Kelvin probe force microscopy for the characterization of semiconductor surfaces in chalcopyrite solar cells

Kelvin probe force microscopy in ultrahigh vacuum is a powerful technique for the quantitative characterization of structural and electronic properties of semiconductor surfaces and interfaces on a nanometer scale. In chalcopyrite heterojunction solar cells the interfaces play a crucial role for the performance of the device. We studied chalcopyrite heterostructures based on epitaxial CuGaSe2 thin films prepared by MOVPE. Lateral variations of the contact potential difference and the surface photovoltage (SPV) were investigated after different process steps, including the deposition of n-CdS or n-ZnSe buffer layers and the n+-ZnO window layer. Measurements on the CuGaSe2 absorber material show terraces with preferential orientation in the [1 1 0] direction in the topographic image. A negative SPV of −300 mV on the as-grown CuGaSe2 absorber could be attributed to a highly doped p+-Cu2−xSe surface layer of a few nm thickness, which was removed by a KCN etch, resulting in a flat band condition. The deposition of the buffer layer alone does not lead to a significant band bending at the CuGaSe2/buffer interface and the deposition of the ZnO window layer seems to be crucial for the development of the band bending within the absorber.