Harvesting Sub-bandgap Photons via Upconversion for Perovskite Solar Cells.

Lanthanide-based upconversion (UC) allows harvesting sub-bandgap near-infrared photons in photovoltaics. In this work, we investigate UC in perovskite solar cells by implementing UC single crystal BaF2:Yb3+, Er3+ at the rear of the solar cell. Upon illumination with high-intensity sub-bandgap photons at 980 nm, the BaF2:Yb3+, Er3+ crystal emits upconverted photons in the spectral range between 520 and 700 nm. When tested under terrestrial sunlight representing one sun above the perovskite's bandgap and sub-bandgap illumination at 980 nm, upconverted photons contribute a 0.38 mA/cm2 enhancement in the short-circuit current density at lower intensity. The current enhancement scales non-linearly with the incident intensity of sub-bandgap illumination, and at higher intensity, 2.09 mA/cm2 enhancement in current was observed. Hence, our study shows that using a fluoride single crystal like BaF2:Yb3+, Er3+ for UC is a suitable method to extend the response of perovskite solar cells to near-infrared illumination at 980 nm with a subsequent enhancement in current for very high incident intensity.