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

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%.