The effect of KF post-deposition treatments on the optoelectronic properties of Cu(In,Ga)Se2 single crystals

The power conversion efficiency boost of Cu(In,Ga)Se2 in the past years has been possible due to the incorporation of heavy alkali atoms. Their addition through post-deposition treatments results in an improvement of the open-circuit voltage, which origin has been associated with grain boundaries. The present work discusses the effect of potassium fluoride post-deposition treatments on the optoelectronic properties of a series of sodium-free Cu(In,Ga)Se2 single crystals with varying Cu and Ga content. Results suggest that improvement of the quasi-Fermi level splitting can be achieved despite 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 KF-treated low-gallium Cu(In,Ga)Se2 is observed, which along a carrier lifetime study demonstrates that potassium can act as a dopant. The fact that potassium by its own can alter the optoelectronic properties of Cu(In,Ga)Se2 single crystals demonstrates that the effect of post-deposition treatments goes beyond grain boundary passivation.