Improving the Back Electrode Interface Quality of Cu₂ZnSn(S,Se)₄ Thin-Film Solar Cells Using a Novel CuAlO₂ Buffer Layer

A novel buffer layer CuAlO2 (CAO) with smooth and compact surface was applied in Cu2ZnSn(S,Se)4-based (CZTSSe) solar cells to optimize back electrode interface (BEI). It is found that introduction of CAO exerts a remarkable effect on the crystalline quality of absorber and the thickness of interfacial layer Mo(S,Se)2 (MSSe) at BEI. When the thickness of CAO buffer layer was optimized to 10.6 nm, CZTSSe film exhibits preferable crystallinity with larger grains without pin holes. Also, MSSe decreases significantly to ∼244 nm, and it is smaller than that (∼463 nm) of the sample without CAO. With this interface optimization, the solar cell with 10.6 nm thick CAO shows the higher shunt resistance, lower reversion saturation current density and smaller series resistance, leading to an increase in short-circuit current density (from 26.91 to 30.66 mA·cm–2) as well as fill factor (from 46.60% to 49.93%) compared to that of the sample without CAO. As a consequence, power conversion efficiency of the corresponding devices increases from 4.12% to 5.10%. The influence mechanism of CAO buffer layer on the photovoltaic properties of CZTSSe solar cell is discussed in detail, and this approach presents a wide range of possibilities for the further development of interface optimization in solar cells.