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(Invited) Photovoltaic Properties of Undoped Discharge-Produced Amorphous Silicon

A brief review is given of the photovoltaic properties of undoped amorphous silicon (a-Si) which has been used in fabricating efficient thin film solar cells. The concepts underlying the photovoltaic characteristics of this material are discussed and related to studies carried out on metal/undoped a-Si Sohottky barrier structures. The contribution of both the junction and bulk region are evaluated in terms of their electronic properties and the optical absorption of the a-Si. Several of the results discussed offer insight into the carrier transport and gap state distributions of discharge produced a-Si. The outstanding photovoltaic characteristics of these undoped a-Si structures can be in large part attributed to the excellent junction properties which can be characterized by the same parameters as single crystal Schottky barriers. The contribution to the photovoltaic characteristics of the quasi-neutral regions present in these structures is correlated with the electronic properties of bulk undoped a-Si. This includes effects not generally found in photovoltaic structures formed on crystalline semiconductors.