A Precursor Stacking Strategy to Boost Open-Circuit Voltage of Cu₂ZnSnS₄ Thin-Film Solar Cells

Solar cells based on Cu₂ZnSnS₄ (CZTS) thin-film light absorber are promising photovoltaic (PV) technologies to achieve cost-effiective solar electricity. One of the challenges in CZTS PV technology is its low open-circuit voltage (V_oc), which is normally less than 600 mV. In this paper, we enhanced V_oc of CZTS solar cells to above 700 mV by constructing a four-layer stacking precursor of Zn/Sn/Cu/Zn (ZTCZ). Compared with the ordinary three-layer Zn/Sn/Cu (ZTC) stacking precursor, the efficiency of CZTS solar cells prepared by the ZTCZ precursor of the same stoichiometry was significantly enhanced. Cross-sectional microscopic characterization revealed that a compact bilayer structured CZTS with Zn-rich surface was formed using the ZTCZ precursor while lots of voids were observed at the bottom of the CZTS film produced using the ZTC precursor. The mechanism is proposed for the high V_oc achieved through a Zn-rich surface in the CZTS. Photoluminescence and time-resolved photoluminescence measurements confirmed that the device made from the four-layer ZTCZ stacking precursor possess higher charge separation efficiency and lower radiative recombination compared with the ZTC stacking precursor. These factors have greatly contributed for the high V _oc and superior efficiency of the CZTS thin-film solar cells.