Structural and Electro-Optical Properties of CdTe Films Used in CdTe/CdS Solar Cells Grown with Substrate Configuration
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Structural and Electro-Optical Properties of CdTe Films Used in CdTe/CdS Solar Cells Grown with Substrate Configuration Helio R. Moutinho, Manuel J. Romero, John Moseley, Ramesh G. Dhere, Chun-Sheng Jiang, Kim M. Jones, Joel N. Duenow, and Mowafak M. Al-Jassim National Renewable Energy Laboratory, Golden, CO ABSTRACT In this work, we study CdTe thin films used in CdTe/CdS solar cells with a substrate configuration, which allows for better control in forming the junction, and the possibility for using flexible non-transparent substrates. We studied the properties of CdTe films grown at 450° and 550°C, with and without a CuxTe layer, and before and after CdCl2 treatment. We analyzed the structural and electro-optical properties using electron backscatter diffraction (EBSD), cathodoluminescence (CL) and X-ray diffraction (XRD), and investigated how the film structure, stress, and defect structure changes with the different growth conditions. INTRODUCTION Most CdTe/CdS solar cells have a superstrate structure (back contact/CdTe/CdS/ transparent conducting oxide/glass), in which light penetrates the device through the glass substrate. In this work, we studied CdTe thin films used in CdTe/CdS cells grown with a substrate configuration, with the following structure: front contact/CdS/CdTe/ CuxTe/Mo/glass. Among the advantages of this configuration are better control in forming the junction, and the possibility of using flexible non-transparent substrates. We analyzed the structural and electrooptical properties of CdTe/CdS cells grown at 450° and 550°C, with different conditions, using EBSD, CL and XRD. In EBSD [1], diffracted electrons form Kikuchi patterns on the screen of a detector, which are a signature of the crystalline structure and crystallographic orientation of the sample. In contrast to XRD, the information comes from the region close to the surface. Furthermore, EBSD has high spatial resolution and is able to reveal the grain structure and characterize different types of boundaries. We analyzed the surface and cross sections of CdTe films, and revealed both how the films grow—from the substrate to the surface—and differences in grain structure for films grown with different parameters. XRD was used to investigate the stress in the films, before and after CdCl2 treatment. CL [2] was used to study the radiative recombination of minority carriers at different locations (e.g., grain boundaries and intragrain material). Our goal was to study the structural and electro-optical properties of the films. For this purpose, we used focused ion beam (FIB) in a scanning electron microscope (SEM) to make micro-size marks on the surface of the samples. We then used these marks as reference points to perform EBSD and CL analyses on exactly the same locations. With EBSD, we identified and characterized the boundaries (e.g., coincidence site lattice [CSL] and grain boundaries), and compared results with the emission spectrum from the CL measurements. EXPERIMENT The structure and deposition parameters for the films used in this
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