift;I s ,ionization energy;evF , measured hole injection barrier;ev8F , calculated holeinjection barrier using Eq.~4!.Interface D ~eV! I s ~eV! evF ~eV! ev8F ~eV!P5V4 / Au 21.060.1 5.6 0.1 1.460.1 1.560.3P5V4 / Ag 20.460.1 5.6 0.1 1.760.1 1.6 0.3MEH-P5V4 / Au 21.260.1 5.260.1 1.260.1 1.360.3MEH-P5V4 / Ag 20.560.1 5.360.2 1.360.2 1.460.4orng.-gfes.n--yn-c-nl.me-is--antyyaland the oligomer, as shown in Fig. 1 (evF), or by using Eq.~4! (ev8F). The differences between the values for the sainjection barrier~see column 4 and 5 in the table! give anindication of the systematic error. The values clearly shthat the misalignment of the vacuum levels strongly inflences the hole injection barrier and acts as to keep the baalmost independent of the work function of the metal sustrate.In conclusion, we reported UPS measurements ofinterface formed by evaporating PPV-like oligomers onmetals ~Ag and Au!. We found for all these interfacesmisalignment between the vacuum levels of the metalthe organic overlayer. This shift of levels, presumabcaused by an interfacial dipole layer, strongly influenceshole injection barrier in such a way as to keep this barrnearly constant and therefore at most weakly sensitive towork function of the metal or the ionization potential of tholigomer. Knowledge of this interfacial dipole layer is therfore crucial for understanding the electrical characteristicsoligomer-based electronic devices.The authors are grateful to J. Wildeman and M. Sefor providing P5V4, P. F. van Hutten for many discussioand J. Kappenburg for his technical support. This reseawas financially supported by the Dutch Organization for Sentific Research~NWO-CW! and the TMR program of theEC.1Y. Park, V. Choong, E. Ettedgui, Y. Gao, B. R. Hsieh, T. Wehrmeistand K. Müllen, Appl. Phys. Lett.69, 1080~1996!.2Y. Park, V.-E. Choong, B. R. Hsieh, C. W. Tang, T. Wehrmeister,Müllen, and Y. Gao, J. Vac. Sci. Technol. A15, 2574~1997!.3W. R. Salaneck, S. Stafstro¨m, and J.-L. Bre´das, inConjugated PolymerSurfaces and Interfaces~Cambridge University Press, New York, 1996!.4P. F. van Hutten, J. Wildemann, A. Meetsma, and G. HadziioannouAm. Chem. Soc.121, 5910~1999!.5L. Ouali, V. V. Krasnikov, U. Stalmach, and G. Hadziiouannou, AdMater.11, 1515~1999!.6H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater.11, 605~1999! andreferences therein.7W. Mönch, in Semiconductor Surfaces and Interfaces, Springer series inSurface Sciences 26~Springer, Berlin 1993!.8J. Hölzel and F. K. Schulte,Solid Surface Physics, Work Function oMetals in Springer Tracts in Modern Physics~Springer, Berlin, 1979!,Vol. 85.9R. Hesper, L. H. Tjeng, and G. A. Sawatzky, Europhys. Lett.40, 177~1997!.10G. Kaindl, T.-C. Chang, D. E. Eastman, and F. J. Himpsel, Phys. RLett. 45, 1808~1980!.11A. Schmidt, M. L. Anderson, D. Dunphy, T. Wehrmeister, K. Mu¨llen, andN. R. Armstrong, Adv. Mater.7, 722 ~1995!.

Proceedings - University of Groningen

   University of GroningenEnergy level alignment at the conjugated phenylenevinylene oligomer/metal interfaceVeenstra, S. C.; Stalmach, U.; Krasnikov, V. V.; Hadziioannou, G.; Jonkman, H. T.; Heeres,A.; Sawatzky, G. A.Published in:Applied Physics LettersDOI:10.1063/1.126312IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Veenstra, S. C., Stalmach, U., Krasnikov, V. V., Hadziioannou, G., Jonkman, H. T., Heeres, A., &Sawatzky, G. A. (2000). Energy level alignment at the conjugated phenylenevinylene oligomer/metalinterface. Applied Physics Letters, 76(16), 2253 - 2255. https://doi.org/10.1063/1.126312CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 01-11-2023APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 16 17 APRIL 2000Energy level alignment at the conjugated phenylenevinyleneoligomer Õmetal interfaceS. C. Veenstra, U. Stalmach, V. V. Krasnikov, and G. Hadziioannoua)Department of Polymer Chemistry, Materials Science Centre, University of Groningen, Nijenborgh 4,9747 AG Groningen, The NetherlandsH. T. Jonkman, A. Heeres, and G. A. SawatzkyLaboratory of Solid State and Applied Physics, Materials Science Centre, University of Groningen,Nijenborgh 4, 9747 AG Groningen, The Netherlands~Received 19 November 1999; accepted for publication 17 February 2000!In this letter we report an investigation of the interfacial electronic structure formed by metals andconjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metalsubstrates for two five-ring phenylenevinylene oligomers: unsubstitutedp-bis@(p-styryl)styryl#benzene~P5V4! and the analogous oligomer with 2-methoxy-5-~2’-ethyl-hexyloxy!substitution on the central ring~MEH-P5V4!. We found for all interfaces a lowering of the energylevels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably dueto interface dipole layers, was always such as to keep the hole injection barrier nearly constant andtherefore at most weakly sensitive to the work function of the metal or the ionization potential of theoligomer. © 2000 American Institute of Physics.@S0003-6951~00!01216-X#nehtaycisveelluuoaghtaeacpi-dthrolwigthooll asaythe.-fionyzertesathatesd-e ofw-relarni-attorsingm,oma of-de istheThe charge transfer process at the interface betweeelectrode and a polymeric semiconductor plays a key rolthe performance of optoelectric devices like polymer ligemitting diodes (pLEDs) and photovoltaic devices~PVDs!.These devices typically consist of one or more polymer lers sandwiched between two electrodes. The transfer proat the interface between the contact and the active layerfunction of the barrier height between the Fermi energy leof the electrode and the frontier orbital levels of the polymlayer. In order to estimate the injection barrier one usuaassumes a rigid band model and an alignment of the vaclevels at the interface. However, these assumptions ignthe possible formation of dipole layers or covalent bondsthe interface and doping of the interfacial region. Althoumeasurements of the vacuum level alignment are imporfor understanding device behavior and optimizing deviceficiency, they have seldom been reported for the interfbetween metals and phenylenevinylene oligomers.1–3Here we report ultraviolet photoelectron spectrosco~UPS! measurements on the interfaces between metals~Auand Ag! and two different five-ring phenylenevinylene olgomers ~P5V4’s!, unsubstituted~P5V4! and 2-methoxy-5-~2’-ethyl-hexyloxy!-substituted P5V4~MEH-P5V4! ~see in-set Figs. 1 and 2, respectively!. These oligomers can be usefor optoelectronic applications and as model systems fortwo most widely used corresponding conjugated polymeThe advantage of using these oligomers instead of the pmers is that their high chemical purity and relatively lomass enables us to prepare thin filmsin situ by deposition ofthe oligomer on a cleaned metal surface under ultrahvacuum~UHV! conditions.In all cases we measured a misalignment betweenvacuum levels of the metal and the oligomer. The levelsthe oligomers shift downward relative to the energy levelsa!Electronic mail: hadzii@chem.rug.nl2250003-6951/2000/76(16)/2253/3/$17.00anin--essalrymretntf-eyes.y-heffthe metal~a negative shift!. The magnitude of this shift isbetween 0.4 and 1.2 eV, depending on the metal as weon the oligomer. The observed energy level alignment mhave a strong effect on the charge injection process atmetal/oligomer interface as it is found in LEDs and PVDsAll experiments were performed in a single UHV chamber with a base pressure of 1029 mbar equipped with a Hedischarge lamp (He I,hn521.22 eV) and an effusion cell oour own design. A hemispherical analyzer, in combinatwith a lens system, was used as electron energy analwith an overall resolution of 150 meV. The metal substrawere cleaned by polishing and washing in an ultrasonic bwith toluene and acetone as solvents. Next, the substrwere placed in a sampleholder on ax,y,z stage. Under UHVconditions, the substrates were Ar1-ion sputtered in order toobtain UPS spectra without features from impurities or asorbed overlayers. During measurements, a bias voltag24.00 V was applied to the sampleholder to clear the lokinetic-energy cutoff of the spectrum. The oligomers wesynthesized and purified as described elsewhere.4,5 Theoligomer/metal interfaces were formed by slow molecubeam deposition monitored by a calibrated thickness motor, after degassing the effusion cell for several hours120 °C. Typical sublimation temperatures range from 225275 °C and the pressure ranges from 0.531027 to 231027 mbar, giving a deposition rate of several monolayeper minute. The oligomer film thickness used in determinthe energy level offset was typically 5–10 nm.Figure 1 illustrates how an interfacial energy diagraincluding the presence of an electric field, is deduced frtwo UPS spectra. The figure shows the He I UPS spectrAu ~left! and P5V4 on Au~right!. Both spectra were recorded while a bias voltage of24.00 V between sample ananalyzer was applied. The energy diagram of the interfacshown in the middle of Fig. 1. They axis represents thekinetic energy of the photoelectrons at the entrance slit of3 © 2000 American Institute of Physicssuoluacrrthrndtointo-o:t adeth:he-in-sst4aretheonheV4heioneeffin1lmcu-iftofa-isha-ralex-ageheethonmxiono-trumndthe–1d bye of2254 Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.analyzer. The total spectrum consists of photoelectronsfering no energy loss processes on their way out of the sproviding information on the binding energies of the occpied states of the solid, and scattered electrons which hsuffered energy loss which contributes to a structured baground. The high-kinetic-energy onset of the spectra cosponds to emission of electrons from the Fermi level ofmetal @Ekmax(Au)# or from the highest occupied moleculaorbital ~HOMO! of the oligomer@Ekmax(P5V4)#. The low-kinetic-energy cutoff@Ekmin(Au),Ekmin(P5V4)# correspondsto electrons having suffered energy loss processes and eup at the minimum kinetic energy in the solid requiredescape into the vacuum.Thus the position of the vacuum level of a materialthe diagram is determined by adding 21.2 eV to the cuenergy to giveEvac(Au) andEvac(P5V4). Now one can determine the work function of Au using:FAu5hn2@Ekmax~Au!2Ekmin~Au!# ~1!and the position of the HOMO level of P5V4 according tI s5hn2@Ekmax~P5V4!2Ekmin~P5V4!#. ~2!By comparison of the two UPS spectra, we can construcenergy diagram as shown in the middle of the plot andtermine the positions of the energy levels at both sides ofinterface. We can determine the difference (D) between thevacuum levels of the metal and the oligomer layer usingD5Ekmin~P5V4!2Ekmin~Au!. ~3!FIG. 1. Method for determining the energy diagram of the metal/oligominterface using the ultraviolet photoelectron spectra of the metal andoligomer. The spectra were recorded with a bias of24.0 V between sampleand analyzer. They axis represents the kinetic energy of the photoelectrat the entrance slit of the analyzer.~a! Photoemission of the Au substrate.~c!photoemission of 5–10 nm of P5V4 on the Au substrate.~b! Energy dia-gram of the Au/P5V4 interface.Ekmax(Au): maximum kinetic energy of aphotoelectron excited from Au withhn: photon energy~21.22 eV!;Ekmax(P5V4): maximum kinetic energy of a photoelectron excited froP5V4; Ekmin(Au): minimum kinetic energy of a scattered photoelectron ecited from Au; Evac(Au): vacuum level of Au;Ekmin(P5V4): minimumkinetic energy of a scattered photoelectron from P5V4;Evac(P5V4):vacuum level of P5V4;FAu : work function of Au;EHOMO : energy level ofhighest occupied molecular orbital of P5V4,evF : hole injection barrier or theenergy difference between the Fermi level of Au and the HOMO level;D:vacuum level shift. The structure of P5V4 is shown at the top, right.f-id-vek-e-eedffn-eSince the vacuum level of the oligomer is lower than tvacuum level of Au, the electric field points from the oligomer (d1) to the metal (d2), makingD,0 ~as defined inRef. 3!. In this case, the vacuum level shift leads to ancrease of the barrier for hole injection (ev8F) according to:ev8F5I s2FAu2D. ~4!This hinders hole injection from Au to P5V4, but facilitateelectron injection from the fermi level of Au to the loweunoccupied molecular orbital~LUMO!.In Fig. 2 we show the interface formation of MEH-P5Von a clean Ag substrate. In the center, the full spectragiven. The bottom spectrum shows the UP spectrum ofAg substrate with a thin layer of MEH-P5V4 evaporatedtop ~in the order of 1 nm!. The subsequent spectra show tphotoemission of the surface with increasing MEH-P5thickness~0.5–1 nm increase per deposition step!. The topline, with the largest offset, shows the Ag spectrum with tFermi edge well resolved~at hn2FAg1Vbias520.6 eV!. Atthe right-hand side, the gradual change in the HOMO regis depicted in more detail. As the film grows thicker, thfeatures arising from the Fermi level of Ag give way to thpure MEH-P5V4 spectrum. The low-kinetic-energy cutoregion is plotted at the left-hand side of Fig. 2. The shiftthe onset (D) is complete after depositing approximatelynm on the substrate, it does not increase further with fithickness. This result indicates that one or very few molelar layers are involved in the process.The upper two spectra of MEH-P5V4 show a small shof 0.1 eV towards lower kinetic energy. A slight chargingthe sample may cause this shift, which is within our mesurement accuracy. The lowering of the vacuum levelcaused by an electric field at the interface due to~partial!charge transfer in the interfacial region. The exact mecnism creating the electric field is so far unknown. Seveprocesses can cause an electric field at an interface, forample, electron transfer from a donor to an acceptor, imeffects, tailing of the electron cloud of the metal towards tres-FIG. 2. UPS spectrum of MEH-P5V4 on Ag as function of the deposittime ~thickness!. All spectra were recorded with a bias of24.0 V betweensample and analyzer. Thex axis represents the kinetic energy of the photelectrons at the entrance slit of the analyzer. The dotted line is the specfrom the polycrystalline Ag substrate. The plots on the right- and left-haside show an enlargement of the central plot. After each spectrum,thickness of the MEH-P5V4 layer was increased by approximately 0.5nm, except for the last two spectra where the thickness was increaseseveral nanometers. The inset in the middle plot shows the structurMEH-P5V4.,PVdinmlecinocioreudi-eaaitaroregoagoeseslrgetmeowu-rrierb-thetoaandlytheiertheee-ofrliensrchci-er,K., J.v.fev.2255Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.adsorbent causing dipoles or metal induced gap stateschemical interactions between metal and overlayer.6–8Charge transfer at the interface formed by similar Poligomers on a Ca substrate was observed by Parket al.1,2The charge transfer caused an overall energy level benof 0.5 eV in a space-charge region extending over 10The energy level bending in their experiment indicates etron transfer from the metal to the organic overlayer makD.0, which is opposite to the values reported here.Image potential effects lower the ionization energyinsulators and semiconductors adsorbed on metal surfaThis stabilizing effect is linearly proportional to 1/d wheredis the distance between the metal surface and the photoized molecule.9,10 Such a distance dependence is notflected in a dependence on the film thickness in this st~see Fig. 2!, indicating that the image potential effect is ether small for even the thinnest layer measured or compsated for by another electro-static effect. Chemical intertions at the interface formed by evaporating Al onsexithiophene layer have been reported3 with a significantcharge transfer from the metal to the oligomer, resultingthe formation of a bond between the oligomer and the meSuch a charge transfer causes an electric field pointing fthe metal towards the organic layer (D.0). For mostmetal–organic interfaces the dipole is negative~s e for ex-ample Fig. 15 in Ref. 6!, similar to our measurements.Table I summarizes the results of the UPS measuments. The shift between vacuum levels at the interfacnegative in all cases, moving the energy levels of the olimer downward relative to the levels of the metal. The mnitude of the shift is larger for Au than for Ag and alsdepends on the oligomer: the substituted oligomer causlarger shift. The ionization energy seems to be independof the metal substrate and compares well with previoureported values for similar P5V4s.11 The hole injection bar-rier can be obtained in two ways: by measuring the enedifference between the high kinetic energy onset of the mTABLE I. UPS measurements on P5V4 and MEH-P5V4 (FAu55.160.1 eV andFAg54.460.1 eV!. D represents the vacuum level shift;I s ,ionization energy;evF , measured hole injection barrier;ev8F , calculated holeinjection barrier using Eq.~4!.Interface D ~eV! I s ~eV! evF ~eV! ev8F ~eV!P5V4 / Au 21.060.1 5.6 0.1 1.460.1 1.560.3P5V4 / Ag 20.460.1 5.6 0.1 1.760.1 1.6 0.3MEH-P5V4 / Au 21.260.1 5.260.1 1.260.1 1.360.3MEH-P5V4 / Ag 20.560.1 5.360.2 1.360.2 1.460.4orng.-gfes.n--yn-c-nl.me-is--antyyaland the oligomer, as shown in Fig. 1 (evF), or by using Eq.~4! (ev8F). The differences between the values for the sainjection barrier~see column 4 and 5 in the table! give anindication of the systematic error. The values clearly shthat the misalignment of the vacuum levels strongly inflences the hole injection barrier and acts as to keep the baalmost independent of the work function of the metal sustrate.In conclusion, we reported UPS measurements ofinterface formed by evaporating PPV-like oligomers onmetals ~Ag and Au!. We found for all these interfacesmisalignment between the vacuum levels of the metalthe organic overlayer. This shift of levels, presumabcaused by an interfacial dipole layer, strongly influenceshole injection barrier in such a way as to keep this barrnearly constant and therefore at most weakly sensitive towork function of the metal or the ionization potential of tholigomer. Knowledge of this interfacial dipole layer is therfore crucial for understanding the electrical characteristicsoligomer-based electronic devices.The authors are grateful to J. Wildeman and M. Sefor providing P5V4, P. F. van Hutten for many discussioand J. Kappenburg for his technical support. This reseawas financially supported by the Dutch Organization for Sentific Research~NWO-CW! and the TMR program of theEC.1Y. Park, V. Choong, E. Ettedgui, Y. Gao, B. R. Hsieh, T. Wehrmeistand K. Müllen, Appl. Phys. Lett.69, 1080~1996!.2Y. Park, V.-E. Choong, B. R. Hsieh, C. W. Tang, T. Wehrmeister,Müllen, and Y. Gao, J. Vac. Sci. Technol. A15, 2574~1997!.3W. R. Salaneck, S. Stafstro¨m, and J.-L. Bre´das, inConjugated PolymerSurfaces and Interfaces~Cambridge University Press, New York, 1996!.4P. F. van Hutten, J. Wildemann, A. Meetsma, and G. HadziioannouAm. Chem. Soc.121, 5910~1999!.5L. Ouali, V. V. Krasnikov, U. Stalmach, and G. Hadziiouannou, AdMater.11, 1515~1999!.6H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater.11, 605~1999! andreferences therein.7W. Mönch, in Semiconductor Surfaces and Interfaces, Springer series inSurface Sciences 26~Springer, Berlin 1993!.8J. Hölzel and F. K. Schulte,Solid Surface Physics, Work Function oMetals in Springer Tracts in Modern Physics~Springer, Berlin, 1979!,Vol. 85.9R. Hesper, L. H. Tjeng, and G. A. Sawatzky, Europhys. Lett.40, 177~1997!.10G. Kaindl, T.-C. Chang, D. E. Eastman, and F. J. Himpsel, Phys. RLett. 45, 1808~1980!.11A. Schmidt, M. L. Anderson, D. Dunphy, T. Wehrmeister, K. Mu¨llen, andN. R. Armstrong, Adv. Mater.7, 722 ~1995!.

NARCIS 

   University of GroningenEnergy level alignment at the conjugated phenylenevinylene oligomer/metal interfaceVeenstra, S. C.; Stalmach, U.; Krasnikov, V. V.; Hadziioannou, G.; Jonkman, H. T.; Heeres,A.; Sawatzky, G. A.Published in:Applied Physics LettersDOI:10.1063/1.126312IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Veenstra, S. C., Stalmach, U., Krasnikov, V. V., Hadziioannou, G., Jonkman, H. T., Heeres, A., &Sawatzky, G. A. (2000). Energy level alignment at the conjugated phenylenevinylene oligomer/metalinterface. Applied Physics Letters, 76(16), 2253 - 2255. https://doi.org/10.1063/1.126312CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 02-06-2022APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 16 17 APRIL 2000Energy level alignment at the conjugated phenylenevinyleneoligomer Õmetal interfaceS. C. Veenstra, U. Stalmach, V. V. Krasnikov, and G. Hadziioannoua)Department of Polymer Chemistry, Materials Science Centre, University of Groningen, Nijenborgh 4,9747 AG Groningen, The NetherlandsH. T. Jonkman, A. Heeres, and G. A. SawatzkyLaboratory of Solid State and Applied Physics, Materials Science Centre, University of Groningen,Nijenborgh 4, 9747 AG Groningen, The Netherlands~Received 19 November 1999; accepted for publication 17 February 2000!In this letter we report an investigation of the interfacial electronic structure formed by metals andconjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metalsubstrates for two five-ring phenylenevinylene oligomers: unsubstitutedp-bis@(p-styryl)styryl#benzene~P5V4! and the analogous oligomer with 2-methoxy-5-~2’-ethyl-hexyloxy!substitution on the central ring~MEH-P5V4!. We found for all interfaces a lowering of the energylevels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably dueto interface dipole layers, was always such as to keep the hole injection barrier nearly constant andtherefore at most weakly sensitive to the work function of the metal or the ionization potential of theoligomer. © 2000 American Institute of Physics.@S0003-6951~00!01216-X#nehtaycisveelluuoaghtaeacpi-dthrolwigthooll asaythe.-fionyzertesathatesd-e ofw-relarni-attorsingm,oma of-de istheThe charge transfer process at the interface betweeelectrode and a polymeric semiconductor plays a key rolthe performance of optoelectric devices like polymer ligemitting diodes (pLEDs) and photovoltaic devices~PVDs!.These devices typically consist of one or more polymer lers sandwiched between two electrodes. The transfer proat the interface between the contact and the active layerfunction of the barrier height between the Fermi energy leof the electrode and the frontier orbital levels of the polymlayer. In order to estimate the injection barrier one usuaassumes a rigid band model and an alignment of the vaclevels at the interface. However, these assumptions ignthe possible formation of dipole layers or covalent bondsthe interface and doping of the interfacial region. Althoumeasurements of the vacuum level alignment are imporfor understanding device behavior and optimizing deviceficiency, they have seldom been reported for the interfbetween metals and phenylenevinylene oligomers.1–3Here we report ultraviolet photoelectron spectrosco~UPS! measurements on the interfaces between metals~Auand Ag! and two different five-ring phenylenevinylene olgomers ~P5V4’s!, unsubstituted~P5V4! and 2-methoxy-5-~2’-ethyl-hexyloxy!-substituted P5V4~MEH-P5V4! ~see in-set Figs. 1 and 2, respectively!. These oligomers can be usefor optoelectronic applications and as model systems fortwo most widely used corresponding conjugated polymeThe advantage of using these oligomers instead of the pmers is that their high chemical purity and relatively lomass enables us to prepare thin filmsin situ by deposition ofthe oligomer on a cleaned metal surface under ultrahvacuum~UHV! conditions.In all cases we measured a misalignment betweenvacuum levels of the metal and the oligomer. The levelsthe oligomers shift downward relative to the energy levelsa!Electronic mail: hadzii@chem.rug.nl2250003-6951/2000/76(16)/2253/3/$17.00anin--essalrymretntf-eyes.y-heffthe metal~a negative shift!. The magnitude of this shift isbetween 0.4 and 1.2 eV, depending on the metal as weon the oligomer. The observed energy level alignment mhave a strong effect on the charge injection process atmetal/oligomer interface as it is found in LEDs and PVDsAll experiments were performed in a single UHV chamber with a base pressure of 1029 mbar equipped with a Hedischarge lamp (He I,hn521.22 eV) and an effusion cell oour own design. A hemispherical analyzer, in combinatwith a lens system, was used as electron energy analwith an overall resolution of 150 meV. The metal substrawere cleaned by polishing and washing in an ultrasonic bwith toluene and acetone as solvents. Next, the substrwere placed in a sampleholder on ax,y,z stage. Under UHVconditions, the substrates were Ar1-ion sputtered in order toobtain UPS spectra without features from impurities or asorbed overlayers. During measurements, a bias voltag24.00 V was applied to the sampleholder to clear the lokinetic-energy cutoff of the spectrum. The oligomers wesynthesized and purified as described elsewhere.4,5 Theoligomer/metal interfaces were formed by slow molecubeam deposition monitored by a calibrated thickness motor, after degassing the effusion cell for several hours120 °C. Typical sublimation temperatures range from 225275 °C and the pressure ranges from 0.531027 to 231027 mbar, giving a deposition rate of several monolayeper minute. The oligomer film thickness used in determinthe energy level offset was typically 5–10 nm.Figure 1 illustrates how an interfacial energy diagraincluding the presence of an electric field, is deduced frtwo UPS spectra. The figure shows the He I UPS spectrAu ~left! and P5V4 on Au~right!. Both spectra were recorded while a bias voltage of24.00 V between sample ananalyzer was applied. The energy diagram of the interfacshown in the middle of Fig. 1. They axis represents thekinetic energy of the photoelectrons at the entrance slit of3 © 2000 American Institute of Physicssuoluacrrthrndtointo-o:t adeth:he-in-sst4aretheonheV4heioneeffin1lmcu-iftofa-isha-ralex-ageheethonmxiono-trumndthe–1d bye of2254 Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.analyzer. The total spectrum consists of photoelectronsfering no energy loss processes on their way out of the sproviding information on the binding energies of the occpied states of the solid, and scattered electrons which hsuffered energy loss which contributes to a structured baground. The high-kinetic-energy onset of the spectra cosponds to emission of electrons from the Fermi level ofmetal @Ekmax(Au)# or from the highest occupied moleculaorbital ~HOMO! of the oligomer@Ekmax(P5V4)#. The low-kinetic-energy cutoff@Ekmin(Au),Ekmin(P5V4)# correspondsto electrons having suffered energy loss processes and eup at the minimum kinetic energy in the solid requiredescape into the vacuum.Thus the position of the vacuum level of a materialthe diagram is determined by adding 21.2 eV to the cuenergy to giveEvac(Au) andEvac(P5V4). Now one can determine the work function of Au using:FAu5hn2@Ekmax~Au!2Ekmin~Au!# ~1!and the position of the HOMO level of P5V4 according tI s5hn2@Ekmax~P5V4!2Ekmin~P5V4!#. ~2!By comparison of the two UPS spectra, we can construcenergy diagram as shown in the middle of the plot andtermine the positions of the energy levels at both sides ofinterface. We can determine the difference (D) between thevacuum levels of the metal and the oligomer layer usingD5Ekmin~P5V4!2Ekmin~Au!. ~3!FIG. 1. Method for determining the energy diagram of the metal/oligominterface using the ultraviolet photoelectron spectra of the metal andoligomer. The spectra were recorded with a bias of24.0 V between sampleand analyzer. They axis represents the kinetic energy of the photoelectrat the entrance slit of the analyzer.~a! Photoemission of the Au substrate.~c!photoemission of 5–10 nm of P5V4 on the Au substrate.~b! Energy dia-gram of the Au/P5V4 interface.Ekmax(Au): maximum kinetic energy of aphotoelectron excited from Au withhn: photon energy~21.22 eV!;Ekmax(P5V4): maximum kinetic energy of a photoelectron excited froP5V4; Ekmin(Au): minimum kinetic energy of a scattered photoelectron ecited from Au; Evac(Au): vacuum level of Au;Ekmin(P5V4): minimumkinetic energy of a scattered photoelectron from P5V4;Evac(P5V4):vacuum level of P5V4;FAu : work function of Au;EHOMO : energy level ofhighest occupied molecular orbital of P5V4,evF : hole injection barrier or theenergy difference between the Fermi level of Au and the HOMO level;D:vacuum level shift. The structure of P5V4 is shown at the top, right.f-id-vek-e-eedffn-eSince the vacuum level of the oligomer is lower than tvacuum level of Au, the electric field points from the oligomer (d1) to the metal (d2), makingD,0 ~as defined inRef. 3!. In this case, the vacuum level shift leads to ancrease of the barrier for hole injection (ev8F) according to:ev8F5I s2FAu2D. ~4!This hinders hole injection from Au to P5V4, but facilitateelectron injection from the fermi level of Au to the loweunoccupied molecular orbital~LUMO!.In Fig. 2 we show the interface formation of MEH-P5Von a clean Ag substrate. In the center, the full spectragiven. The bottom spectrum shows the UP spectrum ofAg substrate with a thin layer of MEH-P5V4 evaporatedtop ~in the order of 1 nm!. The subsequent spectra show tphotoemission of the surface with increasing MEH-P5thickness~0.5–1 nm increase per deposition step!. The topline, with the largest offset, shows the Ag spectrum with tFermi edge well resolved~at hn2FAg1Vbias520.6 eV!. Atthe right-hand side, the gradual change in the HOMO regis depicted in more detail. As the film grows thicker, thfeatures arising from the Fermi level of Ag give way to thpure MEH-P5V4 spectrum. The low-kinetic-energy cutoregion is plotted at the left-hand side of Fig. 2. The shiftthe onset (D) is complete after depositing approximatelynm on the substrate, it does not increase further with fithickness. This result indicates that one or very few molelar layers are involved in the process.The upper two spectra of MEH-P5V4 show a small shof 0.1 eV towards lower kinetic energy. A slight chargingthe sample may cause this shift, which is within our mesurement accuracy. The lowering of the vacuum levelcaused by an electric field at the interface due to~partial!charge transfer in the interfacial region. The exact mecnism creating the electric field is so far unknown. Seveprocesses can cause an electric field at an interface, forample, electron transfer from a donor to an acceptor, imeffects, tailing of the electron cloud of the metal towards tres-FIG. 2. UPS spectrum of MEH-P5V4 on Ag as function of the deposittime ~thickness!. All spectra were recorded with a bias of24.0 V betweensample and analyzer. Thex axis represents the kinetic energy of the photelectrons at the entrance slit of the analyzer. The dotted line is the specfrom the polycrystalline Ag substrate. The plots on the right- and left-haside show an enlargement of the central plot. After each spectrum,thickness of the MEH-P5V4 layer was increased by approximately 0.5nm, except for the last two spectra where the thickness was increaseseveral nanometers. The inset in the middle plot shows the structurMEH-P5V4.,PVdinmlecinocioreudi-eaaitaroregoagoeseslrgetmeowu-rrierb-thetoaandlytheiertheee-ofrliensrchci-er,K., J.v.fev.2255Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.adsorbent causing dipoles or metal induced gap stateschemical interactions between metal and overlayer.6–8Charge transfer at the interface formed by similar Poligomers on a Ca substrate was observed by Parket al.1,2The charge transfer caused an overall energy level benof 0.5 eV in a space-charge region extending over 10The energy level bending in their experiment indicates etron transfer from the metal to the organic overlayer makD.0, which is opposite to the values reported here.Image potential effects lower the ionization energyinsulators and semiconductors adsorbed on metal surfaThis stabilizing effect is linearly proportional to 1/d wheredis the distance between the metal surface and the photoized molecule.9,10 Such a distance dependence is notflected in a dependence on the film thickness in this st~see Fig. 2!, indicating that the image potential effect is ether small for even the thinnest layer measured or compsated for by another electro-static effect. Chemical intertions at the interface formed by evaporating Al onsexithiophene layer have been reported3 with a significantcharge transfer from the metal to the oligomer, resultingthe formation of a bond between the oligomer and the meSuch a charge transfer causes an electric field pointing fthe metal towards the organic layer (D.0). For mostmetal–organic interfaces the dipole is negative~s e for ex-ample Fig. 15 in Ref. 6!, similar to our measurements.Table I summarizes the results of the UPS measuments. The shift between vacuum levels at the interfacnegative in all cases, moving the energy levels of the olimer downward relative to the levels of the metal. The mnitude of the shift is larger for Au than for Ag and alsdepends on the oligomer: the substituted oligomer causlarger shift. The ionization energy seems to be independof the metal substrate and compares well with previoureported values for similar P5V4s.11 The hole injection bar-rier can be obtained in two ways: by measuring the enedifference between the high kinetic energy onset of the mTABLE I. UPS measurements on P5V4 and MEH-P5V4 (FAu55.160.1 eV andFAg54.460.1 eV!. D represents the vacuum level shift;I s ,ionization energy;evF , measured hole injection barrier;ev8F , calculated holeinjection barrier using Eq.~4!.Interface D ~eV! I s ~eV! evF ~eV! ev8F ~eV!P5V4 / Au 21.060.1 5.6 0.1 1.460.1 1.560.3P5V4 / Ag 20.460.1 5.6 0.1 1.760.1 1.6 0.3MEH-P5V4 / Au 21.260.1 5.260.1 1.260.1 1.360.3MEH-P5V4 / Ag 20.560.1 5.360.2 1.360.2 1.460.4orng.-gfes.n--yn-c-nl.me-is--antyyaland the oligomer, as shown in Fig. 1 (evF), or by using Eq.~4! (ev8F). The differences between the values for the sainjection barrier~see column 4 and 5 in the table! give anindication of the systematic error. The values clearly shthat the misalignment of the vacuum levels strongly inflences the hole injection barrier and acts as to keep the baalmost independent of the work function of the metal sustrate.In conclusion, we reported UPS measurements ofinterface formed by evaporating PPV-like oligomers onmetals ~Ag and Au!. We found for all these interfacesmisalignment between the vacuum levels of the metalthe organic overlayer. This shift of levels, presumabcaused by an interfacial dipole layer, strongly influenceshole injection barrier in such a way as to keep this barrnearly constant and therefore at most weakly sensitive towork function of the metal or the ionization potential of tholigomer. Knowledge of this interfacial dipole layer is therfore crucial for understanding the electrical characteristicsoligomer-based electronic devices.The authors are grateful to J. Wildeman and M. Sefor providing P5V4, P. F. van Hutten for many discussioand J. Kappenburg for his technical support. This reseawas financially supported by the Dutch Organization for Sentific Research~NWO-CW! and the TMR program of theEC.1Y. Park, V. Choong, E. Ettedgui, Y. Gao, B. R. Hsieh, T. Wehrmeistand K. Müllen, Appl. Phys. Lett.69, 1080~1996!.2Y. Park, V.-E. Choong, B. R. Hsieh, C. W. Tang, T. Wehrmeister,Müllen, and Y. Gao, J. Vac. Sci. Technol. A15, 2574~1997!.3W. R. Salaneck, S. Stafstro¨m, and J.-L. Bre´das, inConjugated PolymerSurfaces and Interfaces~Cambridge University Press, New York, 1996!.4P. F. van Hutten, J. Wildemann, A. Meetsma, and G. HadziioannouAm. Chem. Soc.121, 5910~1999!.5L. Ouali, V. V. Krasnikov, U. Stalmach, and G. Hadziiouannou, AdMater.11, 1515~1999!.6H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater.11, 605~1999! andreferences therein.7W. Mönch, in Semiconductor Surfaces and Interfaces, Springer series inSurface Sciences 26~Springer, Berlin 1993!.8J. Hölzel and F. K. Schulte,Solid Surface Physics, Work Function oMetals in Springer Tracts in Modern Physics~Springer, Berlin, 1979!,Vol. 85.9R. Hesper, L. H. Tjeng, and G. A. Sawatzky, Europhys. Lett.40, 177~1997!.10G. Kaindl, T.-C. Chang, D. E. Eastman, and F. J. Himpsel, Phys. RLett. 45, 1808~1980!.11A. Schmidt, M. L. Anderson, D. Dunphy, T. Wehrmeister, K. Mu¨llen, andN. R. Armstrong, Adv. Mater.7, 722 ~1995!.

   University of GroningenEnergy level alignment at the conjugated phenylenevinylene oligomer/metal interfaceVeenstra, S. C.; Stalmach, U.; Krasnikov, V. V.; Hadziioannou, G.; Jonkman, H. T.; Heeres,A.; Sawatzky, G. A.Published in:Applied Physics LettersDOI:10.1063/1.126312IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Veenstra, S. C., Stalmach, U., Krasnikov, V. V., Hadziioannou, G., Jonkman, H. T., Heeres, A., &Sawatzky, G. A. (2000). Energy level alignment at the conjugated phenylenevinylene oligomer/metalinterface. Applied Physics Letters, 76(16), 2253 - 2255. https://doi.org/10.1063/1.126312CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 03-06-2022APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 16 17 APRIL 2000Energy level alignment at the conjugated phenylenevinyleneoligomer Õmetal interfaceS. C. Veenstra, U. Stalmach, V. V. Krasnikov, and G. Hadziioannoua)Department of Polymer Chemistry, Materials Science Centre, University of Groningen, Nijenborgh 4,9747 AG Groningen, The NetherlandsH. T. Jonkman, A. Heeres, and G. A. SawatzkyLaboratory of Solid State and Applied Physics, Materials Science Centre, University of Groningen,Nijenborgh 4, 9747 AG Groningen, The Netherlands~Received 19 November 1999; accepted for publication 17 February 2000!In this letter we report an investigation of the interfacial electronic structure formed by metals andconjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metalsubstrates for two five-ring phenylenevinylene oligomers: unsubstitutedp-bis@(p-styryl)styryl#benzene~P5V4! and the analogous oligomer with 2-methoxy-5-~2’-ethyl-hexyloxy!substitution on the central ring~MEH-P5V4!. We found for all interfaces a lowering of the energylevels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably dueto interface dipole layers, was always such as to keep the hole injection barrier nearly constant andtherefore at most weakly sensitive to the work function of the metal or the ionization potential of theoligomer. © 2000 American Institute of Physics.@S0003-6951~00!01216-X#nehtaycisveelluuoaghtaeacpi-dthrolwigthooll asaythe.-fionyzertesathatesd-e ofw-relarni-attorsingm,oma of-de istheThe charge transfer process at the interface betweeelectrode and a polymeric semiconductor plays a key rolthe performance of optoelectric devices like polymer ligemitting diodes (pLEDs) and photovoltaic devices~PVDs!.These devices typically consist of one or more polymer lers sandwiched between two electrodes. The transfer proat the interface between the contact and the active layerfunction of the barrier height between the Fermi energy leof the electrode and the frontier orbital levels of the polymlayer. In order to estimate the injection barrier one usuaassumes a rigid band model and an alignment of the vaclevels at the interface. However, these assumptions ignthe possible formation of dipole layers or covalent bondsthe interface and doping of the interfacial region. Althoumeasurements of the vacuum level alignment are imporfor understanding device behavior and optimizing deviceficiency, they have seldom been reported for the interfbetween metals and phenylenevinylene oligomers.1–3Here we report ultraviolet photoelectron spectrosco~UPS! measurements on the interfaces between metals~Auand Ag! and two different five-ring phenylenevinylene olgomers ~P5V4’s!, unsubstituted~P5V4! and 2-methoxy-5-~2’-ethyl-hexyloxy!-substituted P5V4~MEH-P5V4! ~see in-set Figs. 1 and 2, respectively!. These oligomers can be usefor optoelectronic applications and as model systems fortwo most widely used corresponding conjugated polymeThe advantage of using these oligomers instead of the pmers is that their high chemical purity and relatively lomass enables us to prepare thin filmsin situ by deposition ofthe oligomer on a cleaned metal surface under ultrahvacuum~UHV! conditions.In all cases we measured a misalignment betweenvacuum levels of the metal and the oligomer. The levelsthe oligomers shift downward relative to the energy levelsa!Electronic mail: hadzii@chem.rug.nl2250003-6951/2000/76(16)/2253/3/$17.00anin--essalrymretntf-eyes.y-heffthe metal~a negative shift!. The magnitude of this shift isbetween 0.4 and 1.2 eV, depending on the metal as weon the oligomer. The observed energy level alignment mhave a strong effect on the charge injection process atmetal/oligomer interface as it is found in LEDs and PVDsAll experiments were performed in a single UHV chamber with a base pressure of 1029 mbar equipped with a Hedischarge lamp (He I,hn521.22 eV) and an effusion cell oour own design. A hemispherical analyzer, in combinatwith a lens system, was used as electron energy analwith an overall resolution of 150 meV. The metal substrawere cleaned by polishing and washing in an ultrasonic bwith toluene and acetone as solvents. Next, the substrwere placed in a sampleholder on ax,y,z stage. Under UHVconditions, the substrates were Ar1-ion sputtered in order toobtain UPS spectra without features from impurities or asorbed overlayers. During measurements, a bias voltag24.00 V was applied to the sampleholder to clear the lokinetic-energy cutoff of the spectrum. The oligomers wesynthesized and purified as described elsewhere.4,5 Theoligomer/metal interfaces were formed by slow molecubeam deposition monitored by a calibrated thickness motor, after degassing the effusion cell for several hours120 °C. Typical sublimation temperatures range from 225275 °C and the pressure ranges from 0.531027 to 231027 mbar, giving a deposition rate of several monolayeper minute. The oligomer film thickness used in determinthe energy level offset was typically 5–10 nm.Figure 1 illustrates how an interfacial energy diagraincluding the presence of an electric field, is deduced frtwo UPS spectra. The figure shows the He I UPS spectrAu ~left! and P5V4 on Au~right!. Both spectra were recorded while a bias voltage of24.00 V between sample ananalyzer was applied. The energy diagram of the interfacshown in the middle of Fig. 1. They axis represents thekinetic energy of the photoelectrons at the entrance slit of3 © 2000 American Institute of Physicssuoluacrrthrndtointo-o:t adeth:he-in-sst4aretheonheV4heioneeffin1lmcu-iftofa-isha-ralex-ageheethonmxiono-trumndthe–1d bye of2254 Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.analyzer. The total spectrum consists of photoelectronsfering no energy loss processes on their way out of the sproviding information on the binding energies of the occpied states of the solid, and scattered electrons which hsuffered energy loss which contributes to a structured baground. The high-kinetic-energy onset of the spectra cosponds to emission of electrons from the Fermi level ofmetal @Ekmax(Au)# or from the highest occupied moleculaorbital ~HOMO! of the oligomer@Ekmax(P5V4)#. The low-kinetic-energy cutoff@Ekmin(Au),Ekmin(P5V4)# correspondsto electrons having suffered energy loss processes and eup at the minimum kinetic energy in the solid requiredescape into the vacuum.Thus the position of the vacuum level of a materialthe diagram is determined by adding 21.2 eV to the cuenergy to giveEvac(Au) andEvac(P5V4). Now one can determine the work function of Au using:FAu5hn2@Ekmax~Au!2Ekmin~Au!# ~1!and the position of the HOMO level of P5V4 according tI s5hn2@Ekmax~P5V4!2Ekmin~P5V4!#. ~2!By comparison of the two UPS spectra, we can construcenergy diagram as shown in the middle of the plot andtermine the positions of the energy levels at both sides ofinterface. We can determine the difference (D) between thevacuum levels of the metal and the oligomer layer usingD5Ekmin~P5V4!2Ekmin~Au!. ~3!FIG. 1. Method for determining the energy diagram of the metal/oligominterface using the ultraviolet photoelectron spectra of the metal andoligomer. The spectra were recorded with a bias of24.0 V between sampleand analyzer. They axis represents the kinetic energy of the photoelectrat the entrance slit of the analyzer.~a! Photoemission of the Au substrate.~c!photoemission of 5–10 nm of P5V4 on the Au substrate.~b! Energy dia-gram of the Au/P5V4 interface.Ekmax(Au): maximum kinetic energy of aphotoelectron excited from Au withhn: photon energy~21.22 eV!;Ekmax(P5V4): maximum kinetic energy of a photoelectron excited froP5V4; Ekmin(Au): minimum kinetic energy of a scattered photoelectron ecited from Au; Evac(Au): vacuum level of Au;Ekmin(P5V4): minimumkinetic energy of a scattered photoelectron from P5V4;Evac(P5V4):vacuum level of P5V4;FAu : work function of Au;EHOMO : energy level ofhighest occupied molecular orbital of P5V4,evF : hole injection barrier or theenergy difference between the Fermi level of Au and the HOMO level;D:vacuum level shift. The structure of P5V4 is shown at the top, right.f-id-vek-e-eedffn-eSince the vacuum level of the oligomer is lower than tvacuum level of Au, the electric field points from the oligomer (d1) to the metal (d2), makingD,0 ~as defined inRef. 3!. In this case, the vacuum level shift leads to ancrease of the barrier for hole injection (ev8F) according to:ev8F5I s2FAu2D. ~4!This hinders hole injection from Au to P5V4, but facilitateelectron injection from the fermi level of Au to the loweunoccupied molecular orbital~LUMO!.In Fig. 2 we show the interface formation of MEH-P5Von a clean Ag substrate. In the center, the full spectragiven. The bottom spectrum shows the UP spectrum ofAg substrate with a thin layer of MEH-P5V4 evaporatedtop ~in the order of 1 nm!. The subsequent spectra show tphotoemission of the surface with increasing MEH-P5thickness~0.5–1 nm increase per deposition step!. The topline, with the largest offset, shows the Ag spectrum with tFermi edge well resolved~at hn2FAg1Vbias520.6 eV!. Atthe right-hand side, the gradual change in the HOMO regis depicted in more detail. As the film grows thicker, thfeatures arising from the Fermi level of Ag give way to thpure MEH-P5V4 spectrum. The low-kinetic-energy cutoregion is plotted at the left-hand side of Fig. 2. The shiftthe onset (D) is complete after depositing approximatelynm on the substrate, it does not increase further with fithickness. This result indicates that one or very few molelar layers are involved in the process.The upper two spectra of MEH-P5V4 show a small shof 0.1 eV towards lower kinetic energy. A slight chargingthe sample may cause this shift, which is within our mesurement accuracy. The lowering of the vacuum levelcaused by an electric field at the interface due to~partial!charge transfer in the interfacial region. The exact mecnism creating the electric field is so far unknown. Seveprocesses can cause an electric field at an interface, forample, electron transfer from a donor to an acceptor, imeffects, tailing of the electron cloud of the metal towards tres-FIG. 2. UPS spectrum of MEH-P5V4 on Ag as function of the deposittime ~thickness!. All spectra were recorded with a bias of24.0 V betweensample and analyzer. Thex axis represents the kinetic energy of the photelectrons at the entrance slit of the analyzer. The dotted line is the specfrom the polycrystalline Ag substrate. The plots on the right- and left-haside show an enlargement of the central plot. After each spectrum,thickness of the MEH-P5V4 layer was increased by approximately 0.5nm, except for the last two spectra where the thickness was increaseseveral nanometers. The inset in the middle plot shows the structurMEH-P5V4.,PVdinmlecinocioreudi-eaaitaroregoagoeseslrgetmeowu-rrierb-thetoaandlytheiertheee-ofrliensrchci-er,K., J.v.fev.2255Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.adsorbent causing dipoles or metal induced gap stateschemical interactions between metal and overlayer.6–8Charge transfer at the interface formed by similar Poligomers on a Ca substrate was observed by Parket al.1,2The charge transfer caused an overall energy level benof 0.5 eV in a space-charge region extending over 10The energy level bending in their experiment indicates etron transfer from the metal to the organic overlayer makD.0, which is opposite to the values reported here.Image potential effects lower the ionization energyinsulators and semiconductors adsorbed on metal surfaThis stabilizing effect is linearly proportional to 1/d wheredis the distance between the metal surface and the photoized molecule.9,10 Such a distance dependence is notflected in a dependence on the film thickness in this st~see Fig. 2!, indicating that the image potential effect is ether small for even the thinnest layer measured or compsated for by another electro-static effect. Chemical intertions at the interface formed by evaporating Al onsexithiophene layer have been reported3 with a significantcharge transfer from the metal to the oligomer, resultingthe formation of a bond between the oligomer and the meSuch a charge transfer causes an electric field pointing fthe metal towards the organic layer (D.0). For mostmetal–organic interfaces the dipole is negative~s e for ex-ample Fig. 15 in Ref. 6!, similar to our measurements.Table I summarizes the results of the UPS measuments. The shift between vacuum levels at the interfacnegative in all cases, moving the energy levels of the olimer downward relative to the levels of the metal. The mnitude of the shift is larger for Au than for Ag and alsdepends on the oligomer: the substituted oligomer causlarger shift. The ionization energy seems to be independof the metal substrate and compares well with previoureported values for similar P5V4s.11 The hole injection bar-rier can be obtained in two ways: by measuring the enedifference between the high kinetic energy onset of the mTABLE I. UPS measurements on P5V4 and MEH-P5V4 (FAu55.160.1 eV andFAg54.460.1 eV!. D represents the vacuum level shift;I s ,ionization energy;evF , measured hole injection barrier;ev8F , calculated holeinjection barrier using Eq.~4!.Interface D ~eV! I s ~eV! evF ~eV! ev8F ~eV!P5V4 / Au 21.060.1 5.6 0.1 1.460.1 1.560.3P5V4 / Ag 20.460.1 5.6 0.1 1.760.1 1.6 0.3MEH-P5V4 / Au 21.260.1 5.260.1 1.260.1 1.360.3MEH-P5V4 / Ag 20.560.1 5.360.2 1.360.2 1.460.4orng.-gfes.n--yn-c-nl.me-is--antyyaland the oligomer, as shown in Fig. 1 (evF), or by using Eq.~4! (ev8F). The differences between the values for the sainjection barrier~see column 4 and 5 in the table! give anindication of the systematic error. The values clearly shthat the misalignment of the vacuum levels strongly inflences the hole injection barrier and acts as to keep the baalmost independent of the work function of the metal sustrate.In conclusion, we reported UPS measurements ofinterface formed by evaporating PPV-like oligomers onmetals ~Ag and Au!. We found for all these interfacesmisalignment between the vacuum levels of the metalthe organic overlayer. This shift of levels, presumabcaused by an interfacial dipole layer, strongly influenceshole injection barrier in such a way as to keep this barrnearly constant and therefore at most weakly sensitive towork function of the metal or the ionization potential of tholigomer. Knowledge of this interfacial dipole layer is therfore crucial for understanding the electrical characteristicsoligomer-based electronic devices.The authors are grateful to J. Wildeman and M. Sefor providing P5V4, P. F. van Hutten for many discussioand J. Kappenburg for his technical support. This reseawas financially supported by the Dutch Organization for Sentific Research~NWO-CW! and the TMR program of theEC.1Y. Park, V. Choong, E. Ettedgui, Y. Gao, B. R. Hsieh, T. Wehrmeistand K. Müllen, Appl. Phys. Lett.69, 1080~1996!.2Y. Park, V.-E. Choong, B. R. Hsieh, C. W. Tang, T. Wehrmeister,Müllen, and Y. Gao, J. Vac. Sci. Technol. A15, 2574~1997!.3W. R. Salaneck, S. Stafstro¨m, and J.-L. Bre´das, inConjugated PolymerSurfaces and Interfaces~Cambridge University Press, New York, 1996!.4P. F. van Hutten, J. Wildemann, A. Meetsma, and G. HadziioannouAm. Chem. Soc.121, 5910~1999!.5L. Ouali, V. V. Krasnikov, U. Stalmach, and G. Hadziiouannou, AdMater.11, 1515~1999!.6H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater.11, 605~1999! andreferences therein.7W. Mönch, in Semiconductor Surfaces and Interfaces, Springer series inSurface Sciences 26~Springer, Berlin 1993!.8J. Hölzel and F. K. Schulte,Solid Surface Physics, Work Function oMetals in Springer Tracts in Modern Physics~Springer, Berlin, 1979!,Vol. 85.9R. Hesper, L. H. Tjeng, and G. A. Sawatzky, Europhys. Lett.40, 177~1997!.10G. Kaindl, T.-C. Chang, D. E. Eastman, and F. J. Himpsel, Phys. RLett. 45, 1808~1980!.11A. Schmidt, M. L. Anderson, D. Dunphy, T. Wehrmeister, K. Mu¨llen, andN. R. Armstrong, Adv. Mater.7, 722 ~1995!.

ARTS repository - University of Groningen

   University of GroningenEnergy level alignment at the conjugated phenylenevinylene oligomer/metal interfaceVeenstra, S. C.; Stalmach, U.; Krasnikov, V. V.; Hadziioannou, G.; Jonkman, H. T.; Heeres,A.; Sawatzky, G. A.Published in:Applied Physics LettersDOI:10.1063/1.126312IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Veenstra, S. C., Stalmach, U., Krasnikov, V. V., Hadziioannou, G., Jonkman, H. T., Heeres, A., &Sawatzky, G. A. (2000). Energy level alignment at the conjugated phenylenevinylene oligomer/metalinterface. Applied Physics Letters, 76(16), 2253 - 2255. https://doi.org/10.1063/1.126312CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-09-2023APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 16 17 APRIL 2000Energy level alignment at the conjugated phenylenevinyleneoligomer Õmetal interfaceS. C. Veenstra, U. Stalmach, V. V. Krasnikov, and G. Hadziioannoua)Department of Polymer Chemistry, Materials Science Centre, University of Groningen, Nijenborgh 4,9747 AG Groningen, The NetherlandsH. T. Jonkman, A. Heeres, and G. A. SawatzkyLaboratory of Solid State and Applied Physics, Materials Science Centre, University of Groningen,Nijenborgh 4, 9747 AG Groningen, The Netherlands~Received 19 November 1999; accepted for publication 17 February 2000!In this letter we report an investigation of the interfacial electronic structure formed by metals andconjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metalsubstrates for two five-ring phenylenevinylene oligomers: unsubstitutedp-bis@(p-styryl)styryl#benzene~P5V4! and the analogous oligomer with 2-methoxy-5-~2’-ethyl-hexyloxy!substitution on the central ring~MEH-P5V4!. We found for all interfaces a lowering of the energylevels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably dueto interface dipole layers, was always such as to keep the hole injection barrier nearly constant andtherefore at most weakly sensitive to the work function of the metal or the ionization potential of theoligomer. © 2000 American Institute of Physics.@S0003-6951~00!01216-X#nehtaycisveelluuoaghtaeacpi-dthrolwigthooll asaythe.-fionyzertesathatesd-e ofw-relarni-attorsingm,oma of-de istheThe charge transfer process at the interface betweeelectrode and a polymeric semiconductor plays a key rolthe performance of optoelectric devices like polymer ligemitting diodes (pLEDs) and photovoltaic devices~PVDs!.These devices typically consist of one or more polymer lers sandwiched between two electrodes. The transfer proat the interface between the contact and the active layerfunction of the barrier height between the Fermi energy leof the electrode and the frontier orbital levels of the polymlayer. In order to estimate the injection barrier one usuaassumes a rigid band model and an alignment of the vaclevels at the interface. However, these assumptions ignthe possible formation of dipole layers or covalent bondsthe interface and doping of the interfacial region. Althoumeasurements of the vacuum level alignment are imporfor understanding device behavior and optimizing deviceficiency, they have seldom been reported for the interfbetween metals and phenylenevinylene oligomers.1–3Here we report ultraviolet photoelectron spectrosco~UPS! measurements on the interfaces between metals~Auand Ag! and two different five-ring phenylenevinylene olgomers ~P5V4’s!, unsubstituted~P5V4! and 2-methoxy-5-~2’-ethyl-hexyloxy!-substituted P5V4~MEH-P5V4! ~see in-set Figs. 1 and 2, respectively!. These oligomers can be usefor optoelectronic applications and as model systems fortwo most widely used corresponding conjugated polymeThe advantage of using these oligomers instead of the pmers is that their high chemical purity and relatively lomass enables us to prepare thin filmsin situ by deposition ofthe oligomer on a cleaned metal surface under ultrahvacuum~UHV! conditions.In all cases we measured a misalignment betweenvacuum levels of the metal and the oligomer. The levelsthe oligomers shift downward relative to the energy levelsa!Electronic mail: hadzii@chem.rug.nl2250003-6951/2000/76(16)/2253/3/$17.00anin--essalrymretntf-eyes.y-heffthe metal~a negative shift!. The magnitude of this shift isbetween 0.4 and 1.2 eV, depending on the metal as weon the oligomer. The observed energy level alignment mhave a strong effect on the charge injection process atmetal/oligomer interface as it is found in LEDs and PVDsAll experiments were performed in a single UHV chamber with a base pressure of 1029 mbar equipped with a Hedischarge lamp (He I,hn521.22 eV) and an effusion cell oour own design. A hemispherical analyzer, in combinatwith a lens system, was used as electron energy analwith an overall resolution of 150 meV. The metal substrawere cleaned by polishing and washing in an ultrasonic bwith toluene and acetone as solvents. Next, the substrwere placed in a sampleholder on ax,y,z stage. Under UHVconditions, the substrates were Ar1-ion sputtered in order toobtain UPS spectra without features from impurities or asorbed overlayers. During measurements, a bias voltag24.00 V was applied to the sampleholder to clear the lokinetic-energy cutoff of the spectrum. The oligomers wesynthesized and purified as described elsewhere.4,5 Theoligomer/metal interfaces were formed by slow molecubeam deposition monitored by a calibrated thickness motor, after degassing the effusion cell for several hours120 °C. Typical sublimation temperatures range from 225275 °C and the pressure ranges from 0.531027 to 231027 mbar, giving a deposition rate of several monolayeper minute. The oligomer film thickness used in determinthe energy level offset was typically 5–10 nm.Figure 1 illustrates how an interfacial energy diagraincluding the presence of an electric field, is deduced frtwo UPS spectra. The figure shows the He I UPS spectrAu ~left! and P5V4 on Au~right!. Both spectra were recorded while a bias voltage of24.00 V between sample ananalyzer was applied. The energy diagram of the interfacshown in the middle of Fig. 1. They axis represents thekinetic energy of the photoelectrons at the entrance slit of3 © 2000 American Institute of Physicssuoluacrrthrndtointo-o:t adeth:he-in-sst4aretheonheV4heioneeffin1lmcu-iftofa-isha-ralex-ageheethonmxiono-trumndthe–1d bye of2254 Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.analyzer. The total spectrum consists of photoelectronsfering no energy loss processes on their way out of the sproviding information on the binding energies of the occpied states of the solid, and scattered electrons which hsuffered energy loss which contributes to a structured baground. The high-kinetic-energy onset of the spectra cosponds to emission of electrons from the Fermi level ofmetal @Ekmax(Au)# or from the highest occupied moleculaorbital ~HOMO! of the oligomer@Ekmax(P5V4)#. The low-kinetic-energy cutoff@Ekmin(Au),Ekmin(P5V4)# correspondsto electrons having suffered energy loss processes and eup at the minimum kinetic energy in the solid requiredescape into the vacuum.Thus the position of the vacuum level of a materialthe diagram is determined by adding 21.2 eV to the cuenergy to giveEvac(Au) andEvac(P5V4). Now one can determine the work function of Au using:FAu5hn2@Ekmax~Au!2Ekmin~Au!# ~1!and the position of the HOMO level of P5V4 according tI s5hn2@Ekmax~P5V4!2Ekmin~P5V4!#. ~2!By comparison of the two UPS spectra, we can construcenergy diagram as shown in the middle of the plot andtermine the positions of the energy levels at both sides ofinterface. We can determine the difference (D) between thevacuum levels of the metal and the oligomer layer usingD5Ekmin~P5V4!2Ekmin~Au!. ~3!FIG. 1. Method for determining the energy diagram of the metal/oligominterface using the ultraviolet photoelectron spectra of the metal andoligomer. The spectra were recorded with a bias of24.0 V between sampleand analyzer. They axis represents the kinetic energy of the photoelectrat the entrance slit of the analyzer.~a! Photoemission of the Au substrate.~c!photoemission of 5–10 nm of P5V4 on the Au substrate.~b! Energy dia-gram of the Au/P5V4 interface.Ekmax(Au): maximum kinetic energy of aphotoelectron excited from Au withhn: photon energy~21.22 eV!;Ekmax(P5V4): maximum kinetic energy of a photoelectron excited froP5V4; Ekmin(Au): minimum kinetic energy of a scattered photoelectron ecited from Au; Evac(Au): vacuum level of Au;Ekmin(P5V4): minimumkinetic energy of a scattered photoelectron from P5V4;Evac(P5V4):vacuum level of P5V4;FAu : work function of Au;EHOMO : energy level ofhighest occupied molecular orbital of P5V4,evF : hole injection barrier or theenergy difference between the Fermi level of Au and the HOMO level;D:vacuum level shift. The structure of P5V4 is shown at the top, right.f-id-vek-e-eedffn-eSince the vacuum level of the oligomer is lower than tvacuum level of Au, the electric field points from the oligomer (d1) to the metal (d2), makingD,0 ~as defined inRef. 3!. In this case, the vacuum level shift leads to ancrease of the barrier for hole injection (ev8F) according to:ev8F5I s2FAu2D. ~4!This hinders hole injection from Au to P5V4, but facilitateelectron injection from the fermi level of Au to the loweunoccupied molecular orbital~LUMO!.In Fig. 2 we show the interface formation of MEH-P5Von a clean Ag substrate. In the center, the full spectragiven. The bottom spectrum shows the UP spectrum ofAg substrate with a thin layer of MEH-P5V4 evaporatedtop ~in the order of 1 nm!. The subsequent spectra show tphotoemission of the surface with increasing MEH-P5thickness~0.5–1 nm increase per deposition step!. The topline, with the largest offset, shows the Ag spectrum with tFermi edge well resolved~at hn2FAg1Vbias520.6 eV!. Atthe right-hand side, the gradual change in the HOMO regis depicted in more detail. As the film grows thicker, thfeatures arising from the Fermi level of Ag give way to thpure MEH-P5V4 spectrum. The low-kinetic-energy cutoregion is plotted at the left-hand side of Fig. 2. The shiftthe onset (D) is complete after depositing approximatelynm on the substrate, it does not increase further with fithickness. This result indicates that one or very few molelar layers are involved in the process.The upper two spectra of MEH-P5V4 show a small shof 0.1 eV towards lower kinetic energy. A slight chargingthe sample may cause this shift, which is within our mesurement accuracy. The lowering of the vacuum levelcaused by an electric field at the interface due to~partial!charge transfer in the interfacial region. The exact mecnism creating the electric field is so far unknown. Seveprocesses can cause an electric field at an interface, forample, electron transfer from a donor to an acceptor, imeffects, tailing of the electron cloud of the metal towards tres-FIG. 2. UPS spectrum of MEH-P5V4 on Ag as function of the deposittime ~thickness!. All spectra were recorded with a bias of24.0 V betweensample and analyzer. Thex axis represents the kinetic energy of the photelectrons at the entrance slit of the analyzer. The dotted line is the specfrom the polycrystalline Ag substrate. The plots on the right- and left-haside show an enlargement of the central plot. After each spectrum,thickness of the MEH-P5V4 layer was increased by approximately 0.5nm, except for the last two spectra where the thickness was increaseseveral nanometers. The inset in the middle plot shows the structurMEH-P5V4.,PVdinmlecinocioreudi-eaaitaroregoagoeseslrgetmeowu-rrierb-thetoaandlytheiertheee-ofrliensrchci-er,K., J.v.fev.2255Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Veenstra et al.adsorbent causing dipoles or metal induced gap stateschemical interactions between metal and overlayer.6–8Charge transfer at the interface formed by similar Poligomers on a Ca substrate was observed by Parket al.1,2The charge transfer caused an overall energy level benof 0.5 eV in a space-charge region extending over 10The energy level bending in their experiment indicates etron transfer from the metal to the organic overlayer makD.0, which is opposite to the values reported here.Image potential effects lower the ionization energyinsulators and semiconductors adsorbed on metal surfaThis stabilizing effect is linearly proportional to 1/d wheredis the distance between the metal surface and the photoized molecule.9,10 Such a distance dependence is notflected in a dependence on the film thickness in this st~see Fig. 2!, indicating that the image potential effect is ether small for even the thinnest layer measured or compsated for by another electro-static effect. Chemical intertions at the interface formed by evaporating Al onsexithiophene layer have been reported3 with a significantcharge transfer from the metal to the oligomer, resultingthe formation of a bond between the oligomer and the meSuch a charge transfer causes an electric field pointing fthe metal towards the organic layer (D.0). For mostmetal–organic interfaces the dipole is negative~s e for ex-ample Fig. 15 in Ref. 6!, similar to our measurements.Table I summarizes the results of the UPS measuments. The shift between vacuum levels at the interfacnegative in all cases, moving the energy levels of the olimer downward relative to the levels of the metal. The mnitude of the shift is larger for Au than for Ag and alsdepends on the oligomer: the substituted oligomer causlarger shift. The ionization energy seems to be independof the metal substrate and compares well with previoureported values for similar P5V4s.11 The hole injection bar-rier can be obtained in two ways: by measuring the enedifference between the high kinetic energy onset of the mTABLE I. UPS measurements on P5V4 and MEH-P5V4 (FAu55.160.1 eV andFAg54.460.1 eV!. D represents the vacuum level shift;I s ,ionization energy;evF , measured hole injection barrier;ev8F , calculated holeinjection barrier using Eq.~4!.Interface D ~eV! I s ~eV! evF ~eV! ev8F ~eV!P5V4 / Au 21.060.1 5.6 0.1 1.460.1 1.560.3P5V4 / Ag 20.460.1 5.6 0.1 1.760.1 1.6 0.3MEH-P5V4 / Au 21.260.1 5.260.1 1.260.1 1.360.3MEH-P5V4 / Ag 20.560.1 5.360.2 1.360.2 1.460.4orng.-gfes.n--yn-c-nl.me-is--antyyaland the oligomer, as shown in Fig. 1 (evF), or by using Eq.~4! (ev8F). The differences between the values for the sainjection barrier~see column 4 and 5 in the table! give anindication of the systematic error. The values clearly shthat the misalignment of the vacuum levels strongly inflences the hole injection barrier and acts as to keep the baalmost independent of the work function of the metal sustrate.In conclusion, we reported UPS measurements ofinterface formed by evaporating PPV-like oligomers onmetals ~Ag and Au!. We found for all these interfacesmisalignment between the vacuum levels of the metalthe organic overlayer. This shift of levels, presumabcaused by an interfacial dipole layer, strongly influenceshole injection barrier in such a way as to keep this barrnearly constant and therefore at most weakly sensitive towork function of the metal or the ionization potential of tholigomer. Knowledge of this interfacial dipole layer is therfore crucial for understanding the electrical characteristicsoligomer-based electronic devices.The authors are grateful to J. Wildeman and M. Sefor providing P5V4, P. F. van Hutten for many discussioand J. Kappenburg for his technical support. This reseawas financially supported by the Dutch Organization for Sentific Research~NWO-CW! and the TMR program of theEC.1Y. Park, V. Choong, E. Ettedgui, Y. Gao, B. R. Hsieh, T. Wehrmeistand K. Müllen, Appl. Phys. Lett.69, 1080~1996!.2Y. Park, V.-E. Choong, B. R. Hsieh, C. W. Tang, T. Wehrmeister,Müllen, and Y. Gao, J. Vac. Sci. Technol. A15, 2574~1997!.3W. R. Salaneck, S. Stafstro¨m, and J.-L. Bre´das, inConjugated PolymerSurfaces and Interfaces~Cambridge University Press, New York, 1996!.4P. F. van Hutten, J. Wildemann, A. Meetsma, and G. HadziioannouAm. Chem. Soc.121, 5910~1999!.5L. Ouali, V. V. Krasnikov, U. Stalmach, and G. Hadziiouannou, AdMater.11, 1515~1999!.6H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater.11, 605~1999! andreferences therein.7W. Mönch, in Semiconductor Surfaces and Interfaces, Springer series inSurface Sciences 26~Springer, Berlin 1993!.8J. Hölzel and F. K. Schulte,Solid Surface Physics, Work Function oMetals in Springer Tracts in Modern Physics~Springer, Berlin, 1979!,Vol. 85.9R. Hesper, L. H. Tjeng, and G. A. Sawatzky, Europhys. Lett.40, 177~1997!.10G. Kaindl, T.-C. Chang, D. E. Eastman, and F. J. Himpsel, Phys. RLett. 45, 1808~1980!.11A. Schmidt, M. L. Anderson, D. Dunphy, T. Wehrmeister, K. Mu¨llen, andN. R. Armstrong, Adv. Mater.7, 722 ~1995!.

Energy level alignment at the conjugated phenylenevinylene oligomer/metal interface

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