Effect of Cs+ Fraction on Photovoltaic Performance of Perovskite Solar Cells Based on CsxMA1−xPbI3 Absorption Layers

In this paper, we report the effect of the Cs+ fraction x on the structure, morphology and optical properties of mixed organic–inorganic cation CsxMA1−xPbI3 absorption layers prepared using an anti-solvent-assisted one-step solution deposition method in an ambient environment. By changing the ratios of cesium iodide (CsI) to methyl iodide (MAI) in the precursor solution, the Cs+ fraction x varies from 0 to 1. The hole-transporting-material (HTM)-free perovskite solar cells (PSCs) based on the CsxMA1−xPbI3 absorption layers were fabricated in ambient air, and their photovoltaic performance was studied. Results indicate that when the Cs+ fraction x is less than 0.3, Cs+ ions mainly replace a fraction of MA+ ions in the MAPbI3 lattice and do not lead to a change in the crystal structure of the MAPbI3. When the Cs+ fraction x is between 0.3 and 0.7, one part of Cs+ ions incorporated in the MAPbI3 lattice replaces MA+ ions (α-phase), while the other part forms a yellow photovoltaic-inactive δ-CsPbI3 phase. The CsxMA1−xPbI3 films with x ≥ 0.8 contain much of the photovoltaic-inactive δ-CsPbI3 phase. The band gap of the CsxMA1−xPbI3 films increases from 1.53 eV to 1.65 eV with the increase of the Cs+ fraction x. The PSCs based on the Cs0.1MA0.9PbI3 films exhibit remarkable improvement in device efficiency and stability.