Effect of Fermi Level Position in Intrinsic a -Si:H on the Evolution of Defect States under Light Exposure
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A13.1.1
Effect of Fermi Level Position in Intrinsic a-Si:H on the Evolution of Defect States under Light Exposure. M. Zeman1, V. Nádaždy1,2, R. Durný3, J.W. Metselaar1 1 Delft University of Technology – DIMES, P.O. Box 5053, 2600 GB Delft, The Netherlands 2 Institute of Physics, SAS, Dúbravská cesta 9, 845 11 Bratislava, Slovakia 3 Slovak University of Technology, Ilkovičova 3, 812 19 Bratislava, Slovakia ABSTRACT The evolution of the programmed defect-state distributions in intrinsic hydrogenated amorphous silicon (a-Si:H) due to light soaking was qualitatively determined from charge deeplevel transient spectroscopy. The defect-state distribution in a-Si:H was programmed by applying a particular bias voltage on the metal-oxide-semiconductor structure while annealing the structure above the equilibration temperature. The programmed distributions simulate defectstate distributions in different parts of an actual a-Si:H solar cell, particularly in the intrinsic regions close to the p/i and i/n interfaces. The defect-state distribution in the bulk of the intrinsic layer is characterized by comparable contributions from the positively charged defect states above midgap, Dh, neutral states, Dz, and negatively charged states below midgap, De. In the programmed p-type (n-type) defect-state distribution there is an excess of the Dh (De) states. Light exposure modifies the p-type distribution that evolves to a broad distribution of states with a maximum around midgap. This distribution is dominated by Dz states with substantial contributions from Dh and De states. In case of n-type distribution light soaking only slightly influences the distribution by removing a part of the Dh states and by a small increase of Dz and De states. INTRODUCTION Inherent to hydrogenated amorphous silicon (a-Si:H) are the reversible changes in electronic properties of a-Si:H under light exposure, known today as the Staebler-Wronski effect [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. According to the defect-pool model (DPM) [3] the energy distribution of gap states (EDOS) in a-Si:H is composed by three components, which represent positively charged, Dh, states above midgap, neutral, Dz, states around midgap, and negatively charged, De, states below midgap. The defectpool model predicts that the EDOS is determined by the position of the Fermi level in the band gap at the equilibration temperature. The position of the Fermi level in the intrinsic layer of the pin solar cell varies from near the valence band at the p/i interface to near the conduction band at the i/n interface. This Fermi level dependence has serious implications for the spatial variation of the gap-state distribution in a-Si:H solar cells. In order to understand and simulate the changes in performance of a-Si:H solar cells due to light exposure the knowledge of changes in the spatial energy distribution of gap states in the intrinsic layer is r
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