Defects in solution-processed dithienylsilole-based small-molecule photovoltaic thin-films

Abstract

DTS-(FBTTh2)2 is a prominent solution-processable small-molecule donor for donor-acceptor bulk-heterojunction organic photovoltaics. Power conversion efficiency of DTS-(FBTTh2)2 based photovoltaic devices exceeds 8%. This paper reports on the distribution of sub-bandgap trap states in DTS-(FBTTh2)2. Trap states were probed using admittance spectroscopy and low-frequency capacitance-voltage profiling and analyzed using established theoretical models. Three distributions were revealed in the trap density of states energy spectra. Key observations were (1) thicker solution-processed films with higher drying time had 55% less traps than thinner films that dried relative faster (2) blending of DTS-(FBTTh2)2 with the acceptor PC70BM introduced traps at the center of the donor-acceptor interfacial bandgap. Charge carrier dynamics in DTS-(FBTTh2)2 based thin-films was also characterized using impedance spectroscopy.