Self-assembled nanometer-scale ZnS structure at the CZTS/ZnCdS heterointerface for high-efficiency wide band gap Cu ₂ ZnSnS ₄ solar cells

Despite remarkable progress in the performance of kesterite Cu ₂ ZnSnS ₄ (CZTS)-based photovoltaic technology has been achieved, the interface recombination and associated opencircuit voltage (V _oc ) deficit still dominate the loss mechanism in this technology. To alleviate heterojunction interface recombination in pure sulfide thin film solar cells, passivation structure at the interface is required. In this work, we developed an ultrathin nanometer-scale ZnS dielectric passivation layer which is readily formed in situ at the CZTS/ZnCdS heterointerface during the ZnCdS buffer deposition process via Zn diffusion from the ZnCdS bulk to the interface. With this nanoscale structure, a remarkable open-circuit voltage and fill factor improvement is illustrated, and a total area efficiency of 9.25% is obtained. The formation and features of the nanoscale ZnS layer are investigated by high-resolution scanning transmission electron microscopy and energydispersive X-ray spectroscopy. This self-assembled ZnS layer with dielectric properties passivates defects at the interface while still enabling the electrons to transport across the buffer layer because of the ultrathin thickness, which satisfies the requirement of dielectric passivation layer but requiring no complicated regular patterning. The correlation between the effects of passivation and device performance is investigated by device simulation, presenting a reasonable understanding of the experimental results. The results open a new aspect to passivate the interface recombination and expand the potential of upscaling CZTS technology.