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Title: Structural and chemical evolution of the CdS:O window layer during individual CdTe solar cell processing steps

Abstract

Oxygenated cadmium sulfide (CdS:O) is often used as the n-type window layer in high-performance CdTe heterojunction solar cells. The as-deposited layer prepared by reactive sputtering is XRD amorphous, with a bulk composition of CdS0.8O1.2. Recently it was shown that this layer undergoes significant transformation during device fabrication, but the roles of the individual high temperature processing steps was unclear. In this work high resolution transmission electron microscopy coupled to elemental analysis was used to understand the evolution of the heterojunction region through the individual high temperature fabrication steps of CdTe deposition, CdCl2 activation, and back contact activation. It is found that during CdTe deposition by close spaced sublimation at 600 degrees C the CdS:O film undergoes recrystallization, accompanied by a significant (~30%) reduction in thickness. It is observed that oxygen segregates during this step, forming a bi-layer morphology consisting of nanocrystalline CdS adjacent to the tin oxide contact and an oxygen-rich layer adjacent to the CdTe absorber. This bilayer structure is then lost during the 400 degrees C CdCl2 treatment where the film transforms into a heterogeneous structure with cadmium sulfate clusters distributed randomly throughout the window layer. The thickness of window layer remains essentially unchanged after CdCl2 treatment, butmore » a ~25 nm graded interfacial layer between CdTe and the window region is formed. Finally, the rapid thermal processing step used to activate the back contact was found to have a negligible impact on the structure or composition of the heterojunction region.« less

Authors:
 [1];  [2];  [3];  [3];  [1];  [4]
  1. Loughborough Univ. (United Kingdom)
  2. Colorado School of Mines, Golden, CO (United States); National Renewable Energy Lab. (NREL), Golden, CO (United States)
  3. National Renewable Energy Lab. (NREL), Golden, CO (United States)
  4. Colorado School of Mines, Golden, CO (United States)
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE), Solar Energy Technologies Office (EE-4S), SunShot Foundational Program to Advance Cell Efficiency (F-PACE); USDOE
OSTI Identifier:
1414818
Alternate Identifier(s):
OSTI ID: 1549150
Report Number(s):
NREL/JA-5K00-70708
Journal ID: ISSN 0038-092X
Grant/Contract Number:  
AC36-08GO28308; AC36-08-GO28308; CBET-1706149
Resource Type:
Accepted Manuscript
Journal Name:
Solar Energy
Additional Journal Information:
Journal Volume: 159; Journal Issue: C; Journal ID: ISSN 0038-092X
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; oxygenated CdS; CdTe solar cells; cadmium sulfate; TEM

Citation Formats

Abbas, A., Meysing, D. M., Reese, M. O., Barnes, T. M., Walls, J. M., and Wolden, C. A. Structural and chemical evolution of the CdS:O window layer during individual CdTe solar cell processing steps. United States: N. p., 2017. Web. doi:10.1016/j.solener.2017.11.051.
Abbas, A., Meysing, D. M., Reese, M. O., Barnes, T. M., Walls, J. M., & Wolden, C. A. Structural and chemical evolution of the CdS:O window layer during individual CdTe solar cell processing steps. United States. doi:10.1016/j.solener.2017.11.051.
Abbas, A., Meysing, D. M., Reese, M. O., Barnes, T. M., Walls, J. M., and Wolden, C. A. Fri . "Structural and chemical evolution of the CdS:O window layer during individual CdTe solar cell processing steps". United States. doi:10.1016/j.solener.2017.11.051. https://www.osti.gov/servlets/purl/1414818.
@article{osti_1414818,
title = {Structural and chemical evolution of the CdS:O window layer during individual CdTe solar cell processing steps},
author = {Abbas, A. and Meysing, D. M. and Reese, M. O. and Barnes, T. M. and Walls, J. M. and Wolden, C. A.},
abstractNote = {Oxygenated cadmium sulfide (CdS:O) is often used as the n-type window layer in high-performance CdTe heterojunction solar cells. The as-deposited layer prepared by reactive sputtering is XRD amorphous, with a bulk composition of CdS0.8O1.2. Recently it was shown that this layer undergoes significant transformation during device fabrication, but the roles of the individual high temperature processing steps was unclear. In this work high resolution transmission electron microscopy coupled to elemental analysis was used to understand the evolution of the heterojunction region through the individual high temperature fabrication steps of CdTe deposition, CdCl2 activation, and back contact activation. It is found that during CdTe deposition by close spaced sublimation at 600 degrees C the CdS:O film undergoes recrystallization, accompanied by a significant (~30%) reduction in thickness. It is observed that oxygen segregates during this step, forming a bi-layer morphology consisting of nanocrystalline CdS adjacent to the tin oxide contact and an oxygen-rich layer adjacent to the CdTe absorber. This bilayer structure is then lost during the 400 degrees C CdCl2 treatment where the film transforms into a heterogeneous structure with cadmium sulfate clusters distributed randomly throughout the window layer. The thickness of window layer remains essentially unchanged after CdCl2 treatment, but a ~25 nm graded interfacial layer between CdTe and the window region is formed. Finally, the rapid thermal processing step used to activate the back contact was found to have a negligible impact on the structure or composition of the heterojunction region.},
doi = {10.1016/j.solener.2017.11.051},
journal = {Solar Energy},
number = C,
volume = 159,
place = {United States},
year = {2017},
month = {12}
}

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