Strain release of formamidinium-cesium perovskite with imprint-assisted organic ammonium halide compensation for efficient and stable solar cells

Residual lattice strain in halide perovskites has been recognized as a key factor that affects device efficiency and stability of perovskite solar cells (PSCs). Here, we reveal that the escape of organic ammonium halide and their inhomogeneous distribution after thermal annealing could cause severe residual strain in intrinsic thermal stable formamidinium-cesium (FACs) perovskite films. Thus, we develop an imprint-assisted organic ammonium halide compensation strategy for residual lattice strain relaxation. We demonstrate the residual lattice strain is well-released after post-treatment, along with further perovskite grain growth/coalescence, crystallinity improvement, and defect repair. As a result, the strain-free PSCs with nickel oxide based inverted architecture exhibit a power conversion efficiency as high as 21.30% and the corresponding encapsulated devices retain 98% of their initial efficiencies at 45 °C under 1-sun equivalent white-light light-emitting diode array illumination with maximum power point tracking in the ambient environment for 1000 h.