The Effect of Annealing Treatments on Close Spaced Sublimated Cadmium Telluride Thin Film Solar Cells
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The Effect of Annealing Treatments on Close Spaced Sublimated Cadmium Telluride Thin Film Solar Cells J.M. Walls¹, A. Abbas¹ ², G. D. West², J.W. Bowers¹, P.J.M. Isherwood¹, P. M. Kaminski¹, B. Maniscalco¹, W.S. Sampath3 and K. L. Barth3
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1
CREST (Centre for Renewable Energy Systems and Technology), Loughborough, UK
2
Department of Materials, Loughborough University, Loughborough, United Kingdom
NSF I/UCRC for Next Generation Photovoltaics, Colorado State University, Fort Collins, Colorado, United States
ABSTRACT It is well known that the cadmium chloride annealing treatment is an essential step in the manufacture of efficient thin film cadmium telluride solar cells. It has been recognized that the combination of annealing at ~4000C together with the addition of cadmium chloride at the surface induces re-crystallisation of the cadmium telluride layer and also affects the n-type cadmium sulfide. We have applied advanced micro-structural characterization techniques to distinguish the effect of the annealing and the cadmium chloride treatments on the properties of the cadmium telluride deposited via close space sublimation (CSS) and relate these observations to device performance. Transmission electron microscopy (TEM) has shown a variation in stacking fault density with annealing temperature and annealing time. Stacking faults observed within the cadmium telluride grains in TEM were partially removed post annealing; these findings show that temperature alone has a role in the reduction of stacking faults. However, since we have previously observed almost complete removal of stacking faults with annealing in combination with cadmium chloride, the cadmium chloride is essential to defect removal and high efficiency cells. INTRODUCTION The thin film cadmium telluride/cadmium sulfide system is an important photovoltaic technology with cell efficiencies now over 17% [1]. Cadmium telluride is a p-type material with a band gap of 1.45 eV, ideal for photovoltaic conversion. Also it has a very high absorption coefficient, so that only a thin layer (~2 μm) is required for almost total absorption. The p-type cadmium telluride and n-type cadmium sulfide can be deposited using a variety of methods. It has been found that the as-deposited films contain a high density of stacking faults which correspond to low cell efficiency [2,3]. The cadmium chloride annealing treatment causes a significant increase in cell performance and is known to radically change the structural and electrical characteristics of the cell [4-7]. It consists of two parts, first the introduction of cadmium chloride onto the cadmium telluride surface and second, the heating of the device stack. The aim of this investigation is to determine the change in the microstructure of the cadmium telluride caused by annealing alone. We have changed the annealing time and temperature without the presence of cadmium chloride. This has allowed us to distinguish the role of annealing during the cadmium chloride treatment.
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EXPERIMENTAL PROCEDURES The thin film cad
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