Abstract
A bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaic (OPV). We investigated femtosecond charge dynamics after D (at 633 nm) and A (at 400 nm) excitations in a prototypical low-band-gap and highly efficient OPV, i.e., poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM). The photoinduced absorption (PIA) spectra of the blend film were decomposed into three PIAs, i.e., donor exciton (D*), acceptor exciton (A*), and carrier (D+) components. The analysis revealed that D*→D+ conversion predominates in 633 nm excitation, while A*→D+ conversion predominates in 400 nm excitation. The carrier formation times of both processes are nearly the same (τrise∼0.2–0.3 ps).
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