Microstructure and performance of thin film cadmium telluride solar cells

The work presented in this thesis has investigated the microstructural development of CdTe thin film solar cells. A range of cadmium telluride (CdTe) thin film and solar cells produced using a wide range of deposition techniques, substrate configurations and activation treatments have been characterised. Devices made in range of research laboratories have been investigated including the Centre for Renewable Energy Systems Technology (CREST) at Loughborough University, University of Swansea, Colorado State University (CSU), University of Toledo, National Renewable Energy Laboratory (NREL) and from industrial sources such as CTF Solar GmbH.

Analysis of CdTe films involved the use of a range of advanced characterisation techniques to help understand how processing impacts the absorber microstructure. To obtain statistically accurate microstructural data of grain boundaries, grain sizes and texture, electron backscatter diffraction (EBSD) has been used in 2D cross-sections, planar view and in 3D with the milling assistance from a Xenon-Plasma Focused Ion Beam (PFIB). EBSD has revealed several consistent observations from a range of CdTe/CST devices that texture randomisation is connected to improvements in the performance of devices. This is a useful indicator of assessing how far the activation process has progressed and whether the device is undertreated or overtreated.

3D-EBSD is an advanced characterisation technique that has produced a reconstructed absorber and allowed its detailed analysis as a function of depth. This involved identifying reduced coincident site lattice 3 boundary changes as a function of depth and texture change with randomisation when transitioning between the CdTe and CST layers. CdTe microstructure is significantly impacted by the deposition and activation process conditions, so a range of devices was necessary to be analysed. With the use of 2D backscatter images (BSE) in a rapid milling PFIB, alongside EBSD allowed database of microstructures to produce for comparison. Large crosssectional surveys have opened the possibility of identifying features of interest from a statistically sufficient area to then extract features for subsequent high resolution TEM analysis.

Initial work using EBSD of thick CdSe layers in devices revealed hexagonal phase CdSe following cadmium chloride activation. Further TEM analysis confirmed the presence of remnant hexagonal CdSe and an interdiffused CdSeTe layer and the presence of chlorine. TEM-Cathodoluminescence (CL) revealed that the individual CdSe grains are photoactive, but that the n-type CdSe remnant moves the position of p-n junction and acts as a parasitic absorber. This limits use of thick CdSe unless further and more aggressive activation is used. This becomes a problem however, since it had been found that overactivation is not optimal for performance and can lead to the development of dendrites which are detrimental to device performance.