The aim of this investigation is to apply advanced microstructural characterization techniques to study the effect of the cadmium chloride treatment on the physical properties of cadmium telluride solar cells deposited via close-spaced sublimation (CSS) and relate these to cell performance. A range of techniques have been used to observe the microstructural changes as well as the chemical changes before and after cadmium chloride treatment. Electrical measurements that link the device performance with the microstructural properties of the cells have also been undertaken. Transmission Electron Microscopy (TEM) has revealed high densities of stacking faults in the as-grown CdTe samples. Further, it has been observed that these stacking faults are removed during the cadmium chloride treatment. These observations show that the presence of chlorine plays an important role in the removal of these defects and the subsequent production of high efficiency thin film CdTe solar cells. Elemental analysis in the TEM indicates chlorine rich regions appearing at the CdTe/CdS interface as well as at grain boundaries after the treatment.

We present a study of low-energy ion surface cleaning treatments and their impact on the surface electronic structure of an air-exposed CdTe thin film treated with CdCl 2 . In order to determine the electronic structure using surface-sensitive photoemission, surfaces need to be free of contaminants. This is achieved by subsequent low-energy ion treatment steps, carefully monitoring the chemical and electronic surface structure. We present data on the valence band maximum (VBM), and core-level binding energies, that suggest that neither preferential sputtering occurs nor metallic states are formed using our cleaning procedure. For a clean CdTe surface, the VBM is determined to be (0.8 ± 0.1) eV below the Fermi energy.

A continuous, in-line process suitable for high throughput manufacturing of CdS/CdTe photovoltaic devices has been demonstrated. Utilizing this process, devices with efficiencies of 13% has been fabricated with a low iron soda lime glass (3″×3″) with ant-reflection coatings. The process has been extended to large area devices (16″ ×16″ substrate size). After CdCl2 treatment, devices showed Voc ≫ 700 mV and Jsc ≫ 20 mA/cm2. This performance is similar to the performance of small area devices which showed good stability. Also we have employed Spectroscopic Ellipsometry (SE) as a nondestructive tool to characterize CdS/CdTe heterojunction specifically studying the effects of chemical treatment on the optical properties of the thin-film layers.