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 Cs x MA 1− x PbI 3 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 Cs x MA 1− x PbI 3 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 MAPbI 3 lattice and do not lead to a change in the crystal structure of the MAPbI 3 . When the Cs + fraction x is between 0.3 and 0.7, one part of Cs + ions incorporated in the MAPbI 3 lattice replaces MA + ions (α-phase), while the other part forms a yellow photovoltaic-inactive δ-CsPbI 3 phase. The Cs x MA 1− x PbI 3 films with x  ≥ 0.8 contain much of the photovoltaic-inactive δ-CsPbI 3 phase. The band gap of the Cs x MA 1− x PbI 3 films increases from 1.53 eV to 1.65 eV with the increase of the Cs + fraction x . The PSCs based on the Cs 0.1 MA 0.9 PbI 3 films exhibit remarkable improvement in device efficiency and stability.