Rear‐Side ZnS Passivation With Spontaneous Local Point Contacts for Boosting Efficiency and Bifaciality in Ultrathin Semitransparent CuInSe ₂ Solar Cells

ABSTRACT

Bifacial semitransparent (BST) solar cells utilizing transparent conducting oxides are advantageous in various applications because of their controllable transparency, ease of use, and potential in bifacial power generation using the albedo effect. However, their power conversion efficiency (PCE) is limited owing to the ultrathin absorber layer, where rear‐side recombination may dominate. To suppress carrier recombination at the rear‐interface in ultrathin BST CuInSe ₂ (CISe) solar cells, a direct passivation method is demonstrated, that is, a nanoscale ZnS layer is deposited between the rear‐side of the CISe absorber and an indium tin oxide (ITO) back contact using atomic layer deposition. The ZnS layer spontaneously forms localized holes by recrystallization during CISe co‐evaporation, which act as local direct conduction paths from the CISe to the ITO. The PCE of the BST‐CISe solar cells increases dramatically from 5.29% to 9.79% after applying the optimal 20‐nm ZnS layer. In addition, the passivation layer significantly reduces rear‐side carrier recombination and increases the bifaciality factor. The bifacial power‐generation density with rear‐side ZnS passivation increases to 11.4% and 17.8% under total illumination of 1.2 and 2.0 sun (considering the albedo effect), respectively.