Grain boundaries and potassium post-deposition treatments in chalcopyrite solar cells

Over the last years, alkali post-deposition treatments (PDT) have been attributed as the main driver for the continuous improvements in the power conversion efficiency (PCE) of Cu(In,Ga)Se2 (CIGSe) solar cells. All the alkali elements have shown beneficial optoelectronic effects, ranging from sodium to cesium, with many reports linking the improvements to grain boundary (GB) passivation. The most common process for alkali incorporation into the CIGS absorber is based on the thermal evaporation of alkali fluorides in a selenium atmosphere. Besides the demonstrated improvements in performance, disentangling the individual contributions of the PDTs on the GB, surface, and bulk is very challenging because of the many concurring chemical reactions and diffusion processes.
This thesis aims to investigate how pure metallic potassium interacts with CIGSe epitaxially grown on GaAs (100) and multi-crystalline GaAs. Surface sensitive Kelvin probe force microscopy (KPFM) and X-ray photoelectron spectroscopy (XPS) measurements are used to, in-situ, analyze changes in workfunction and compositional changes before and after each deposition step. Inert gas transfer systems and ultrahigh vacuum (UHV) are used to keep the pristine surface properties of the CIGSe.
An in-depth understanding of how different KPFM operation modes and environments influence the measured workfunction is discussed in detail in this thesis. It is shown that AM-KPFM, the most common KPFM operation mode, leads to misinterpretations of the measured workfunction at GBs on rough samples. Frequency modulation KPFM (FM-KPFM), on the other hand, turns out to be the most suitable KPFM mode to investigate GB band bending.
Pure metallic potassium evaporation on CIGSe epitaxially grown on GaAs (100) leads to diffusion of K from the surface down to the CIGS/GaAs interface even in the absence of GBs. Evaporation of metallic K is performed using a metallic dispenser, in which the evaporation rate can be controlled to deposit a few monolayers of K. The deposition is done in UHV, and an annealing step is used to diffuse K from the surface to the bulk.
Pure metallic potassium is also evaporated on CIGSe epitaxially grown on multicrystalline GaAs substrate, where well-defined GBs are present. Negligible workfunction changes at the GB were observed. XPS shows a strong Cu depletion after K deposition followed by annealing. Interestingly, the amount of K on the absorber surface after the K-deposition and subsequent annealing is almost equal to the amount of Cu that diffused into the bulk, suggesting a 1:1 exchange mechanism and no KInSe2 secondary phase.