Amorphous silicon solar cells obtained by Hot-Wire Chemical Vapour Deposition

Abstract

The photovoltaic conversion of sun light energy into electricity constitutes a viable, clean and renewable alternative for electric energy production. A wide variety of different materials have been proposed over the last decades for photovoltaic applications, although silicon seems to be the most suitable one for global implementation. Crystalline silicon, c-Si, is the most commonly employed material, although the elevated costs associated to its production have given place to new technologies based on thin film materials. Among thin film materials, hydrogenated amorphous silicon (a Si:H), either solely or in combination with other thin film materials, has been proven to be a very promising alternative due to the suitable properties and to the low costs associated to its production. Taking into account the promising properties of a-Si:H, its deposition and characterization was carried out at Universitat de Barcelona in the frame of the presented work. Plasma Enhanced Chemical Vapour Deposition (PECVD) is the most widely employed technique for the deposition of thin silicon layers. Nevertheless, certain limitations have led to the investigation of alternative deposition techniques, such as Hot-Wire Chemical Vapour Deposition (Hot-Wire CVD), which features higher deposition rates, simpler and cheaper deposition geometry and easier scalability to large area deposition than PECVD. Hot-Wire CVD has been analysed at Universitat de Barcelona since 1993 for the growth of nanocrystalline silicon (nc-Si:H) layers, this being the first work concerning a-Si:H based devices. Moreover, our research was focused on the low substrate temperature regime (Ts ~ 200ºC), thus allowing the future use of low cost substrates, such as flexible plastic substrates. In the present work, the influence of the different deposition parameters on the structural and optoelectronic features of both intrinsic and doped a-Si:H is presented and carefully analysed. The ability to grow intrinsic a-Si:H exhibiting satisfactory structural and optoelectronic properties after suitably tuning the deposition parameters is described. Subsequently, the properties of doped layers are shown, these results giving evidence of the satisfactory behaviour of the n-type layers and the limited properties of the p-type ones. Finally, the performance of our first a-Si:H based solar cells completely obtained by Hot-Wire CVD at low substrate temperatures is shown. Promising results are presented, especially when dealing with the properties of both the intrinsic and the n-type material, thus making the employed technique a promising alternative for industrial application.