Alkali metal doping, typically Li⁺ doping, is beneficial for CZTSSe solar cells. Herein, aiming at the low solubility of conventional LiF which is hard to directly introduce into the precursor solution, an organic lithium salt (LiTFSI, lithium bis(trifluoromethanesulfonyl)imide) has been introduced into fabricating CZTSSe solar cells for the first time. 13.2% PCE (power-conversion-efficiency) with 12.7% certified PCE has been achieved with relatively higher V_OC and FF, which is the highest efficiency of lithium-doping/alloying devices so far. A comprehensive investigation on the influence of LiTFSI on the formation of precursor and selenized absorber films has been carried out. The existence forms of LiTFSI in different processes are revealed as well as the relevant mechanism. That is, during selenization, LiTFSI decomposes to LiF at early stage, then LiF combines with Se to afford the Li-Se liquid fluxing intermediate, which can facilitate the phase transition and crystal growth to give compact CZTSSe films with larger grains and less pinholes, thus leading to better surface morphology and electric properties of LiTFSI doped CZTSSe absorber films. Li⁺ doping is thus found to enable reducing interfacial defects, improving band bending and lowering V_OC deficit. This work provides an easy way to precisely modify Li⁺ salt amount in the precursor solution and subsequent CZTSSe selenization process by using organic lithium salts for high quality CZTSSe crystals and high-performance devices.

碱金属掺杂，通常是 Li ⁺掺杂，有利于 CZTSSe 太阳能电池。在此，针对传统 LiF 溶解度低且难以直接引入前驱体溶液的问题，首次将有机锂盐（LiTFSI，双（三氟甲磺酰）亚胺锂）引入制备 CZTSSe 太阳能电池。13.2% 的 PCE（功率转换效率）和 12.7% 的认证 PCE 已实现，V _OC相对较高FF，这是迄今为止锂掺杂/合金器件的最高效率。对 LiTFSI 对前驱体和硒化吸收体薄膜形成的影响进行了全面研究。揭示了LiTFSI在不同过程中的存在形式以及相关机制。也就是说，在硒化过程中，LiTFSI 在早期分解为 LiF，然后 LiF 与 Se 结合提供 Li-Se 液态助熔中间体，这可以促进相变和晶体生长，得到具有更大晶粒和更少针孔的致密 CZTSSe 薄膜，从而导致 LiTFSI 掺杂的 CZTSSe 吸收膜具有更好的表面形貌和电性能。因此发现Li ⁺掺杂能够减少界面缺陷，改善能带弯曲并降低V_OC赤字。这项工作提供了一种简单的方法，可以通过使用有机锂盐来精确修改前体溶液中的Li ⁺盐量和随后的 CZTSSe 硒化过程，从而获得高质量的 CZTSSe 晶体和高性能器件。