Microstructure and performance of CdTe solar devices

One of the most critical processes in CdTe device production is the activation process induced by cadmium chloride (CdCl2). In this thesis, the CdCl2 treatment has been optimized using both wet and thermal evaporation methods for close-spaced sublimated (CSS) devices. Maximum cell efficiencies of η=7.24% and η=9.37% respectively have been measured without the use of copper in the back contact. A clear link has been established between treatment conditions, electrical measurements and microstructure, where parameters such as the dwell annealing temperature for evaporated CdCl2 and the concentration of the solution for the wet treatment are varied. It has been shown that a certain concentration of chlorine is necessary to remove high densities of planar defects present in the as-deposited material. The CSS CdTe is deposited in a dual layer structure with smaller grains at the CdS interface and with larger grains developing towards the surface. The defects are initially removed in the smaller grains at the CdS interface. When the temperature and concentration increase, more grains recrystallize with the total removal of stacking faults. At a critical temperature and Cl concentration, the entire CdTe film recrystallizes into large grains with no stacking faults. The CdS grains and the interface with the CdTe also changes with sulphur migration into the CdTe. The results indicate that the recrystallization actually initiates at the CdS/CdTe junction. This has been observed clearly for both sputtered and electrodeposited CdTe. The recrystallization process gradually propagates towards the surface as the concentration of the CdCl2 solution in methanol is increased. This observation is not intuitive because the solution is initially in contact with the outer surface of the CdTe. Finally, the use of different chlorine containing compounds has been used as an alternative to CdCl2 and to further understand the role of chlorine in the process. All the samples treated with Cl containing compounds have shown the elimination of the dual layer structure and the recrystallization of the small grains at the interface. Tellurium tetrachloride (TeCl4) and zinc chloride (ZnCl2) have shown the most promising increase in conversion efficiency. The maximum efficiencies measured using these two solutions were 4.58% and 5.05% respectively. TeCl4 has shown an encouraging open circuit voltage of 594 mV, while the open circuit voltage using ZnCl2 was 494 mV. However, TeCl4 has shunting issues and low current density (17.9 mA/cm2), whereas ZnCl2 has the promising current density of 20.8 mA/cm2. This work has shown that alternatives to CdCl2 treatment exist, however further work is required to optimize the performance of these treatments to enable them to be competitive. Advanced materials characterization techniques are essential to understand and then enhance photovoltaic cell and module performance. New and improved tools are being developed to deliver fast, accurate and non-destructive characterization. One of these tools is coherence correlation interferometry (CCI) which has been developed by Taylor Hobson Ltd. This is a particular variant of scanning white light interferometry used for surface metrology with a high vertical resolution. In this thesis, it has been shown that the capability of the CCI can be extended to perform accurate thin film thickness measurements using the Helix Complex Field (HFC) function. The main attraction of this technique for thin film PV applications is that it allows surface metrology and thin film thickness measurements to be obtained simultaneously from the same area of the sample in the same system. The results obtained from CCI on a variety of materials, used in thin film PV, correlate very well the results obtained from other techniques such as ellipsometry, electron microscopy and atomic force microscopy. The CCI has also been used in the optimization of a new one-step interconnect process (OSI) for thin film PV module interconnects.