An effective air heat-treatment strategy of precursor films in search of high open-circuit voltage for efficient CZTSSe cells

Abstract

Rationally controlling the electronic property of the absorbers is effective but challenging in overcoming the drawbacks of large open-circuit voltage (VOC) deficit for high efficient Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Herein, selecting chlorides as metal precursor, for the ambient-air processed devices, an approach of heat-treating the Cu2ZnSnS4 (CZTS) precursor films to regulate the electronic properties of CZTSSe absorbers is proposed to effectively increase the VOC towards high efficient cells. It is found that though the temperature of heat-treatment can lead to similar CZTSSe absorber films in composition and crystallinity, the final photovoltaic performance of the cells are dramatic difference. An efficiency of 9.20% with a large VOC of 464 mV was achieved from the 400 oC treated precursor film, whereas the non-treated cells obtained 6.38% efficiency and 399 mV VOC, realizing the VOC deficit decreased from 711 to 594 mV. Detailed investigations indicate a proper heat-treating temperature can significantly improve the electric property of the absorber layers, making it possess reduced band-tailing effects, lower Urbach energy, larger recombination resistance and longer carrier lifetime for enhancing the device efficiency. Such effective approach of directly heat-treating the precursor films to rationally regulate electric properties of the absorbers might provide a new way to identify the origin of the VOC deficit for high efficient CZTSSe solar cells.