Characterization of microstructure and interfaces in cadmium telluride thin film cells

The efficiency of cadmium telluride thin film solar cells is drastically influenced by the microstructure and interfaces of the whole layer stack consisting of transparent conductive oxide (TCO), n-type cadmium sulfide layer, p-type cadmium telluride absorber layer and metallic back contact. In the frame of the present work CdS and CdTe layers are deposited by close-space sublimation (CSS) on a commercial available SnO2:F (FTO) coated float glass. The layer thickness of cadmium sulfide was varied in the range between 70 and 150 nm, whereas the subsequent CdCl2 activation procedure was carried out at temperatures between 390 and 430 °C with a constant treatment time of 20 min. As back contact a gold layer was sputtered after nitric and phosphoric acid etching of the CdTe layer surface. Especially for detailed characterization and imaging of interfaces between the layers a high quality cross section preparation of the solar cells is necessary. For this purpose an ion beam preparation technique is applied. This preparation technique enables the subsequent investigation of the cross sections by high-resolution field emission scanning electron microscopy (FE-SEM). For imaging mainly backscattered electrons are used. The observed contrast of the backscattered electrons is influenced by crystal orientation (channelling contrast) and by atomic number (material contrast), whereas topography contrast is suppressed by the ion polishing of the cross sections. Additionally, surface and interface roughness are characterized by high resolution FE-SEM and atomic force microscopy (AFM). As a result an optimized CdS layer thickness could be deduced for the used FTO coated float glass. Further the effect of CdCl2 activation temperature on CdS grain growth, agglomeration effects and void formation could be elucidated. The microstructure results are discussed in relationship to the IV characteristics of the solar cells.