Computer modeling of electronic properties in thin film silicon-hydrogen - alloy and its application to solar cells

A self-consistent numerical model for hydrogenated amorphous silicon(a-Si:H) has been developed to aid in the understanding of the details of the electronic behavior of silicon-hydrogen alloy material and the characteristic features of devices made from it. A gap state model incorporating exponential tail states and Gaussian-distributed dangling bond states and doping states based on the experimental results and theoretical background is proposed. Detailed transport equations including charge trapping and recombination processes are formulated, and solved numerically in one-dimension. Since a large number of material and geometrical parameters are involved, it is possible to fit experimental data with more than one parameter set. Therefore the consistency of the proposed model was tested by fitting diverse experiments with the same material parameters. The detailed model calculations are compared with published experimental results for the dependence of dark conductivity on doping and temperature, and dependence of sweep-out charge on doping. It is also used to evaluate a one-to-one relationship between four-fold coordinated doping atoms and dangling bonds, as well as the dangling bond energy levels and distribution. The dependence of the photoconductivity on light-intensity, temperature, and spin density was investigated to understand the recombination processes and transport mechanism in a-Si:H material. The capture cross-sections for tail states and dangling bonds are determined by comparing the model calculated photoconductivity results with corresponding experimental results. An example of the use of the program TFSSP (Thin Film Semiconductor Simulation Program) for the analysis of solar cell parameters, (open-circuit voltage, short-circuit current, fill factor, collection efficiency, and conversion efficiency) as function of cell thicknesses for an a-SiC:H p-/a-Si:H i-n structure is presented and compared with corresponding experimental results. The model program is also implemented to design optimum solar cells. In conclusion, a self-consistent numerical model for thin film silicon-hydrogen alloy materials and devices has been developed which includes the one-to-one relationship between doping and dangling bonds. The model turns out to be an excellent tool for the analysis of dark conductivity, photoconductivity, and the characteristics of a-SiC:H p-/a-Si:H i-n solar cells, and for cell design as well