Efficient additive-free FAPbI₃ perovskite solar cells achieved by promoting homogeneity

Formamidinium lead iodide (FAPbI(3)) has received substantial research interest for photovoltaic applications due to its excellent thermal stability and narrow bandgap close to the Shockley-Queisser limit. Unfortunately, the compositionally pure FAPbI(3)-based perovskite solar cells (PSCs) have unsatisfactory efficiency and the common approach of incorporating additives into FAPbI(3) has been used to improve their properties. However, commonly used additives generally contribute to the complexity of the FAPbI(3) composition, which could pose potential risks to the long-term operational stability of PSCs. Here, we report a strategy for the fabrication of additive-free FAPbI(3) films using a low pressure assisted (LPA) method. By adjusting the pressure reduction rate, we achieved compositionally pure FAPbI(3)-based PSCs with power conversion efficiency (PCE) exceeding 20%, along with enhanced thermal stability and long-term stability under working conditions. The improved optoelectronic properties can be attributed to the excellent homogeneity of the FAPbI(3) film with low defect density and better carrier extraction at the interface between the FAPbI(3) film and carrier-transport layers. This further leads to better thermal and long-term operational stability of the FAPbI(3) PSCs.