Dissertation
ELECTRICAL PROPERTIES OF CdTe SINGLE CRYSTALS, DEFECTS AND THEIR CORRELATION TO SOLAR CELL PERFORMANCE
Doctor of Philosophy (PhD), Washington State University
01/2015
Handle:
https://hdl.handle.net/2376/112071
Abstract
CdTe-based thin film photovoltaics constitute a major part of the world’s solar energy supply. However, its efficiency, in particular the open circuit voltage (VOC), has remained low due to material qualities such as short minority carrier lifetime and low acceptor density (NA). The defects in CdTe play a critical role in impacting electrical properties of crystals and related device performance. In this thesis, a systematic study was performed to investigate correlating key material properties of CdTe and device performance.
Eleven CdTe boules or crystals were grown using vertical Bridgman method including undoped, phosphorous-doped, and arsenic-doped. Overall, single crystals showed high purity, high lifetime, and improved NA compared to typical CdTe thin-films reported in literatures. The lifetimes are as high as ~300 ns in undoped single crystals and 5-50 ns in P-doped single crystals. We demonstrated high acceptor density, in the range of mid-1016 to low-1017 cm-3, with relatively high bulk minority carrier lifetimes of ~20-50 ns, which translated into exceptional device performance. Device research at WSU showed device performances were strongly related to material properties. The best performance resulted from the P-doped material, which has the highest NA, lowest resistivity and relatively high lifetime. The device work performed in collaboration with the National Renewable Energy Laboratory (NREL) resulted in cell performance with VOC >1000 mV from P-doped crystals, owing to superior material quality. None of the device fabrication processes on single crystals used CdCl2 treatment.
Defects in CdTe were characterized by thermoelectric-effect spectroscopy (TEES) and studied systematically. Comparative TEES analysis on different types of crystals reveals that around 7-8 defects were observed in all types of crystals which were attributed to native defects. Although relatively high NA was obtained from p-type doped growths, it was observed in crystals from all five P-doped and two As-doped growths that the NA was several orders of magnitude less than the dopant concentration, indicating strong self-compensation. The compensations appear to be linked to self-compensating donors and/or defect complexes with cadmium vacancies (VCd). Formation of secondary phases such as Cd3P2 seems to be responsible for the compensation as well in P-doped growth.
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Details
- Title
- ELECTRICAL PROPERTIES OF CdTe SINGLE CRYSTALS, DEFECTS AND THEIR CORRELATION TO SOLAR CELL PERFORMANCE
- Creators
- Abulikemu Tuerxunjiang
- Contributors
- Kelvin G. Lynn (Advisor)Jeanne McHale (Committee Member)Yi Gu (Committee Member)Louis Scudiero (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Materials Science and Engineering Program
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Number of pages
- 158
- Identifiers
- 99900581639101842
- Language
- English
- Resource Type
- Dissertation