Nanophase semiconductors embedded within transparent conductive oxides matrices as optical sensitizers for photovoltaic applications

ABSTRACT The optical absorption of a transparent condu ctive oxide (TCO), which is often used as the basis for junction or contact layers in thin film photovoltaics, can be tailored by incorporating a nanophase semiconductor (SC) component. Using a, dual-source, sequential R.F. magnetron sputter deposition tec hnique, we manipulate the opti cal and electronic properties of SC:TCO composites by varying the local and extended nanophase assembly and composition. The present study explores nanocomposite systems based on Ge:ZnO and Ge:I TO. The impact of host material (ITO vs. ZnO) on the evolution of nanostructure is investigated. Heat treatment of th e as-deposited films results in an increased crystallinity of the TCO and SC components, confirmed by X-ray diffraction and Raman spectro scopy studies. The presence of the SC phase is found to influence TCO grain growth and crystallographic orientation, and modification of the SC phase distribution is coincident with the morphological development of the TCO phas e in both composite systems. Upon heat-treatment, the high-energy optical absorption edge of the nanocomposite is blue-shifted compared to that of the corresponding as-deposited material. This indicates the development of quantum-confinement conditions for photocarriers within the Ge phase which leads to an increase d energy gap over that expected for the more bulk-like, as-deposited Ge material. Under the deposition and thermal treatment conditions used in the present study, the spectral absorption response is consistent between the ZnO and ITO-based thin films examined. This suggests that carrier confinement conditions are med iated by the development of similar Ge-phas e local spatial extent and Ge:TCO interfacial structures in both systems, regardless of TCO identity. Keywords: Nanocomposite; thin-film; transp arent conductive oxides; quantum-confined semiconductor