Potassium Treatments for Solution-Processed Cu(In,Ga)(S,Se)₂ Solar Cells

Cu(In,Ga)(S,Se)2 (CIGSe, CIGSSe) has emerged as an attractive thin-film solar cell absorber material owing to its high light absorption coefficient and tunable bandgap. In CIGSSe processing and fabrication, the use of alkali treatments has been implemented as sodium doping is considered a requirement for obtaining high-efficiency CIGSSe solar cell devices and has been used extensively. One of the more significant developments in recent years has been the discovery of the beneficial effects that potassium post-deposition treatments have on vacuum-processed CIGSSe solar cells as they are responsible for a major increase in CIGSSe solar cell performance. Here, we conduct a study of the effect of potassium treatments on solution-processed CIGSSe films grown from colloidal sulfide-based nanoparticle inks. By adding potassium through e-beam evaporation of potassium fluoride (KF) prior to selenization and grain growth, we find that the grain growth of CIGSSe is enhanced with potassium addition and that a larger-grained film results compared to the untreated selenized CIGSSe film, similar to that observed in sodium-treated films. We also observe through X-ray photoelectron spectroscopy (XPS) that films treated with K show the presence of the high-bandgap K–In–Se surface phase. Upon fabricating the devices, we find that films that have been subjected simultaneously to both sodium and potassium treatments have enhanced optoelectronic performance, mainly manifested in the higher open-circuit voltage and higher short-circuit current.