Robin Nicholas

Abstract:

This paper deals with the design, implementation and assessment of single-junction solar cells using the quaternary semiconductor material InGaAsP. Two bandgaps of 1.0 and 1.2eV were considered in n/p and p/n structure configurations lattice-matched to InP. These solar cells are being investigated for their subsequent integration, together with single-junction 0.74eV bandgap InGaAs thermophotovoltaic (TPV) cells, as double-junction three-terminal photovoltaic (PV) devices. The devices were grown using MOVPE on InP substrates, with layer thicknesses and doping concentrations optimised through prior modelling. The assessment of the cells was undertaken using current-voltage measurements with a calibrated TPV simulator on mesa diodes of different sizes. Open-circuit voltages of up to 695 and 655mV, and fill factors of 0.48 and 0.6 were measured for 1.2 and 1.0eV solar cells, respectively. The best energy conversion efficiency recorded was 7% on a 1.0eV bandgap, n/p device. These cells were further investigated using spectral response measurements and the results are presented, compared and discussed. (c) 2007 Elsevier B.V. All rights reserved.

Abstract:

Combinations of different aromatic polymers and organic solvents have been studied as dispersing agents for preparing single-walled carbon nanotubes solutions, using optical absorbance, photoluminescence-excitation mapping, computer modeling, and electron microscopic imaging to characterize the solutions. Both the polymer structure and solvent used strongly influence the dispersion of the nanotubes, leading in some cases to very high selectivity in terms of diameter and chiral angle. The highest selectivities are observed using toluene with the rigid polymers PFO-BT and PFO to suspend isolated nanotubes. The specific nanotube species selected are also dependent on the solvent used and can be adjusted by the use of THF or xylene. Where the structure has more flexible conformations, the polymers are shown to be less selective but show an enhanced overall solubilization of nanotube material. When chloroform is used as the solvent, there is a large increase in the overall solubilization, but the nanotubes are suspended as bundles rather than as isolated tubes which leads to a quenching of their photoluminescence.

Abstract:

Single-walled carbon nanotubes have been efficiently dispersed in an organic solvent using the semiconducting polymers MEHPPV and PFO. It has been found that energy is transferred to the carbon nanotubes when the polymer is photo-excited across its minimum energy gap. This is shown using photoluminescence excitation mapping in the range of both the polymer's and the nanotube's optical absorbance. Possible mechanisms for the energy transfer are discussed.

Abstract:

Using magneto-luminescence (ML) measurement, we have observed the formation of magneto-excitons with either type-I or type-II band alignment depending on the strain distribution in the Si/SiGe quantum well structures. This observation is consistent with the analysis of strain-induced band shifting and exciton-transition energy. (c) 2007 Elsevier B.V. All rights reserved.