Optoelectronic Properties of Amorphous Silicon the Role of Hydrogen: from Experiment to Modeling

Abstract

Amorphous silicon, and its more useful alloy form, hydrogenated amorphous silicon (a-Si:H), has been the subject of investigation for more than three decades. A-Si:H is a lowcost, efficient material which is used extensively for electronic devices. Indeed, most recent electronic device textbooks contain a comprehensive review of the physics of amorphous materials and amorphous silicon in particular (Baranovski, 2006; Kasap, 2005; Street, 2000). The advantages of a-Si:H are particularly evident when considering the photovoltaic application context for the preparation of solar cells: in fact, a-Si:H has a large optical absorption coefficient (about 0.5 micron of the material will absorb 90% of the incident sunlight); the energy gap can be modulated to allow for near optimum conversion efficiency for sunlight; it can be alloyed with other elements (carbon, germanium) to create multijunction structures with increased energy conversion efficiency for sunlight. Finally, it is plentiful and can be deposited on a variety of materials (at low temperature, over large areas, and on flexible substrates). However, the presence of metastable defects in a-Si:H adversely affects the performance of photovoltaic cells and thin film transistors. Electrical conductivity, photoconductivity and luminescence degradation have been linked to defect formation, such as dangling bonds (DBs) in the a-Si:H film (Akkaya & Aktas, 1995; Street, 1980). Staebler and Wronski (1977) found that defects can be created by illuminating a-Si:H. The creation of these light-induced defects (LID) is therefore referred to as the Staebler-Wronski (SW) effect. The presence of these defects, or dangling bonds, is the major factor responsible for the deterioration of the optical and electronic properties of a-Si:H. On the other hand, these defects are metastable and can be cured. Indeed, we could define a SW process that can be described as a two-step reversible process: i. Exposure to sunlight leads to an increase in the density of states (dangling bonds) in the energy gap of a-Si:H; this represents the SW effect proper; ii. Subsequent annealing at elevated temperatures (150-200 OC) reduces the density of states back to the original value, thus restoring the optoelectronic properties. It has been shown experimentally that both optical and electronic properties of amorphous silicon, such as refractive index, optical gap, absorption coefficient, electron and hole