Microcrvstalline and Mixed Phase Si:H : Preparation. Potential and Properties for Devices
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Abstract We discuss the preparation, properties and device potential of microcrystalline Si:H (ic-Si:H) material deposited using plasma-CVD techniques. We have been able to deposit high quality jic-Si:H using both glow discharge and ECR plasma CVD techniques. In both cases, the critical parameter was the presence of a high flux of H, and high power. The quality of the material was measured using sub-gap photo-conductivity, in both films and devices. We find that the absorption coefficient in uc-Si:H films follows the c-Si curve very closely, with little excess absorption. We have also fabricated p-i-n devices in these materials. We find that the properties of the devices depend critically upon the p-i interface and on the deposition technique used to make the devices. We use quantum efficiency techniques to study the devices in detail. We find that often, a good device characteristic corresponds to a case where the undoped layer is a mixed phase(amorphous-microcrystalline) and not just microcrystalline. In contrast, using just the microcrystalline phase for i layers in devices leads to severe interface problems, which lead to inflexion points in I(V) curves. Using the mixed phase for i layer, we have made solar cells with 0.46 V open circuit voltage, and a fill factor of 0.58. In contrast, using a microcrystalline phase can lead to devices with high voltages (0.6), but poor fill factors. We also discuss the device potential of microcrystalline devices, and we find that it may be possible to fabricate devices with 8 % solar conversion efficiency in this material once all the interface problems are solved. Such devices may have applications as low-gap cells in an a-Si:HI/uc-Si:H cell structure.
I.Introduction Recently, there has been considerable interest in using microcrystalline Si:H as material for solar cells and for thin film transistors. [1]. Work at the University of Neuchatel has shown that solar cells of 4.5% efficiency can be made in this material, and that these cells can be incorporated as the low gap cell in a tandem a-Si;H-/lc Si:H cell structure. Efficiencies of 9% in a tandem structure have been reported.[2]. The previous work relied on depositing this material using VHF glow-discharge techniques. In this work, we report on the deposition and properties of bcSi:H using both standard 13.56 M-Hz RF glow discharge,and also using low pressure ECR glow discharge techniques. We show that the electronic properties of this material are quite good when either of these techniques is used. An important technical issue is the ultimate device potential of gc-Si:H for solar cells. In this paper,we use realistic design parameters to suggest that an ultimate solar conversion efficiency
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Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
of about 8% may be achieved in this material when appropriate back reflectors etc. are used. A critical issue in developing such cells is the appropriate design of interfaces between the different layers in a cell. In this paper,we discuss these interfacial i
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