Role of CdTe Deposition Temperature in the Fabrication and Optimization of Sputtered CdTe Solar Cells

Thin film CdTe solar cells have been fabricated through RF magnetron sputtering on soda lime glass substrates coated with transparent conducting oxide layers. The performance of such devices is expected to be sensitive to the CdTe absorber layer deposition parameters, such as the target power, which controls the flux of depositing species, gas pressure, which controls the energetic bombardment of the growing film, and deposition temperature, which controls the surface diffusion of the depositing species. Recent studies of the Urbach absorption tails of both sputtered and close space sublimated CdTe applied in solar cells have suggested that the broader band tails found in the sputtered form of CdTe provides a route for photogenerated carrier recombination [1]. Such broader band tails appear to be associated with smaller grains and residual stresses that exist even after an optimized CdCl₂ thermal treatment. These results motivates the study of sputtered CdTe solar cells prepared with absorbers fabricated at elevated temperatures in order to explore the role of deposition temperature in controlling the grain size, Urbach tail breadth, and band tail recombination for both as-deposited and CdCl₂ treated materials. The CdCl₂ treatment may play the dominant role, however, compared to deposition temperature, in controlling the CdTe grain size and the resulting optoelectronic properties. Although the effect of the CdCl₂ treatment on the quality of CdTe films and devices has been studied extensively, few studies have investigated the role of the as-deposited material properties which in the case of low temperature sputtered CdTe are more strongly modified by the treatment [1]. In this work, we have fabricated CdS/CdTe solar cell devices on 15 cm x 15 cm TEC-15/HRT glass substrates in the superstrate configuration with the CdTe deposition temperatures varied from 150 to 330 °C. The structural and the optical characteristics of the as-deposited and treated CdTe materials are investigated using X-ray diffraction, scanning electron microscopy, photothermal deflection spectroscopy, and spectroscopic ellipsometry (SE) in order to compare the impact of deposition temperature and the CdCl₂ treatment on the CdTe film structure and optoelectronic properties. Mapping spectroscopic ellipsometry (M-SE) in conjunction with specialized calibration procedures have been applied to optimize the window layer and back contact layer thicknesses independently for each deposition temperature [2]. In this process, it has been found that at the low sputtering pressures an appreciable thickness of the CdS window layer evaporates prior to the CdTe sputter deposition at the elevated temperature, and this possibly unrecognized effect has a significant influence on solar cell process optimization at elevated temperatures.<br/><br/>[1] J. J. Andrews, M. Beaudoin, S. K. O’Leary, P. Koirala, B. Ramanujam, X. Tan, M. A. Razooqi Alaani, P. Pradhan, N. J. Podraza, and R. W. Collins, <i>Journal of Applied Physics</i> <b>129</b>, 165302 (2021).<br/>[2] M. A. Razooqi Alaani, P. Koirala, P. Pradhan, A. B. Phillips, N. J. Podraza, M. J. Heben, and R. W. Collins, <i>Solar Energy Materials and Solar Cells</i> <b>221</b>, 110907 (2021).