Illumination Dependence of Microcrystalline PIN Diodes
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Illumination Dependence of Microcrystalline PIN Diodes Torsten Brammer, Franz Birmans, Mathias Krause, Helmut Stiebig and Heribert Wagner Institut für Photovoltaik, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany ABSTRACT Numerical simulations of the current-voltage characteristics of PECVD-microcrystalline silicon based p-i-n diodes were performed to study the affect of defect density and mobility on solar cell performance. Depending on the combination of both parameters the ideality factor increases or decreases with applied forward bias. The reason is the variable contribution of volume recombination to the total diode current and space charge stored in defect states. The decrease in dark current with reduced hydrogen dilution can partly be attributed to a decrease in recombination centers by the same factor as predicted for midgap defect states by the analytic diode theory. Microcrystalline silicon solar cells deposited in the highly crystalline regime (high H-dilution) are limited by recombination of photogenerated carriers and high dark current. Both can be attributed to a large number of recombination centers. The fill factor of our state-of-theart solar cell is limited by the dark current for small illumination intensities, by series resistance for high illumination levels and by both at its maximum under AM1.5 illumination. Short-circuit current and open-circuit voltage pairs measured under intensities from 10-6 to 30 suns reveal a diode characteristic corresponding to an ideality factor of one at large forward bias. INTRODUCTION The combined results of various experimental material characterization methods (e.g. electron spin resonance, Raman spectroscopy, x-ray diffractometry (XRD), conductivity and mobility measurement) performed on microcrystalline silicon (µc-Si:H) need further analysis. The interpretation is complicated due to the anisotropic and heterogeneous nature of µc-Si:H. Variation of the silane concentration (SC = [SiH4]/([SiH4]+[H2]) changes crystallinity [1, 2, 3], mobility-lifetime product, spin density [4], microstructure [2, 3] and solar cell performance [1, 2, 5]. In this contribution we apply device simulation to show principle links between material properties (defect density and mobility) and pin diode behavior (dark and illuminated case). The focus is on the highly crystalline deposition regime (SC = 1.5%) and on the regime near the transition region to amorphous growth where the highest efficiencies are achieved (SC = 5%). Current efficiencies of µc-Si:H single junction solar cell in our lab are beyond 8 % with opencircuit voltages above 540 mV. EXPERIMENT The pin solar cells (glass/ZnO/pin/ZnO/Ag) examined in this study were deposited by PECVD at 95 MHz [1]. The i-layers of the pin structures with a thickness of 1 µm were deposited for 1 % < SC < 7.2 %. The crystallinity decreases from its maximum value > 90 % down to 80 % at SC = 5 % and < 30 % at SC = 7.2 % (XRD). The preparation conditions for the doped layers with a thickness of around 20 nm were kept constant. More deta
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