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.