Identifying Electronic Properties Relevant To Improving Stability In A Si

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ISBN 13 :
Total Pages : 5 pages
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Download or read book Identifying Electronic Properties Relevant to Improving the Performance and Stability of Amorphous Silicon-based Mid-gap and Low-gap Cells written by and published by . This book was released on 2000 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: An overriding theme of the work described in this report has been the effect of partial crystallinity, or the approach to partial crystallinity, on the electronic properties of a-Si:H. This includes, of course, how degradation or the relative stability of these films is affected by the approach to, or onset of, microcrystallinity. The authors first discussed the results on a set of samples produced by dc reactive magnetron sputtering, obtained in collaboration with John Abelson's group at the University of Illinois, for which they demonstrated the existence of a small, but significant, microcrystalline component. For these films, the degradation kinetics was found to be quite unusual; however, it could be well accounted for by a model that postulated two phases of degrading material. One was a-Si:H host material of good quality and the other was a more defective component associated with boundary regions near the microcrystallites. The sub-band-gap photocapacitance measurements on these films also indicated the existence of a distinct feature (a ''shoulder'' with a threshold near 1.1 eV) that signaled the presence of the microcrystalline phase. The second set of samples investigated were produced by Uni-Solar, deposited under conditions of high hydrogen dilution, very close to but just below the microcrystalline phase boundary. Here they found that the defect density following light-induced degradation decreased as the film thickness increased. Corroborating their findings with X-ray diffraction results obtained by Don Williamson on sets of similar films, the authors concluded that the films were becoming more ordered and less defective just prior to the onset of a detectable microcrystalline component. Furthermore, they found that at almost exactly the conditions that Williamson found XRD evidence for the onset of microcrystallinity, they found the appearance of the distinctive ''shoulder'' in the sub-band-gap photocapacitance spectra. Third, they investigated two sets of samples where the deposition rate had been varied to include samples grown at moderate to high rates. In one set of samples, produced at ETL, samples deposited under H2 dilution were found to exhibit extremely low deep defect densities and narrow Urbach tails, indicating films of exceptional quality. The photocapacitance spectra for these films were found to contain evidence for a small degree of microcrystallinity. In another set of samples, produced at UniSolar, they found evidence for increasing defect densities plus somewhat larger Urbach energies for the films deposited at higher rates. This is consistent with the fact that the photovoltaic device performance is significantly poorer for the higher deposition rate material. Finally, they discussed the general issue of deep defect densities in the a-Si, Ge:H alloys. They again demonstrated how well the deep defect densities in such samples from several sources could be fit using the spontaneous bond-breaking model of Martin Stutzmann. This implies that such state-of-the-art alloy films have been optimized in a quantifiable sense. They also found that the increase in deep defect density with small amounts of P and B dopants could also be reproduced reasonably well by modifying the spontaneous bond-breaking model to include the extra energy terms associated with charged defects.