Photophysics of semiconducting polymer-C60 composites: A comparative study

I IITRIkK ELSEVIER

Synthetic Metals 70 (1995) 1349-1352

PHOTOPHYSICS OF SEMICONDUCTING POLYMER-C60 COMPOSITES: A COMPARATIVE STUDY N. S. Sariciftci and A. J. Heeger Institute for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, CA 93106

Abstract Experimental results on the metastable, reversible, ultrafast photoinduced electron transfer between semiconducting, conjugated polymers and Buckminsterfullerene, C60, are summarized. The results are discussed in terms of opportunities for solar energy conversion, for photodetector devices and for a variety of other applications which use photoinduced charge separation like in the photosynthesis.

1. INTRODUCTION Recently, we reported the evidence for a photoinduced electron transfer and metastable charge separation in semiconducting, conjugated polymer/C60 composites [1-8]. This forward transfer occurs in less than 1 picosecond, thereby inhibiting the luminescence as well as intersystem crossing to the triplet manifold [4, 10]. Thus, the quantum efficiency of this molecular photoelectron transfer is near unity t. Using this i n t e r m o l e c u l a r photoinduced charge transfer from semiconducting conjugated polymers (as donors) onto C60 at the interface of a bilayer device we were able to fabricate diodes with rectification ratios of approximately 104 which operated both as photodiodes and as photovoltaic cells [2]. The power conversion efficieny of the initial heterojunction devices was 0.04%. Today, solar cells of electron to photon efficiency of about 20% and a power conversion efficieny of 1 percent have been realized [ 11 ]. These Schottky devices based on the photoinduced electron transfer sensitized photogenerated carrier efficiency perform well as photodiodes with quantum yields around 80% at -15V and 540nm competetive to UV sensitized Si photodetectors [11]. Thus, the molecular photoeffect discovered in these supramolecular composites has a promising technological potential. The photoinduced electron transfer between conjugated polymers and C60 has been studied in detail. The photophysics of the donor plays a major role as discussed for degenerate ground state polymers (like polyacetylene PA) [3] as well as excitonic systems such as polydiacetylenes (PDA) [9]. Furthermore, the energetics of the donor and the acceptor plays a crucial role as described by the Marcus theory [ 12]. The effect of the surrounding medium is another important parameter for molecular charge transfer phenomena [ 12]. How do all these effect the photophysics of the supramolecular composite between conjugated polymers and 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02874-X

C6o? This question will be addressed below with comparative studies. All experimental details are listed in references [1-10],

2. RESULTS AND DISCUSSION 2.1 Time resolved absorption (PIA) studies

femtosecond

photoinduced

a.) In poly(3-octylthiophene) (P3OT) there are two P1A bands at 1.2 and 1.9 eV which decay with 1.2 ps lifetime as the inital component. Upon adding 1% C60 into the polymer the spectrum completely changes rendering a single broad PIA band centered at around 1.55 eV. This peak has a much longer lifetime compared to the PIA bands in the pristine polymer spectra decreasing only a factor of 3 after 500 ps [4, 10]. This broad band shifts to 1.45 eV within 500 ps and locks into that spectral position where it persists upto milliseconds as observed with chopped photoexcitation spectroscopy [3]. This observation clearly shows that the photoexcited species which are created within 1 ps persist up to millisecond where spin resonance experiments show the two radical ions [ 1, 3]. The rise time of this 1.55 eV PIA band is not resolved in the spectra in [4]. To actually resolve the forward electron transfer a one wavelength dichroic ratio experiments has been carried out [10]. The dichroic ratio is defined as the ratio of the photoinduced absorption with the pump and probe polarization parallel to that with the polarization perpendicular. For electron-hole pairs initally excited on single chains dichroic ratio should be 3 and as contributions from interchain excitations increase and the polarization memory gets lost, the dichroic ratio should approach unity. The results show an initial dichroic ratio of 2.5 to 2.7 for all the samples under investigation [10]. The pristine P3OT shows a

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decrease of the dichroic ratio down to 1.5 within 10 ps. Upon adding 1% C60 this process is heavily quenched and the polarization memory is lost at around 300 fs (down to 1.5)[10]. This is the first direct observation of the forward photoinduced electron transfer in conjugated polymer/C60 composites and determines the electron transfer time at 300 femtoseconds [10]. Due to the electron transfer the intrachain excitations are quenched by transferring an electron onto C6o and rendering polarization transfer perpendicular to the excitation. b.) In the case of soluble derivatives of poly(para phenylene vinylene), e.g. MEH-PPV and BCHA-PPV the dichroic ratio shows similar behaviour as discussed above for the P3OT. Starting with values as high as 2.7 initially, the dichroic ratio decreases to 1.5 within 20 ps in pristine polymers and within 300 fs for MEH-PPV/C60 composite. For the BCHA-PPV/C6o composite, however, the dichroic ratio decreases slower than in the case of MEH-PPV/C60. This difference of the side chain functionalizationon the photophysics of the composite may arise from the bulky side chains of the BCHA-PPV compared to MEHPPV [10]. Furthermore, a decrease in dimensionality for the BCHA-PPV is more pronounced due to lower interchain coupling as a result of these bulky sidechains. The spectral differences between pristine MEH-PPV and BCHAPPV and their composites with C60 further support an ultrafast photoinduced electron transfer. c.) In polydiacetylenes (PDA) the femtosecond photoinduced absorption spectrum shows no detectable difference between the pristine polymer and its composite with C60 [9]. Our efforts to detect any changes of the photophysical properties upon adding C60 into PDA with picosecond photoconducitivity, millisecond photoexcitation spectroscopy experiments failed, so far [9]. A slight increase in steady state photocunductivity has been observed due to increased disorder in the composite. The solubility of C60 in films of PDA's PPV's and P3OT's is unknown and a possible decreased solubility in PDA's compared to other conjugated polymers might be relevant. On the other hand, in heavily C60 loaded samples of PPV's and P3OT's which exhibit partial phase segregation due to lower solubility of the C60 component, the ultrafast photoinduced electron transfer is still clearly observable. Therefore, we cannot attribute the complete absence of the electron transfer in PDA/C60 composites to sample morphology effects. Moreover, since all the data from the PDA's were obtained from disordered films cast from solution (rather than from single crystals) disorder in PPV's and the P3OT's can no longer be invoked as the origin of the qualitatively different phenomena 19[.

Why is the photoinduced electron transfer so strongly inhibited in polydiacetylenes? The energetics of the donor and the acceptor components and their relative positions of the various energy levels are important parameters of the photoinduced electron transfer phenomena [12]. The ionization potentials of the PDA's are around 5.5 eV, nearly identical to the ionization potential of polythiophenes (5.2 eV) and very close to PPV's (5.1 eV)[9]. The optical gaps are comparable. Furthermore, the effect of the dielectric constant cannot provide a decisive difference; the real parts of the dielectric constants of the PPV's and PDA's have nearly identical magnitudes [9]. Thus, the extraordinary difference --ultrafast photoinduced electron transfer in the PPV's and the complete inhibition of ultrafast photoinduced electron transfer in PDA's-- must have its origin in the photophysics of polydiacetylenes [91. We proposed that the strong exciton binding energy in PDA's to be the photophysical origin of the observed hindrance of the photoinduced electron transfer in these systems [9]. To achieve a complete charge separation the exciton binding energy should be overcome. The 0.5 eV exciton binding energy in PDA's is sufficiently large to inhibit the photoinduced electron transfer. The origin of the relatively large binding energy in the PDA's is somewhat of a puzzle, since the n-electron densities are comparable to PPV's [9]. We note, however, that valence bond arguments imply that the triple bonds in the PDA's tend to confine the charged excitations and thereby prevent separation of the electron-hole pair [9]. Such confinement effects will increase the magnitude ot the exciton binding energy and thereby inhibit the photoinduced electron transfer. d.) In degenerate ground state polymers (PHDK, soluble derivative of polyacetylenes [3, 10]) the photoinduced electron transfer also seems to be absent. We could not observe any difference in pristine polymer and its composite with C60 in femtosecond PIA, picosecond photoconductivity, millisecond photoexcitation spectroscopy studies. A slight increase in steady state photocunductivity has been observed due to increased disorder in the composite. We attribute this effect also to the photophysics of the donor conjugated polymer and in this case to the rapid formation of solitons with energy eigenstates deep in the gap, and thus inhibiting the electron transfer due to energetic stabilization [3]. Since the soliton formation is an ultrafast process (<1 O0 fs [13]) this process will cut off the photoinduced electron transfer [3].

N.S. Sariciflci, A.J. Heeger / Synthetic Metals 70 (1995) 1349-1352

2.2 Photoinduced Electron Transfer in Solution:

Recently, triplet excitations have been demostrated as the primary photoexcitations in poly(3 alkylthiophenes) (P3AT) in millisecond photoinduced absorption studies [8, 14]. The triplet state has been characterized as a single P1A band which is readily observable in various solvents 18, 14]. In contrast to previous studies in films, however, the effect of adding C60 in non-polar solutions of P3AT's is an efficient energy transfer rather than an electron transfer [8, 14]. The results clearly demonstrate that the mixtures in p-xylene as solvent result in an efficient quenching of the P3AT triplet-triplet absorption and a strong enhancement in C60 triplet-triplet absorption. This effect can be explained through an efficient triplet energy transfer from conjugated polymer triplet state onto C6o which then is excited into its own triplet state [8, 14]. On the other hand in more polar solvents such as ortho-dicholorobenzene (ODCB), photoexcitation of P3AT in mixture solutions result in charge separation as in the films with clear PIA signatures of the positive polaron on the P3AT backbone and C6o-[8, 14]. This observation demonstrates that the nature of the solvent (or medium) can play an important role. A solvent with high polarity will stabilize the charged reaction products. This observation, on the other hand, opens up the discussion of the medium effects on the highly efficient photoinduced electron transfer in films of P3AT's, PPV's mixed with C60. In this case, conjugated polymer not only acts as an electron donating group but also as a host matrix with very high polarizability. Thus, any photophysical reaction which produces charges, will be highly favored due to strong polarization of the medium around these charges and the resulting screening effect. This may not only favor the charged photoexcitations but stabilize them as well, in agreement with the observed metastability of the observed photoinduced charge separation in conjugated polymer/C60 composites at low temperatures. 2.3 Photoinduced Mixtures

Electron T r a n s f e r in O l i g o m e r / C 6 0

We extended our studies to oligomers of thiophene with well defined chemical structure [15, 16]. Comparative studies with oligomers of different chain lengths (n=6, 7, 9, 11) in various solvents and with different acceptors like C60 and tetracyanoethylene (TCNE) clearly demostrate that an efficient triplet energy transfer (e.g. triplet sensitization of C60) occur with oligothiophenes. The efficieny of the energy transfer somehow depends on the chain length of the oligomer [15, 16]. For the six membered oligomer (T6) a complete quenching of the triplet state is observed upon adding C6o in p-xylene solutions, whereas at the same conditions triplet signale of T 9 and T I I remain to a some

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extent [15, 16]. Furthermore, adding C60 does not alter the photoluminescence of the oligomer solutions in p-xytene, clearly demonstrating that the singlet excited state is unaltered upon adding C6o I15, 16]. In ODCB solutions, on the other hand, charged excitations are clearly favored and adding C6o quenches the triplet state rendering charged species as evidenced in light induced electron spin resonance experiments [15]. Stronger electron acceptors like TCNE almost always yield a photoinduced charge separation in solutions. Triplet energy transfer is not favored due to higher lying triplet state of the acceptor [ 15]. 3. CONCLUSION & O U T L O O K In accordance with studies in other groups [17-201 the photoinduced electron transfer from semiconducting, conjugated polymers onto C60 is ultrafast, reversible and metastable. Comparative studies yield valuble information about several parameters: i.) Photophysics of the donor polymer is important. Ultrafast soliton stabilization in degenerate ground state polymers as well as exciton binding energy as a barrier in PDA's inhibit the photoelectron transfer heavily. ii.) The effect of the conjugated polymer as a highly polarizable matrix in addition to its role as donor unit is shown to be important for the efficiency as well as stabilization of the charge separated state. iii.) Oligomers act as donors, similar to conjugated polymers. iv.) Photoinduced electron transfer from conjugated polymers is not limited to C60 as an acceptor. Stronger electron acceptors like TCNE may increase the efficiency. For devices applications, however, an acceptor unit which becomes highly conducting upon adding an electron is important in heterojunction diodes and solar cells [2]. For Schottky devices which consist of a conjugated polymer/acceptor composite sandwiched between metal electrodes of different workfunction, this restriction is less important. In these devices a high efficieny photoinduced electron transfer readily sensitizes the photoconducivity and inhibits the geminate recombination, yielding high collection efficiencies [ 11 ]. As discussed above this photoinduced electron transfer is also observed in oligomer/C60 composites. Since vacuum sublimation

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method can be utilized for both oligomers of conjugated polymers as well as C60. it is conceiveable that one can grow oligomer (donor) and C60 (acceptor) multiple quantum well heterostructures by alternating source in vacuum deposition process or by fabrication of alternating Langmuir-Blodgett films. The photoinduced electron transfer from semiconducting polymers (as donors) to C6o (as acceptor) summarized here provide opportunities for utilizing the charge separated state for photoelectrochemistry and for molecular optoelectronics; virtual excitations (in the sense of perturbation theory) may provide a mechanism for nonlinear optical response. The photoinduced charge separation may result in large second harmonic generation on oriented, ordered interfaces. As interdisciplinary research, which includes organic chemistry, organic solid state physics, polymer chemistry, electronic engineering and many overlapping areas among the traditional research areas, the study of photoinduced charge transfer between semiconducting polymers and C60 represents both an important scientific opportunity and a difficult challenge.

Acknowledgements This work is supported by Department of Energy (DOE No: DEFG0393ER12138). We gratefully acknowledge our coworkers at the Institute for Polymers & Organic Solids, F. Wudl, G. Srdanov, L. Smilowitz, D. Braun, B. Kraabel, C. H. Lee, G. Yu, K. H. Lee, R. A. J. Janssen, M. P. T. Christiaans, D. Moses, K. Pakbaz.

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