The Effect of Potassium Fluoride Postdeposition Treatments on the Optoelectronic Properties of Cu(In,Ga)Se2 Single Crystals

The power conversion efficiency boost of Cu(In,Ga)Se-2 in the past years has been possible due to the incorporation of heavy alkali atoms. Their addition through postdeposition treatments results in an improvement of the open-circuit voltage, the origin of which has been associated with grain boundaries. Herein, the effect of potassium fluoride postdeposition treatments on the optoelectronic properties of a series of sodium-free Cu(In,Ga)Se-2 single crystals with varying Cu and Ga content is discussed. Results suggest that improvement of the quasi-Fermi level splitting can be achieved in the absence of grain boundaries, being greater in low-gallium Cu-poor absorbers. Secondary ion mass spectrometry reveals the presence of potassium inside the bulk of the films, suggesting that transport of potassium can occur through grain interiors. In addition, a type inversion from n to p in potassium fluoride-treated low-gallium Cu(In,Ga)Se-2 is observed, which along with study of the carrier lifetime demonstrates that potassium can act as a dopant. The fact that potassium on its own can alter the optoelectronic properties of Cu(In,Ga)Se-2 single crystals demonstrates that the effect of postdeposition treatments goes beyond grain boundary passivation.