Scanning Kelvin probe measurements on As-doped CdTe solar cells

Abstract

Scanning Kelvin probe microscopy (SKPM) has been used to study the Fermi level shift in arsenic (As) doped cadmium telluride (CdTe) photovoltaic devices. The contact potential difference (CPD) between probe tip and sample surface revealed that increasing As concentrations in CdTe led to a decrease in CPD. This highlighted a downward shift in the CdTe Fermi level and an increase in the CdTe work function. Using a highly oriented pyrolytic graphite sample in ambient conditions as a reference, the absolute work functions of the CdTe samples were estimated to vary from 3.88 to 4.09 eV. High-resolution SKPM measurements revealed localized shifts in CPD at CdTe grain boundaries. This was directly correlated to As doping concentrations, and indicated the segregation of As to grain boundaries. A mechanism is proposed where localized band bending at grain boundaries channels minority carriers away from the grain boundary, leading to reduced carrier recombination.