Defects in Undoped p-type CdTe Single Crystals

Abstract

Recently, the device performance of Cadmium Telluride (CdTe) solar cells have improved significantly. However, little progress has been made in understanding the defected structures responsible for carrier trapping, which have limited further cell efficiency improvements. In order to better understand the underlying electrical compensation mechanism, an undoped CdTe bulk crystal boule was grown from melt with Te-rich stoichiometry using the Vertical Bridgman technique, where the growth process was controlled to maximize net acceptor density. The samples prepared from the boule indicate low resistivity of 100–500 Ω·cm, and Hall measurements provided net acceptor density of (1–2) $\times \,{\text 10^{15}}\,{\text{cm}^{- 3}}$ , which are comparable to relevant values reported from the CdTe films after CdCl2 treatment. Infrared microscopy suggests there are significant amounts of secondary phases (SP's) present in the samples, where the SP's are believed to be Te inclusions and precipitates. The point defects are characterized by thermoelectric effect spectroscopy (TEES), where we observed six TEES current peaks associated with the defect levels ranging from 110 to 700 mV within the bandgap.