Temperature Dependent Transport in Microcrystalline PIN Diodes
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Temperature Dependent Transport in Microcrystalline PIN Diodes T. Brammer, H. Stiebig, A. Lambertz, W. Reetz and H. Wagner Institut für Photovoltaik, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany ABSTRACT The optoelectronic behavior of diodes deposited by plasma enhanced chemical vapor deposition was investigated for a series of different silane concentrations in the gas phase. The purpose of this work was to correlate device characteristics with inherent properties of microcrystalline silicon by experiments and numerical simulations. Dark diode characteristics and, therefore, the open circuit voltage behavior of this series were dominated by the bulk properties of the i-layer (equilibrium carrier concentration) as shown by numerical modeling. Measurement of the solar cell output parameters as a function of the temperature showed that the fill factor of solar cells with small silane concentrations is dominated by the dark diode characteristics. This is in contrast to the temperature dependent fill factor of solar cells with large silane concentration which is limited by the extraction efficiency of the photogenerated carriers. Interface effects dominated the temperature dependent blue response. The gain in blue response increased with temperature and silane concentration by up to 200 % which revealed transport limiting material properties in the vicinity of the p/i-interface. This behavior was attributed to the nucleation region. INTRODUCTION Microcrystalline silicon is regarded as a promising material for application in stacked thin film solar cells [1, 2, 3, 4] and sensor applications [5]. State-of-the art single junction pin diodes have a VOC above 500 mV and a FF exceeding 70 % [3]. Stacked solar cells with µc-Si:H as middle and bottom cell show stable efficiencies above 10 % [2, 4]. Despite the good performance of the devices the transport and recombination behavior is still under discussion. Deposition near the transition region between microcrystalline and amorphous growth has proven to yield optimum solar cell performance [3]. In doing so the highest efficiency is mainly a trade off between reduction of the dark saturation current and decreasing extraction efficiency of the photogenerated charge carriers. This work examines the optoelectronic properties of µc-Si:H diodes with absorption layers deposited with different silane concentrations. EXPERIMENT The pin solar cells examined in this study were deposited in a multi-chamber glow discharge deposition system under very high frequency condition (VHF) of 95 MHz on glass substrates coated with non-textured transparent conductive oxide (TCO). Typical deposition conditions were a substrate temperature of 200 °C, deposition pressure of 300
A32.3.1
mTorr and power of 50-80 mW/cm2. The i-layers of the pin structures were deposited at different silane concentrations (SC) in hydrogen in the gas phase ([SiH4]/([SiH4]+[H2]) varying from 2 % to 7.2 % with a thickness of 1 µm. The preparation conditions for the doped layers were kept constant. More details co
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