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Photoluminescence and Extended X-ray Absorption Fine Structure Studies on CdTe Material
| Title: | Photoluminescence and Extended X-ray Absorption Fine Structure Studies on CdTe Material |
| Author: | Liu, Xiangxin |
| Description: | The direct-band-gap semiconductor CdTe is an important material for fabricating high efficiency, polycrystalline thin-film solar cells in a heterojunction configuration. The outstanding physical properties of this material such as its good band-gap match to the solar spectrum, ease of fabrication of stoichiometric films, and easy grain boundary passivation make it an important candidate for large area, thin-film solar cells. However, there are several poorly understood processing steps that are commonly utilized in cell fabrication. One of these is a CdCl2 treatment near 400C in the presence of oxygen, which can improve the cell efficiency a factor of two or more. Another factor is the role of copper in cell performance. In high performance CdS/CdTe thin-film solar cells, copper is usually included in the fabrication of low-resistance back contacts to obtain heavy p-type doping of the absorber CdTe at the contact. However, most of the copper is not electrically active. For example, secondary ion mass spectroscopy (SIMS) on typical CdTe cells has shown Cu concentrations of 1019 atoms/cm3 and even higher, although capacitance-voltage (C-V) measurements indicate typical ionized acceptor levels on the order of 1014 /cm3. Thus, there is great interest in the location and role of this inactive copper in CdTe photovoltaic (PV) devices. In this thesis, I will describe results obtained on magnetron-sputtered CdTe films that were diffused with copper following the procedure used for creating a cell back contact. Extended x-ray absorption fine structure (EXAFS) measurements identified the chemical environment of the majority of the copper and show major differences depending on whether the CdTe film has been treated with chloride prior to the Cu diffusion. Photoluminescence of CdTe crystals doped with two elements, Cl and P were studied individually. I have identified band-to-band transitions, free-to-bound transitions and bound-exciton lines related to these species. Through the properties of these emission lines, the ionization energy of the Cl donor and the P acceptor in CdTe are also identified to be 53 meV and 70 meV, respectively. |
| Permanent Link: |
http://rave.ohiolink.edu/etdc/view?acc_num=toledo1148408018
http://hdl.handle.net/2374.OX/19377 |
| Date: | 2006 |
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