Enhanced Bifaciality Factor with Sb₂Se₃ Devices Modeling Cu₂O Back Buffer

We have investigated bifacial Ge-incorporated Sb₂Se₃ thin-film solar cells by modeling and simulating a back buffer layer including Cu₂O to optimize the performance of bifacial devices. We proposed Ge-incorporated Sb₂Se₃ absorber layers for bifacial devices, which were used as modeling input parameters after characterizing the absorber film. With extracted parameters (bandgap, 1.23 eV and the absorption coefficient, 3 × 10⁵ cm^-1at 600 nm), we investigated the impact of various back buffer layers on bifacial device performance. The simulated device structure is ZnO:Al/i-ZnO/CdS/Ge-Sb₂Se₃/back buffer layer/TCO. First, the impact of conduction and valence band offsets was investigated, showing the significant dependence of device performance on valence band offset. To minimize the Schottky hole barrier at the back contact region, high back buffer doping concentration (1 × 10¹⁵cm^-3or higher) is required to improve device performance, whereas low absorber doping concentration (<1 × 10¹⁴cm^-3) enables higher efficiency due to the enlarged energy band bending. With a Cu₂O back buffer layer, 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 %.