Electronic structure of secondary phases in Cu-rich CuGaSe2 solar cell devices

Kelvin probe force microscopy in ultrahigh vacuum was used to image the electronic structure of thin-film solar cell cross sections based on as-grown Cu-rich CuGaSe2 absorbers. We observe different secondary phases in the absorber film. A p-type degenerate Cu2-xSe phase is identified by a higher work function (Phisimilar to5.35 eV) than CuGaSe2 (Phisimilar to5.1 eV), allowing good contrast mappings of both phases within the absorber film. Besides entire Cu2-xSe crystallites we also observed this secondary phase segregated as an interfacial layer along CuGaSe2 grain boundaries. An additional high-work function phase at the CuGaSe2/window junction is attributed to the formation of an improper CuS buffer layer during chemical bath processing. The detrimental effect of these secondary phases on the solar cell performance is discussed. (C) 2004 American Institute of Physics.