Femtosecond transient absorption dynamics in low bandgap polymer solar cell materials including poly(thienylenevinylene) derivative and benzothiadiazole moiety

•Ultrafast dynamics in specific low bandgap polymer solar cell materials was studied.•Charge transfer occurred in bulk heterojunction (BHJ) materials.•We observed an evidence of photo induced charge separation at PEBTBT/fullerene interface.•PC70BM blended BHJ showed slower geminate recombination than PCBM blended BHJ.

We report dynamics of photo-generated charge carriers in (E)-poly[2,2′-(1,2-ethenediyl)bisthiophene-alt-4,7-(2,1,3-benzothiadiazole)] (PEBTBT) donor/acceptor copolymer, and in PEBTBT:fullerene bulk heterojunctions (BHJs) blended with [6,6]-phenyl-C61-butylic acid methyl ester (PCBM) and with [6,6]-phenyl-C71-butylic acid methyl ester (PC70BM) by using femtosecond transient absorption (TA) spectroscopy. The TA decay dynamics appeared that both photo-induced bleaching and absorption of the PEBTBT/fullerene BHJs exhibited slower lifetimes compared to the decay of the pristine copolymer. This can be an evidence of photo induced charge separation at PEBTBT/fullerene interface. When the pump fluence exceeded over the threshold for exciton-carrier (and exciton-exciton) annihilation, within short time delays, the PEBTBT:PC70BM BHJ exhibited a bit faster decay by bimolecular process than the PEBTBT:PCBM BHJ. With regard to the decay property of interfacial charge transfer states, slower geminate recombination was expected in the PC70BM blended film compared with PCBM blended film.

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