Prospective CaSnS3/Si tandem solar cells: assessing material aspects and quantifying temperature-induced efficiency losses

Abstract

The selection of an appropriate perovskite top cell material for a perovskite/Si tandem device is always critical. CaSnS₃, a Sn-based perovskite suitable for the top cell, has a ∼1.74 eV bandgap and contains Sn⁴⁺, which lowers Sn oxidation. When temperature is taken into consideration through bandgap renormalization, CaSnS₃'s bandgap falls by ∼207 meV up to 400 K compared to 0 K, resulting in a red shift according to the Varshni relation. A CaSnS₃/Si tandem device with a 310 nm thick CaSnS₃ top cell absorber achieves 36.14% power conversion efficiency (PCE) with a V_oc of ∼1.82 V under matched current and control test conditions at 300 K, AM 1.5G spectrum, and 100 mW cm⁻² power. The temperature coefficient is used to assess temperature influence, and the tandem solar cell's T_Voc (Tandem), T_Jsc (Tandem), and T_PCE (Tandem) values are expected to be −0.24% K⁻¹, 0.07% K⁻¹, and −0.22% K⁻¹, respectively. Temperature changes the properties of CaSnS₃ and Si, e.g., CaSnS₃'s bandgap, like Si's, shrinks with temperature, resulting in a 9.68% decrease in V_oc (tandem cell) as the temperature is increased from 300 K to 340 K under controlled test conditions. Despite this, increasing the temperature to 340 K only decreased the device's PCE by 8.88% (from 36.14% to 32.93%). Because of the changing solar spectra and average photon energy (APE) throughout the year, for an APE value of 1.9 eV, the PCE drops by 13.83% from 300 K to 340 K (34.10% to 29.38%); however, for an APE value of 1 eV, performance drops by 4.56% (17.75% to 16.94%), demonstrating that the device is more effective with blue-rich APE than with red-rich APE. Overall, our findings shed light on the impact of critical parameters such as temperature and spectrum variation in Sn-based CaSnS₃/Si tandem cells, which have important implications for the design of perovskite/Si tandem cells using a Sn-based absorber as a top cell.