Strain Relaxation and Light Management in Tin–Lead Perovskite Solar Cells to Achieve High Efficiencies

Tin–lead (Sn–Pb)-based perovskite solar cells (PSCs) still exhibit inferior power conversion efficiency (PCE) compared to their pure Pb counterparts because of high voltage loss (VL) and high photocurrent loss in the infrared region. This study explores that a small amount of cesium ion (Cs+) incorporation in the lattice of Sn–Pb perovskite can reduce the relative lattice strain, which in turn decreases the VL less than 0.50 V. Moreover, surface and bulk trap densities also seem to be reduced by Cs+ addition, as concluded by thermally stimulated current measurements and increased carrier lifetime by photoluminescence study. It was discovered that a small amount of Cs+ lowered the Urbach energy, which can be used as a signature to optimize the optoelectronic and the photovoltaic properties of multication perovskite materials. This study further demonstrates that a high external quantum efficiency (∼80% at 900 nm) can be obtained with fluorine-doped tin oxide (FTO) glass rather than frequently used indium tin oxide (ITO) glass. The strategies employed in the work improved the open-circuit voltage to 0.81 V and gave a photocurrent density of >30 mA/cm2 and a PCE of >20% using a band gap of 1.27 eV.