Optical Studies of Photoexcitations in Polymer/Fullerene Blends for Organic Photovoltaic Applications

Abstract

We used a variety of optical probe techniques including broadband femtosecond transient and continuous wave (cw) photomodulation spectroscopies and electroabsorption for studying the photophysics in two typical π-conjugated polymers, namely regio-regular poly (3-hexyl thiophene) (RR-P3HT) with self-organized π-stacked two-dimensional lamellae, and 2-methoxy-5-(2′-ethylhexyloxy) poly(p-para-phenylene-vinylene) (MEH-PPV) with amorphous nanomorphology; both polymers in pristine and blend with fullerene molecules. In the pristine forms we identified singlet excitons as the primary photoexcitations, having typical photoinduced absorption (PA) band that is correlated with stimulated emission. In contrast, in polymer/fullerene blends the photogenerated excitons quickly decay giving rise to a novel photoexcitation species, and the stimulated emission is absent. For the blends we provide strong evidence for the existence of charge transfer complex (CTC) manifold that is formed inside the optical gap of the polymer and fullerene constituents, which is clearly revealed in the electro-absorption spectrum. Because the lowest energy CTC lies below the optical gap then it is possible to directly generate polarons in the blends without involving intrachain excitons in the polymer phase, when using below gap pump excitation. When using excellent quality materials we present evidence that the CTC states are populated in RR-P3HT/fullerene blend within 20 ps following exciton photogeneration in the polymer chains; but no charge polarons are generated on their expense up to ~2 ns. Interestingly the CTC states are photogenerated much faster in D-A blends having smaller domain size such as in regio-random P3HT/PCBM and MEH-PPV/C60; however the CTCs do not easily dissociate in these blends because of the large binding energy. Our findings indicate that the CTC state and film morphology play a crucial role in carrier photogeneration in donor-acceptor blends. More thorough investigation of the CTC and its interaction with free polaron excitations may improve the power conversion efficiency of organic solar cells and drive the development of novel photoactive materials.