We establish a framework to examine the feasibility of using local vegetation for bioenergy power systems in small-scale applications and remote settings. The framework has broad application, and we present a specific case here to demonstrate the process. Our case study is the Tiwi Islands in northern Australia, where a large Acacia mangium plantation is a potential source of biofuel feedstock. Two types of technology were considered: 1. Bio-oil from pyrolysis in diesel generators and 2. Direct combustion coupled with a steam turbine. The biomass was characterized and found to have adequate properties for an energy crop, with a lower heating value of about 18 MJ/kg and entire tree ash content of 2%. Measurements from trees that were damaged from wildfires had similar results, showing potential value recovery for a plantation after unplanned fire. In comparison to a petroleum diesel-based generator, the bio-oil system was 12% more expensive. The direct combustion system was found to be the most economical of those explored here, costing as low as 61% of the bio-oil system. Additional social and environmental benefits were identified, including local employment opportunities, improved energy security and reduced greenhouse gas emissions. Our findings of high techno-economic potential of bioenergy systems, especially through direct combustion, are widely applicable to on-demand renewable energy supply in remote communities.

Motivated by its potential properties and applications, the energy band alignment of the amorphous-crystalline Ge₂Sb₂Te₅ heterojunction in thermal equilibrium is explored. An approximate analytic model based on the exact solution to Poisson’s equation is constructed to describe the electrostatics of the heterojunction between the amorphous phase and the face-centred cubic crystalline phase of Ge₂Sb₂Te₅. The model captures the physics of accumulation in the crystalline layer, as well as that of depletion and inversion due to the deep defect distribution in the amorphous layer. Without introducing fitting parameters, the model approximates the influence of the density of states parameters of each phase on the electric potential distribution across the heterojunction. It is then validated against the exact solution obtained numerically using solar cell capacitance simulations. Apart from the small inaccuracy in modelling the electric potential distribution in the depletion region, simulation results reveal that the approximations are successful in modelling the electrostatics of the heterojunction.

The transport of holes through a representative isotype amorphous-crystalline Ge₂Sb₂Te₅ heterojunction under forward bias is explored for the first time. An approximate analytic model, based on the exact solution to Poisson’s equation, the Continuity equation, and the Transport equation, is proposed to describe the forward current-voltage characteristic and hole quasi-Fermi level distribution across the heterojunction with a reduced set of material and device parameters. The proposed model incorporates thermionic emission across the heterojunction interface as well as drift and diffusion across the quasi-neutral regions of the heterojunction layers, but neglects drift and diffusion across the space-charge regions of the heterojunction layers, as well as recombination. Solar cell capacitance simulation results demonstrate general agreement between the approximate and exact solutions. Therefore, the approximate model is effective in capturing the physics of thermionic emission-limited transport at low applied bias and drift-diffusion-limited transport through the quasi-neutral region of the amorphous layer at larger applied biases. However, simulation results also show that an extremely narrow subregion of the space-charge region within the amorphous layer, which has been neglected within the proposed model, limits the transport of holes at very low bias and inhibits transport at all other biases. Nevertheless, the proposed model provides improved accuracy across the entire bias range compared to the individual thermionic emission or drift-diffusion models.

Motivated by future data storage requirements, Ge₂Sb₂Te₅ is studied for application in phase-change random access memory. The currently accepted density of states (DOS) models for the cubic crystalline phase, based on first-principles calculations, are reviewed. An alternative DOS model, which incorporates band tails and an antimony vacancy multivalent defect, is proposed. Solar cell capacitance simulation results reveal that the alternative model is successful in predicting a free hole concentration and Fermi level position consistent with previous Hall effect and thermopower measurements respectively. The conduction band tail, which has not previously been incorporated within the DOS model of the crystalline phase, is shown to contribute to this success.

To predict possible minority-carrier effects in multi-level phase change memory devices, minority-carrier transport through an isotype amorphous-crystalline Ge₂Sb₂Te₅ heterojunction under forward bias is studied for the first time. Electron thermionic emission, thermal generation, drift, diffusion, radiative recombination, Auger recombination, Schockley–Read–Hall recombination via conduction band tails, valence band tails, acceptor-type mid-gap, donor-type mid-gap and multivalent defect distributions, as well as surface recombination are considered in the construction of the steady-state Continuity Equation relevant to the representative amorphous-crystalline Ge₂Sb₂Te₅ heterojunction, which is then numerically solved at 0.15 V and 0.40 V using solar cell capacitance simulations. Provided that radiative recombination is negligible and defect distributions within the band gap of either layer are energetically localised, the simulated electron concentration, electron current density and electron quasi-Fermi level distributions across the heterojunction reveal that transport through the amorphous layer limits electron flow through the device. At low applied bias, net recombination and diffusion within the quasi-neutral region (QNR) of the amorphous layer dominate, whereas at larger applied bias, drift across the QNR, due to the electric field induced by the significant majority-carrier current density, as well as surface recombination at the amorphous layer contact contribute significantly. Within the crystalline layer, net generation of electrons supplies the amorphous layer at all biases, assuming that the crystalline layer contact does not limit electron transport. Thus, the effect of forward bias on the dominant transport mechanisms through the amorphous-crystalline Ge₂Sb₂Te₅ heterojunction demonstrated herein represents the key contribution of this paper.