Insight into the energy loss in organic solar cells based on benzotrithiophene copolymers: A dark current analysis at low temperature

Abstract

Owing to the formation of the charge transfer (CT) state, the open-circuit voltage (Voc) of organic photovoltaic (OPV) devices commonly suffers an energy loss of 0.8–1.3 eV from the effective bandgap. Benzotrithiophene (BTT)-based low-bandgap polymers that we have recently reported showed deep HOMO levels (−5.4 to −5.6 eV) and moderate optical bandgaps of 1.7–1.8 eV, which resulted in high Voc’s of 0.78–0.98 V and relatively low energy losses when blended with methano[60]fullerene (PCBM). Here, we report the temperature-dependent dark current analysis of organic solar cells of BTT copolymers:PCBM blends. Shockley diode analyses revealed the dominant contribution of CT energy and concomitant pre-exponential factor of dark saturation current density associated with charge recombination. The findings could establish a fundamental aspect to draw a design rule in BTT-based polymers towards their evolutions in OPV devices.