Surface potential of chalcopyrite films measured by KPFM

Atomic force microscopy is widely used to characterize the surface topography of a variety of samples. Kelvin probe force microscopy (KPFM) additionally allows determining images of the surface potential with nanometer resolution. The KPFM technique will be introduced and studies on surfaces of chalcopyrite semiconductors for solar cell absorbers will be presented. It is shown that operation in ultra-high vacuum (UHV) is required to obtain meaningful work function values. Different methods for obtaining UHV-clean surfaces are presented and KPFM studies on these are compared. Surfaces where prepared by in-vacuum deposition, inert-gas transfer, in-vacuum decapping of a protective Se-cap and a peel-off method. Finally, a sputter-annealing cycle also allows to obtain well-suited surfaces for KPFM studies. Employing KPFM, variations in the local surface potential at grain boundaries of polycrystalline CuGaSe2 films were observed. A potential drop indicates the presence of charged defects at grain boundaries. Furthermore, different electronic activity was found for different grain boundaries, as concluded from studies under illumination. Using laterally resolved surface photovoltage, a Cu2−xSe impurity phase could be observed in CuGaSe2.