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

Abstract

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-C<inf>61</inf>-butylic acid methyl ester (PCBM) and with [6,6]-phenyl-C<inf>71</inf>-butylic acid methyl ester (PC<inf>70</inf>BM) 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:PC<inf>70</inf>BM 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 PC<inf>70</inf>BM blended film compared with PCBM blended film. © 2015 Elsevier B.V. All rights reserved.