Achieving High Open‐Circuit Voltages up to 1.57 V in Hole‐Transport‐Material‐Free MAPbBr3 Solar Cells with Carbon Electrodes

Abstract An open‐circuit voltage ( V oc ) of 1.57 V under simulated AM1.5 sunlight in planar MAPbBr 3 solar cells with carbon (graphite) electrodes is obtained. The hole‐transport‐material‐free MAPbBr 3 solar cells with the normal architecture (FTO/TiO 2 /MAPbBr 3 /carbon) show little hysteresis during current–voltage sweep under simulated AM1.5 sunlight. A solar‐to‐electricity power conversion efficiency of 8.70% is achieved with the champion device. Accordingly, it is proposed that the carbon electrodes are effective to extract photogenerated holes in MAPbBr 3 solar cells, and the industry‐applicable carbon electrodes will not limit the performance of bromide‐based perovskite solar cells. Based on the analysis of the band alignment, it is found that the voltage (energy) loss across the interface between MAPbBr 3 and carbon is very small compared to the offset between the valence band maximum of MAPbBr 3 and the work function of graphite. This finding implies either Fermi level pinning or highly doped region inside MAPbBr 3 layer exists. The band‐edge electroluminescence spectra of MAPbBr 3 from the solar cells further support no back‐transfer pathways of electrons across the MAPbBr 3 /TiO 2 interface.