Materials studies related to the CuxS/ZnyCd1-yS solar cell

A study was conducted of CuₓS and its interaction with the substrate and ambient. The goals of these CuₓS on CdS and Zn_yCd_1-yS substrates were to find the differences in materials related properties, if any. Cadmium and zinc compositions in CuₓS formed on Zn_yCd_1-yS films (O < y < 0.25) by means of ion exchange were measured using Auger Electron Spectroscopy (AES), Atomic Absorption Spectroscopy (AAS), and Electron Spectroscopy for Chemical Analysis (ESCA). Net concentrations of Cd and Zn in as-formed Cu₂S are generally in the 10¹⁸-10¹⁹ cm⁻³ range. Heat treatments in both oxidizing and reducing ambients raise the concentrations by over an order of magnitude, with the Zn concentration increasing more so than those of Cd. Large increases in Zn at or near the CuₓS surface were measured subsequent to heat treatment, accompanied by increased oxygen. Following heat treatments, Cd and Zn concentrations in the CuₓS"bulk" are found to be less than 10¹⁹ and 10²⁰ cm.⁻³, respectively, for all substrate compositions used. It is proposed that the presence of Cd and Zn can adversely effect the minority carrier lifetimes. These effects would tend to reduce the light generated current. The effects of heat treating CuₓS/Zn_yCd_1-yS and CuₓS/CdS in reducing and various oxidizing ambients are also reported. Structural changes taking place in CuₓS as a result of these heat treatments were monitored by using x-ray diffraction. The principal physical mechanism responsible for phase changes in CuₓS appears to x be copper diffusion through the copper sulfide layer to the top surface as well as into the substrate. Changes in CuₓS stoichiometry were correlated with the sheet resistance of the CuₓS layer. Results indicate that heat treatment in a hydrogen atmosphere causes an increase in resistivity (corresponding to an increase in stoichiometry) while heat treatment in air causes the reverse effect. Wet air heat treatment tended to decrease the resistivity much more as compared to dry air. It was observed that CuₓS formed on Zn_yCd_1-yS tended to degrade in stoichiometry much faster as compared to CuₓS formed on CdS. The resistivity of evaporated CuₓS on plain glass seemed to be linked to the amount of free copper and sulfur present in the as-deposited film. Argon heat treatment tended to decrease the resistivity by an order of magnitude. Heat treatment tended to react the free copper and sulfur, giving CuₓS. Free copper and sulfur can increase the resistivity by acting as neutral impurity scattering centers. As-deposited films were always Cu rich as evidenced by x-ray diffraction and EDAX. Argon heat treatment tended to decrease the amount of free copper present. X-ray photoelectron spectroscopy (XPS) was applied to the surface chemical characterization of chemiplated CuₓS on Zn_yCd_1-yS and CdS. CuₓS was also vacuum evaporated onto glass substrates for this purpose. The effects of ambient (oxygen and water vapor in particular) on chemical species present at or near the CuₓS surface were investigated. Subsequent heat treatments of CuₓS/Zn_yCd_1-yS and CuₓS/CdS promoted migration of Zn and Cd toward the Cu₂S surface. When formed on CdS, the CuₓS surface was found to contain CdO (or Cd (OH)₂) CuO, CuSO₄.nH₂O and CdSO₄.nH₂). Cu₂S formed on Zn_yCd_1-yS was found to contain ZnO as the predominant chemical species, with the Cu and Cd compounds present in lesser amounts. Some interesting characteristics of powder standards used in the XPS studies, some of which have not appeared in the literature, are presented in Appendix 2. The above effects can account for key differences in the properties of CuₓS formed on Zn_yCd_1-yS and CdS films. This provides information on the possible degradation mechanisms for these types of junctions.

Name thesis:degree_name

Ph. D.

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Materials Engineering Science

Department dc:contributor.department

Materials Engineering Science

Grantor dc:publisher

Virginia Polytechnic Institute and State University

Year dc:date.issued

1983

Author dc:creator

• Uppal, Parvez Nasir

Chair dc:contributor.committeechair

• Burton, Larry C.

Committee members dc:contributor.committeemember

• Ijaz, M.A.
• Leinhardt, T.E.
• Dwight, David W.
• Hasselman, D.P.H.

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en_US

Handle dc:identifier.uri

http://hdl.handle.net/10919/94542

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/94542