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

Journal Article · 30 November 2017 · Solar Energy

DOI: https://doi.org/10.1016/j.solener.2017.11.051 · OSTI ID:1414818

Abbas, A. ^[1]; Meysing, D. M. ^[2]; Reese, M. O. ^[3]; Barnes, T. M. ^[3]; Walls, J. M. ^[1]; Wolden, C. A. ^[4]

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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)

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.

• Research Organization: National Renewable Energy Laboratory (NREL), Golden, CO (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE), Solar Energy Technologies Office (EE-4S), SunShot Foundational Program to Advance Cell Efficiency (F-PACE)
• Grant/Contract Number: AC36-08GO28308
• OSTI ID: 1414818
• Alternate ID(s): OSTI ID: 1549150
• Report Number(s): NREL/JA--5K00-70708
• Journal Information: Solar Energy, Journal Name: Solar Energy Journal Issue: C Vol. 159; ISSN 0038-092X
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English