Optimization of bifacial Ge-incorporated Sb2Se3 thin-film solar cells by modeling Cu2O back buffer layer

Abstract

We have investigated bifacial Ge-incorporated Sb2Se3 thin-film solar cells by modeling and simulating a back buffer layer including Cu2O to optimize the bifacial device performance. A Ge-incorporated Sb2Se3 absorber layer was grown using Vapor Transport Deposition to extract the input parameters for the simulation, followed by exploring the properties of the transparent back buffer layer. After selecting a Cu2O back buffer layer, we conducted the optimization study of bifacial Ge-doped Sb2Se3 devices by varying a set of devices and materials parameters. The optical bandgap is characterized as 1.23 eV for Ge-incorporated Sb2Se3 absorber and the absorption coefficient is 3 × 105 cm−1 at 600 nm. Using these input parameters, we simulated the device configuration of ZnO:Al/i-ZnO/CdS/Ge-Sb2Se3/back buffer layer/transparent conductive oxide. Firstly, we checked the impact of conduction and valence band offsets on device performance, indicating the valence band offset of the back buffer layer plays a significant role. In particular, zero or small positive valence band offset minimizes a Schottky hole barrier, which could reduce hole current clamping near the back buffer region. As for back buffer doping concentration, high back buffer doping concentration (1 × 1015 cm−3 or higher) is required to improve device performance even with zero valence band offset, whereas low absorber doping concentration (<1 × 1014 cm−3) enables higher efficiency due to the enlarged energy band bending. For bifacial device performance, a Cu2O back buffer layer is simulated by varying critical input parameters such as absorber thickness, doping concentration, and defect concentration near the back buffer interface. The best efficiency of front-side illumination is 19.7%, Voc (744.4 mV), Jsc (40.14 mA/cm2), and FF (66.1%) and that of the rear-side illumination is 13.0%, Voc (724.5 mV), Jsc (31.6 mA/cm2), and FF (56.7%). Consequently, the bifaciality factor is 66%.