The Effect of Source Microstructure on the Close-Space Sublimation of Cdte Thin Films for Solar Cell Applications
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Fig. 1 CdTe Superstrate Device Structure
Glass Substrate
Several fabrication techniques have been used to achieve conversion efficiencies exceeding 10%. These include evaporation, electrodeposition, spray pyrolysis, and close-space sublimation (CSS). CdTe thin films grown by CSS have produced the highest efficiency polycrystalline CdTe solar cells to date [2,3,4]. In this process, a source of CdTe is heated to a temperature of about 600-660 'C in a low pressure ambient (1 to 30 torr) consisting of He and 02. The CdTe decomposes according to the reaction:
CdTe(s) --> Cd(g) + 0.5 Te 2 (g)
(1)
and is transported to, and condenses upon a slightly cooler substrate which is placed in close proximity to the source, thus the designation close-space sublimation. For solar cell device fabrication, substrates consist of glass pre-coated with Sn0 2 and CdS layers in a CdS/Sn0 2/glass 45 Mat. Res. Soc. Symp. Proc. Vol. 410 0 1996 Materials Research Society
configuration. The source is typically a thick CdTe film pre-grown (again by CSS) on either bare 7059 glass or 7059 glass coated with a Sn0 2 layer used to enhance nucleation. These latter source plates are used repeatedly for between 15-20 CdTe depositions depending upon the initial amount (i.e., thickness) of source material present. Heating of both the substrate and source material is accomplished with the use of individually controlled IR-lamps. The arrangement of source, substrate, and heating elements are shown in Figure 2. H2
IRLamp
graphite blocks
He
N2
A\-i
1
--
--
H-I Exhaust
substrate
-'---
Fig. 2 Drawing of the CSS process
H
showing arrangement of the source, substrate, and heating
_U Exhaust VIR Lampz
elements.
source
The growth of CdTe by CSS involves a combination of various series-parallel growthlimiting mechanisms. Evaporation of CdTe from the source involves the parallel combination of evaporation kinetics, as determined by the vapor pressure of CdTe and microstructure of the source, and possible chemical reactions of the source with the ambient atmosphere, in this case, most notably, oxidation [5]. The latter oxidation is apparant to the visible eye as the formation of a yellowish powder on the CdTe source surface with repeated source use. CdTe growth on the substrate is similarly affected by the condensation kinetics of nucleation and growth, and again, chemical reactions with the substrate, in this case the reaction between CdTe and CdS. Transport of CdTe from source to substrate, is diffusion-controlled, and any deviation between predicated and actual deposition rates has been attributed to barriers associated with either evaporation or condensation [6 ]. In this paper, we present data discussing the first of these mechanistic steps, evaporation of CdTe from CdTe sources grown on Corning 7059 glass, with and without a Sn02 nucleating layer. EXPERIMENT The growth of CdTe sources on both bare and Sn0 2 -coated 7059 glass was performed in the same CSS system used for depositing CdTe on CdS/Sn0 2/Glass substrates for solar cell devic
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