Li Incorporation Induces Surface Chemical Disorder in Solution-Processed Cu₂ZnSn(S,Se)₄

Lithium incorporation into Cu2ZnSn(S,Se)4 (CZTSSe) absorbers has led to improvements in power conversion efficiencies (PCEs), often linked to changes in doping density and improvement in grain growth. In this study, we shed new light onto the influence of Li on CZTSSe absorbers, specifically on a transparent support, by investigating the effect of adding lithium bis(trifluoromethane)sulfonimide (LiTFSI) from 0% to 5% Li/Cu ratios to molecular precursor solutions. Secondary ion mass spectrometry analysis confirmed both the uniform distribution of Li in annealed CZTSSe absorber, while there were also detectable changes to grain microstructure and surface electronic landscape. Upon the addition of 1% Li, the work function (WF) distribution, analysed by energy-filtered photoemission electron microscopy, broadens significantly from 85 to 200 meV, allowing for direct visualisation of the increase in surface electronic disorder of CZTSSe with Li incorporation for the first time. The Li additive was also found to induce the diffusion of Sn to the surface. A champion PCE of 5.07% was measured for F:SnO2 (FTO)-based CZTSSe with 1% Li, revealing the balance between microstructure, chemical disorder and Sn elemental distribution. This research demonstrates the importance of understanding alterations of nanoscale properties of CZTSSe when varying precursor solution conditions.