Polycrystalline silicon thin film solar cells

SCHMIDT, J. A.

La Plata

Conferencia; Humboldt Kolleg - International Conference on Physics; 2011

Fundación Alexander von Humboldt

The photovoltaic industry has had an important development in the last years, growing more than 30 % annually. The photovoltaic market is currently dominated by the technology based on crystalline silicon (c-Si), with a market share in terms of modules production of around 78 %. However, the need to lower the prices of photovoltaic electricity pushes towards a reduction in the production costs of silicon solar cells. To achieve this goal, approaches that use less material and lower temperatures are needed, known as thin films approaches. Polycrystalline silicon (poly-Si), deposited as a thin film on glass, offers the possibility to combine the low-cost potential of thin films with the proven stability and high efficiency of crystalline silicon. In this presentation, we give an overview of different deposition and processing techniques that can be used to obtain thin poly-Si films on glass substrates. We discuss the direct deposition of poly-Si on glass by thermal CVD at intermediate temperatures, starting from chlorosilane precursors. This is an attractive technique since the substrate, the chemical precursors and the process are all of low cost. We also present the technique of metal-induced crystallization of amorphous silicon, which allows producing large crystalline grains (sizes over 100 microns) at low temperatures (below 600 ºC) by means of thermally activated solid phase transformations. Specifically, we focus on the use of nickel as the metallic mediator for inducing silicon crystallization, and its influence on grain nucleation and growth. We analyze the relevant concept of solid phase epitaxial growth of a poly-Si film on a previously crystallized seed layer. The pre- or post-crystallization doping possibilities, together with their own advantages and disadvantages, are addressed. We also present results of different techniques for the structural characterization of thin poly-Si films. Raman spectroscopy, reflectance in the UV region, X-rays diffraction, optical microscopy, SEM and AFM are shown as valuable tools to follow the crystal growth and to test the crystalline quality of the resulting material. Finally, we discuss the application of the introduced techniques in the development and design of polycrystalline silicon thin film solar cells. Among others, we present the process used by the German company CSG Solar A.G. (CSG º Crystalline Silicon on Glass), based on solid phase crystallization. This process is at industrial level, producing photovoltaic modules with efficiencies of up to 8 % and mini-modules with efficiencies slightly larger than 10 %. Some concluding remarks about the prospective utilization of solar energy as an alternative energy source are also presented.