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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 ArticleThe passivated emitter, rear locally diffused (PERL) cells, fabricated in our laboratory, reach an efficiency of 24.0%, the highest value for any silicon-based solar cell under terrestrial illumination. In an attempt to improve the rear surface passivation, which is usually obtained by a thermally grown oxide, we add a floating (i.e., noncontacted) p-n junction at the rear surface, resulting in the passivated emitter, rear floating p-n junction (PERF) cell design. Although these cells exhibit record 1-sun open-circuit voltages of up to 720 mV, their efficiency is degraded by nonlinearities ("shoulders") in the logarithmic I-V curves. In order to understand and manipulate such nonlinearities, this paper presents a detailed investigation of the internal operation of PERF cells by means of numerical modelling based on experimentally determined device parameters. From the model, we derive design rules for optimum cell performance and develop a generalized argumentation that is suitable to compare the passivation properties of different surface structures. For example, the oxidized rear surface of the PERL cell is treated as an electrostatically induced floating junction in this approach and analogies to the diffused floating p-n junction are drawn. Our simulations indicate that optimum rear surface passivation can be obtained in three different ways. (i) The floating junction of the PERF cell should be very lightly doped, resulting in a sheet resistivity of 5000 Omega/[D'Alembertian], and losses due to shunt leaking paths between the p-n junction and the rear metal contacts must be avoided. (ii) The rear surface of the PERL cell should be passivated by chemical vapor deposition of a silicon nitride film containing a larger positive interface charge density than exists in thermally grown oxides. (iii) An external gate can be added at the rear with low leakage currents and gate voltages of around 15 V.
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(1995)ThesisAspects of the pollination ecology of selected mass-flowering rainforest trees and shrubs (42 species in total) were investigated between 1990 - 1994 in subtropical lowland rainforest remnants in the Manning Valley on the North Coast of New South Wales. A subset of 14 species was studied in detail. The period of study coincided with a prolonged and widespread drought which may have influenced the results. The floral morphology of the studied species is described; this extends the information previously available for these species. Phenology, flower longevity, breeding system and pollen structure were investigated providing additional information, not previously available, for the species. Experimental exclusion of pollinators from flowers by bagging indicated that the majority of species studied had some ability for self-fertilization. This has important implications for the survival of isolated remnant plant populations. Approximately 60,000 insects were collected from inflorescences during the study. These were identified at least to order and assigned to morphospecies (which in many cases could be specifically identified). The data were subjected to multivariate analysis to explore spatial and temporal variation in the collections. The collections indicate that most of the plants studied recruit potential pollinators from an unspecialised guild comprising principally Diptera, Coleoptera and Hymenoptera. Membership of the pollinator guild is not constant but varies spatially (in relation to site, tree species and rainforest subformation) and temporally (during the day, seasonally and between years). However, the analyses consistently suggest that the assemblage in littoral rainforest is distinct from the assemblage in lowland subtropical rainforest. Although generalist strategies predominated, a number of specialist interactions were recorded. The trees and shrubs with close relationships to particular insects were mainly ecotonal or successional species rather than being restricted to mature rainforest. Plant-pollinator mutualisms for Wilkiea huegeliana (Monimiaceae), Daphnandra micrantha (Monimiaceae), Rapanea howittiana (Myrsinaceae) and Eupomatia laurina (Eupomatiaceae) were studied in greater detail. R. howittiana and W huegeliana are dependent on thrips for pollination. Large native insects (>9mm) amounted to fewer than 5% of total insects collected. Although it has been claimed that large beetles are poorly adapted to carry pollen, SEM studies consistently revealed large pollen loads. Large insects have the potential to undertake relatively long distance movements and this may be important in achieving outcrossing in spatially disjunct tree species. Introduced honey bees were the most numerous large sized insects and were most abundant at flowers during peak phase flowering. The possible influence of honey bees on native pollinators is discussed. Native bees constituted only a small percentage of flower visitors, but were taxonomically diverse, and many species recorded represented a substantial extension of the previously known range. The implications of the findings on guild structure for the conservation and management of rainforest remnants are discussed. The diversity and lack of constancy of guild composition suggests that the pollination system is robust and impacts of fragmentation may not limit the ability of even small plant populations to reproduce.
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(1997)ThesisGrowing evidence describes the global extent of human-induced environmental degradation. The Australian continent has suffered considerably, more so following European settlement just 200 years ago. Large expanses of the landmass have been cleared of native vegetation, animal habitats have been lost, soils degraded and waterways clogged with sediments and overloaded with nutrients from domestic, agricultural and industrial runoff. Despite being subjected to exploitation longer than any other Australian catchment, the Hawkesbury-Nepean contains significant areas which, due to their geography, represent remnants of near pre-contact landscapes. Whilst it is understood these ecosystems are worthy of preservation, little quantitative knowledge has been collected from them. Current management strategies have only been recently co-ordinated, and they are based on little scientific knowledge. This is of some concern given an increase in urban, agricultural, industrial and recreational pressures within the catchment today. This study aims to improve the management of ecosystems in the lower Hawkesbury River valley, by collecting quantitative information, to ensure the area's sustainability. The project concentrates on Mill Creek, a tributary of the Hawkesbury-Nepean River, located 70 km north west of Sydney. During the late Holcene this area was inhabited by small numbers of Aboriginal people who utilised simple technologies to exploit terrestrial and aquatic resources. With European settlement, Aboriginal land management practices were disrupted. Broad-scale removal and alteration of terrestrial and aquatic ecosystems ensued. Today this catchment forms part of Dharug National Park, yet it is influenced by land degradation and declining water quality in the greater Hawkesbury-Nepean catchment.
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