Toward High Efficient Cu 2 ZnSn(S x ,Se 1- x ) 4 Solar Cells: Break the Limitations of V OC and FF.

Increasing the fill factor (FF) and the open-circuit voltage (V OC ) simultaneously together with non-decreased short-circuit current density (J SC ) are a challenge for highly efficient Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells. Aimed at such target in CZTSSe solar cells, a synergistic strategy to tailor the recombination in the bulk and at the heterojunction interface has been developed, consisting of atomic-layer deposited aluminum oxide (ALD-Al 2 O 3 ) and (NH 4 ) 2 S treatment. With this strategy, deep-level Cu Zn defects are converted into shallower V Cu defects and improved crystallinity, while the surface of the absorber is optimized by removing Zn- and Sn-related impurities and incorporating S. Consequently, the defects responsible for recombination in the bulk and at the heterojunction interface are effectively passivated, thereby prolonging the minority carrier lifetime and increasing the depletion region width, which promote carrier collection and reduce charge loss. As a consequence, the V OC deficit decreases from 0.607 to 0.547 V, and the average FF increases from 64.2% to 69.7%, especially, J SC does not decrease. Thus, the CZTSSe solar cell with the remarkable efficiency of 13.0% is fabricated. This study highlights the increased FF together with V OC simultaneously to promote the efficiency of CZTSSe solar cells, which could also be applied to other photoelectronic devices.