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

journal contribution
posted on 20.03.2018, 14:13 by Ali Abbas, Daniel M. Meysing, M.O. Reese, Teresa M. Barnes, Michael Walls, Colin A. Wolden
© 2017 Elsevier Ltd 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 CdS 0.8 O 1.2 . 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 de position, CdCl 2 activation, and back contact activation. It is found that during CdTe deposition by close spaced sublimation at 600 °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 °C CdCl 2 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 CdCl 2 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.

Funding

AA and JMW were funded by UKERC through the EPSRC Supergen SuperSolar Hub. DMM, MOR, and TMB gratefully acknowledge funding from the U.S. Department of Energy through the SunShot Foundational Program to Advance Cell Efficiency (F-PACE) under Contract No. DE-AC36-08-GO28308. CAW was funded by the Bay Area Photovoltaic Consortium (BAPVC, Award No. DE-EE0004946).

History

School

  • Mechanical, Electrical and Manufacturing Engineering

Published in

Solar Energy

Volume

159

Pages

940 - 946

Citation

ABBAS, A. ...et al., 2017. Structural and chemical evolution of the CdS:O window layer during individual CdTe solar cell processing steps. Solar Energy, 159, pp. 940-946.

Publisher

© Elsevier

Version

AM (Accepted Manuscript)

Publisher statement

This work is made available according to the conditions of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) licence. Full details of this licence are available at: https://creativecommons.org/licenses/by-nc-nd/4.0/

Acceptance date

20/11/2017

Publication date

2017

Notes

This paper is in closed access.

ISSN

0038-092X

Language

en

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