Control of morphology for enhanced electronic transport in PECVD-grown a-Si : H Thin Films

Abstract

Solar cells have become an increasingly viable alternative to traditional, pollution causing power generation methods. Although crystalline silicon (c-Si) modules make up most of the market, thin films such as hydrogenated amorphous silicon (a-Si:H) are attractive for use in solar cell modules because of the capacity to fabricate cells with much less material. However, several challenges exist in making this material a more practical alternative to c-Si; despite having superior optical absorption properties, a-Si:H suffers in electronic transport, having a hole mobility 3-7 orders of magnitude less than that of c-Si. In the MOSFET transistor industry, carrier speeds and thus mobilities of c-Si were improved through the application of stress in the material. This work hypothesizes that a similar application of stress on a-Si:H thin films can enhance this material's hole mobility. A comprehensive study of the parameter space for a plasma enhanced chemical vapor deposition technique used to produce a-Si:H is performed. This enables the control of stress within the deposited film, from compressive to tensile; the mechanical limits of the material resulting in buckling and delamination failure are observed. Further characterization of a-Si:H thin films with different levels of engineered stress was performed; an analysis of the films' surface using AFM measurements to calculate a fractal dimension for each did not result in a significant descriptor of the surfaces' domain distribution. This work includes a detailed analysis of the theory of time-of-flight for measuring carrier mobility in thin film materials, and the system requirements needed to perform them.