Selective-Area Growth of ZnCdTe on Patterned SiO2/CdS via Close-Space Sublimation

D. Zubia, B.A. Aguirre, D. Pete, K.L. Jungjohann, J. Michael, D. Marrufo, A. Vidana, J.C. McClure, J.L. Cruz-Campa
University of Texas at El Paso, US

Keywords: CdTe, selective-area growth, close space sublimation

Summary:

Cadmium telluride (CdTe) thin-film photovoltaics has surpassed crystalline silicon in cost-efficiency in the multi-kilowatt market as a result of its favorable material properties and relatively low manufacturing cost. CdTe has a high absorption coefficient (>104 cm-1) and ideal bandgap (~1.5 eV) with a theoretical maximum efficiency of ~30%. However, the highest reported efficiency is 20.4% for laboratory scale devices. In this paper, we present a comprehensive approach that combines nanoheteroepitaxy (nano-patterned epitaxy) with alloy compositional grading of ZnxCd(1-x)Te to achieve arrays of isolated, nano-scale, and single-crystal solar cells with the purpose to reduce the defect density and achieve high open-circuit voltage. Selective growth of CdTe on patterned CdS/ITO/glass substrates was performed using closed space sublimation. We present the morphology and corresponding grain orientation of the ZnCdTe island array via SEM and EBSD. Good uniformity was obtained and the CdTe grains appear to be single crystal as indicated by the lack of contrast in SEM and EBSD images. A study to understand the correlation between microstructure and electrical performance was performed. The electrical I-V performance of isolated ZnCdTe/CdS cells is measured using conductive AFM. TEM is used to directly compare the atomic structure of individual nanocells to their electrical I-V performance.