Design and Growth of Band-GAP Graded a-SiGe:H Solar Cells
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ABSTRACT The i-layers of the middle and bottom cells in stable triple-junction amorphous silicon solar cells are composed of a-SiGe:H alloys which are graded in composition to enhance performance. We compare modeling and experimental results for three i-layer band gap grading schemes to determine the optimal profile. We find a good correlation between model trends and measured device parameters for all grading schemes. This is encouraging for the use of the model in predictive device design. We find that the highest white and redL light performance do not necessarily have the same cell parameter set. Modeling and experiment indicate that thin cells without band gap profile and with suitably designed p/i and n/i buffer layers, have the best red light performance. INTRODUCTION Several numerical models have been developed for the simulation of a-Si:H based solar cellsl,2, 3 . These models are becoming increasingly accurate in predicting both the as-deposited and light-soaked performance of a-Si:H and a-SiGe:H solar cells 4 . Despite these advances, numerical modeling has not been reported extensively in the design of solar cells. A robust parameter set for the physical properties of the cell materials is needed before a numerical model can be used for cell design. We developed design rules and a material parameter set for the simulation of a-SiGe:H cell performance by iteratively matching measured and simulated data 4 ,5 . With this parameter set, the numerical model is used to evaluate and optimize
cell design. The predicted performances are compared to measured results from the cells grown in our laboratory without any parameter fitting in the model. This procedure lets us evaluate the reliability of the numerical model and boost cell efficiency at the same time. The cells reported in this paper were grown in our new three-chamber deposition system, which is operated in the dc diode mode (ref: Solarex model # S900). The a-SiGe:H alloy deposition with this system has not yet been optimized, so that the present paper should be taken as reporting cell performance trends rather than absolute values. COMPUTER MODELING We use the numerical model AMPS (developed at Penn State University 3 ) to design the ilayers of the a-SiGe:H solar cells. The material parameter set used for a-Si:H and a-SiGe:H alloys was developed by comparing AMPS results to measured material and device performance parameters 4 ,5 . End-of-life efficiency simulations are carried out 4 for all the solar cell designs and emphasis is placed on improving the end-of-life performance under red light. In this section we compare three possible i-layer grading schemes using a-SiGe:H alloys; the cells with the i-layer profiles showing improved end-of-life performance are then optimized further to achieve maximum conversion efficiency. The following sections discuss the cells having these i-layer profiles, and compare simulation to experiment to establish the validity of the design and the accuracy of our input parameter set. 663 Mat. Res. Soc. Symp. Proc. Vol. 3
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