Boosting the Conversion Efficiency Over 20% in MAPbI₃ Perovskite Planar Solar Cells by Employing a Solution-Processed Aluminum-Doped Nickel Oxide Hole Collector

Recently, nickel oxide (NiO_x) thin films have been used as an efficient and robust hole transport layer (HTL) in inverted planar perovskite solar cells (IP-PSCs) to replace costly and unstable organic transport materials. However, the power conversion efficiency (PCE) of most IP-PSCs using NiO_x HTLs is rather limited below 20% due to insufficient electronic conductivity of the NiO_x. In this work, solution-processed Al-doped NiO_x (ANO) films are suggested as HTLs for low-cost and stable IP-PSCs. The electrical conductivity of the NiO_x film is significantly enhanced by Al doping, which effectively reduces the nonradiative recombination losses at the HTL–perovskite interfaces and boosts hole extraction/transportation. The device with undoped NiO_x shows the best PCE of 16.56%, whereas ANO HTL (5% doping) contributes to achieving a PCE of 20.84%, which outperforms other CH₃NH₃PbI₃ IP-PSCs with NiO_x-based HTLs reported to date. Moreover, a reliability test (1728 h storage) shows that the performance stability is enhanced by approximately 11% by employing ANO HTLs. This investigation into ANO HTLs provides a new guideline for the further development of highly efficient and reliable IP-PSCs using low-cost and robust metal oxide HTLs.