Double-sided annealing to reverse the crystallization direction for efficient and stable flexible FACs-perovskite solar modules

The application of methylammonium cations makes flexible perovskite solar modules (<i>f</i>-PSMs) unstable in long term. Here, we report flexible p-i-n structure perovskite modules made of methylammonium-free formamidinium cesium lead iodide (FA_1-_<i>x</i>Cs<i>_x</i>PbI₃, <i>x</i> ≤ 0.1) to improve their stability. Interfacial voids between FA_1-_<i>x</i>Cs<i>_x</i>PbI₃ layer and hole-transport layers, caused by the limited annealing temperature for <i>f</i>-PSMs, severely disrupt the film structural continuity. A double-sided annealing strategy is used to address this challenge, which markedly reduces the voids at the buried interface and yields homogeneous perovskite films with enlarged grains. The double-sided annealing can flip the phase transition direction from traditional top-down to bottom-up, because of the much higher air temperature above perovskite films, allowing a quick dimethyl sulfoxide evaporation. The <i>f</i>-PSMs achieve an aperture-area power conversion efficiency (PCE) of 19.1% and show ~10% efficiency loss after 107 thermal cycles between -40° and 85°C. The small-area devices retained over 98% of their initial PCE after 1176 hours under one Sun illumination at 85°C.