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Spatially resolved characterisation of CdTe photovoltaic solar cells

Brooks, William (2012) Spatially resolved characterisation of CdTe photovoltaic solar cells. PhD thesis, Prifysgol Bangor University.

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Abstract

Spatially resolved measurements of CdTe thin film photovoltaic solar cells were performed using both laser beam induced current (LBIC) and scanning probe microscopy (SPM) techniques. The triple wavelength LBIC system was used to assess the thickness uniformity of Cd1-xZnxS window layers incorporated into CdTe solar cells. A blue laser was used to reveal window layer absorption and transmission characteristics. This was observed to influence photoresponse at longer wavelengths where lateral variations in minority carrier lifetime were leading to variable carrier collection. This was found to be caused by localised regions of ~ 50 nm thin Cd1-xZnxS forming a defective depletion region. The moderate to high clustering of pin-holes in both thick and thin regions of Cd1-xZnxS and CdTe layers was found to contribute to shunt resistance losses independently of the Cd1-xZnxS thickness distribution. Quantum dot (QD) luminescent down shifting layers incorporated into Cd1-xZnxS/CdTe devices were studied using the LBIC technique where, using a 405 nm excitation wavelength, QD isotropic emission was observed to increase the overall lateral carrier collection area of the cell. Scanning Kelvin probe microscopy (SKPM) was used to study the Fermi level shift in Arsenic doped CdTe devices where the contact potential difference (CPD) between probe tip and sample surface revealed that increasing As concentrations in CdTe led to a decrease in CPD. This highlighted a downward shift in the CdTe Fermi level and an increase in CdTe work function. Absolute CdTe work function values between 3.88 and 4.09 eV were calculated using a highly oriented pyrolytic graphite reference sample. A localised shift in CPD at grains boundaries with increased As doping was observed. This was proposed to reduce carrier recombination by channelling minority carriers away from the grain boundary. Conductive atomic force microscopy revealed differences in bulk grain and grain boundary conductivity. The localised CdTe Ef and the barrier formed at the tip/surface interface was observed to determine the measured current.

Item Type: Thesis (PhD)
Subjects: Degree Thesis
Departments: College of Physical and Applied Sciences > School of Electronic Engineering
Degree Thesis
Date Deposited: 21 Aug 2015 13:38
Last Modified: 10 Jan 2017 14:03
URI: http://e.bangor.ac.uk/id/eprint/5197
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