Thermally stable methylammonium-free inverted perovskite solar cells with Zn2+ doped CuGaO2 as efficient mesoporous hole-transporting layer

Highlights: • Inorganic mesoporous hole transporting layer (HTL) has been designed. • Zn²⁺ doping significantly increase the conductivity of CuGaO₂ HTL. • 20.67% efficient cesium-formamidinium (Cs-FA) double-cation perovskite based device has been achieved. • The Zn:CuGaO₂ based unsealed device exhibits superior long-term thermal stability. -- Abstract: Despite incredible success has been achieved for perovskite solar cells (PSCs) in pursuing high power conversion efficiency (PCE), their practical application is prevented by the low stability issues, especially an accelerating stability test at high temperature still needs to be demonstrated. Herein, we present an inverted mesoscopic PSCs with Zn²⁺ doped CuGaO₂ (Zn:CuGaO₂) as both the scaffold and hole transporting materials (HTM). Both theoretical and experimental results indicate that the carrier density and conductivity of CuGaO₂ is significantly improved via Zn²⁺ doping, which is beneficial for the hole transfer. Moreover, the mesoporous structure combined with the well matched energy levels between Zn:CuGaO₂ and perovskite can effectively extract holes from perovskite, reduce charge transfer barrier, and depress the charge-carrier recombination. As a result, the champion device with Zn:CuGaO₂ as HTM gives a power conversion efficiency of 20.67% from reverse scan and a stabilized efficiency of 20.15%, which is among the best results for PSCs based on methylammonium-free, cesium-formamidinium (Cs-FA) double-cation perovskite and inorganic HTM. Moreover, PCE of the unencapsulated device retains over 85% after thermal annealing at 85 °C for 1000 h in a nitrogen atmosphere, demonstrating the superior thermal stability of the present PSCs with the metal doped inorganic HTM.