Fengxia Yang,Xiaofei Dong,Lei Yu,Xudong Sun,Jiangtao Chen,Yun Zhao,Yan Li
The cation ratio in Cu-poor and Zn-rich CZTSSe absorbers is key to decide the photovoltaic performance of CZTSSe solar cells, while its toleration and the influence mechanism is still not clear. In this work, for single-layer-structured CZTSSe absorbers spin-coated from metal chloride precursors with constant Cu and Zn values, the Zn/Sn ratio is regulated in the range of 0.8∼1.6. It is found that the Zn/Sn ratio has great influence on the morphology and electrical properties of the CZTSSe absorbers and thus the performance parameters of the CZTSSe cells. Comparatively, the results show that a 1.2 Zn/Sn ratio can boost the photovoltaic efficiency to 10.65% with a relatively high open-circuit voltage (VOC) of 481 mV and a reduced VOC-deficit of 585 mV. Detailed studies indicate that the 1.2 Zn/Sn ratio could favor the large-grain growth, increase the carrier concentration, suppress the carrier recombination, and reduce the band-tail effects and Urbach energy, thus leading to the highest efficiency with superior photovoltaic performance. The optimization of the Zn/Sn ratio in chloride precursor-coated CZTSSe absorbers would be beneficial for in-depth understanding of the effect mechanism of the element ratio on device performance.
Lu Cao,Jie Wang,Jia-Rui Rao,Guang-Zhe Ma,Min Gao,Dong Shi
Metal halide perovskites (MHPs) are attracting ever-growing interest across diverse optoelectronic subdisciplines. Yet, the physical mechanism underlying photoexcitation and subsequent photocarrier dynamics remains poorly understood, due to the apparent spectroscopic diversities observed for any certain MHP. Here, diverse spectroscopic characteristics, including static spectral line-shapes and time-resolved photoluminescence (TRPL) traces shown by polycrystalline versus single-crystalline CH 3 NH 3 PbBr 3 perovskites, are restudied. Key photophysical merits, including exciton binding energy ( E B ), Sommerfeld factor ( ξ ), and Urbach energy ( E U ) that account for the diversities, are discussed within the established framework of semiconductor optics. The value of ξ , which increases with E B , determines how much the interband absorption is enhanced beyond that expected for free carriers. The intrinsic band-edge luminescence is identified, with its asymmetric spectral line-shape linked to E U via the van Roosbroeck–Shockley relation. Excited phase evolution, accompanied by rapid electron–hole (e–h) pairs dissociation and subsequent occurrence of e–h plasma, is indicated by the two-stage TRPL traces that are only observable for high-quality single crystals. With all the spectroscopic analysis and interpretations rooted in the established semiconductor optical theorems, the mechanistic merits revealed in this study are informative for plausible identification between the interband and excitonic photophysical processes of MHPs.
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