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Title: CdCl2 Treatment, S Diffusion, and Recombination in Polycrystalline CdTe

Abstract

Time-resolved photoluminescence measurements on glass/SnO{sub 2}/CdTe and glass/SnO{sub 2}/CdTe/CdS structures indicate that the CdCl{sub 2} process, without any S present, significantly reduces recombination. However, S diffusion is required for lifetimes comparable to those observed in high-efficiency solar cells. Low-temperature photoluminescence, cathodoluminescence, and scanning electron images indicate how defect chemistry, grain-boundary passivation, and morphology are affected by S diffusion and the CdCl{sub 2} treatment.

Authors:
; ; ; ;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
976980
DOE Contract Number:
AC36-08GO28308
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 99; Journal Issue: 10, 2006; Related Information: Article No. 103703
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; CATHODOLUMINESCENCE; CHEMISTRY; DEFECTS; DIFFUSION; ELECTRONS; MORPHOLOGY; PASSIVATION; PHOTOLUMINESCENCE; RECOMBINATION; SOLAR CELLS; Solar Energy - Photovoltaics

Citation Formats

Metzger, W. K., Albin, D., Romero, M. J., Dippo, P., and Young, M. CdCl2 Treatment, S Diffusion, and Recombination in Polycrystalline CdTe. United States: N. p., 2006. Web. doi:10.1063/1.2196127.
Metzger, W. K., Albin, D., Romero, M. J., Dippo, P., & Young, M. CdCl2 Treatment, S Diffusion, and Recombination in Polycrystalline CdTe. United States. doi:10.1063/1.2196127.
Metzger, W. K., Albin, D., Romero, M. J., Dippo, P., and Young, M. Sun . "CdCl2 Treatment, S Diffusion, and Recombination in Polycrystalline CdTe". United States. doi:10.1063/1.2196127.
@article{osti_976980,
title = {CdCl2 Treatment, S Diffusion, and Recombination in Polycrystalline CdTe},
author = {Metzger, W. K. and Albin, D. and Romero, M. J. and Dippo, P. and Young, M.},
abstractNote = {Time-resolved photoluminescence measurements on glass/SnO{sub 2}/CdTe and glass/SnO{sub 2}/CdTe/CdS structures indicate that the CdCl{sub 2} process, without any S present, significantly reduces recombination. However, S diffusion is required for lifetimes comparable to those observed in high-efficiency solar cells. Low-temperature photoluminescence, cathodoluminescence, and scanning electron images indicate how defect chemistry, grain-boundary passivation, and morphology are affected by S diffusion and the CdCl{sub 2} treatment.},
doi = {10.1063/1.2196127},
journal = {Journal of Applied Physics},
number = 10, 2006,
volume = 99,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • For decades, polycrystalline CdTe thin films for solar applications have been restricted to grain sizes of microns or less whereas other semiconductors such as silicon and perovskites have produced devices with grains ranging from less than a micron to more than 1 mm. Because the lifetimes in as-deposited polycrystalline CdTe films are typically limited to less than a few hundred picoseconds, a CdCl2 treatment is generally used to improve the lifetime; but this treatment may limit the achievable hole density by compensation. Here, we establish methods to produce CdTe films with grain sizes ranging from hundreds of nanometers to severalmore » hundred microns by close-spaced sublimation at industrial manufacturing growth rates. Two-photon excitation photoluminescence spectroscopy shows a positive correlation of lifetime with grain size. Large-grain, as-deposited CdTe exhibits lifetimes exceeding 10 ns without Cl, S, O, or Cu. This uncompensated material allows dopants such as P to achieve a hole density of 1016 cm-3, which is an order of magnitude higher than standard CdCl2-treated devices, without compromising the lifetime.« less
  • Cited by 6
  • In this work, the authors describe procedures to prepare the surface of close-spaced sublimation CdTe thin films necessary for producing good electron backscatter diffraction (EBSD) data. They found that polishing resulted in an amorphous layer on the surface and no Kikuchi pattern; however, ion-beam milling produced a relatively flat and good-quality surface, resulting in high-quality patterns and, consequently, excellent EBSD data. The authors used a combination of polishing and ion-beam milling or etching to study the crystalline structure of the CdTe film at different depths. They also used EBSD, in conjunction with other analytical techniques, to investigate the effects ofmore » the CdCl{sub 2} treatment, performed at different temperatures and times, on the recrystallization process of physical vapor deposition CdTe thin films. The authors found that the untreated films were <111> oriented, with grain sizes smaller than 1 {micro}m. The CdCl{sub 2} at 350 C produced partially recrystallized films, whereas treatments at 400 C or 420 C produced completely recrystallized films, with no texture, and grains with grain sizes varying from about 1 {micro}m to more than 40{micro}m. These films were so flat that good EBSD data could be obtained without any surface preparation. Atomic force microscopy and scanning electron microscopy showed that large grains had different morphologies than smaller grains, and EBSD showed that these large grains had <111> texture. These results indicate that the (111) surface is the lowest energy surface in these films and, consequently, <111>-oriented grains grow at the expense of grains oriented in less-favorable directions. Regardless of the deposition method and treatment, the CdTe films have a high density of 60{sup o} <111> twin boundaries.« less
  • We report on a transmission electron microscopy and energy-dispersive x-ray spectroscopy study of S diffusion in polycrystalline CdS/CdTe heterojunctions. We find that grain boundaries significantly assist S diffusion in the CdTe layer when the CdTe is grown without the presence of oxygen, i.e., the S diffuses more easily along the grain boundaries than in the grains. However, grain boundaries do not enhance the S diffusion in CdTe when it is grown in the presence of oxygen. The reason is likely to be the formation of Cd--O bonds at the grain boundaries, which are resistance to the S diffusion.