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Real Time and Mapping Spectroscopic Ellipsometry for CdTe Photovoltaics

  • Prakash Koirala
  • Jian Li
  • Nikolas J. PodrazaEmail author
  • Robert W. Collins
Chapter
Part of the Springer Series in Optical Sciences book series (SSOS, volume 212)

Abstract

Real time spectroscopy ellipsometry (RTSE) has been implemented in studies of the structural evolution during magnetron sputtering of the polycrystalline CdS and CdTe thin film components of CdS/CdTe solar cells on glass substrates coated in turn with transparent conducting oxide (TCO) top contact layers. RTSE provides detailed information on various structural features identified during CdS and CdTe film growth. These include (i) formation of the TCO/CdS interface as the surface modulations of the high resistivity transparent (HRT) layer are filled with the depositing CdS during its initial growth; (ii) evolution of the CdS bulk layer thickness and associated surface roughness layer thickness; (iii) formation of the CdS/CdTe interface as the depositing CdTe fills the modulations of the CdS surface, resulting in the heterojunction; and (iv) evolution of the CdTe bulk layer and surface roughness layer thicknesses. The structural and optical models established in the analysis procedures for RTSE data acquired from the film side at a single location on the solar cell surface are also applied in the analysis of ex situ mapping spectroscopic ellipsometry (M-SE) data acquired in through-the-glass mode over the entire area of the completed solar cell structure. In fact, M-SE has been applied to generate maps in the effective thicknesses (or volumes/area) of layer components over the 225 cm2 areas of the solar cell structures for spatial correlation with the performance of arrays of 256 dot cells fabricated over the same areas. The results of such M-SE studies are presented in detail including those from several investigations designed for process optimization of the CdS/CdTe solar cell on TCO-coated glass. In the solar cell fabrication process, the CdS/CdTe structure is subjected to various processing steps after deposition of the heterojunction materials. The first steps include an anneal of the structure with simultaneous CdCl2 exposure of the CdTe film surface, followed by a deposition of ultrathin Cu by thermal evaporation on the CdTe surface. The additional steps that complete the device include a deposition of a metal back contact layer and a final anneal to promote the diffusion of Cu into the underlying CdTe. M-SE studies are described for CdS/CdTe solar cells fabricated with different thicknesses of the CdTe absorber layer within the range from 0.5 to 2.5 μm, focusing initially on the influence of the CdCl2 treatment duration which ranges from 5 to 30 min. In addition, processing-property-performance relationships are identified and presented for the CdS top window layer and the Cu back contact layer; the effective thickness of each layer is critical for the optimization of the solar cells. Relationships between the solar cell performance parameters and the effective thicknesses from M-SE analysis for solar cells fabricated under different processing conditions are demonstrated to facilitate process optimization.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  • Prakash Koirala
    • 1
  • Jian Li
    • 1
  • Nikolas J. Podraza
    • 1
    Email author
  • Robert W. Collins
    • 1
  1. 1.Department of Physics & Astronomy and Center for Photovoltaics Innovation & CommercializationUniversity of ToledoToledoUSA

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