This book continues the long-standing and highly successful series on amorphous silicon science and technology. The opening article honors the pioneering use of photons to probe silicon films and provides an historical overview of optical absorption for studies of the Urbach edge and disorder. Additional invited presentations focus on new approaches for the fabrication of higher stability amorphous silicon-based materials and solar cells, and on the characterization of materials and cells both structurally and electronically. The book includes topics relevant to solar cells, including the role of hydrogen in metastability phenomena and deposition processes, and the application of atomistic material simulations in elucidating film growth mechanisms and structure as characterized by in situ probes. Chapters are devoted to nanostructures, such as quantum dots and wires, and to nano/microcrystalline and poly/single crystalline films, the latter involving new concepts in crystalline grain growth and epitaxy. Device applications are also highlighted, such as thin-film transistors, solar cells, and image sensors, operable on the meter scale, to memories, operable on the nanometer scale.