Probing the key roles of the back interface in the performance of carbon-based hole-transport-layer free perovskite solar cells

Carbon electrodes have gained widespread attention as a sustainable, stable, and low-cost alternative to metal electrodes in perovskite solar cells (PSCs). However, the power conversion efficiency (PCE) of carbon electrode-based PSCs (C-PSCs) without the hole-transport-layer (HTL) lags far behind their metal-electrode-based counterparts (M-PSCs), and the key factors causing this PCE downgrading have not been comprehensively elucidated. Herein, we study the photovoltaic performance of various HTL-free C-PSCs employing four typical absorbers, namely MAPbI 3 (MA = CH 3 NH 3 ), FAPbI 3 (FA = CH(NH 2 ) 2 ), one-step processed FA 0.85 MA 0.15 PbI 3 (FA/MA-OS), and two-step processed FA 1− x MA x PbI 3 (FA/MA-TS). Unexpectedly, we found that the PCE of C-PSCs follows the order MAPbI 3 > FAPbI 3 > FA/MA-TS > FA/MA-OS, quite different from that of devices with the Ag-electrode (FAPbI 3 > FA/MA-TS > FA/MA-OS > MAPbI 3 ). The in-depth studies reveal that the remarkable differences in surface roughness, surface potential (SP) distribution, and local built-in potential ( V bi ) of the four absorber films directly affect both the physical and electrical contacts between the perovskite and carbon electrode, which finally determine the efficiency of C-PSCs. Among them, the MAPbI 3 films possess the smallest roughness and minimum SP gaps between the grain boundaries (GBs) and the grain interiors (GIs), which enable compact contact at the perovskite/carbon interface and higher V bi within the C-PSCs for fast charge transfer, significantly suppressed nonradiative recombination, and thus the highest PCE (15.42%). Based on these findings, we provide some promising approaches for the development of high-efficiency C-PSCs, especially for the ones employing FA-based perovskite absorbers which have performed excellently in M-PSCs.