Photoresponse of donor/acceptor blends in organic transistors: a tool for understanding field-assisted charge separation in small molecule bulk heterojunction solar cells.

Photoresponse and ambipolar charge transport in organic bulk heterojunctions (BHJ) is investigated using field-effect transistors (FET) based on two donors, poly(3-hexylthiophene) (P3HT) and 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu)2) blends with [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) acceptor. Upon 100 mW/cm2 AM 1.5 G illumination, P3HT:PC70BM shows an equivalent hole and electron current together with a largely enhanced photoresponse in the FET. The DPP(TBFu)2:PC70BM blends display an electron-dominating transport along with showing a relatively poor photoresponse in FETs upon irradiation. By comparing the two systems, it suggests that DPP(TBFu)2:PC70BM possesses a less-efficient charge separation assisted by electric fields after exciton dissociation. The FET results correlate well to the solar cell device performance and provide further understanding and optimizing of solution-processed DPP small molecule solar cells.