Enhancement of open circuit voltage for CuSCN-based perovskite solar cells by controlling the perovskite/CuSCN interface with functional molecules

Abstract

CuSCN, a low-cost inorganic hole transporting material (HTM), exhibits notably high hole-mobility and material stability, but turns out to show significantly lower open circuit voltage (V_OC) than organic hole-conductors in its application as an HTM in perovskite solar cells (PSCs). Herein, for the enhancement of V_OC, various functional molecules were introduced on the surface of a CH₃NH₃PbI₃ (MAPbI₃) layer to passivate the defects and to improve the contact between MAPbI₃ and CuSCN layers. In particular, by introducing a mixture of 3-pyridyl isothiocyanate (Pr-ITC) and phenylene-1,4-diisothiocyanate (Ph-DITC), the V_OC of the CuSCN-based PSC (PSC-CuSCN) was increased over 40 mV, due to their unique molecular structures possessing two separate functional groups that can interact with MAPbI₃ as well as CuSCN. By this interfacial treatment, the fabricated CuSCN-based PSC device exhibits an average photovoltaic conversion efficiency (PCE) of 18.57% (PCE of 19.17% for the champion device), which is very close to the photovoltaic performance of the PSC with spiro-OMeTAD (PSC-spiro, average PCE of 19.03%). Also, the hysteresis in J–V measurements has been minimized, suggesting no appreciable charge accumulation at the perovskite/CuSCN interface by successful passivation of defects. Moreover, the long-term stability of PSC-CuSCN devices was further improved: after 200 days at a relative humidity of 30 ± 5%, 86% of the initial PCE remained for the PSC-CuSCN with surface treatment, whereas only 73% and 56% remained for the bare PSC-CuSCN and PSC-spiro, respectively.