Electronic Properties of Microcrystalline Silicon investigated by Photoluminescence Spectroscopy on Films and Devices
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Electronic Properties of Microcrystalline Silicon investigated by Photoluminescence Spectroscopy on Films and Devices R. Carius, T. Merdzhanova, F. Finger Institut für Photovoltaik, Forschungszentrum Jülich GmbH D-52428 Jülich ABSTRACT Photoluminescence spectroscopy has been applied to investigate localized states in microcrystalline silicon (µc-Si:H) films and to address the problem of the changes of the electronic properties of this material upon changes of the hydrogen dilution during film growth. By a comparison of photoluminescence and Raman spectra on device grade sample series prepared at different silane concentration in hydrogen (SC) by PE-CVD and HW-CVD a correlation between the microstructure and the photoluminescence energy is found. It is proposed that the density of band tail states is reduced with increasing SC leading to the increase of the PL energy as well as to the increase of Voc of solar cells. The reason for the tails and their reduction is not clear but strain might play a crucial role and the amorphous hydrogenated phase might be effective for strain reduction. INTRODUCTION Plasma enhanced chemical vapor deposition (PE-CVD) is at present the well established method for preparation of microcrystalline silicon (µc-Si:H) as this technique is highly successful for manufacturing device grade amorphous silicon films. Recently, a significant progress in the material and device quality prepared by hot wire (HW)-CVD has been achieved by lowering the substrate temperature to values typically used in PE-CVD processes [1,2]. Microcrystalline solar cells with this material as intrinsic layer show similar efficiencies as for PE-CVD material [1,2]. Moreover, very similar electronic and device properties are observed, such as the decrease of the dark conductivity and an increase of the open circuit voltage (Voc ) with decreasing crystalline volume fraction [l]. These effects are not understood and particularly the high Voc of more than 590 mV obtained for a HW-CVD cell at a still high fill factor and current density [2] is of current interest. We address this issues by investigation of the states at the band edges using photoluminescence spectroscopy (PL). Already in the early stages of research on microcrystalline silicon PL measurements indicated similarities of microcrystalline and amorphous silicon, e.g. a broad featureless PL band at energies significantly below the band gap of crystalline silicon and a distribution of recombination lifetimes. The interpretation was based on transitions involving distributions of defect states [3]. In a later investigation a shift of the PL energy as a function of the plasma excitation frequency was observed but no shift of the band gap derived from absorption measurements was found [4]. Although transitions between tail states were discussed as a possible source for the PL band the similarities with well known defect bands (D1-D4) in crystalline silicon was taken as evidence to favor the defect model. More recent work revealed a systematic decrease of the PL en
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