Effect of CdCl 2 Treatment on the Interior of CdTe Crystals
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Effect of CdCl2 Treatment on the Interior of CdTe Crystals Kent J. Price Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606 ABSTRACT An essential processing step in CdTe/CdS polycrystalline solar cells is heat treatment in CdCl2. We present photoluminescence results from single crystals of CdTe that have been exposed to CdCl2 treatments at 387 C similar to those used in actual cell fabrication. Using sub band gap excitation from a tunable diode laser, we probe states in the interior of the crystal. We show that high-purity (99.998 percent) CdCl2 treatment results in the appearance of a 1.45 eV donor-acceptor transition that is likely due to a Cl-Cu center. Low purity (99.7 percent) CdCl2 treatment results in the appearance of the 1.45 eV line and a 1.555 eV Cu-related emission. These results indicate that the CdCl2 treatment has an effect on the interior of CdTe grains, in addition to its already well established effect on grain boundaries in polycrystalline CdS/CdTe devices. They also imply that CdCl2 treatment may result in the incorporation of Cu into the CdTe grains. The results will be related to the effects of CdCl2 on polycrystalline CdS/CdTe devices that have been observed by other groups. This work is supported by NREL INTRODUCTION CdTe/CdS polycrystalline solar cells show great promise as a low-cost renewable energy source. An essential step in the fabrication of these devices is the annealing of the CdTe layer in the presence of CdCl2. The effects of this process are not fully understood. It is generally accepted that the CdCl2 process promotes grain boundary growth and passivation, and facilitates interdiffusion of S and Te near the CdTe/CdS junction [1-5]. Recently, evidence has been presented suggesting that CdCl2 treatment affects doping and carrier barrier height at the grain boundaries [6,7] and results in the incorporation of Cu into the CdTe layer [8]. The above effects are not fully understood, and it is not known how they work in tandem to improve solar cell performance. One of the reasons for the incomplete understanding is the complicated nature of polycrystalline materials as compared to their single crystal counterparts due to the presence of grain boundary effects. In order to better understand the role of CdCl2 treatment on CdTe, we have performed photoluminescence (PL) and electrical measurements on single crystal CdTe that has been annealed in CdCl2 vapor under conditions similar to those used in the fabrication of polycrystalline CdTe/CdS solar cells. This allows us to determine the effects of CdCl2 on the bulk of the CdTe in the absence of grain boundaries. We show that a 1.550 eV emission, known to be related to a cadmium vacancy (VCd) center, is not eliminated by high-purity CdCl2 treatment despite the presence of excess Cd. When the CdCl2 used is of low purity, the 1.550 eV emission disappears, and an additional emission at 1.555 eV appears that is known to be related to Cu. We also show that both high and low purity CdCl2 treatment creates a 1.45 eV donor-a
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