Amorphous Silicon-Carbon Alloys for Solar Cells
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AMORPHOUS SILICON-CARBON ALLOYS FOR SOLAR CELLS Yuan-Min Li, Solarex Corporation, Thin Film Division, 826 Newtown-Yardley Road, Newtown, Pennsylvania, 18940 USA ABSTRACT Recent efforts to optimize undoped, glow-discharge hydrogenated amorphous siliconcarbon alloys (a-SiC:H) with 1.9-2.0 eV bandgaps for solar cell applications are reviewed. Hydrogen dilution coupled with relatively low substrate temperatures (below 200 'C) have led to great improvements in the optical and phototransport properties of a-SiC:H films. The issue of alternative carbon feedstocks other than methane (CH4) will be explored. The improved a-SiC:H alloys have resulted in solar cells with high open circuit voltages (V. > 1.0 volt) and high fill factors (> 0.7). Further, the a-SiC:H solar cell instability upon prolonged light exposure has been much reduced. Correlation will be made between the properties of bulk undoped a-SiC:H films and the performance of p-i-n single junction solar cells using corresponding a-SiC:H thin i-layers. INTRODUCTION In recent years there has been great interest in hydrogenated amorphous silicon (a-Si:H) based wide-bandgap allos for opto-electronic applications. Hydrogenated amorphous siliconcarbon alloys (a-SiC:H), in particular, have received much attention for their applications in, among other things, a-Si:H based solar cells.2 The benefit of a-SiC:H for a-Si:H based photovoltaic technology was first demonstrated in the form of p-type (boron doped) a-SiC:H alloy, now standard window layer material, in a-Si:H based p-i-n type solar cells. 3 In comparison, because of the rapid deterioration in film quality (particularly in stability with prolonged light soaking) as the carbon content is increased, the solar cell applications of undoped a-SiC:H alloys have been largely limited to thin buffers or graded interface layers at the p/i interface. These layers can produce a significant gain in open circuit voltage (V.) of a-Si:H solar cells when the carbon content of these layers is fairly low.4 Further increase in V. (using the same doped layers), however, requires that the bulk of the i-layer have larger bandgap (E,) without degrading bipolar phototransport and stability. To enhance the V.. of multijunction solar cells, 2 it is desirable to use widegap undoped a-Si alloy with 1.9- 2.0 eV as the i-layer (the active layer) in the top junction, and as p/i interface layer(s) in the second (and possibly the third) junction. There is also appeal in aSiC:H based semi-transparent solar cells for architectural window applications. Of course, by incorporating more hydrogen, the bandgap of a-Si:H itself can be raised. The addition of carbon will simply further increase E,. Note that other widegap alloys such as a-SiO:H, a-SiN:H, aSiSe:H, a-SiS:H, have been suggested as alternatives to a-SiC:H. However, none of these has been demonstrated to work well as i-layers in a-Si:H based solar cells. This is not surprising since, compared to a-SiC:H (and a-Si:H), these 'undoped' alloys are substantially more n-type, exhibiting high electron p
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