Enhanced stability of alpha-phase FAPbI(3) perovskite solar cells by insertion of 2D (PEA)(2)PbI4 nanosheets

Methylammonium (MA) is one of the main obstacles that hold back the commercialization of perovskite solar cells (PSCs). Formamidinium (FA)-based perovskite is a promising photovoltaic material due to its higher thermal stability and smaller bandgap. However, despite the introduction of Cs+ in FAPbI(3), the photoactive alpha-phase FAPbI(3) can quickly transform into a non-perovskite hexagonal phase delta-FAPbI(3), which limits its use in perovskite solar cells. In this work, we show that the dispersion of 2D (PEA)(2)PbI4 nanosheets into a Cs(0.1)FA(0.9)PbI(3) thin film successfully prevents the transformation of the alpha-phase Cs(0.1)FA(0.9)PbI(3) to the delta-phase. Because of the 2D (PEA)(2)PbI4 nanosheets, a higher quality perovskite thin-film was obtained with a longer carrier lifetime, lower trap state density, and enhanced stability. The resulting device achieves a high power conversion efficiency of 20.44%, which is one of the highest for MA-free perovskites and retains 82% of its initial efficiency after 800 h of aging study.