The Influence of Light-Soaking and Atmospheric Adsorption on Microcrystalline Silicon Films studied by Coplanar Transien
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A9.11.1
The Influence of Light-Soaking and Atmospheric Adsorption on Microcrystalline Silicon Films studied by Coplanar Transient Photoconductivity V. Smirnov1, S. Reynolds1, F. Finger2, C. Main1 and R. Carius2 1 EPICentre, School of Computing and Advanced Technologies, University of Abertay Dundee, Bell Street, Dundee DD1 1HG, U.K. 2 IPV, Forschungszentrum Jülich, D-52425 Jülich, Germany. ABSTRACT A study of the effects of light-soaking and atmospheric adsorption (aging) on the dark- and photo-conductivity of a series of microcrystalline silicon films of varying crystallinity is presented. Light-soaking in vacuum slightly reduces photoconductivity in films close to the amorphous – microcrystalline transition, and there is also a reduction in dark current. Aging increases the dark current, and thus unless due care is taken during light-soaking experiments to eliminate or compensate for aging, the apparent effect of light-soaking may be reduced or even reversed in sign. Transient photocurrent decays confirm the presence of a large density of metastable light-induced defects. A shift in the apparent distribution of defects occurs on prolonged aging, which may be due either to changes in the DOS or a shift in the Fermi level.
INTRODUCTION Microcrystalline silicon is potentially superior to amorphous silicon as the active layer in thin film solar cells, due to its increased optical absorption and resistance to light-induced degradation (Staebler-Wronski effect). By varying deposition conditions, e.g. silane concentration (SC), it is possible to obtain a wide range of materials, from highly crystalline to amorphous. Solar cell efficiency is strongly correlated with absorber layer deposition conditions, and optimal performance is obtained near the transition from microcrystalline to amorphous growth [1]. However, as a significant fraction of amorphous material is present, an increased susceptibility to light-induced degradation might be anticipated, and has indeed been demonstrated [2]. An accurate assessment of such effects, and how they might be minimised, is therefore important if stable high efficiencies are to be realised. Amorphous and microcrystalline silicon are also known to be sensitive to atmospheric gas adsorption (referred to here as aging). Both coplanar [3-5] and sandwich [2,6] configurations show changes in optoelectronic properties, such as dark conductivity and open-circuit voltage, due to these effects. Films may be classified into two groups, depending on deposition conditions, microstructure and properties [4]: porous, (so-called type I material), and compact (type II). Samples with high crystalline volume fraction (generally deposited at low SC) show a conductivity decrease after storage in room air (type I behaviour). For type II materials (generally films with large amorphous fraction), coplanar dark conductivity increases with time. We present a study of a series of microcrystalline silicon films of varying crystallinity, controlled by varying the gas concentration ratio r = [silane] : [silane +
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