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Brooks W.S.M.,Center for Solar Energy Research | Irvine S.J.C.,Center for Solar Energy Research | Barrioz V.,Center for Solar Energy Research
Energy Procedia | Year: 2011

An assessment of Cd 0.9Zn 0.1S window layer thickness and its impact on photoresponse uniformity in CdTe thin film photovoltaic (PV) devices is presented. A triple-wavelength laser beam induced current (LBIC) system provided a spatially resolved photocurrent mapping technique. Three diode lasers; λ = 405, 658, 810nm gave photon absorption and carrier generation characteristics over the spectral range of the Cd 0.9Zn 0.1S/ CdTe devices. Two contrasting device structures were grown by metal-organic chemical vapour deposition (MOCVD), where the uniformity of the Cd 0.9Zn 0.1S window layer was known to vary: 1. uniform 240 nm Cd 0.9Zn 0.1S/ 2 μm CdTe and 2. a poorly nucleated 40 - 300 nm Cd 0.9Zn 0.1S/ 2 μm CdTe. Calculated photon penetration depths, δ p allowed for the separation of identified defects within the device cross-section. Cd 0.9Zn 0.1S pin holes were identified in 240 nm Cd 0.9Zn 0.1S/ 2 μm CdTe where 405 nm photon 'punch-through' into the absorber material was observed. These pin holes also led to a localised reduction in photoresponse at λ = 658 and 810 nm. In the device structure where the Cd 0.9Zn 0.1S window layer thickness was known to vary from 40 to 300 nm, CdTe pin holes were identified where localised ∼ 50 μm regions of reduced photoresponse, at all wavelengths were observed. Local variations in both Cd 0.9Zn 0.1S and CdTe thickness were also identified where variable absorption led to a distribution of LBIC photoresponse. It was demonstrated that reduced photoresponse uniformity at all incident wavelengths was related to reduced device shunt resistance, R sh and open-circuit voltage, V oc. © 2011 Published by Elsevier Ltd.

Barrioz V.,Center for Solar Energy Research | Kartopu G.,Center for Solar Energy Research | Irvine S.J.C.,Center for Solar Energy Research | Monir S.,Center for Solar Energy Research | Yang X.,University of Science and Arts of Iran
Journal of Crystal Growth | Year: 2012

A study was undertaken to assess the efficiency of precursors usage during deposition of cadmium telluride (CdTe) layers via atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD) for thin film photovoltaic solar cells. Precursors were released from a showerhead assembly normal to the glass substrate 0.7 mm thick (5×7.5 cm 2) being deposited which was kept stationary or moved under the showerhead assembly, with speed of upto 2.25 cm/min. In order to estimate the effective precursor utilisation, the weight deposit (layer) was compared against the theoretical values calculated for ideal molar supply. The layer thickness, composition, morphology, and crystallinity were also measured using profilometry, energy dispersive X-ray (EDX), scanning electron microscopy (SEM), and X-ray diffraction (XRD), respectively. It is shown that over 40% material utilisation can be achieved depending on the deposition parameters of substrate temperature and speed, partial pressure of precursors and total gas flow. The activation energy derived from an Arrhenius plot of deposition rate equals 49 kJ mol -1 and is consistent with previous reports of MOCVD CdTe using a horizontal reactor. This confirms that, despite the very different reactor geometry, the alkyl radical homolysis and reaction mechanism applies in the case of the inline injector geometry in the work presented here. These results demonstrate an alternative path to high throughput processing of CdTe thin film solar cells by inline AP-MOCVD. © 2012 Elsevier B.V.

Brooks W.S.M.,Center for Solar Energy Research | Irvine S.J.C.,Center for Solar Energy Research | Barrioz V.,Center for Solar Energy Research | Clayton A.J.,Center for Solar Energy Research
Solar Energy Materials and Solar Cells | Year: 2012

The role of the window layer in Cd 1-xZn xS/CdTe photovoltaic (PV) device structures was investigated using a triple wavelength laser beam induced current (LBIC) technique. Full pn structures were deposited using metal organic chemical vapour deposition (MOCVD). The triple wavelength LBIC technique allowed for the diagnostic analysis of carrier collection uniformity at 3 wavelength dependent photon penetration depths, δ p. An assessment of the Cd 1-xZn xS window layer film thickness and surface coverage showed a correlation between lateral device performance and window layer thickness distribution. Islands of thick Cd 1-xZn xS were shown to increase localised blue absorption, correlating to isolated regions of increased red and IR photoresponse. Overall device open-circuit voltage, V oc is shown to demonstrate a linear dependence on thick (∼300 nm) Cd 1-xZn xS surface coverage. V oc and shunt resistance R sh losses attributed to the window layer were found to jointly contribute to the overall cell efficiency losses. A non-uniform pin-hole distribution, leading to R sh loss, was found to be independent of the overall Cd 1-xZn xS thickness distribution; thus the triple wavelength LBIC measurement was able to identify causes for the observed losses in both V oc and R sh. © 2012 Elsevier B.V. All rights reserved.

Taylor A.A.,Durham University | Taylor A.A.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Major J.D.,University of Liverpool | Kartopu G.,Center for Solar Energy Research | And 7 more authors.
Solar Energy Materials and Solar Cells | Year: 2015

The role of CdCl2 activation in the production of high quality CdTe-based photovoltaic devices remains a subject of much debate. In this study, CdTe-based cells produced in three independent laboratories using different device fabrication technologies are investigated before and after CdCl2 activation with regard to structural changes (recrystallisation and grain growth) and sulphur out-diffusion. Using scanning transmission electron microscopy (STEM) and X-ray diffraction it is demonstrated that CdCl2 activation of the investigated cells produces no statistical structural changes to the CdTe. Additionally, energy dispersive spectrometry (EDS) performed in the STEM on the same samples illustrates that the change in sulphur diffusion following activation is more limited than expected from previous studies; no change is detectable when the thermal budget for CdTe deposition is significantly greater than that for activation. This suggests that the efficiency enhancement during CdCl2 treatment is not due to sulphur out-diffusion. Lastly, cathodoluminescence microscopy is used to demonstrate in two dimensions how sulphur diffuses into a model sample and the results are found to be consistent with STEM-EDS. Some spectroscopic evidence for enhanced sulphur diffusion along grain boundaries is also observed. © 2015 Elsevier B.V.

Hodgson S.D.,Center for Solar Energy Research | Brooks W.S.M.,Center for Solar Energy Research | Clayton A.J.,Center for Solar Energy Research | Kartopu G.,Center for Solar Energy Research | And 2 more authors.
Nano Energy | Year: 2014

Semiconductor quantum dots (QDs) have been encapsulated in poly(methyl, methacrylate) (PMMA) to prepare luminescent down-shifting films. The concentration of the QDs within each film has been varied, up to a maximum of 240μg/mm3 and the optical properties have been characterised in detail. The QD/PMMA films have been placed over thin-film CdS/CdTe photovoltaic devices and improvements to cell performance have been recorded via external quantum efficiency and current-voltage measurements. The best overall improvement to performance, compared to the uncoated device, was 1.7%. This was obtained with a QD density of 48μg/mm3, while the largest improvement to short-wavelength photocurrent was 30.30%, with a QD density of 192μg/mm3. To further explain these results laser beam induced current measurements were carried out using laser wavelengths of 405 and 658nm on individual cells with and without QD/PMMA films attached. © 2013 Elsevier Ltd.

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