Solution-processed, silver-doped NiO_x as hole transporting layer for high-efficiency inverted perovskite solar cells

NiO_x is as a promising hole transporting layer (HTL) for perovskite solar cells (PSCs) due to its good stability, large bandgap, and deep valence band. The use of NiO_x as a HTL for "inverted" PSC as part of a monolithic silicon/perovskite tandem solar cell is also suitable when the processing temperature is suitably low. Solution-processed NiO_x at low temperature for PSCs remains to be improved due to the relatively low short-circuit current density (J_sc) and fill factor (FF) of reported devices. In this work, the use of Ag-doping is reported for solution-processed NiO_x film at 300 °C for inverted planar PSCs. We have shown that Ag-doping has no negative effect on the optical transmittance and morphology of the NiO_x film and the overlying perovskite film. In addition, Ag-doping is effective in improving conductivity, improving carrier extraction, and enhancing the p-type property of the NiO_x film confirmed by electrical characterization, photoluminescence measurements, and ultraviolet photoelectron spectroscopy. These improvements result in better devices based on the ITO/Ag:NiO_x/CH₃NH₃PbI₃/PCBM/BCP/Ag structure with improved average FF (from 69% to 75%), enhanced average J_SC (by 1.2 mA/cm² absolute) and enhanced average V_OC (by 29 mV absolute). The average efficiency of these devices is 16.3% while the best device achieves a PCE of 17.3% with negligible hysteresis and a stabilized efficiency of 17.1%. In comparison, devices that use undoped NiO_x have an average efficiency of 13.5%. This work demonstrates that silver is a promising doping material for NiO_x by a simple solution process for high-performance inverted PSCs and perovskite tandems.