Bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaics (OPV). In order to improve the incident photon-to-current efficiency (IPCE) of the BHJ solar cell, a comprehensive understanding of the ultrafast dynamics of excited species, such as singlet exciton (D*), interfacial charge-transfer (CT) state, and carrier (D+), is indispensable. Here, we performed femtosecond time-resolved spectroscopy of two prototypical BHJ blend films: poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend film and poly(9,9′-dioctylfluorene-co-bithiophene) (F8T2)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) blend film. We decomposed differential absorption spectra into fast, slow, and constant components via two-exponential fitting at respective probe photon energies. The decomposition procedure clearly distinguished photoinduced absorptions (PIAs) due to D*, CT, and D+. Based on these assignments, we will compare the charge dynamics between the F8T2/PC70BM and P3HT/PCBM blend films. 1. Introduction Organic photovoltaics (OPV) is an environmentally friendly and low-cost technology, which converts the solar energy into electric one. The incident photon-to-current efficiency (IPCE) of the bulk heterojunction (BHJ) solar cell [1, 2] is governed by the three processes: (1) charge formation process at the donor (D)-acceptor (A) interface, (2) charge transport process within the organic semiconductor, and (3) charge collecting process on the Al and indium tin oxide (ITO) electrodes. The femtosecond time-resolved spectroscopy is one of the powerful tool to reveal the (1) charge formation dynamics, because we can trace the ultrafast dynamics of excited species, such as singlet exciton (D*), interfacial charge-transfer (CT) state, and carrier (D+) [3–5]. The photoirradiation of the D polymer (A molecule) excites an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). We call such a photoexcited D (A) state as excitons D* (A*). The D* (A*) state migrates within the D domain (or A domain) to reach the D-A interface. At the interface, the charge transfer between D and A produces an intermediate state (CT state). The CT state consists of electrostatically bound charge pairs, where the hole is primarily localized on the D HOMO and the electron on the A LUMO. Finally, the charge separation takes place to produce free carriers D+ (A？). Historically, extensive spectroscopic investigations [3–12] have been carried out on the
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