Advanced material system for the design of an intermediate band solar cell: type-II CdTe quantum dots in a ZnCdSe matrix*

Photovoltaics based on intermediate band (IB) absorption have the potential to overcome the Shockley-Quiesser limit and maintain a large V_oc. Quantum dots (QDs) or impurities can be used to form the IB. We propose a material system of submonolayer CdTe QDs embedded a ZnCdSe matrix that is optimum for the formation of an ideal IB and has other advantages to materials previously considered. Some unique attractive features of this material are its binary composition, simplifying growth; the absence of a deleterious interfacial layer, in spite of the lack of common ion; and the presence of strain, which allows for strain engineering of the IB. A superlattice structure of alternating QD and spacer layers is analyzed by X-ray diffraction (XRD) and photoluminescent (PL) spectroscopy. Simple arguments are used, following continuum elastic theory, to deduce the size of the dots and the strain within the superlattice from XRD data. Results of structural and optoelectronic characterization of both active layer and full device structures, using XRD, PL, photocurrent and contactless electro-reflectance measurements will be presented. The results suggest that the optimized materials are very well suited for potential high-efficiency IB solar cells.