Structural and Chemical Evolution of the CdS:O Window Layer During Individual CdTe Solar Cell Processing Steps

Matthew Reese, Teresa Barnes, A. Abbas, D. Meysing, J. Walls, C. Wolden

Research output: Contribution to journalArticlepeer-review

16 Scopus Citations

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 °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 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.

Original languageAmerican English
Pages (from-to)940-946
Number of pages7
JournalSolar Energy
Volume159
DOIs
StatePublished - 1 Jan 2018

Bibliographical note

Publisher Copyright:
© 2017 Elsevier Ltd

NREL Publication Number

  • NREL/JA-5K00-70708

Keywords

  • Cadmium sulfate
  • CdTe solar cells
  • Oxygenated CdS
  • TEM

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