Ultrafast Spectroscopy of Charge Generation in Polythiophene/Fullerene Blends for Organic Photovoltaic Applications

We present a detailed spectroscopy study of ultrafast photogeneration of charges in polymer/fullerene blends, such as regio-regular polythiophene (RR-P3HT), regio-random polythiophene (RRa-P3HT) and fullerene derivatives such as C$_{61}$ (PCBM), using the transient pump-probe photomodulation (PM) spectroscopy with $\sim $ 100 fs resolution. These blends serve as active layers in organic photovoltaic devices with high power conversion quantum yield (up to 6 {\%}), due to a photoinduced charge transfer (PCT) reaction between the polymer and the fullerene molecules. Our transient PM spectrum spans a broad energy range from 0.1-2.4 eV, and this allows us to monitor the transient behavior of the various photoinduced absorption (PA) bands of polarons and excitons; as well as photobleaching (PB) of the ground state. In order to understand the nano-morphology of these active layers, which also affects their photophysics, TEM images are also presented. Various processes such as PCT, geminate recombination, and energy transfer from the polymer to the fullerene phase are unraveled.