Nanoscale sharp bandgap gradient for efficiency improvement of Cu2ZnSn(S, Se)4 thin film solar cells

Due to the difficulty in controlling the post-selenization/sulfurization process accurately, single-graded structure with a lower bandgap on the surface is easy to form in the Cu2ZnSn(S, Se)4 absorber. This will lower the built-in potential and lead to the formation of undesirable “cliff” type band alignment in the device, and hence a drastic Voc-deficit and lower efficiency are expected. Herein, an effective and versatile method to improve the Voc and efficiency of Cu2ZnSn(S, Se)4 solar cells through bandgap engineering was presented in this work. A sharp surface bandgap gradient with larger surface bandgap was constructed simply by sputtering a nanoscale Cu2ZnSnS4 layer on the surface of Cu2ZnSn(S, Se)4 absorber. Under the coactions of surface sharp bandgap gradient and the single bandgap gradient existed in the Cu2ZnSn(S, Se)4 absorber naturally, a quasi-U-shaped bandgap gradient was constructed inside the absorber. Benefited from the enlarged surface bandgap, favorable U-shaped bandgap gradient and appropriate band alignment at the absorber/buffer interface, a best efficiency of 9.9% with a Voc of 450 mV, a Jsc of 35.26 mA/cm2 and a fill factor (FF) of 62.73% was achieved.