Zinc‐Tin‐Oxide Buffer Layer and Low Temperature Post Annealing Resulting in a 9.0% Efficient Cd‐Free Cu₂ZnSnS₄Solar Cell

Abstract

Zn 1−x Sn x O y (ZTO) has yielded promising results as a buffer material for the full sulfur Cu 2 ZnSnS 4 (CZTS), with efficiencies continuously surpassing its CdS‐references. ZTO can be deposited by atomic layer deposition (ALD), enabling tuning of the conduction band position through the choice of metal ratio or deposition temperature. Thus, an optimization of the conduction band alignment between ZTO and CZTS can be achieved. The ZTO bandgap is generally larger than that of CdS and can therefore yield higher currents due to reduced losses in the short wavelength region. Another advantage is the possibility to omit the toxic Cd. In this study, the ALD process temperature was varied from 105 to 165 °C. Current‐blocked devices were obtained at 105 °C, while the highest open‐circuit voltage and device efficiency was achieved for 145 °C. The highest fill factor was seen at 165 °C. The best efficiency reached in this study was 9.0%, which, to our knowledge, is the highest efficiency reported for Cd‐free full‐sulfur CZTS. We also show that the effect of heat needs to be taken into account. The results indicate that part of the device improvement comes from heating the absorber, but that the benefit of using a ZTO‐buffer is clear.