Bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaics\r\n(OPV). In order to improve the incident photon-to-current efficiency (IPCE) of the BHJ solar cell, a comprehensive understanding\r\nof the ultrafast dynamics of excited species, such as singlet exciton (D*), interfacial charge-transfer (CT) state, and\r\ncarrier (D+), is indispensable. Here, we performed femtosecond time-resolved spectroscopy of two prototypical BHJ blend films:\r\npoly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend film and poly(9,9-dioctylfluorene-cobithiophene)\r\n(F8T2)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) blend film. We decomposed differential absorption\r\nspectra into fast, slow, and constant components via two-exponential fitting at respective probe photon energies. The decomposition\r\nprocedure clearly distinguished photoinduced absorptions (PIAs) due to D*, CT, and D+. Based on these assignments, we\r\nwill compare the charge dynamics between the F8T2/PC70BM and P3HT/PCBM blend films.
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