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The efficiency of a solar cell can be substantially increased by opening new energy gaps within the semiconductor band gap. This creates additional optical absorption pathways which can be fully exploited under concentrated sunlight. Here we report a new approach to opening a sizeable energy gap in a single junction GaAs solar cell using an array of small InAs QDs that leads directly to high device open circuit voltage. High resolution imaging of individual QDs provides experimentally obtained dimensions to a quantum mechanical model which can be used to design an optimised QD array. This is then implemented by precisely engineering the shape and size of the QDs resulting in a total area (active area) efficiency of 18.3% (19.7%) at 5 suns concentration. The work demonstrates that only the inclusion of an appropriately designed QD array in a solar cell has the potential to result in ultra-high efficiency under concentration.

Laterally coupled semiconductor quantum dot (QD) arrays emerged recently as promising structures for the next generation of high efficiency intermediate band solar cell (IBSC).We present here a method based on multi-band k·p Hamiltonian combined with periodic boundary conditions, applied to predict the electronic and optical properties of InAs/GaAs QDs based lateral QD arrays. The absorption coefficients were used to estimate the corresponding photovoltaic conversion efficiencies using the drift-diffusion transportation theory. Special attention was paid on the transition between IB to continuum states in conduction band. By comparing the experimental and theoretical results we have confirmed operation of the InAs/GaAs QD array based solar cell in the intermediate band regime.

We present a theoretical method for calculations of exciton and bi-exciton energies in type-II colloidal quantum dots. Our methodology is based on an 8-band k · p Hamiltonian of the zinc-blend structure, which incorporates the effects of spin-orbit interaction, strain between the core and the shell and piezoelectric potentials. Exciton states are found using the configuration interaction (CI) method that explicitly includes the effects of Coulomb interaction, as well as exchange and correlation between many-electron configurations. We pay particular attention to accurate modelling of the electrostatic interaction between quasiparticles. The model includes surface polarization and self-polarization effects due to the large difference in dielectric constants at the boundary of the QD.

Laterally coupled semiconductor quantum dot (QD) arrays emerged recently as promising structures for the next generation of high efficiency intermediate band solar cell (IBSC), due to their ability to facilitate the formation of in-plane minibands with large vertical inter-layer distance to prevent the strain field from influencing its neighboring QD array layers. The lateral quantum coupling effect, which exists between states in QDs of an array, influences the electronic and optical properties of such structures. We present here a method based on multi-band k·p Hamiltonian combined with periodic boundary conditions, applied to predict the electronic and optical properties of InAs/GaAs QDs based lateral QD arrays. The absorption coefficients under different in-plane coupling strength were derived and the corresponding photovoltaic conversion efficiencies were also estimated using drift-diffusion transportation theory. Special attention was paid on the transition between IB to continuum states in conduction band.

We present a theoretical model for the design and analysis of semiconductor quantum dot array based intermediate-band solar cells. Information on the electronic structure and wave functions are used to estimate the most relevant radiative as well as non-radiative (Auger effect related) times between the intermediate (IB), conduction (CB) and valence (VB) bands. Analysis of dynamical processes in GaAs/InAs QD array reveal that: (i) the radiative transition time between CB→IB is at least one order of magnitude longer than the radiative time between IB→VB, and (ii) the most detrimental non-radiative time that might prevent quasi-Fermi level separation between CB and IB upon external illumination, required for proper IBSC operation, is Auger electron cooling. It is ~3 orders of magnitude faster than any other scattering time and is in the ps time domain. In order to improve the dynamical conditions for possible formation of quasi-Fermi level separation between the CB and IB, we employ methods of quantum engineering to design the type II alignment, using a GaAsSb barrier buffer. By changing the Sb amount in the buffer region, we predict an increase of the IB→VB radiative time to the same time scale as CB→IB radiative time with simultaneous increase of the Auger electron cooling to ~ 0.1 ns.

Excitons and biexcitons in GaN/AlN quantum dots (QD) were investigated with special emphasis on the use of these QDs for single photon source applications. The theoretical methodology for the calculation of single-particle states was based on 8-band strain-dependent envelope function Hamiltonian, with the effects of spin-orbit interaction, crystal-field splitting, piezoelectric and spontaneous polarization taken into account. Ex-citon and biexciton states were found using the configuration interaction method. Optimal QD heights for their use in singlephoton emitters were determined for various diameter to height ratios. The competition between strong confinement in GaN QDs and internal electric field, generally reported in wurtzite GaN, was also discussed, as well as its effect on appearance of bound biexcitons.

Intermediate band solar cells (IBSC) have emerged as an alternative design for third generation solar cells that could lead to dramatical improvements of the power conversion efficiencies. For this concept to work the intermediate band (IB) has to be located in the forbidden energy gap of the barrier material and to be separated by zero density of states from the valence and conduction band of the barrier material. We have demonstrated that a k · p multiband theory with periodic boundary conditions can easily be applied to predict electronic and absorption characteristics, as well as radiative and non-radiative carrier life-times between IB induced by semiconductor quantum dot (QD) arrays. We have identified that the most detrimental effect that might affect proper operation of the IBSC is caused by very fast, ∼ps, Auger electron cooling non-radiative process. We discuss possible QD array designs that can suppress fast Auger electron cooling.

We investigate the effects of linear and non linear piezoelectricity in wurtzite III-N semiconductors and their influence on the electronic properties of low dimensional quantum dots. By studying the dependence of the biexciton on structural and geometrical parameters of the nanostructure, we show second order to be important particularly when the strain in the nanostructure is reduced

Interconduction band gain without global inversion is obtained by engineering conduction band dispersion relations with dilute N resonant centres. We recover the expected dispersive gain shape for similar masses and contrasting results if the effective masses characterizing the relevant subbands are very different.

In this paper we discuss inter-conduction band gain without global inversion by engineering the conduction band effective masses. We recover the expected dispersive gain shape for similar masses and contrasting results if the effective masses characterizing the relevant subbands are very different. The relevance of many-body effects is investigated and numerical results are present for different structures.

We used high hydrostatic pressure techniques to understand the deteriorating temperature performance with decreasing wavelength of short wavelength quantum cascade lasers. Influence of inter-valley scattering and distribution of the electron wave functions will be discussed.

We show that by decreasing the lasing wavelength of InAs/AlSb-based quantum cascade lasers below 3mum, carrier leakage into the L-minimum of conduction band is responsible for their temperature sensitivity and limits their maximum operating temperature.

We present a theoretical study of the optical properties of GalnNAs quantum dot (QD) structures, emitting at 1.55 μm wavelength. The theoretical model is based on a 10 x 10 k.p band-anti-crossing Hamiltonian, incorporating valence, conduction and nitrogen-induced bands. We have investigated the interplay between the nitrogen to the conduction band mixing, and piezoelectric field on the ground state optical matrix element. With a reduced amount of indium and an increased amount of nitrogen in the QD the optical matrix element becomes on the average larger and less sensitive to the variation of both the QD shape and size than is the case of an InNAs QD. The ideal optical characteristics at room temperature and 1.55 μm wavelength are discussed.

We have modelled the temperature and pressure dependence of the threshold current of GaInNAs-based VCSELs and compared the results with measured data.

By measuring the spontaneous emission from normally operating /spl sim/1.3 /spl mu/m GaInNAs/GaAs-based lasers we have quantitatively determined the variation of each of the current paths present in the devices as a function of temperature from 130 K to 370 K.

Summary form only given. We have shown that it is necessary to introduce a modified 10-band k.p Hamiltonian to describe the band dispersion in GaInN/sub x/As/sub l-x/, adding two spin-degenerate nitrogen states to the conventional 8-band k.p Hamiltonian to account for the band gap reduction and modified conduction band dispersion. We use the modified Hamiltonian here to undertake a theoretical analysis of gain as a function of carrier density and radiative current density in ideal 1.3 /spl mu/m GaInN/sub x/As/sub 1-x//GaAs QW structures. We then use the model to analyse recent experimental data on the variation of threshold current with pressure in a GaInNAs/GaAs laser structure.