Impact of KF Post-Deposition Treatment on Aging of the Cu(In,Ga)Se2 Surface and Its Interface with CdS

Recent world record efficiencies for thin film solar cells based on Cu(In,Ga)Se-2 (CIGS) have been realized with absorbers subject to alkali fluoride post deposition treatments (PDT). We investigated the effect of ambient air exposure on the electronic properties of CIGS with KF-PDT in a combined time-dependent Kelvin probe force microscopy and X-ray photoelectron spectroscopy study. We also studied the early stage formation of the absorber/buffer interface after the initial deposition of CdS in the chemical bath. Our study shows that the KF-PDT, as well as the CdS deposition process induce an increase in the overall surface work function, as compared to bare CIGS. A K-In-Se compound forms after the KF-PDT, accompanied by a stable In oxide which explains the remarkable stability of the contact potential difference to air exposure, confirming phenomenological observations in many laboratories. In clear contrast to the untreated CIGS, the KF-treated CIGS/CdS interface shows a significant variation in the surface potential (similar to 360 mV) over approximately 7 h air-exposure. We attribute this variation to a Cd-In intermixing at interface, whose chemical stability is susceptible to air-exposure. Our results contribute to the understanding of the electronic properties of the KF-treated and untreated CIGS/CdS junction during the early stages of the interface formation, which impact the overall device properties.