Hydrogenated amorphous silicon thin film

Abstract

From solar cell application point of view, this chapter reviews the aspects of hydrogenated amorphous silicon (a-Si:H) based materials. Spear and LeComer made the first a-Si:H films with glow discharge by decomposing hydrogen containing gases such as SiH4, in which hydrogen atoms terminate the Si dangling bonds and reduce the defect density significantly. The reduction of defect density lead to the possibility of doping a-Si:H to form N-type and P-type films and made it possible to make p-n junction, which is the foundation of thin film silicon-based electronic devices such as solar cells and thin film transistors. Since then several methods have been developed to deposit a-Si:H materials and devices. From solar cells point of view, a-Si:H is one of the useful materials with several unique properties: (1) high absorption coefficients in the visible range allowing to use a thin absorber layer, (2) large area uniform deposition for low-cost mass productions, and (3) deposition on various foreign substrates for rigid and flexible solar modules. The main disadvantage of a-Si:H is the lower carriers mobility and lifetime than the crystal copartners, which result in the lower energy conversation efficiency in a-Si:H solar cells than c-Si solar cells. In addition, the so-called Staebler-Wronski effect causes a light-induced degradation in a-Si:H solar cell efficiency. Over the years, a-Si:H science and technology have been very well developed driving by the applications in thin film silicon photovoltaic (PV) solar energy and large area display industries. In the PV applications, a-Si:H solar cell has been one of the most important technologies for the so-called second-generation PV solar energy. However, with the significant cost reduction of poly-Si and c-Si solar panels in the last few years, a-Si:H PV industry has been shrunk to the sideline and can only provide products in the niche market. Fortunately, new application as the surface passivation layer on c-Si to form a-Si:H/c-Si heterojunction solar cells gives a-Si:H a new life in the PV industry.