Atomic-layer-deposited ZnSnO buffer layers for kesterite solar cells: Impact of Zn/(Zn+Sn) ratio on device performance

Atomic layer deposition (ALD) has been used to synthesize earth-abundant and non-toxic ZnSnO thin films as potential replacements for CdS buffer layers in thin-film solar cells. In this study, ALD ZnSnO films with various Zn/(Zn+Sn) ratios ranging from 0.66 to 0.96 were synthesized and their application as buffer layers in CZTSSe solar cells was investigated. The solar cells fabricated with ZnSnO buffer layers exhibit a significant improvement in short-circuit current density (Jsc) compared to the CdS-buffered reference device, primarily due to the wide band gap and high transparency of ZnSnO films. The CZTSSe/ZnSnO solar cell results also demonstrated that the device parameters, particularly the open-circuit voltage (Voc) and fill factor (FF), were significantly affected by the Zn/(Zn+Sn) ratio. A cell efficiency of 8.54% with a Voc of 0.436 V, Jsc of 32.98 mA cm−2, and FF of 0.59 was obtained with ALD ZnSnO buffer layer with an optimal Zn/(Zn+Sn) ratio of 0.76. Increasing or decreasing the Zn/(Zn+Sn) ratio leads to a gradual decrease in the FF and Voc values, which eventually deteriorates device performance. Additionally, the uniform and conformal ALD process results in superior cell-to-cell uniformity for ZnSnO-buffered devices compared to chemical-bath-deposited CdS buffer layers.