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C: Energy Conversion and Storage; Energy and Charge Transport
Effect of the Polymer Chain Arrangement on Exciton and Polaron Dynamics in P3HT and P3HT:PCBM Films Oleg Petrovich Dimitriev, David A Blank, Christian Ganser, and Christian Teichert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05155 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018
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The Journal of Physical Chemistry
Effect of the Polymer Chain Arrangement on Exciton and Polaron Dynamics in P3HT and P3HT:PCBM Films Oleg P. Dimitriev&*, David A. Blank&, Christian Ganser#, Christian Teichert# *
V. Lashkaryov Institute of Semiconductor Physics, Natl. Acad. of Sci. of Ukraine. Prospect Nauki, 41, Kiev-03028, Ukraine. E-mail:
[email protected]; &
Department of Chemistry, University of Minnesota, 207 Pleasant St SE, Minneapolis, Minnesota 55455, USA #
Institute of Physics, University of Leoben, Franz-Josef Strasse 18, A-8700 Leoben, Austria
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Abstract. Arrangement of polymer chains at the interface with an acceptor is one of the key issues which influence the interfacial charge transfer. Films of P3HT and P3HT:PCBM blends with the different polymer chain arrangement have been prepared from one-component and binary solvents representing a mixture of chloroform and different poor solvents with high boiling temperature. Electronic absorption and photoluminescence spectra evidenced in favor of reduced disorder of P3HT films as a result of use of poor solvents; the ordering was displayed through structuring and narrowing the spectral bands, indicative of decreasing width of Gaussian distribution of molecular transition frequencies. At the same time, transient absorption of singlet exciton showed that exciton decay in highly ordered P3HT films slows down as compared to the disordered film and this effect was reproduced for the different poor solvents used. A more pronounced effect was revealed in P3HT:PCBM blends where much faster decay of exciton was found in disordered as-prepared P3HT:PCBM film from chloroform as compared to the annealed film or films prepared from the binary solvents. A complex behavior of polarons in the different regions of the blend film, i.e., within crystalline domains of P3HT and at the interface of P3HT/PCBM, was observed. The conclusion was drawn that chain disorder induces easier exciton dissociation and charge transfer at the P3HT/PCBM interface.
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The Journal of Physical Chemistry
1. Introduction One of the most important issues in construction of the efficient organic solar cells (OSCs) based on conductive polymers is the arrangement of the polymer chains. Inter- and intramolecular arrangement of polymer chains is defined by structural organization of the polymer system, such as polymer chain regularity, intramolecular conformational changes, intermolecular aggregation and packing, which are a critical factor influencing the absorption and emission spectrum, as well as charge transport in the system. Particularly, excitation in poly(3-hexylthiophene) (P3HT) is known to depend on its chain regularity; in regiorandom chains the primary excitations are intrachain excitons which give rise to a moderately strong photoluminescence (PL) band, whereas in regioregular chains the excitons are more delocalized through interchain interaction which results in lack of stimulated emission, vanishing intersystem crossing, and very weak PL band 1,2. There is good understanding of importance of structural organization of polymer chains in photovoltaic (PV) cells based on polymer donor - acceptor bulk heterojunctions, including those based on the blend of P3HT and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Particularly, it has been shown that morphology of the film has a great influence on ultrafast free carrier generation in the OSCs 3,4,5
. A parallel ordering of regioregular P3HT chains into the lamellar structure in thin films results in
improved performance of the corresponding PV cells. At the same time, although lamellar arrangement leads to better charge transport in the system, there is an open question how the structural arrangement of the polymer chains influences the electron transfer rate from the donor polymer to the acceptor PCBM component. On the one hand, it was reported that charge transfer (CT) efficiency at P3HT/PCBM interface does not depend on specific regularity of P3HT chains, whereas charge dissociation is better for regioregular P3HT chains 6. On the other hand, it was shown recently that the use of regiorandom instead of regioregular P3HT leads to more effective electron transfer from P3HT to PCBM 7, which means that disorder can facilitate CT processes 8. This suggestion was also supported earlier by theoretical models 9,10. While it is well known that structural and energetic disorder normally suppresses CT along the polymer chain
11,12,13
, the effect of chain disorder on electron transfer from
donor to acceptor moiety has not been studied thoroughly and to our knowledge only few experimental 14,15,16
and theoretical 17,18 works dealt with this problem. The problem is complicated with the fact that
structural disorder in molecular systems is dynamic and sensitive to photoexcitation. For conjugated polymers, whose chains are usually torsionally disordered around nonplanar equilibrium conformation in the ground state, photoexcitation gives rise first to “hot” exciton which has a limited delocalization within the disordered chain, but which then rapidly converts to a more delocalized “cold” exciton after a torsional relaxation into planar geometries on a picoseconds time scale (1-10 ps) 19,20. The lifetime of the “cold” excitons is longer, in the 0.1-1 ns time scale, but these can be self-trapped into geometrically relaxed aggregate state in clusters of macromolecules as a result of chain-chain interactions 21,22,23. As a 3 ACS Paragon Plus Environment
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result, quantum efficiency of photoluminescence of P3HT aggregates, for example, is about one order of magnitude lower than from P3HT isolated chains 24; that means that chain aggregation, even into the ordered lamellar structure, quenches excitation energy over the chains without its effective transfer to the acceptor counterpart because of self-trapping of excitons on the polymer aggregate. Therefore, in such a structure only “hot” excitons which arise due to initially generated non-equilibrium excitation should be effective to contribute to the electron transfer process to the acceptor counterpart. On the other hand, CT itself in the efficient PV devices should be able to proceed very fast, within a picosecond lifetime of the “hot” excitons. Therefore, one of the main problems in OSCs is the selection of appropriate “fast” electronacceptors which could be able to accept an electron within the short lifetime of the “hot” exciton. Application of “fast” electron-acceptors, such as fullerene derivative PCBM as an acceptor in P3HT PCBM solar cells is very effective, since this fullerene derivative is able to accept an electron within less than a picosecond time scale 25,26,27, which guarantees that the quantum efficiency for the CT process at this interface approaches unity
28,29,
resulting in electrons left on the PCBM network and holes on the
polymer network, respectively. On the other hand, the response speed of CT from donor to some inorganic electron acceptors, such as CdS, TiO2, etc., can extend into a nanosecond scale 30,31. Therefore, extension of material range application in solar cells, particularly due to application of “slow” acceptors, requires involvement of more slow CT processes as well, i.e., utilization of not only “hot” but also “cold” excitons whose lifetime constants are within the range of 0.1-1 ns 32. This problem can be solved by use of factors which could delay relaxation of photoexcited donor polymer chain into undesirable planar conformational configuration. The goal of this work is to study the influence of spatial organization of the donor polymer material in general terms of the polymer chain ordering on the exciton dynamics and decay, where the latter proceeds mainly through CT processes at the donor P3HT - acceptor PCBM interface. The method to vary the polymer chain arrangement here is the addition of a solvent with high boiling temperature during film casting. Such a method has already been used for preparation of polymer solar cells which was shown to assist in optimal organization of donor polymer and acceptor PCBM components in bulk heterostructure: on one hand, it was reported that the solvents with high boiling temperature result in a higher degree of organization in the structure of P3HT formation of PCBM crystalline domains
34
33
, on the other hand, such solvents facilitate
, resulting in overall improvement of power conversion
efficiency (PCE) of OSCs 35. In this work, we clarify how the effect of polymer chain ordering influences exciton decay and polaron dynamics in the neat P3HT and blend P3HT:PCBM films.
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The Journal of Physical Chemistry
2. Experimental 2.1. Sample preparation. P3HT, PCBM and solvents used were obtained from Sigma-Aldrich. Initial stock solutions of regioregular P3HT (~93% RR, 99,995% trace metal basis, number-average molecular weight Mn ~ 15-45 kDa) and P3HT:PCBM (1:1 weight ratio) were prepared in chloroform (CF) with concentration of 0.5 wt%. Binary mixtures were prepared by addition of a small amount of toluene (20 vol%), aniline (10 vol%) or dimethyl-aniline (DMA, 10 vol%) to the CF stock solution of P3HT or P3HT:PCBM. Toluene, aniline and DMA are known to be poor solvents for P3HT (solubility
