Application of the Defect Pool Model in Modelling of a-Si:H Solar Cells
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MIROSLAV ZEMAN*, GUOQIAO TAO*, MARCEL TRIJSSENAAR*, JOOST WILLEMEN*, WIM METSELAAR*, AND RUUD SCHROPP** *Delft University of Technology, Electronic Components, Technology and Materials, Mekelweg 4, 2628 CD Delft, the Netherlands ** Utrecht University, Department of Atomic and Interface Physics, Princetonplein 5, P.O. Box 80000, 3508 TA Utrecht, the Netherlands
ABSTRACT The increasing complexity of hydrogenated amorphous silicon (a-Si:H) based solar cells requires continuous extending and testing of the computer models which are used for their simulation. The calculation of the defect states density in the bandgap of a-Si:H based on the defect pool model (DPM) has been fully implemented in the ASA (Amorphous Semiconductor Analysis) computer program developed at Delft University of Technology. We used the technique of inverse modelling to verify the DPM and to calibrate it by fitting the dark and illuminated J-V characteristics of a single-junction a-Si:H solar cell fabricated at Utrecht University. The DPM was also used in simulating the absorption coefficient of a-Si:H layers. Using the DPM for the defect states distribution, a good agreement between simulated and measured data was obtained. The values of the material parameters needed for obtaining the best fits are more realistic than using a conventional model for the defect states distribution. INTRODUCTION Computer modelling is gaining importance for analysing and optimizing the design and performance of a-Si:H based solar cells. At present, the main effort in the electrical modelling of a-Si:H solar cells is focused on two issues. The first is improving, extending, and testing of the computer models to reflect the increasing complexity and understanding of the solar cells. The second issue is establishing reliable values for material and device parameters which describe the solar cells. Most of the parameters are still not known precisely and this represents the main limitation to accurate modelling. The modelling of the recombination-generation (R-G) mechanism represents the main difference between crystalline and amorphous device modelling. The R-G rate depends on the distribution and charge state of the density of states (DOS) in the mobility bandgap of a-Si:H. Most computer programs use a standard defect model (SDM) for the representation of the dangling bond (DB) defect states around midgap. In this model, the DB states are represented by two equal Gaussian distributions which are separated by a correlation energy. Usually, the parameters which describe the DB states in the SDM are taken to be constant in the separate layers of the solar cell. A recent theory of defect formation in a-Si:H, known as the defect pool concept [1], [2], [3], offers the calculation of DB states distribution based on the structural properties of a-Si:H. We have implemented the calculation of the defect states distribution in the device according to the defect pool model (DPM) in the ASA computer program [4]. In this article we report on the results from simulations using the D
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