The Role of Charged Gap States in Light-Induced Degradation of Single-Junction a-Si:H Solar Cells
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The Role of Charged Gap States in Light-Induced Degradation of Single-Junction a-Si:H Solar Cells. M. Zeman, V. Nádaždy*, and J.W. Metselaar Delft University of Technology – DIMES, P.O. Box 5053, 2600 GB Delft, The Netherlands ABSTRACT Computer simulations of single-junction hydrogenated amorphous silicon (a-Si:H) solar cells with different thickness of the intrinsic layer were carried out in order to study the role of charge gap states in their light-induced degradation. It is demonstrated that it is the decrease of positively charged states above midgap, Dh, and the increase of neutral states around midgap, Dz, and negatively charged states below midgap, De in the intrinsic layer that result in a drop of performance of the solar cells due to light soaking. These changes in the gap states are in accordance with our recent experimental results from the charge deep-level transient spectroscopy on undoped a-Si:H. The experimentally observed changes in the dark and illuminated J-V curves and spectral response could not be simulated with the same set of input parameters by only increasing the defect-state density in the intrinsic layer. INTRODUCTION Inherent to hydrogenated amorphous silicon (a-Si:H) are the reversible changes in electronic properties of a-Si:H under light exposure. This is known as the Staebler-Wronski effect (SWE) [1]. It is generally accepted that light soaking leads to the creation of additional dangling-bond defects [2], which deteriorate the performance of a-Si:H devices such as solar cells. Our recent experimental results from the charge deep-level transient spectroscopy (Q-DLTS) on undoped a-Si:H [3,4] reveal that in the initial stage of light soaking the annihilation of positively charged, Dh, states above midgap takes place. The annihilation of the Dh states is followed by creation of neutral states around midgap, Dz, and negatively charged states below midgap, De. The increase of the Dz states dominates the process of light soaking in the later stages. The p/i and i/n interfaces are the most sensitive parts of a single junction a-Si:H solar cell and strongly influence its performance. According to the predictions of the defect-pool model (DPM) [5,6] the gap-state distribution in the region of the intrinsic layer next to the p-type and n-type doped layers is dominated by the Dh and De states, respectively. The annihilation or creation of the charge gap states due to light soaking would strongly effect the charge distribution and electric field at the p/i and i/n interfaces and thus the performance of the solar cell. In the present study, simulations of dark and illuminated J-V characteristics and external quantum efficiency, QE, of single junction a-Si:H solar cells with different thicknesses of the intrinsic a-Si:H absorber layer were carried out for as-prepared and light-soaked states. The role of three different types of defect states in the absorber layer of the solar cells during light soaking was investigated by matching the simulation and experimental results. *
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