Reducing carrier transport barrier in anode interface enables efficient and stable inverted mesoscopic methylammonium-free perovskite solar cells

Exploitation of novel inorganic hole-transport materials (HTM) is one of the most promising approaches that achieve stable and efficient perovskite solar cells (PSCs). However, the low conductivity and large carrier transport barriers of the commonly used HTMs/perovskite system severely limit the device performance. Herein, Mg²⁺ doped CuCrO₂ (M:CCO) nanocrystals are synthesized using a hydrothermal method and further adopted as the HTM for inverted PSCs. The hole conductivity of CuCrO₂ is substantially enhanced upon ion doping. To address the anode/perovskite interfacial contact issues, the resultant M:CCO is designed to be a mesoporous structure atop an ultrathin compact NiO_x film (5 nm). This mesoscopic structured bilayer hole-transport layers (HTLs) with well-matched graded energy levels effectively shorten the charge transport pathway, reduce carrier transport barrier in HTL/perovskite interface, and improve the light harvesting efficiency. Combined with the design of a top electron-transport layer and the usage of methylammonium (MA)-free perovskites, a champion PCE of 21.64% has been achieved, which is among the highest efficiencies for inverted MA-free, cesium/formamidinium-based PSCs to date. Moreover, the unsealed device shows excellent long-term stability: more than 91% and 80% of their initial efficiencies were retained after light soaking under 1 sun illumination for 400 h and thermal aging at 85 °C for 1000 h, respectively. This study provides a potential strategy for developing highly efficient and stable inverted PSCs.