Kinetic Studies of CdS Formation for a Better Understanding of Chemical Buffer Layer Deposition
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Kinetic Studies of CdS Formation for a Better Understanding of Chemical Buffer Layer Deposition Krzysztof Wilchelmi1, Diana Förster1, Axel Neisser1, Reinhard Schomäcker2 1 Sulfurcell Solartechnik GmbH, Barbara-McClintock-Str. 11, 12489 Berlin, Germany 2 Department of Chemistry, Technical University Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany ABSTRACT CdS is an important part of CIGS-based photovoltaics as a buffer layer between the absorber and the window layer. Chemical Bath Deposition (CBD) is the most common way of depositing CdS. Due to the nature of the CBD process CdS forms not only on the absorber layer surface but also in the bulk solution. In order to control the deposition of CdS at the surface and to avoidCdS formation in the bulk solution the kinetics of these reactions must be understood. In this work we study the formation of CdS in the chemical bath by using in situ spectroscopic extinction measurements. By monitoring the absorbance of the solution we are able to observe the bulk phase reaction (chemical formation of CdS in solution). Deposition of CdS (growth of the CdS layer at the surface) is investigated by measuring the CdS layer thickness after the reaction. The kinetics of both reactions were investigated by a systematic variation of temperature, initial reactant concentration, Reynold number as well as time. Finally the results of our study suggest a model of growth behaviour based on molecule-by-molecule deposition, which explains especially the dependency on temperature and hydrodynamics. INTRODUCTION CdS is currently the most efficient buffer layer for thin film Cu-chalcopyrite solar cells. In an aqueous medium with thiourea as the sulphur source, cadmium salt as the source for cadmium and ammonia as the r buffer in the solution, CdS will be produced and grown on the surface in the method known as chemical bath deposition. Growth kinetics of CdS layers on surfaces have been studied [1-3]. Currently, there are three different models for growing CdS on a surface that are distinguished and discussed in the literature (see figure 1). These different models vary in principle by size of the deposited particles and the place of CdS formation. The first model is known as ion-by-ion deposition in which the layer is grown via a heterogeneous reaction of reactants at the surface directly and constitutes a parallel reaction toa homogeneous CdS formation in solution. In the second model, molecule-by-molecule deposition, the layer grows subsequent to the formation of CdS molecules in solution. This deposition is an adsorption-reaction interaction of CdS molecules with the surface. In the third model, cluster-bycluster deposition, the layers grow by an adsorption-reaction interaction of CdS clusters of different sizes.
Figure 1: Diagram of various methods CdS formation in the bulk solution (left, bright background) and all possible growth mechanisms of CdS on the surface (right, dark underground). The mechanism of CdS adsorbtion differs by deposition time and by size of the depos
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