Studies of Basic Electronic Properties of CdTe-Based Solar Cells and Their Evolution During Processing and Stress; Annual Technical Report, 1 November 2005 - 31 October 2006

Abstract

We describe the results of our continuing study of deep electronic states controlling open-circuit voltage in CdTe/CdS thin-film solar cells (Task 1). The study includes: (1) analysis of factors affecting trap signatures derived from admittance spectroscopy and capacitance transients measurements, such as activation-energy capture cross-sections and trapdensity estimates, and (2) comparative studies of cells received from four different sources and prepared with significant variations in cell structure and processing procedures.

Analysis of a novel CuCl2 back contact process for improved stability in CdTe solar cells

“…In this range, the defects are attributed to the Cu at Cd substitutional sites impurities Cu − Cd. These were also detected by Beach et al: they reported that the concentration of these defects was higher in CdCl 2 ‐treated samples, implying that the defects induced by CdCl 2 (probably Cl‐induced donor) increased the solubility of Cu − Cd. …”

“…The VE‐Cu sample shows two deeper dominant defects (C) in the range of 0.43 to0.45 eV above the valence band, usually detected in our standard samples, which are not clearly attributed. Beach et al as well as Wei et al consider plausible to assign these values to the U‐centre Te 2− i : a deep acceptor level. Also, Beach et al highlight that these defects have a higher density for CdCl 2 ‐treated samples, implying that CdCl 2 contributes to their formation; this has been previously observed also in our samples …”

“…It has been reported that the Cu substitution in Cd vacancy Cu − Cd acts as an acceptor, by increasing the p‐type doping, while the interstitial copper Cu i is a shallow donor, which compensates the doping . For this reason, the formation of the Cu − Cd defects is desirable.…”

“…Only for the CCWD sample the dominant defects, identified by AS, are in a range of energies that have been attributed to the substitutional Cu impurities Cu − Cd , known as acceptors in CdTe. Because all the different samples have been CdCl 2 treated, but only CuCl 2 ‐treated samples show this type of defects, it can be supposed that the introduction of copper bounded to chlorine helps the solubility of the Cu − Cd acceptors because chlorine increases their solubility . As a consequence, the p‐type doping of CdTe is improved .…”

“…Beach et al suggested that the defects induced by CdCl 2 , probably Cl‐induced donor, increased the solubility of the Cu at Cd substitutional sites impurities Cu − Cd , which are considered acceptors …”