Deep-level impurities in CdTe/CdS thin-film solar cells

Abstract

We have studied deep-level impurities in CdTe/CdS thin-film solar cells by capacitance–voltage $\mathrm{(C–V),}$ deep-level transient spectroscopy (DLTS), and optical DLTS (ODLTS). CdTe devices were grown by close-spaced sublimation. Using DLTS, a dominant electron trap and two hole traps were observed. These traps are designated as E1 at $E_C\text{−0.28\hspace{0.05em}}\mathrm{eV},$ H1 at $E_V\text{+0.34\hspace{0.05em}}\mathrm{eV},$ and H2 at $E_V\text{+0.45\hspace{0.05em}}\mathrm{eV}.$ The presence of the E1 and H1 trap levels was confirmed by ODLTS. The H1 trap level is due to Cu-induced substitutional defects. The E1 trap level is believed to be a deep donor and is attributed to the doubly ionized interstitial Cu or a Cu complex. The E1 trap is an effective recombination center and is a lifetime killer.