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Title: A study of recombination centres in electron irradiated silicon solar cells
Author: Smith, Robert Roland
ISNI:       0000 0001 3461 4420
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2002
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Solar cells in space are subject to a harsh radiation environment that reduces the efficiency of these cells by introducing point defects with energy levels in the forbidden bandgap. The lifetime of minority-carriers is reduced by these deep level defects that can act as recombination centres and annihilate electron-hole pairs. These recombination centres are investigated in this report using a variety of characterisation techniques. Firstly the basic principles involved in the generation and recombination of electron-hole pairs is reviewed and the physics of particle irradiation is discussed. The properties of shallow and deep levels are also discussed and these deep level defects are then examined in detail. The characterisation technique known as Deep Level Transient Spectroscopy (DLTS) and some related techniques are then reviewed and the silicon solar cells used for this investigation are described. The electrical properties of silicon solar cells doped with either boron, gallium, indium, aluminium or phosphorus formed on a variety of substrates are then determined before and after electron irradiation. The reduction in minority-carrier diffusion length caused by particle irradiation is deduced from these results by fitting the experimental data to a theoretical model. DLTS is then used to identify and categorise the deep levels present in these samples after particle irradiation. A new version of the forward bias DLTS technique is then defined and used to determine that the minority-carrier capture cross-section of the positive charge state of the divacancy (VV⁺/⁰) is between 1 and 5x10⁻¹³ cm². The Shockley-Read-Hall (SRH) model is then used to determine the minority-carrier lifetime of each of the observed defects in the gallium-doped cells. This model is then extended to show that the VV⁺/⁰ defect is mainly responsible for the lifetime degradation observed in irradiated czochralski silicon solar cells. The damage coefficients are then determined for all of the cells examined in this study and they were compared to the literature data. Finally it is shown that the commonly observed relationship between the doping density and damage coefficient in silicon solar cells can be explained simply in terms of the divacancies VV⁺/⁰ capture cross-section, introduction rate, and its probability of occupation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Perl solar cells