Using an electrochemically gated transistor, we achieved controlled and
reversible doping of poly(p-phenylene vinylene) in a large concentration range.
Our data open a wide energy-window view on the density of states (DOS) and
show, for the first time, that the core of the DOS function is Gaussian, while
the low-energy tail has a more complex structure. The hole mobility increases
by more than four orders of magnitude when the electrochemical potential is
scanned through the DOS.Comment: 4 pages, 4 figure

Brom, H. B.

Houtepen, A. J.

Hulea, I. N.

Kelly, J. J.

Meulenkamp, E. A.

Vanmaekelbergh, D.

English

arXiv

Hulea, I.N.

Brom, H.B.

Houtepen, A.J.

Kelly, J.J.

Meulenkamp, E.A.

Leiden University Scholary Publications

VOLUME 93, NUMBER 16 P H Y S I C A L R E V I E W L E T T E R Sweek ending15 OCTOBER 2004Wide Energy-Window View on the Density of Statesand Hole Mobility in Poly(p-Phenylene Vinylene)I. N. Hulea,1,2 H. B. Brom,1 A. J. Houtepen,3 D. Vanmaekelbergh,3 J. J. Kelly,3 and E. A. Meulenkamp41Kamerlingh Onnes Laboratory, Leiden University, POB. 9504, 2300 RA Leiden, The Netherlands2Dutch Polymer Institute (DPI), POB. 902, 5600 AX Eindhoven, The Netherlands3Debye Institute, Utrecht University, POB. 80000, 3508 TA Utrecht, The Netherlands4Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands(Received 7 May 2004; published 13 October 2004)166601-1Using an electrochemically gated transistor, we achieved controlled and reversible doping ofpoly(p-phenylene vinylene) in a large concentration range. Our data open a wide energy-windowview on the density of states (DOS) and show, for the first time, that the core of the DOS function isGaussian, while the low-energy tail has a more complex structure. The hole mobility increases by morethan 4 orders of magnitude when the electrochemical potential is scanned through the DOS.DOI: 10.1103/PhysRevLett.93.166601 PACS numbers: 72.80.Le, 71.20.Rv, 72.20.Ee, 73.61.PhCharge transport in disordered conjugated polymerslike poly(p-phenylene vinylene) (or PPV), polypyrroleand polythiophene [1], and disordered systems in general[2–6] usually proceeds via thermally activated hoppingallowing the charge carriers to move from one site to thenext. In this process, the energy-dependent density ofstates (DOS) and the charge mobility () [or the diffu-sion constant (D)] are two key parameters. The energydistribution of the DOS is often assumed to be Gaussian[3,7,8] (for dipolar interaction [9] the width will be pro-portional to the strength of the dipoles [2,4,10]) or ex-ponential [11] and both shapes have been applied withsuccess to explain transport properties under differentconditions [12–15]. In devices such as polymeric lightemitting diodes (LEDs), where the carrier concentrationis low, only the tail of the DOS is directly involved in thecharge injection. In field-effect transistors (FETs) thecarrier concentration c is orders of magnitude larger andthe DOS further towards the center of the level distribu-tion is important. These examples show that, for a properunderstanding of the electronic properties of such mate-rials, a direct experimental determination of the shape ofthe DOS function over a large range of energy (or chargeconcentration) is essential. Until now, only a few mea-surements have been reported. One attempt to determinethe DOS (from now on only valence states are considered)in OC1C10-PPV, the workhorse in light emitting diodes,has used the temperature and concentration dependence ofthe hopping conductivity with FeCl3 as a dopant [16]. Inanother approach it is assumed that the DOS is Gaussianshaped. It then follows [3,8,9,17,18] that the carrier mo-bility  depends on temperature via ln / d=kBT2.From the experimental determination of the mobility as afunction of temperature the width d of the distributionwas determined[17,18] (for a discussion of some of thesimplifying assumptions see [8,19]). Recently, concentra-tion dependent DOS and  data were obtained in a FET0031-9007=04=93(16)=166601(4)$22.50 and a LED configuration [18]. All of the above mentionedexperiments showed the mobility to be strongly concen-tration dependent. In the charge carrier concentrationrange covered by chemical doping with FeCl3 the DOSincreased linearly with c [16], while for the analysis ofthe FET and LED experiments [17,18] an exponential andGaussian DOS were assumed, respectively. It can beconcluded that there is still no consensus on the DOS ofPPV in a broad energy range.This letter reports on the DOS and conductivity ofthin spin-coated PPV films in a wide, well defined en-ergy range. The data are obtained using an Electro-chemically Gated Transistor (EGT), which was recentlydeveloped . Using the impressive energy range of theEGT, we are able to confirm the previous data and toshow for the first time that the core of the DOS functionis Gaussian, while the flank has an exponential energydependence. The low-energy tail has a more complexstructure. Conductance measurements were performedover a wide carrier concentration range and show thatthe hole mobility is strongly energy-dependent, as alsoseen in results obtained with a classical FET [18]. Themaximum value of the mobility exceeds previously re-ported values by a factor of 40. The mobility is found tovary over more than 4 orders of magnitude when theFermi-level is scanned over less than half an eV. Inaddition, our measurements allow an energy calibrationof the DOS and the mobility.The electrochemically gated transistor offers a uniquepossibility of studying the transport properties of poly-mers as a function of doping in a reversible way. Theoperating principle is based on changing the electro-chemical potential ~e of the sample (OC1C10-PPV) withrespect to a Ag pseudoreference electrode (RE) by meansof a potentiostat, see Fig. 1. The PPV layer is in electro-chemical equilibrium with the Au source-drain elec-trodes. Any change in potential with respect to the Ag2004 The American Physical Society 166601-1FIG. 2. Electrochemical injection of holes into the PPV film.(a) The differential charge Q ~e stored in PPV when suc-cessive 10 mV steps are applied. The potential is defined withrespect to the Ag pseudoreference. The arrows show the twoscanning directions: doping and dedoping while changing ~e.(b) Doping per monomer c ~e calculated from the data in (a).The reversible doping range extends over 4 orders of magni-tude.RE CEELECTROLYTESOURCE DRAINPPVTLGLASS−− −−++++++−IsdVsdPOTENTIOSTATFIG. 1. Schematic picture of the electrochemically gatedtransistor or EGT. The sample (PPV) is placed in an electrolytesolution. The electrochemical potential of the sample is con-trolled with respect to a reference electrode using a potentio-stat. The conductivity was measured by applying a small dcbias between source and drain. The enlargement shows sche-matically how the holes on the polymer chain are compensatedby the anions in the film.VOLUME 93, NUMBER 16 P H Y S I C A L R E V I E W L E T T E R Sweek ending15 OCTOBER 2004electrode is followed by charge transfer from the Auelectrodes to the PPV or vice versa and current flow tothe Pt counter electrode (CE). When the potential isincreased holes are injected into the PPV [23]. The holecharge is counterbalanced by anions (ClO4 or PF6 ) fromthe electrolyte solution (0.1 M TBAP (tetrabutylammo-nium perchlorate), TBAPF6 (tetrabutylammonium hexa-fluorophosphate) or LiClO4 in acetonitrile) [24] whichpermeates the PPV. The number of holes stored in thePPV film is determined by monitoring the differentialcapacitance. Important advantages of electrochemicalgating over the conventional field-effect transistor arethe uniform charging[22] of the PPV film together witha wider doping range. At a given doping level the con-ductance is measured with a very small source-drain bias(10 mV, ISD / VSD) supplied by a Keithley 2400 sourcemeter. Another advantage of the EGT is that the electro-chemical potential can be correlated with the vacuumlevel. To achieve this the potential of the Ag RE wasmeasured with respect to the electrochemical potentialof the ferrocenium/ferrocene redox reaction (0.68 V).From the literature value of the latter[25] with respectto vacuum (5.14 V), the Ag RE was calculated to be at4.47 V below the vacuum level.The PPV samples with a typical thickness T of 180 nmwere spin-coated on glass substrates with Au electrodesseparated by a gap L of 1.25 up to 10 m and with aneffective transistor length of 1 up to 50 cm (interdigi-tated electrodes). The electrochemical potential was con-trolled with a Princeton Applied Research Potentiostat/Galvanostat 273A and all experiments were carried outunder Ar atmosphere. The volume VP of the variouspolymer films was between 0:03mm3 to 0:1mm3 (accu-racy 5%). From the specific weight of the PPV one candetermine the density N of PPV monomers [26]. The166601-2density of states g ~e, was obtained from the chargeQ stored in the PPV film per ~e  10 meV increaseof ~e, i.e.g ~e Qe~eNVP(1)with e the elementary charge [see Fig. 2(a)]. The DOS istherefore determined directly; no assumptions are re-quired. From 0 to 0.6 eV the amount of injected chargeis small. After this point a strong increase is observed. Ataround 0.8 eV a shoulder is visible and above 1.1 eV,Q ~e starts to decrease. With decreasing ~e the maxi-mum and the shoulder are well reproduced. Measure-ments on different samples, with different potential stepsand other ions in solution gave very similar results. Thenumber of holes per monomer c ~e at a given electro-chemical potential is obtained by integration of gE:c ~e R ~e0 gEdE [see Fig. 2(b)]. It can be seen thatthe doping level can be varied by electrochemical gatingin a controlled and reversible manner between 105 and0.4 holes per monomer.The density of states g(E) is plotted in Fig. 3, where theenergy on the vertical axis is given with respect to thevacuum level. The values for gE on the horizontal axisrange from 104 to one states/monomer eV. At E 5:55 0:02 eV, g has a maximum. This shows thatdue to disorder and dispersion the highest occupied mo-lecular orbital (HOMO) levels are spread in energy andwe are able to access the center of the distribution. Theshoulder at about 5:3 eV reveals some additional struc-ture in the measured DOS. By fitting the data to a166601-2FIG. 4. (a) The hole mobility () calculated from the con-ductivity (- the inset) as function of doping. Because themeasurements had to be performed in a two-point contactconfiguration, the flattening of  at high c might be causedby contact resistances. Hence, actual values of  or  might beeven higher. (b) The dependence of hole mobility on energy orelectrochemical potential ~e. At lower doping, correspondingto energies E>5:2 eV, we have included  (closed symbols)determined in a solid-state FET and LED by Tanase et al. [18].The deviations are discussed in the text. Both plots are on alogarithmic-linear scale.FIG. 3. The E dependence of the experimentally determinedDOS (gE) where E is with respect to the vacuum level. Thehorizontal dashed line marks the HOMO position found fromcyclic voltammetry [28,29]. (a) E vs gE using PF6 and ClO4as anions. A Gaussian function with   0:19 eV (area A) fitsthe data well. (b) E vs gE for PF6 on a linear-log scale. Belowg  0:1states=eVmonomer deviations from the Gaussian fitare visible. The filled squares are the PPV FET-data by Tanaseet al. [18], which are well described by an exponential function.At the lowest values of gE additional structure appears, whichin a limited energy range would allow a description with aGaussian with a width of 0.11 eV (area B) as used for PPV LEDsby Martens et al. [9] and Tanase et al. [18].VOLUME 93, NUMBER 16 P H Y S I C A L R E V I E W L E T T E R Sweek ending15 OCTOBER 2004Gaussian distributiongE Nm2pdexpE Ect2pd2; (2)we find a width d  0:19 0:01 eV, centered at Ect 5:55 0:02 eV (diagonally shaded area A in Fig. 3).The number of states per monomer Nm RgEdE has avalue of 0:52 0:01 when we integrate over the wholeGaussian [27]. The measured DOS validates the assump-tion of a Gaussian distribution [3] in the core [g > 101states/(eV monomer)]. For g between 103 and 101states/(eV monomer) the data follow an exponential de-pendence, see the logarithmic-linear plot of Fig. 3(b). TheDOS in this range measured by Tanase et al. in a solid-state FET configuration [18] (solid squares in Fig. 3(b)] isin very good agreement with our data.In general ions in the dielectric will influence the en-ergy landscape and broaden the DOS [14]. The data pre-sented here compare well with those of Tanase et al. [18],where no ions are present in the film. Hence, the intrinsiclevel distribution of the investigated OC1C10-PPV must besufficiently broad, such that an extra contribution by theions is not observable in our experiments. There is noguarantee that this also holds for the data at higherdoping; the intrinsic width of PPV might therefore belower than 0.19 eV. At very low doping the influence ofthe ions is expected to be small. Here, around g104states=eVmonomer, the data allow for fitting to aGaussian with d  0:11 eV (horizontally shaded area B166601-3in Fig. 3(b)], as reported in Refs. [17,18]. Note, seeFig. 3(b), that the Gaussian distribution in this dopingregime is a tail effect, which is not representative for themain distribution.On inspection of the DOS it is clear that the assign-ment of the HOMO level at 5:2 eV [28,29] (horizontaldashed line in Fig. 3), as determined by cyclic voltam-metry (location at 10% of the peak value), is relativelyarbitrary. The value of 5:2 eV corresponds to the pointat which a strong increase in DOS is observed. However,many states are already available for values of E up to4:5 eV. This means that in LED applications a devia-tion of the ’’HOMO level’’ of 0.7 eV from the cyclicvoltammetric value is possible. This has to be includedin quantitative descriptions of charge injection and car-rier mobility in devices [14].When a small voltage difference is applied between thesource-drain contacts, the conductivity  of the PPVlayer can be measured as a function of doping regulatedby ~e, see Fig. 4. The conductivity, which was checked tobe ohmic up to the highest field of 104V=m, increases by 5orders of magnitude, as the doping is increased fromabout 0.02 to 0.30 holes per monomer. A similar strongincrease was previously reported for FeCl3 doped PPV[16]. The values of  are calculated with a constantthickness of PPV, not taking into account effects of swell-ing. Because the measurements had to be performed in atwo-point contact configuration, the flattening of  athigh c might be caused by contact resistances, whichmeans that actual values of  or  might be even higher.From the measured conductivity the hole mobility ()can be determined via the relation:   Npte with pt166601-3VOLUME 93, NUMBER 16 P H Y S I C A L R E V I E W L E T T E R Sweek ending15 OCTOBER 2004the number of holes per monomer participating in trans-port and e the elementary charge. The states involved inhopping are those within kBT of the chemical potential[30], pt ~e R ~e	kBT=2~ekBT=2gEdE. The results, presentedin Fig. 4, confirm that the mobility is concentration, orbetter, energy-dependent [16,18]. For a doping level of 0.3a mobility of 2 106 m2=Vs is obtained, more than afactor of 10 higher than the highest value found by Tanaseet al. [18] for comparable field values, but at 0.02 holes permonomer. The lower mobilities obtained in the EGTcompared to the solid-state FETat the same doping levelsare likely due to the presence of the anions that tend tolocalize the mobile holes. In addition, in the solid-stateFET transport takes place primarily in the first PPVlayers close to the interface, where a possible higherstructural order will have a favorable effect on themobility.In conclusion, by using an EGT the DOS of a polymercan be determined over more than 1 eVor a doping rangeextending over 4 orders of magnitude. A comparison withthe DOS obtained with a solid-state FET [18] reveals thatdata below 0.1 states/(eV monomer) (doping levels below102 holes per monomer) are representative for the in-trinsic level distribution of OC1C10-PPV. The shape of theDOS function is complex.While the core is well describedby a Gaussian function (its width of 0.19 eV has to beconsidered an upper boundary for the intrinsic width), theflank decays exponentially and the tail at very low den-sities allows a description with another Gaussian. Thehole mobility is found to vary from 1010m2=Vs to106m2=Vs, as the chemical potential changes from5:3 eV to 5:6 eV. We have shown that PPV can beelectrochemically doped with a high degree of controland reversibility. The accessible energy window is muchlarger than that of previously used methods. Thus electro-chemical gating appears to be a powerful method forstudying charging and transport properties of conductingpolymers.We wish to thank Frank Janssen (TUE) for preparingthe samples and Peter Liljeroth (UU) for experimentalassistance and we acknowledge fruitful discussions withPaul Blom and Cristina Tanase (RUG) about the solid-state FET-data. This work forms part of the researchprogram of the Dutch Polymer Institute (DPI), projectDPI274.16660[1] A. B. Kaiser, Rep. Prog. Phys. 64, 1 (2001).[2] L. Kador, J. Chem. Phys. 95, 5574 (1991).[3] H. Bässler, Phys. Status Solidi B 175, 15 (1993).[4] R. H. Young, Philos. Mag. B 72, 435 (1995).[5] J. Dong and D. A. Drabold, Phys. Rev. Lett. 80, 1928(1998).1-4[6] A. L. Roest, P. E. de Jongh, and D.Vanmaekelbergh, Phys.Rev. B 62, 16926 (2000).[7] A. Hirao and H. Nishizawa, Phys. Rev. B 56, R2904(1997).[8] S.V. Novikov, D. H. Dunlap, V. M. Kenkre, P. E. Parris,and A.V. Vannikov, Phys. Rev. Lett. 81, 4472 (1998).[9] H. C. F. Martens, P.W. M. Blom, and H. F. M. Schoo, Phys.Rev. B 61, 7489 (2000).[10] S.V. Novikov and A.V. Vannikov, J. Phys. Chem. 99,14573 (1995).[11] M. C. J. M. Vissenberg and M. Matters, Phys. Rev. B 57,12964 (1998).[12] D. Wang and J. Shen, Synth. Met. 111, 349 (2000).[13] R. Schmechel, Phys. Rev. B 66, 235206 (2002).[14] V. I. Arkhipov, P. Heremans, E.V. Emelianova, G. J.Adriaenssens, and H. H. Bässler, Appl. Phys. Lett. 82,3245 (2003).[15] S. Shaked, S. Tal, Y. Roichman, A. Razin, S. Xiao,Y. Eichen, and N. Tessler, Adv. Mater. 15, 913 (2003).[16] H. C. F. Martens, I. N. Hulea, I. Romijn, H. B. Brom, W. F.Pasveer, and M. A. J. Michels, Phys. Rev. B 67, 121203(2003).[17] H. C. F. Martens, J. N. Huiberts, and P.W. M. Blom, Appl.Phys. Lett. 77, 1852 (2000).[18] C. Tanase, E. J. Meijer, P.W. M. Blom, and D. M. deLeeuw, Phys. Rev. Lett. 91, 216601 (2003).[19] S. D. Baranovskii, H. Cordes, F. Hensel, and S. Yamasaki,Synth. Met. 119, 57 (2001).[20] E. A. Meulenkamp, J. Phys. Chem. B 103, 7831 (1999).[21] X. Jiang, Y. Harima, K. Yamashita, Y. Tada, J. Ohshita,and A. Kunai, Chem. Phys. Lett. 364, 616 (2002).[22] A. L. Roest, J. J. Kelly, D. Vanmaekelbergh, and E. A.Meulenkamp, Phys. Rev. Lett. 89, 036801 (2002).[23] Attempts to dope the PPV with electrons failed becauseof irreversible reactions with the PPV; double valent ionscould not be dissolved.[24] TBAP–Fluka Chemica, electrochemical grade 99%,TBAPF6–Fluka Chemica, electrochemical grade 99%,LiClO4–Aldrich, ACS reagent 95%, acetonitrile–Aldrich, anhydrous 99.8%.[25] I. Noviandri, K. N. Brown, D. S. Fleming, P. T. Gulyas,P. A. Lay, A. F. Masters, and L. Phillips, J. Phys. Chem. B103, 6713 (1999).[26] N  =mmon with   0:989g=cm3 the polymer densityand mmon  288:47 the monomer mass in atomic units.[27] The Nm value can be lower than one per monomer, seeKubatkin et al., Nature (London) 425, 698 (2003).[28] J. K. J. van Duren,V. D. Mihailetchi, P.W. M. Blom, T. vanWoudenbergh, J. C. Hummelen, R. A. J. Janssen, andM. M. Wienk, J. Appl. Phys. 94, 4477 (2003).[29] I. N. Hulea, R. F. J. van der Scheer, H. B. Brom, B. M.W.Langeveld-Voss, A. van Dijken, and K. Brunner, Appl.Phys. Lett. 83, 1246 (2003).[30] By using only the carriers located within the thermalwindow (kBT) around the Fermi-level, we find D= kBT=e, in contrast to the value of about ten kBT=eobtained by Roichman et al., Appl. Phys. Lett. 80,1948 (2002).166601-4

Wide energy-window view on the density of states and hole mobility in poly(p-phenylene vinylene)

hulea, I.N.

Vanmaekelbergh, D.A.M.

Chemie van de vaste stof: Luminescentie en elektrochemie

Universiteit Utrecht

Dep Scheikunde

NARCIS 

Utrecht University Repository