Device Modeling of a-Si:H Alloy Solar Cells: Calibration Procedure for Determination of Model Input Parameters
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ABSTRACT A calibration procedure for determining the model input parameters of standard a-Si:H layers, which comprise a single junction a-Si:H solar cell, is presented. The calibration procedure consists of: i) deposition of the separate layers, ii) measurement of the material properties, iii) fitting the model parameters to match the measured properties, iv) simulation of test devices and comparison with experimental results. The inverse modeling procedure was used to extract values of the most influential model parameters by fitting the simulated material properties to the measured ones. In case of doped layers the extracted values of the characteristic energies of exponentially decaying tail states are much higher than the values reported in literature. Using the extracted values of model parameters a good agreement between the measured and calculated characteristics of a reference solar cell was reached. The presented procedure could not solve directly an important issue concerning a value of the mobility gap in a-Si:H alloys.
INTRODUCTION Modeling of electrical and optical behavior of semiconductor devices has been established world-wide as an essential tool for both the improvement of existing devices and the development of new ones. There is no doubt that the role of modeling will increase in the future. In case of hydrogenated amorphous silicon (a-Si:H) alloy solar cells a further improvement of their performance is expected not only from technological efforts, but also from computer simulations. The effective use of the simulation results and their predictive power strongly depend on the reliability of the input parameters which describe the a-Si:H solar cell. A typical device simulator requires more than twenty input material parameters to define the properties of a separate a-Si:H alloy layer. For the simplest single junction a-Si:H solar cell, which comprises three layers, it means a set of sixty input material parameters. Most of these parameters are usually not known precisely and this represents the main limitation to accurate modeling. Additional difficulty is introduced by the inherent nature of amorphous silicon that this material grown at different laboratories is not the same and a minimum set of characteristic parameters has to be determined to qualify it. Therefore widely accepted default values for most of the model parameters are still missing. The simulation results published by different groups are difficult to compare because they use different sets of input material parameters that are based on their own measurements and/or on literature. No comprehensive procedure has been published which shows a link between the minimum set of material properties determined from the measurements and the set of input material parameters for a computer model. Some general comments on obtaining the input parameters for modeling solar cell structures on a-Si:H are to be found in ref. [1]. In this article we present a calibration procedure that we have used for the determination of the model input paramete
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