Structure optimization of CH₃NH₃PbI₃ by higher-valence Pb in perovskite solar cells with enhanced efficiency and stability

The crystal structure of perovskite has a significant influence on the photovoltaic performance and stability of perovskite solar cells (PSCs). Pb⁴⁺ is introduced into CH₃NH₃PbI₃ (MAPbI₃)-based PSCs by participating the octahedral [PbI₆]^4- structure, then to induce the formation of stronger Pb-I bond and reduce [PbI₆]^4- octahedron distortion, which would improve structural symmetry, decrease the degree of disorder and be beneficial to the crystallization of perovskites. Synchrotron based X-ray absorption fine spectroscope (XAFS) revealed that the existence of higher valence state lead could be realized by doping Pb⁴⁺ directly or induced by electron withdrawing group, in consequence the [PbI₆]^4- octahedral structure becomes more stable. Grazing incidence X-ray diffraction (GIXRD) especially demonstrates that Pb⁴⁺ with appropriate proportion may well replace part of Pb²⁺ to form an uniform phase in the primal perovskite structure to improve the crystallization on the surface and homogeneous out-of-plane (OOP) ordered crystal accumulation in the bulk, which is also important for improving the efficiency and stability of PSCs. As a result, a power conversion efficiency (PCE) exhibits a 42.1% increase with the doping of 0.03% PbF₄ and 0.075% PCBM compared with a pristine device and its stability improves markedly after 30 days of storage in ambient atmosphere.