Femtosecond time-resolved two-photon photoemission spectroscopy is employed to
study the dynamics of an excited state in a thin regioregular
poly(3-hexylthiophene) (RR-P3HT) film deposited on a conducting polymer
poly(3,4-ethylene-dioxythiophene): poly-(styrenesulfonate) (PEDT:PSS)
electrode following optical excitation at 2.1 eV. We found that the
biexponential decay of this excited state has a fast component (2.6 ps)
assigned to bound polaron pairs which recombine quickly or separate to be
added to the slow component (7.6 ps). The latter is attributed to polarons
generated via charge transfer between adjacent polymer chains

Tegeder, Petra

Varene, Erwan

Institutional Repository of the Freie Universität Berlin

Applied Physics A manuscript No.(will be inserted by the editor)Polaron dynamics in thin polythiophene lmsstudied with time-resolved photoemissionErwan 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 spectroscopyis employed to study the dynamics of an excited state in a thin regioregularpoly(3-hexylthiophene) (RR-P3HT) lm deposited on a conducting polymerpoly(3,4-ethylene-dioxythiophene): poly-(styrenesulfonate) (PEDT:PSS) elec-trode following optical excitation at 2.1 eV. We found that the biexponen-tial decay of this excited state has a fast component (2.6 ps) assigned tobound polaron pairs which recombine quickly or separate to be added tothe slow component (7.6 ps). The latter is attributed to polarons generatedvia charge transfer between adjacent polymer chains.Send oprint requests to: Petra Tegeder; email: tegeder@physik.fu-berlin.de; fax:+49-30-838560592 Erwan Varene and Petra Tegeder1 IntroductionOrganic photovoltaic cells (OPVC) are one of the most auspicious alterna-tive for traditional silicon-based solar cells [1], [2]. This new generation ofsolar cells is fabricated using either small molecules or polymers as activesemiconductor layers. The latter are favored because of their higher poten-tial for real low cost and large area production as thin homogenous polymerlms can be easily made from solution. The best performances for poly-mer based OPVCs are obtained using regioregular poly(3-hexylthiophene)(RR-P3HT) as donor material (up to  5% eciency [3]) and poly(3,4-ethylene-dioxythiophene): poly-(styrenesulfonate) (PEDT:PSS) (see Fig. 1)is the electrode of choice for almost all OPVCs due to its transparencyin the visible spectrum [4], its high conductivity [5] and work function. InP3HT electrons are usually delocalized on the -conjugated backbone of thepolymer, resulting in a high conductivity in the organic lm, which can beincreased up to 1000 S/cm when doped with iodine [5]. In this work we areusing regioregular RR-P3HT showing self-organization of the chains into alamellae structure resulting in relatively strong interchain interactions com-pared to regiorandom (RRa) P3HT that keeps a more chainlike morphologyin the lm.In order to optimize device performance, good knowledge of the elec-tronic properties of the polymers is needed. In this study we utilized time-resolved two-photon photoemission (2PPE) in order to gain insight intothe excited states dynamics in the polymer lm deposited on PEDT:PSS.Polaron dynamics in thin polythiophene lms 3PEDTPSSP3HTFig. 1 Chemical structure of a) the conducting polymer mixture PEDT:PSS andb) P3HT.At present, studies on the dynamics of electronic excitations and chargetransfer processes at the adsorbate/substrate interface are limited to rathersimple molecular systems (small molecules) adsorbed on single crystal metalor semiconductor surfaces [9], [10], [11], [13], [12], [14], [15], [16], [17], [18],[19]. In the literature only a few 2PPE studies using nanosecond laser pulseson conjugated polymer lms on metal substrates [20], [21] exist. However,interfaces of direct relevance to OPVCs, i.e., conjugated polymer/ polymerelectrode have so far not been addressed. Here we show that 2PPE is a suit-able tool to probe excited states dynamics in thin polymer lms, namelythe polaron dynamics in thin P3HT lms.4 Erwan Varene and Petra Tegeder2 ExperimentalTwo-photon photoemission spectroscopy: The experimental setup com-bines a tunable femtosecond laser system with an ultrahigh vacuum (UHV)chamber for time-resolved two-photon photoemission spectroscopy and sur-face science techniques. For the 2PPE measurements, femtosecond laserpulses are generated by a 300 kHz Ti:Sapphire laser system which pumpsan optical parametric amplier (OPA). The visible output with photon ener-gies between 1.7 eV and 2.7 eV is frequency doubled to generate ultravioletpulses. The frequency doubled pulses h2 and the visible fundamental h1are delayed with respect to each other (pump and probe pulse) and focusedonto the sample. Thereby a pulse duration of around 60 fs is achieved [18].While the pump pulse excites an electron from below the Fermi level EFto intermediate unoccupied states at energies E = Ekin +  - h2 (with the work function), the probe pulse photoionizes the sample by liftingthe electron above the vacuum level (Evac). Photoelectrons are detected inan electron time-of-ight (TOF) spectrometer and analyzed with respect totheir kinetic energy Ekin. The energy resolution of the TOF spectrometerdepends on the electron energy, in the range of Ekin  1 eV it is around 20meV.Film preparation: As substrates indium-tin-oxide (ITO) coated glasssubstrates (sheet resistance 15-30) were used. ITO samples were cleanedby ultrasonication in acetone and isopropanol. PEDT:PSS (AI4083, H.C.Stark GmbH and Co. KG) was deposited by spin coating at 1500 rpm andPolaron dynamics in thin polythiophene lms 5dried at 200 C for 5 min at ambient atmosphere. P3HT (Aldrich Mn 64.000) was spin coated (3000 rpm) in inert atmosphere from chloroformsolution with various concentrations (0.5 mg/ml, 1 mg/ml and 2 mg/ml).3 Results and DiscussionIn the following we discuss the results on the charge carrier dynamics inthin regioregular RR-P3HT lms on PEDT:PSS obtained by time-resolved2PPE. In Figure 2, a false color representation of the photoemission in-tensity of a 24 nm RR-P3HT lm on PEDT:PSS recorded with photonenergies of h1 = 2.1 eV for the visible and h2 = 4.2 eV for the UV laserpulses represents the electron distribution as a function of both time delay(horizontal axis) and energy (vertical axis). Depending on the sequence oflaser pulses (or formally the sign of the pump-probe delay), the electrons areprobed either near EF or at higher intermediate state energies. Negative de-lays correspond to populating intermediate electronic states with UV lightpulses (h2), while the visible pulses (h1) subsequently probe the excitedelectron distribution. On the other hand, for positive delays the electronsare excited to intermediate 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 the two-dimensional data set by extraction of the time-dependent 2PPE photoelectron yield integrated over the energy intervalscentered on the state of interest, e.g. unoccupied intermediate state labeled6 Erwan Varene and Petra Tegederas A, provides the temporal evolution of the electron population, referredto as cross correlation traces as shown in the bottom panel of Fig. 2.2000150010005000E- E[eV]FinalF3.84.34.85.35.86.3-300Hot e-state AE- E[eV]FinalF3.84.34.85.35.86.324 nm P3HT / PEDT:PSSh = 2.1 eVh = 4.2 eVnn12Pump-Probe Delay [fs]t  =12.6 pst  = 7.62 pslog 2PPE intensity2PPE intensity [a.u.]State A100% 0%2PPE intensityDt = 0Fig. 2 2D-plot of 2PPE spectra as a function of pump-probe delay of a 24 nmP3HT lm on PEDT:PSS. For positive delays, the VIS pulse (h1 = 2.1 eV) ar-rives at the surface before the UV pulse (h2 = 4.2 eV), therefore the unoccupiedintermediate state labeled as state A is VIS-pumped and UV-probed. The spec-trum on the right corresponds to a cut at zero time delay. The bottom panel showscross correlation trace for the energy region of state A.A broad photoemission feature located around EFinal- EF = 4.9 eV isobserved labeled as A showing a lifetime towards positive pump-probe delay.Hence this unoccupied intermediate state is pumped by the visible photons(h1 = 2.1 eV) and probed by the UV pulses (h2 = 4.2 eV) therefore itsenergetic position is  0.7 eV above EF (note that the energy scale is refer-Polaron dynamics in thin polythiophene lms 7enced to the Fermi level of PEDT:PSS). The cross correlation trace for theenergy region of state A is shown in the bottom panel of Fig. 2. Fitting thecross correlation trace with a biexponential decay results in time constantsof 2.6 ps and 7.6 ps. In order to assign this unoccupied intermediate statewe have to consider both its energetic position and lifetime. From UPS mea-surements it is known that the upper valence band (VB) edge of P3HT islocated 0.2 eV below the Fermi level of PEDT:PSS [22]. Surprisingly we donot observe the bottom edge of the conduction band in the 2PPE spectrum.However the commonly accepted optical band gap of P3HT is 1.9-2.1eV [23].Thus the unoccupied state labeled as A which we observe in the 2PPE islocated 0.9 eV above the VB edge (see Fig. 3), corresponding to 1.0-1.2 eVbelow the bottom edge of the conduction band (CB).EFEVBP3HTCB~2 eV0.2 eVEvacpolaron5.1 eV~1 eVPEDT:PSS0.9 eVFig. 3 Energetic position of the polaron in P3HT observed in the present study.The Fermi level of PEDT:PSS serves as reference.8 Erwan Varene and Petra TegederIt is know that the photoexcitation of -electrons with photon ener-gies greater than the band gap of P3HT will induce the formation of exci-tons [25]. An exciton can either meet another exciton and undergo exciton-exciton annihilation or decay to the ground state. Moreover, if the excitonis created close to a dissociation center, for instance another polymer chain,it can undergo a charge transfer and produce a bond polaron pair with thenegative and positive polaron on adjacent chains. Ruseckas et al. [26] havereported that this process is very ecient in polythiophene and takes placewithin 100 fs after photoexcitation. This bound polaron pair can recombine,be trapped by a defect state or dissociate into two charge carriers.The binding energy of the exciton in RR-P3HT reported by Sakamotoet al. [25] is 0.7 eV and its lifetime in the order of a few hundred femtosec-onds [27]. Moreover it is known from literature that two delocalized polaronbands located at 0.1 eV and 1.85 eV, as well as two localized polaron bandsat 0.35 eV and 1.25 eV, respectively, are existent in RR-P3HT [24], viz.in the energy regime where we nd the broad 2PPE feature. Therefore weassign the fast component (2.6 ps) to bound polaron pairs, in which posi-tive and negative polarons are bound to each other on adjacent chains byCoulomb interaction. The bound polaron can recombine quickly or separateand be added to the slow component (7.6 ps) assigned to polarons generatedby charge transfer between adjacent polymer chains. Due to electronic andgeometric disorder in the P3HT lms, the broadening of the polaron peakdoes not allow to dierentiate energetically the polarons from the boundPolaron dynamics in thin polythiophene lms 9polaron pairs. Note that, varying the lm thickness from 5 nm to 30 nmdid not lead to signicant changes in the polaron dynamics. In order tosupport our assignment a comparison between the observed charge carrierdynamics in RR-P3HT lms forming a lamellae structure with non negligi-ble interchain interactions and the dynamics in regiorandom (RRa) P3HTlms keeping a chainlike morphology could certainly be useful to gain fur-ther insight into the nature of the observed processes. However, our studyshows that time-resolved 2PPE is a powerful tool to investigate the excitedstate dynamics in polymer lms.AcknowledgmentsFunding by the Deutsche Forschungsgemeinschaft (DFG) through the pri-ority program SPP 1355 "Elementary Processes in Organic Photovoltaics"is gratefully acknowledge.References1. Y. Shirota and H. Hageyama, Chem. Rev. 107 (2007) 953.2. P. Peumans, A. Yakimov and S. R. Forrest, J. Appl. Phys. 93 (2003) 3693.3. M. T. Dang, L. Hirsch, G. Wantz, Adv. Mat. 31 (2011) 3597.4. D. Wakizaka, T. Fushimi, H. Ohkita, S. Ito, Polymer 45 (2004) 8561.5. R.D. McCullough, S. Tristram-Nagle, S.P. Williams, R.D. Lowe, M. 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Polaron dynamics in thin polythiophene films studied with time-resolved
photoemission

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Polaron dynamics in thin polythiophene films studied with time-resolved photoemission

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