Femtosecond time-resolved two-photon photoemission spectroscopy is employed to
study the dynamics of the non-equilibrium electron distribution in the
conducting polymer poly(3,4-ethylene-dioxythiophene): poly-(styrenesulfonate)
(PEDT:PSS) film following optical excitation at 2.1 eV. We found that the
electron thermalization occurs on a ultrafast timescale of around 60 fs
analogous to the relaxation times of optically excited electrons in Au(111)

Tegeder, Petra

Varene, Erwan

Institutional Repository of the Freie Universität Berlin

Applied Physics A manuscript No.(will be inserted by the editor)Dynamics of optically excited electrons in theconducting polymer PEDT:PSSErwan Varene and Petra TegederFreie Universitat Berlin, Fachbereich Physik, Arnimallee 14, D-14195 Berlin, Ger-manyReceived: date / Revised version: dateAbstract Femtosecond time-resolved two-photon photoemission spectros-copy is employed to study the dynamics of the non-equilibrium electrondistribution in the conducting polymer poly(3,4-ethylene-dioxythiophene):poly-(styrenesulfonate) (PEDT:PSS) lm following optical excitation at 2.1eV. We found that the electron thermalization occurs on a ultrafast timescale of around 60 fs analogous to the relaxation times of optically excitedelectrons in Au(111).Send oprint requests to: Petra Tegeder; email: tegeder@physik.fu-berlin.de; fax:+49-30-838560592 Erwan Varene and Petra Tegeder1 Introduction-Conjugated polymers are promising as low-cost and easily processable al-ternatives to inorganic semiconductors or metals in electronic applications.Utilizing intrinsic conducting polymers (ICP) as electrodes and hole injec-tion layers in optoelectronic devices based on organic materials is thereby akey for a successful implementation of organic electronics. One of the pro-totypical ICP is poly(3,4-ethylene-dioxythiophene):poly(styrene-sulfonate)(PEDT:PSS; see Fig. 1), which has been used for instance for the fabri-cation of very ecient organic light emitting diodes (OLED) [1], [2] andeld-eect transistors [3], [4]. In PEDT, double bonds are alternating withsingle bonds along the polymer chain. Thereby the unhybridized pz-orbitalof one carbon atom overlaps with the pz-orbital of the neighboring carbonatom to form a -bond, in which the electrons are weakly bound and there-fore are relatively delocalized. However, due to the dierent bond lengthof the single and double bond the Peierls instability splits this band intotwo sub-bands, the completely lled valence band and an empty conduc-tion band, separated by an energy gap. Hence the material is a semicon-ductor, but through doping with PSS the polymer become conductive. Ithas been shown that charge carriers created after absorption of light in -conjugated polymer are polarons [5]. The strong electron-phonon couplingof these quasi-particles gives rise to new energy states in the band-gap, thusthe polymer absorbs in the IR region and is transparent [6]. Moreover vary-ing the ratio of PEDT:PSS induces changes in the micromorphology whichDynamics of optically excited electrons in PEDT:PSS 3facilitates adjustment of the overall conductivity of thin lms [7]. In OLEDsPEDT: PSS is one of the most frequently used anode material due to itshigh work function and accordingly the low hole injection barrier. It hasbeen shown that the work function possesses values between 5.0 and 5.6 eVwhich was mainly attributed to the amount of residual water in the thinlms [8]. Thus the work function () can be much higher than the valuesknown for atomically clean gold surfaces (e.g., (Au(111)) = 5.4 eV [9]).It is known that the charge transport in this class of polymers is dom-inated by disorder leading to charge localization that is dominant at allpractical temperature, i.e. temperatures where no damage of the materialoccurs. In this material, polarons are formed in less ordered regions of thedoped polymers [10]. Interacting with phonons, charge carriers hop fromlocalized sites to another. Upon transport, the charge carriers can eithertransfer their energy to the lattice by exciting phonons or radiative recom-bination gives rise to photoluminescence.poly(ethylene-dioxythiophene):poly(styrenesulfonate) (PEDT:PSS)SO3H SO3H SO3H SO3HSO3-SO3-( )nO OO OO OSSSSSSO O O O O O( )m2 +PSSPEDTFig. 1 Chemical structure of the conducting polymer mixture PEDT:PSS.4 Erwan Varene and Petra TegederThe dynamics of electronic excitations in organic molecules or poly-mers with potential application in optoelectronic devices plays an impor-tant rule. Up to now the dynamics of electrons in PEDT:PSS has onlybeen studied by infrared-induced transient adsorption in the sub-picosecondtime-domain [11]. Femtosecond (fs) time-resolved two-photon photoemission(2PPE) spectroscopy has been used to study electron thermalization in var-ious metals, metallic lms and semiconductors [12], [13], [14],[15], [16] [17],[18], [19] but interestingly so far this technique has not been utilized to inves-tigate the dynamic behavior of charge carriers in conducting polymer lms.In metals, light is adsorbed near the surface within the optical penetrationdepth of typically 10-20nm. This leads to the creation of free charge car-riers, which relax predominantly by electron-electron (e-e)-scattering on afemtosecond timescale. Subsequent energy transfer to the lattice by electron-phonon (e-ph)-scattering occurs on a picosecond timescale. In addition thenascent carrier distribution is also depleted by transport of excited carriersinto the bulk. An important dierence between metal and polymer lms isthe penetration depth of light. For the polymer lms the cross section forphoton absorption is not very high. Moreover, the charge carrier density inPEDT:PSS is in the order of 1020 cm 3 [20], while in metals it is around1022 cm 3.In this contribution we investigate the relaxation dynamics of photoex-cited electrons in PEDT:PSS and Au(111) in the fs time domain using2PPE. The comparison of the charge carrier dynamics between this twoDynamics of optically excited electrons in PEDT:PSS 5classes of material is motivated by the potential use of PEDT:PSS as anelectrode material instead of metals in opto-electronic applications . For thepolymer we observed electron relaxation times in the order of 60 fs, akin tothe time scales determined for Au(111).2 ExperimentalThe present experimental setup combines a tunable femtosecond laser sys-tem with an ultrahigh vacuum chamber for two-photon photoemission (2PPE)spectroscopy.For the 2PPE measurements, femtosecond laser pulses are generated bya 300 kHz Ti:Sapphire laser system, which pumps an optical parametricamplier (OPA). The OPA delivers pulses with a tunable photon energybetween 1.7 eV and 2.7 eV output. These pulses can be frequency-doubledin a BBO crystal to generate ultraviolet pulses (3.4{5.4 eV photon energy).After a thorough temporal compression pulses with a duration of about 60fs are obtained. The laser pulses are incident on the surface with an angle of45 with respect to the surface normal. While the pump pulse h1 excites anelectron from below the Fermi level (EF ) to intermediate unoccupied statesat energies E EF = Ekin+ h2 (with  the work function), the probepulse h2 photoionizes the sample by lifting the excited electron above thevacuum level (Evac). Photoelectrons are detected in a custom-built time-of-ight spectrometer and are analyzed with respect to their kinetic energy(Ekin) (for details see Ref. [21]).6 Erwan Varene and Petra TegederThe commercial PEDT:PSS dispersion with a PEDT:PSS ratio of 1:6(Baytronr P AI4083) was used. Thin lms were prepared by spin castingthe dispersion on freshly cleaned and UV/ozone-treated (for 30 min) indiumtin oxide (ITO; on glass) substrate in ambient. After heating the sample atapproximately 200 C for 5 min there were transferred into the ultrahighvacuum system for the time-resolved photoemission experiments.3 Results and DiscussionTo gain insight into the dynamics of photoexcited hot electrons after excita-tion of a PEDT:PSS lm with short laser pulses, fs time-resolved 2PPE hasbeen used. In Figure 2, a two-dimensional false color representation of thephotoemission intensity of PEDT:PSS recorded with photon energies of h1= 2.1 eV for the visible and h2 = 4.2 eV for the UV laser pulses representsthe electron distribution as a function of both time delay (horizontal axis)and energy (vertical axis). The spectrum is displayed as a function of nalstate energy above the Fermi level, EFinal   EF = Ekin + , thus the en-ergy of the secondary edge corresponds to the work function of PEDT:PSSwhich is 5.1 eV. Depending on the sequence of laser pulses (or formally thesign of the pump-probe delay), the dynamics are probed either near EF orat higher intermediate state energies. Negative delays correspond to popu-lating intermediate electronic states with UV light pulses (h2), while thevisible pulses (h1) subsequently probe the excited electron distribution.On the other hand, for positive delays the electrons are excited to interme-Dynamics of optically excited electrons in PEDT:PSS 70 1000 1500500 2000-300Pump-probe delay [fs]5.15.35.55.75.9E- E[eV]FinalFPEDT:PSSh = 2.1 eVh = 4.2 eVnn12100% 0%2PPE intensityFig. 2 2PPE intensity (color map) as a function of pump-probe delay (horizon-tal axis) and nal state energy above the Fermi level (EF ) (vertical axis) fromPEDT:PSS recorded with photon energies of h1 = 2.1 eV for the visible and h1= 4.2 eV for the UV laser pulses. The excited electron distribution is probed withh1 at negative delays, while at positive delays it is probed with h2.diate states close to the Fermi level (hot electrons) with the visible pulses(h1) and are subsequently probed by the UV pulses (h2). Analyzing thetwo-dimensional data set by extraction of the time-dependent 2PPE pho-toelectron yield integrated over the energy intervals centered on the stateof interest, i.e., hot electron distribution, provides the temporal evolutionof the electron population, referred to as cross correlation traces as shownin Fig. 3(a). The cross correlation curves are recorded at intermediate stateenergies E-EF between 0.9 and 1.8 eV.8 Erwan Varene and Petra TegederPump-probe delay [fs]Log 2PPE Intensity [arb. units](a)(b)Intermediate state energy [eV]Relaxationtime[fs]8006004002000-2001.7 eV1.1 eVPEDT:PSSAu(111)30020010001.81.61.41.21.00.8Fig. 3 (a) Cross correlation curves for PEDT:PSS recorded at intermediate stateenergies E-EF between 1.1 and 1.7 eV. (b) Relaxation times of the photoexcitedelectron distribution for PEDT:PSS and Au(111).In general, several processes contribute to the observed population decayin 2PPE. Relaxation out of a given energy interval above E-EF is aected byelectron-electron (e-e) scattering, electron-phonon (e-ph) or electron-defectscattering and by transport out of the detection volume. The rate of e-escattering (i.e., the inverse inelastic lifetime) is given by a screened Coulombinteraction and the available phase space depending on the energy aboveDynamics of optically excited electrons in PEDT:PSS 9the Fermi level [22]. However generation of secondary electrons by Augerdecay of electrons and holes may also increase the population in particularin energy intervals close to EF . Therefore the measured relaxation timescan can not be simply identied with electron lifetimes at least for energiesclose to EF .The relaxation times are determined by tting the cross correlationcurves with a convolution of the pump probe cross correlation with a sin-gle exponential decay. For energies above 1.2 eV the t describes the datawell. For lower energies the electron population is more and more domi-nated by relling processes with secondary electrons. The relaxation timesas a function of intermediate state energy (E-EF ) are displayed in Fig. 3(b)together with the corresponding values for Au(111). Two main point wewould like to point out: (i) The lifetime, and hence the inelastic mean freepath is strongly energy-dependent. The curve shows a clear increase in re-laxation time as EF is approached. This faster relaxation for higher energiesabove E-EF is due to availability of more phase space (see above). (ii) Thecomparison with Au(111) shows that for intermediate state energies aboveapproximately 1.5 eV the relaxation times for PEDT:PSS and Au are verysimilar. The relaxation time for PEDT:PSS is less than a factor 2 higher.For metals, the e-e interaction is eciently reduced by screening, whichattenuates Coulomb interactions. For PEDT:PSS, despite a lower chargecarrier density the screening of carrier-carrier Coulomb interaction seemsalso ecient. Moreover, due to the polaronic nature of the charge carriers10 Erwan Varene and Petra Tegederin the polymer, electron-phonon interaction is much more ecient than inthe case of a metal, leading to comparable relaxation times in the two ma-terials. For intermediate state energies below 1.5 eV the relaxation times inboth materials dier signicantly, in PEDT:PSS the electrons exhibit muchlonger lifetimes. We attribute this to relling eects, viz. the creation ofsecondary electrons by decay of excited electrons into lower energetic states(cascade electrons) and Auger electrons. Furthermore, polaronic hoping toneighboring weakly localized states reduces the probability of radiative re-combination by meeting a polaron of the opposite charge at low carrierexcitation density. Note that the small slope change in the relaxation timeof electrons in Au(111) around 1.5 eV has been associated with photoexci-tation of d-electrons as for copper [15].AcknowledgmentsThis work has been supported by the Deutsche Forschungsgemeinschaftthrough the SPP1355. We thank Norbert Koch and Johannes Frisch (Hum-boldt Universitat zu Berlin) for helping us with the sample preparation.References1. S. A. Carter, M. Angelopoulos, S. Karg, P. J. Brock, and J. C. Scott, Appl.Phys. Lett. 70, 2067 (1997)2. X. Yang, D. C. Mller, D. Neher, and K. Meerholz, Adv. Mater. 18, 948 (2006)3. C. W. Sele, T. von Werne, R. H. Friend, and H. Sirringhaus, Adv. Mater. 17,997 (2005)Dynamics of optically excited electrons in PEDT:PSS 114. P. Cosseddu and A. Bonglio, Appl. Phys. Lett. 88, 023506 (2006)5. Y. Nishihara, A. Matsuda, A. Fujii, M. Ozaki, E.L. Frankevich, K. Yoshino,Synthetic Metals, 154, 102 (2005).6. H. Neugebauer, J. of Electroana. Chem. 563, 153 (2004).7. N. 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Dynamics of optically excited electrons in the conducting polymer PEDT:PSS

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Dynamics of optically excited electrons in the conducting polymer PEDT:PSS

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