Reducing interface recombination losses is one of the major challenges in developing Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Here, we propose a CZTSSe solar cell with an atomic layer deposited Al2O3 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 Al2O3 film presents notable antireflection (AR) properties over a broad range of wavelengths (350-1000 nm) for CZTSSe solar cells. With increasing Al2O3 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 Al2O3. The Al2O3 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 Al2O3 films. Therefore, the J sc and V oc of the CZTSSe solar cell with 2 nm-Al2O3 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 Al2O3 films show the dual functions of AR and surface passivation for photovoltaic applications.
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