Strain-relaxed tetragonal MAPbI₃ results in efficient mesoporous solar cells

In mesoporous organo-halide perovskite solar cells, the coexistence of tetragonal phase and cubic phase in the perovskite layer is found to result in strained crystal structure at the mesoporous TiO₂ (mp-TiO₂)/perovskite heterointerface. This results in structural defects pinholes, grain boundaries, and interfacial porosity which is detrimental towards photovoltaic performance and device stability. The strain at the interface originates from the lattice mismatch between the mp-TiO₂ nanoparticles (NPs) and perovskite. Here, a transformative approach is demonstrated to realize relaxed and high purity tetragonal phase MAPbI₃ (RP) near the mp-TiO₂ interface region. This approach involves inserting a ~2 nm lattice matched buffer layer of cubic CsPbBr₃ between mp-TiO₂ NPs and MAPbI₃, which serves as a template for epitaxial growth for top MAPbI₃. The solar cell with relaxed MAPbI₃ shows a power conversion efficiency (PCE) of 22.12% with significantly enhanced environmental, light, and thermal stability.