20. Hydrogenated Amorphous Silicon Thin Film

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.