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(1996)Conference PaperAn analytical model is developed to decribe recombination currents arising from recombination at grain boundaries (GBs) in the depletion region of a p-n junction solar cell. Grain boundaries are modelled as having a single energy evel in the energy gap, and partial occupancy of these stats gives raise to a chage on the GB. The analytical model is compared to a complete numerical simulation package (DESSIS) and found to be in excellent agreement. Additionally,. cross sectional EBIC images of a multilayer device containing vertical GBs are presented. The experimental data is comared qualitatively with results derived from numerical modelling.
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(1996)Journal ArticleThin film crystalline silicon solar cells can only achieve high efficiencies if light-trapping can be used to give a long optical path lengtrh, while simulatneously achieving near unity collection probabilities for all generated carriers. This necessitates a supporting substrate of a foreign material, with refractive index compatible with light trapping schemes for silicon. The resulting inability to nucleate growth of crystalline silicon films of good crystallographic quality on such foreign substrates, at present prevents the achievement of high efficiecny devices using conventional single junction solar cell structures. The parallel multijunction solar cell preovides a new approach for achieving high efficiencies from very poor quality material, with near unity collection probabilities for all generated carriers achieved through appropriae junction spacing. Heavy doping is used to minimise the dark saturation current contribution from the layers, therefore allowing respectable voltages. The design strategy, corresponding advantages, theoretical predictions and experimental results are presented.
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(1996)Journal ArticleThis paper provides a theoretical investigation of recombination at grain boundaries in both bulk and p-n junction regions of silicon solar cells. Previous models of grain boundaries and grain boundary properties are reviewed. A two dimensional numerical model of grain boundary recombination is presented in this paper. This numerical model is compared to existing analytical models of grain boundary recombination within both bulk and p-n junction regions of silicon solar cells. This analysis shows that, under some conditions, existing models poorly predict the recombination current at grain boundaries. Within bulk regions of a device, the effective surface recombination velocity at grain boundaries is overestimated in cases where the region around the grain boundary is not fully depleted of majority carriers. For vertical grain boundaries (columnar grains), existing models are shown to underestimate the recombination current within p-n junction depletion regions. This current has an ideality factor of about 1.8. An improved analytical model for grain boundary recombination within the p-n junction depletion region is presented. This model considers the effect of the grain boundary charge on the electric field within the p-n junction depletion region. The grain boundary charge reduces the p-n junction electric field, at the grain boundary, enhancing recombination in this region. This model is in agreement with the numerical results over a wide range of grain boundary recombination rates. In extreme cases, however, the region of enhanced, high ideality factor recombination can extend well outside the p-n junction depletion region. This leads to a breakdown of analytical models for both bulk and p-n junction recombination, necessitating the use of the numerical model.
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(1997)Journal ArticleAcoustic resonances of the vocal tract give rise to formants (broad bands of acoustic power) in the speech signal when the vocal tract is excited by a periodic signal from the vocal folds. This paper reports a novel instrument which uses a real-time, non-invasive technique to measure these resonances accurately during phonation. A broadband acoustic current source is located just outside the mouth of the subject and the resulting acoustic pressure is measured near the lips. The contribution of the speech signal in the pressure spectrum is then digitally suppressed and the resonances are calculated from the input impedance of the vocal tract as a function of frequency. The external excitation signal has a much smaller harmonic spacing than the periodic signal from the vocal folds and consequently the resonances are determined much more accurately due to the closer sampling. This is particularly important for higher pitched voices and we demonstrate that this technique can be markedly superior to the curve-fitting technique of linear prediction. The superior frequency resolution of this instrument which results from external vocal tract excitation can provide the precise, stable, effective, articulatory feedback considered essential for some language-learning and speech therapy applications.
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(1997)Journal ArticleThe magnetization of melt-textured Y-123 and single crystal La-214 has been measured in magnetic field up to 5.5 T using a SQUID magnetometer. Partial and nearly complete flux jumps were observed at about 10 K in Y-123 and 5 K in La-214 single crystals, respectively.
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(1997)Conference PaperThe formants of speech sounds are usually attributed to resonances of the vocal tract. Formant frequencies are usually estimated by inspection of spectrograms or by automated techniques such as linear prediction. In this paper we measure the frequencies of the first two resonances of the vocal tract directly, in real time, using acoustic impedance spectrometry. The vocal tract is excited by a carefully calibrated, broad band, acoustic current signal applied outside the lips while the subject is speaking. The sound pressure response is analysed to give the resonant frequencies. We compare this new method (Real-time Acoustic Vocal tract Excitation or RAVE) with linear prediction and we report the vocal tract resonances for eleven vowels of Australian English. We also report preliminary results of using feedback from vocal tract excitation as a speech trainer, and its effect on improving the pronunciation of foreign vowel sounds by monolingual anglophones.
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(1998)Conference Paper
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(1997)Journal ArticleThe authors propose an optical fibre narrow bandpass filter using a grating in transmission mode. This is achieved using the enhanced dispersion effect instead of selective reflection. The highly dispersive coupling achievable with this proposal can produce pass bandwidths of < 1 nm for WDM systems.
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