Engineering

Publication Search Results

Now showing 1 - 6 of 6
  • (1996) Sproul, Alistair; Edminston, Sean; Puzzer, Tom; Heiser, Gernot; Wenham, Stuart; Green, Martin; Young, Timothy
    Conference Paper
    An 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.

  • (1996) Wenham, Stuart; Green, Martin; Edminston, Sean; Campbell, Patrick; Koschier, L; Thorpe, D; Honsberg, Christiana; Shi, Z; Heiser, Gernot; Sproul, Alistair
    Journal Article
    Thin 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.

  • (1996) Edminston, Sean; Heiser, Gernot; Sproul, Alistair; Green, Martin
    Journal Article
    This 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.

  • (1999) Malomed, Boris; Wang, Zheng; Chu, Pak; Peng, Gang-Ding
    Journal Article
    We consider a model of a nonlinear planar waveguide with a sinusoidal modulation of the refractive index in the transverse direction, which gives rise to a system of parallel troughs that may serve as channels that trap solitary beams (spatial solitons). The model can also be considered as an asymptotic one describing a dense planar array of parallel nonlinear optical fibers, with the modulation representing the corresponding effective Peierls–Nabarro potential. By means of the variational approximation and by direct simulations we demonstrate that the one-soliton state trapped in a channel has no existence threshold and is always stable. In contrast with this a stationary state of two beams trapped in two adjacent troughs has an existence border, which is found numerically. Depending on the values of the parameters, the two-soliton states are found to be dynamically stable over an indefinitely long or a finite but large distance. We consider the possibility of switching the beam from a channel where it was trapped into an adjacent one by a localized spot attracting the beam through the cross-phase modulation. The spot can be created between the troughs by a focused laser beam shone transversely to the waveguide. By means of the perturbation theory and numerical method we demonstrate that the switching is possible, provided that the spot’s strength exceeds a certain threshold value.

  • (1998) Ankiewicz, A; Wang, Zheng; Peng, Gang-Ding
    Journal Article
    In this analysis, we show that, using a Bragg grating in transmission mode, an optical filter based on an asymmetric twin-core fibre can operate with excellent narrow bandpass characteristics (e.g., pass bandwidths less than 1 nm) for optical wavelength division multiplexing systems. The grating in this scheme operates outside the band-gap. This ensures that the grating functions are in ‘transmission mode', i.e., that most of the light is transmitted through the device. More importantly, the grating introduces a periodic modulation in propagation constant which produces resonantly enhanced dispersion in coupling, and this leads to a significant reduction in pass bandwidth.

  • (2000) Wang, Zheng; Peng, Gang-Ding; Chu, Pak
    Journal Article
    An improvement has been made to Rouard's method for the analysis of fiber and waveguide gratings. The real reflectivity of each interface is used instead of an approximate value obtained by coupled-mode theory. The improved Rouard's method becomes a complete Rouard's method and an exact method. It is simple and independent on coupled-mode theory for no calculation of coupling coefficient required. Numerical examples of uniform and nonuniform gratings have been given.