Realistic simulation of polycrystalline CIGS absorbers and experimental verification
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Realistic simulation of polycrystalline CIGS absorbers and experimental verification C. Maragliano1, M. Stefancich*2,3, S. Rampino2, L. Colace1 *
[email protected]
1
NooEL-Nonlinear Optics and OptoElectronics Lab., Department of Electronic Engineering, INFN, CNISM, University “Roma Tre,” Via della Vasca Navale 84, 00146 Rome, Italy 2
CNR-IMEM, Parco Area delle Scienze 37/A - 43124 Parma, Italy - Phone: +39052126911 - Fax +390521269206 3
Masdar Institute of Science and Technology, Masdar City, Abu Dhabi, United Arab Emirates
Abstract: Cu(InGa)Se2 solar cells modeling is challenging due to their complex electronic structure, to the presence of interface states between layer and grains and to the microcrystalline structure of the absorber. Here we present a ISE-TCAD based realistic absorber 3D model, with the specific objective to take into account, among several effects, these challenging aspects. The CdS/Cu(InGa)Se2 solar cell is modeled as an array of columnar microcells, connected in parallel, mimicking the polycrystalline nature of the absorber. The model optical and electrical parameters are optimized based on a review of available experimental material characterization and realization results. Simulation outcomes are compared with experimental data in order to validate the model.
Introduction Chalcogenide based thin film solar cells are considered a promising class of solar cell for costeffective power generation1. This is mainly due to advantages of thin films for low-cost and high-rate semiconductor deposition over large areas. Since these cells have demonstrated promising results in terms of efficiency2 and long-term stability3, a physical model able to simulate electrical and optical behavior of the solar cell would be very useful for further improvements. Unfortunately, the modeling of Cu(InGa)Se2 (CIGS) solar cells poses some challenges4, 5, mainly caused by the microcrystalline nature of the absorber that affects both electrical and optical material properties. It has been demonstrated by Herberholz et al. 6 that band bending induced by surface charges drives electromigrating Cu into the bulk, leaving the surface depleted of Cu. This depletion is stopped when the composition is that of CuIn3Se5. This phenomenon has also been correlated with type conversion of the chalcopyrite material 7. However, the band bending as well as the CuIn3Se5-phase formation disappears when the material is exposed to atmosphere for some time as oxides form on the surface. It has been
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demonstrated that the surface oxidation is enhanced also by the presence of Na diffusing from soda lime glass 8. Thus, even if the surface passivation is performed during the growing process, the electrical properties (i.e. mobility) are strongly dependent on crystals size and can vary significantly between bulk and grain boundaries regions. Moreover chalcogenide based thin film solar cells have a complex electronic structure (for example glass/Mo/CIGS/CdS/Al-ZnO) where the line-up of the bands shows discontinuities, possibly l
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