Simulations of the gas flux distribution for different gas showers and filament geometries on the large-area deposition of amorphous silicon by hot-wire CVD

For the scaleup of hot-wire CVD for the deposition of a-Si:H based solar cells, simulations regarding the influence of gas supply and filament geometry on the uniformity of a-Si:H films are described to support the design of hot-wire CVD reactors. We simulated the spatial distribution of the gas flux generated by different kinds of gas showers and changes in this spatial distribution with increasing distance from the gas outlet. Regarding the influence of the filaments, we calculated temperature gradients across the substrate caused by the radiative heat and the gas flux, based on the assumption that the gas extends collision-free, from the filament grid. For both simulations, the distances between the filaments and between substrate and filament grid were varied. The simulations of the gas flux from both gas shower and filament grid were combined and compared to experimental results obtained in our large-area deposition system. With an optimized setup, we could achieve good quality a-Si:H films with high thickness uniformity of ±2.5% and p–i–n solar cells with initial efficiencies of η=(6.1±0.2)% on an area of 20×20 cm2.