Cadmium Chloride Assisted Re-Crystallization of CdTe: The Effect of Varying the Annealing Time
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Cadmium Chloride Assisted Re-Crystallization of CdTe: The Effect of Varying the Annealing Time A. Abbas¹ ², G. D. West¹, J.W. Bowers², P. M. Kaminski², B. Maniscalco², J.M. Walls² Kurt L. Barth3 and W.S. Sampath3 1
Department of Materials, 2CREST (Centre for Renewable Energy Systems and Technology), Loughborough University, Loughborough, LE11 3TU, United Kingdom 3
NSF I/UCRC for Next Generation Photovoltaics, Colorado State University, Fort Collins, CO 80523, United States
ABSTRACT Although the cadmium chloride treatment is an essential process for high efficiency thin film cadmium telluride photovoltaic devices, the precise mechanisms involved that improve the cadmium telluride layer are not well understood. In this investigation we apply advanced microstructural characterization techniques to study the effect of varying the time of the cadmium chloride annealing treatment on the micro-structure of cadmium telluride solar cells deposited by close spaced sublimation (CSS) and relate this to cell performance. A range of techniques has been used to observe the morphological changes to the micro-structure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the micro-structural properties of the cells have also been undertaken. Techniques used include Transmission Electron Microscopy (TEM) for sub-grain analysis and XPS for composition-depth profiling. The study provides a new insight in to the mechanisms involved in the initiation and the subsequent complete re-crystallization of the cadmium telluride layer. INTRODUCTION Thin film cadmium telluride photovoltaics is an important energy technology especially for solar power utilities. Recent improvements in small area research cell devices have resulted in a reported efficiency of 19.6% [1] while modules may now be produced with efficiencies up to 16.1% efficiency [1]. CdTe is used as a p-type material with a band gap of 1.45eV which is optimum for photovoltaic conversion. The material has a very high absorption coefficient, so that only a thin layer (~2µm) is required for almost total absorption of the solar spectrum. The CdTe can be deposited using a variety of deposition methods and close spaced sublimation is used here. In previous studies it has been found that the as-deposited films contain a high density of stacking faults [2]. These as-deposited cells exhibit low cell efficiency [2]. A key process step in the production of high efficiency cells is the activation annealing treatment using cadmium chloride. This annealing treatment causes a dramatic improvement in cell performance and is known to change the structural and electrical characteristics of the device [3]. A previous investigation by the present authors has revealed that by annealing without the presence of cadmium chloride, the stacking fault density in the CdTe grains reduces [4]. The temperature and time used for the annealing process has an effect, but defects remain and cell efficiencies are low. Howeve
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