Enhancing Surface Properties for Electrodeposited Cu(In,Ga)Se₂ Films by (NH₄)₂S Solution at Room Temperature

Fabricating chalcopyrite Cu(In,Ga)Se2 (CIGS) films from electrodeposition is one of the most effective ways to lower the production cost and energy consumption compared with that from vacuum methods. However, this process often produces a pronounced rough surface with impurity phases, induces the accumulation of gallium, and results in a low surface band gap. In this work, ammonium sulfide ((NH4)2S, AS) solution treatment is used as a single way to realize the optimization of the distribution of elements on the CIGS surface. It is revealed that this room-temperature process not only significantly reduces the surface roughness (Ra) but also incorporates sulfur (S) into the CIGS film successfully. An appropriate AS treatment duration can effectively remove the impurity phases and reduce the series resistance, and thus the dominant recombination moves from the CdS/CIGS heterojunction interface to the bulk. Consequently, the open-circuit voltage (VOC) and fill factor (FF) of solar cells have been greatly improved (from 484 to 532 mV, from 62.52 to 68.15%), leading to higher efficiency of 11.92%. This work focuses on revealing the etching mechanism of the CIGS films by the AS treatment at room temperature.