Enhanced efficiency of Cu 2 ZnSn(S,Se) 4 solar cells via anti-reflectance properties and surface passivation by atomic layer deposited aluminum oxide.

Reducing interface recombination losses is one of the major challenges in developing Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells. Here, we propose a CZTSSe solar cell with an atomic layer deposited Al 2 O 3 thin film for surface passivation. The influence of passivation layer thickness on the power conversion efficiency (PCE), short-circuit current density ( J sc ), open-circuit voltage ( V oc ) and fill factor (FF) of the solar cell is systematically investigated. It is found that the Al 2 O 3 film presents notable antireflection (AR) properties over a broad range of wavelengths (350-1000 nm) for CZTSSe solar cells. With increasing Al 2 O 3 thickness (1-10 nm), the average reflectance of the CZTSSe film decreases from 12.9% to 9.6%, compared with the average reflectance of 13.6% for the CZTSSe film without Al 2 O 3 . The Al 2 O 3 passivation layer also contributes to suppressed surface recombination and enhanced carrier separation. Passivation performance is related to chemical and field effect passivation, which is due to released H atoms from the Al-OH bonds and the formation of Al vacancies and O interstitials within Al 2 O 3 films. Therefore, the J sc and V oc of the CZTSSe solar cell with 2 nm-Al 2 O 3 were increased by 37.8% and 57.8%, respectively, in comparison with those of the unpassivated sample. An optimal CZTSSe solar cell was obtained with a V oc , J sc and η of 0.361 V, 33.78 mA and 5.66%. Our results indicate that Al 2 O 3 films show the dual functions of AR and surface passivation for photovoltaic applications.