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Short Wavelength Response in a-Si:H p-i-n Diodes: A Simple Method to Minimize Interface Recombination

Published online by Cambridge University Press:  26 February 2011

A. Catalano  and
G. Wood
A. Catalano
Affiliation:
Solarex Corporation, Thin Film Division, 826 Newtown-Yardley Road, Newtown PA 18940
G. Wood
Affiliation:
Solarex Corporation, Thin Film Division, 826 Newtown-Yardley Road, Newtown PA 18940
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Abstract

Although single junction a-Si:H p-i-n solar cells with a conversion efficiency of about 12% have been demonstrated [1], the photo response of devices is limited by several factors. At short wavelengths, the most obvious limitation is imposed by the optical bandgap of the topmost doped layer. However further limitations to the short wavelength response have also been noted. Homojunction p-i-n devices of a-Si:H suffer from poor short wavelength response owing not only to high optical absorption in the p-layer, but also recombination at the p/i interface. The latter effect is enhanced due to the “back diffusion” of electrons towards the p/i interface and recombination that further reduces the short wavelength response. The short wavelength response of devices is improved by the use of a heterojunction front contact such as p-type a-Si1-x Cx :H. The resulting conduction band discontinuously reduces in part the back diffusion and recombination and reduces optical losses. Despite these improvements, however, the short wavelength response of p-i-n devices is frequently lower than expected on the basis of optical absorption within the topmost layers. Evidence for these losses comes from measurement of devices in reverse bias, where improvements in the short wavelength response are indicative of recombination losses. For example, Figure 1 compares the normalized photoresponse of two a-Si1-x Cx p-i-n devices. The photoresponse is normalized by taking the ratio of the quantum efficiency at -3V and zero bias at each wavelength. Cells with a photoresponse ratio greater than unity have additional, nonoptical losses. In Figure 1 the “normal” devices is a standard a-Si1-x Cx :H p-i-n solar cell while the graded interface device has a silicon-carbon alloy grading extending several hundred Angstroms away from the p/i interface. It is clear from the photoresponse data that recombination lowers the short wavelength response of the ungraded device.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 118: Symposium E – Amorphous Silicon Technology , 1988 , 581
Copyright
Copyright © Materials Research Society 1988

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References

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