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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