Transparent conductors for thin film solar cells

Devices such as thin film photovoltaic panels, smartphones and touch screen television displays rely on a thin film transparent conducting oxide (TCO) to make electrical contact. The transparent conductor is usually deposited on the inside glass face and is protected by being encapsulated within the device. However, in some solar cell applications it is part of a thin film stack that is exposed to the environment. In both cases, it is important that the TCO retains its performance over time.
The performance of a TCO is determined by its optical, electrical and structural properties. The most important TCO's for solar applications are Indium Tin Oxide (ITO), Aluminium doped Zinc Oxide (AZO) and Indium doped Zinc Oxide (IZO). Using combinatorial sputtering a range of ITO compositions will be fabricated and mapped for their optical and electrical properties. These properties will be matched to the requirements of solar applications. One application is a coating that is anti-reflecting in the visible wavelengths but reflecting in the infra-red. Reflecting the infra-red helps to keep the module cool, improving efficiency and module reliability. The refractive index dispersion and absorption are the key performance parameters. Another potentially important application is the development of a bifacial thin film solar cell where the electrical properties of sheet resistance and carrier concentration are important.
The stability of transparent conductors when exposed to the atmosphere has not been established. This project will use a combination of Laboratory environmental stress tests and real outdoor exposure tests to determine if degradation occurs. The Laboratory tests will include damp heat (85C, 85% humidity), UV exposure, temperature cycling, humidity freeze etc. In addition to electrical tests of sheet resistance, the chemical and structural coating degradation will be investigated using Scanning Electron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).