  By application of the ac admittance spectroscopy method, the defect state
energy distributions were determined in CdTe incorporated in thin film solar
cell structures concluded on ZnO, ZnSe, and ZnS buffer layers. Together with
the Mott-Schottky analysis, the results revealed a strong modification of the
defect density of states and the concentration of the uncompensated acceptors
as influenced by the choice of the buffer layer. In the solar cells formed on
ZnSe and ZnS, the Fermi level and the energy position of the dominant deep trap
levels were observed to shift closer to the midgap of CdTe suggesting the
mid-gap states may act as recombination centers and impact the open-circuit
voltage and the fill factor of the solar cells. For the deeper states, the
broadening parameter was observed to increase indicating fluctuations of the
charge on a microscopic scale. Such changes can be attributed to the
grain-boundary strain and the modification of the charge trapped at the
grain-boundary interface states in polycrystalline CdTe
