Investigations of radio-frequency, capacitively-coupled large area industrial reactor : cost-effective production of thin film microcrystalline silicon for solar cells

The constant energy consumption and world-wide demography expansion add to the potential risks of ecological and human disaster associated with global warming. This makes it a necessity to develop renewable energy technologies such as photovoltaic energy. These technologies already exist, but their cost has to be reduced in order to compete with well-established energies based on natural resources such as oil, coal or natural gas. A first step has been performed successfully in recent years in the field of low cost photovoltaic (PV) solar cells. Manufacturing equipment for large area (> 1 m2) amorphous silicon thin film solar cell production is now available. Even if this type of PV cell has lower energy conversion efficiencies (≈ 9 – 10 %) than other types of cells such as crystalline silicon cells (≈ 25 %), it has lower financial and ecological costs. Nevertheless, the next generation of large area silicon thin film PV cells promise higher conversion efficiencies (≈ 12 %) and stability under light exposure. This new type of PV cell is based on microcrystalline silicon grown on large glass substrates by plasma-enhanced chemical vapor deposition from silane (SiH4) and hydrogen (H2) gas, as for the previous amorphous silicon generation. Amorphous/microcrystalline silicon PV multi-junction cells require a thick (≈ 2 µm) microcrystalline intrinsic light absorber layer because of the need to fit the photo-generated current of the two stacked cells. This increases the cost of the final product since the deposition rate of microcrystalline silicon achieved nowadays is limited to a few Å/s, making the processing time very long. The enhancement of the deposition rate while maintaining a good material quality, i.e. at the boundary between amorphous and microcrystalline growth, is difficult because the phenomena involved in the deposition of microcrystalline silicon are not well understood, and the optimization is then generally performed empirically. The plasma composition is measured using Fourier transform infrared absorption spectroscopy and optical emission spectroscopy. It is shown that the deposited films can be classified into three categories (amorphous, transitional and microcrystalline) as a function of silane concentration in the plasma, while it is impossible to do so as a function of all other process parameters (silane input concentration, RF power, pressure, etc...) if they are all varied simultaneously. This means that the common way to deposit microcrystalline silicon by strongly diluting the silane with hydrogen (< 5 %) is not unique. This is because the plasma composition does not depend only on the gas composition, but also on the fraction of silane depleted in the plasma. Analytical and numerical plasma chemistry modeling show that this is because the silane concentration in the plasma determines the species flux towards the growing film surface. Hence, in agreement with existing phenomenological models of microcrystalline growth, the