Preparation Temperature Effects in Microcrystalline Silicon Thin Film Solar Cells
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Preparation Temperature Effects in Microcrystalline Silicon Thin Film Solar Cells O. Vetterl, A. Dasgupta1,*, A. Lambertz, H. Stiebig, F. Finger and H. Wagner Institut für Photovoltaik, Forschungszentrum Juelich GmbH, 52425 Juelich, Germany 1 Energy Research Unit, IACS, Jadavpur, Kolkata-700032, India; * present address: Physical Metallurgy Section, Indira Gandhi Centre for Atomic Research, Kalpakkam, India ABSTRACT Microcrystalline silicon solar cells were prepared at various substrate temperatures using a plasma enhanced chemical vapor deposition technique at 95 MHz. Devices in superstrate configuration, i.e. prepared on transparent glass/ZnO substrates with deposition sequence p-i-n, suffer from a reduction of short wavelength response upon increasing substrate temperature. As underlying mechanism adverse effects on the p-i interface region are discussed. For devices in substrate configuration (deposition sequence n-i-p on Ag/ZnO back-reflectors) a pronounced efficiency maximum with a highest value of 8.7 % is observed at substrate temperatures of about 250 °C. Comparing the dark J-V characteristics obtained for different device thicknesses at substrate temperatures of 200 °C and 250 °C, respectively, improved i-layer material and transport properties are suggested in the latter case. The results illustrate the sensitivity of microcrystalline silicon devices with respect to the employed substrate temperature by effects on the absorber layer material properties on the one hand and by effects related to the device design, e.g. the specific deposition sequence of the individual layers, on the other hand. INTRODUCTION In recent years the conversion efficiencies of microcrystalline silicon (µc-Si:H) thin film solar cells have been continuously improved [1-4]. An important prerequisite for this advance is the ability to produce high quality intrinsic µc-Si:H material by optimized preparation processes. However, the connections between material properties on the one hand and the device performance on the other hand are presently not well explored and must be further investigated. In previous studies, we already reported on such connections, including the important dependence of the open-circuit voltage VOC on the silane concentration SC=[SiH4]/[SiH4+H2], used for the preparation of the i-layer [4-6]. Since VOC increases towards higher SC values, the most efficient solar cells are obtained under microcrystalline growth conditions close to the transition zone between crystalline and amorphous growth and not in the highly crystalline growth regime at low SC values, as was often supposed in the past. The material prepared under the latter conditions rather exhibits high recombination losses in the devices, which is probably due to a high density of defect states. Important to note, the optimized material prepared ‘close to the transition’ is still highly crystalline and of homogeneous structural appearance [4]. Based on the findings of the previous works, we now extend the investigations to a variation of substrate temperat
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