Microcrystalline Si and (Si,Ge) Solar Cells
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Microcrystalline Si and (Si,Ge) Solar Cells Vikram L. Dalal,* Tim Maxson+ and Kay Han+ * Dept. of Electrical and Computer Engr., Iowa State University, Ames, Iowa 50011 + Microelectronics Research Center, Iowa State University, Ames, Iowa 50011 ABSTRACT We report on the growth of microcrystalline Si and (Si,Ge) cells on stainless steel substrates. The devices were grown using a remote, low pressure, ECR growth technique at low temperatures (250-350 C). The precursor gases were silane, germane and hydrogen. The devices were of the p-i-n type, with light incident on the p layer. The p layer was a-(Si,C). A novel interface buffer layer, consisting of an amorphous alloy whose bandgap was graded from 1.3 eV to 1.9 eV was used to match the crystalline base layer with the higher gap p layer. It was found that the properties of this buffer layer were critical in determining the properties of the resulting device. The buffer layer was found to increase the voltage by almost 20%. Cells with high fill factors were made using this technique. The quantum efficiency data indicated that the base layers had absorption characteristic of crystalline materials. INTRODUCTION Microcrystalline Si (µc-Si:H) is becoming an important electronic material with applications in solar cells[1-3] and thin film transistors[4-6]. This material is characterized by small grain sizes and yet, the transport seems to be not by field-aided drift alone, but rather, by a combination of drift and diffusion.[7]. The grain boundaries are passivated by H during growth, and hence, recombination at these boundaries is reduced. Most of the previous work in this material has been done using VHF glow discharge deposition at high frequencies (~60-70 MHz), or by using hot-wire CVD techniques. Most of the devices also used crystalline layers for all three layers of the device, n+, n and p+. In this paper, we grow the devices using a remote ECR ( electron cyclotron resonance) plasma technique, with significant hydrogen dilution[8], and also use a novel interface between the p junction and the n base layers so as to improve the device properties. We show that the design of this interface is critical in determining the performance of the device. In addition to growing good µc-Si:H devices, we also fabricated µc-(Si,Ge):H devices, also using the remote hydrogen rich ECR plasma deposition technique. DEVICE FABRICATION The devices were grown using successive depositions of n+, n , buffer and p layers using the remote ECR plasma deposition system described previously[8]. The precursor gases were silane, germane diluted in hydrogen, and hydrogen. The dilution ratio of hydrogen to silane, or hydrogen to (silane + germane) was > 20:1. Growth temperatures were in the range of 275-350 C. Growth pressures were kept low, around 5 mT, in order to obtain a high hydrogen radical and ion flux density at the growing surface. It is believed that such high reactive ion (H) flux
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densities contribute to improved crystallinity in the film, by etching away the weakest (amorphous) bo
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