Constructing graded heterojunctions for rigid and flexible inverted photovoltaics under outdoor and indoor illumination

Abstract

All inorganic perovskites (AIPs) have been widely used as an absorber for outdoor photovoltaics due to their high resistance toward thermal/light stress. Recently, CsPbI2Br with a wide bandgap of 1.93 eV has been considered optimal for indoor photovoltaics due to the higher efficiency limit than others. However, more often, inverted AIP devices suffer from serious charge recombination and inefficient carrier extraction, hence presenting unsatisfactory performance. In this work, solution-processable Bi2WO6 nanosheets (Nano-BWO) are prepared and deposited at the CsPbI2Br/PC61BM interface to form graded heterojunctions. A combined experimental and computational study reveals significant electron transfer from AIP to Nano-BWO and PC61BM due to their appropriate band structure offsets, leading to remarkably enhanced electron extraction in inverted devices. In addition, due to the defects passivation effect of Nano-BWO, the interfacial charge recombination is considerably reduced; meanwhile the electronic conductivity of the AIP absorber is remarkably enhanced. The resulting photovoltaics deliver a champion efficiency of 15.71% or 30.43% under one-sun (AM 1.5G) or white light emitting diode (WLED, 1000 lux) illumination. Furthermore, flexible devices with Nano-BWO achieve a high efficiency up to 13.74% (one-sun) or 24.50% (WLED). This work thus provides the first case of inverted AIP devices promising for indoor applications.