Regulating crystal growth via organic lithium salt additive for efficient Kesterite solar cells

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 VOC 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 VOC 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.