Real time polarization spectroscopies: Applications in thin film growth and surfaces for photovoltaics

Robert W. Collins
University of Toledo

Photovoltaics (PV) technologies based on thin films of hydrogenated amorphous silicon (a-Si:H) and polycrystalline cadmium telluride (pc-CdTe) have met with considerable success over the past few years. These thin film PV devices are deposited by chemical and physical vapor deposition methods on low cost substrates. Optimized devices have become increasingly complex, and in the case of a-Si:H technology include as many as a dozen major layers and several minor layers, as well, designed to capture a broad range of the solar spectrum. In addition, post-deposition processing may be required, and in the case of pc-CdTe this includes a critical anneal under CdCl2 vapor. For a?Si:H-based PV, in-situ real time polarization spectroscopies including spectroscopic ellipsometry (SE) have played a key role in establishing principles that guide the fabrication of the optimized devices that are being used widely in research and production. In this seminar, an overview will be provided of the useful information that can be extracted from such polarization spectroscopies. Multichannel optical instrumentation will be described that measure polarization state changes occuring upon light reflection from materials and devices. The focus is on instruments that can extract characteristics including complex dielectric functions at high speed over a wide spectral range for in-situ real time applications. One application example to be described is the study of complex optical anisotropy spectra for the surfaces of cubic crystalline semiconductors that provide a fingerprint of the surfaces. A second example involves device development in which the evolution of nanocrystallites in amorphous silicon thin films is controlled for the optimization of solar cells. In a final example, losses due to plasmon resonances in Ag/ZnO reflector structures are analyzed to identify approaches for efficiency enhancement in a second pass through PV devices.