The electronic properties of MoO and reduced molybdenum oxide phases are studied by density functional theory (DFT) alongside characterization of mixed phase MoO films. Molybdenum oxide is utilized in compositions ranging from MoO to MoO with several intermediary phases. With increasing degree of reduction, the lattice collapses and the layered MoO structure is lost. This affects the electronic and optical properties, which range from the wide band gap semiconductor MoO to metallic MoO. DFT is used to determine the stability of the most relevant molybdenum oxide phases, in comparison to oxygen vacancies in the layered MoO lattice. The non-layered phases are more stable than the layered MoO structure for all oxygen stoichiometries of MoO studied where 2 ≤ x < 3. Reduction and lattice collapse leads to strong changes in the electronic density of states, especially the filling of the Mo 4d states. The DFT predictions are compared to experimental studies of molybdenum oxide films within the same range of oxygen stoichiometries. We find that whilst MoO is easily distinguished from MoO, intermediate phases and phase mixtures have similar electronic structures. The effect of the different band structures is seen in the electrical conductivity and optical transmittance of the films. Insight into the oxide phase stability ranges and mixtures is not only important for understanding molybdenum oxide films for optoelectronic applications, but is also relevant to other transition metal oxides, such as WO, which exist in analogous forms.

A detailed balance model for the intermediate band (IB) solar cell has been developed. The model allows the electron concentration in the IB to vary and assumes a linear relation between this concentration and the absorption coefficients related to transitions over the subband gaps. Numerical results show that for IBs with densities of states typical for quantum dot-superlattices it is possible to sustain a useful population of photogenerated electrons in the IB when the cell is exposed to concentrated light. For unconcentrated light the IB must be partially filled by means of doping to achieve high efficiencies within reasonable optical path lengths. The filling of the IB is shown to vary with light intensity, cell voltage, density of IB-states, and the positioning of the IB in the main band gap both for cells that are partially filled by doping and for photofilled cells.

A novel drift-diffusion model for intermediate band solar cells (IBSC) is presented. The model differs from previous driftdiffusion models by allowing the carrier concentrations in all three bands to vary. It is developed for the idealized case where only radiative recombination occurs and where the IB has zero width. The model is used to compare the performance of IBSCs where the IB-region is doped to get a partially filled IB (prefilled IBSC) to IBSCs where the IB-region is not doped to partially fill the IB (photofilled IBSC). Numerical results show that a photofilled IBSC can achieve high efficiencies when operated under concentrated light. In fact, for some particular cases, a photofilled cell will perform better than a prefilled cell. The optimal degree of prefilling, i.e. the ratio of the concentration of doping atoms to the total number of IB-states, is found for a particular example. It is also examined how the carrier concentrations in all three bands, the conduction, the intermediate and the valence bands, vary in prefilled and photofilled IBSCs. Finally, the band diagrams of a prefilled and photofilled IBSC are discussed.

We report current-voltage and spectral response characteristics of high density InAs/GaAs quantum dot (QD) solar cells with different positions where dots are located. The short circuit current density (Jsc), open circuit voltage (Voc), and external quantum efficiency of these cells under air mass 1.5 are presented and compared with a GaAs reference cell. An extended photoresponse in contrast to the GaAs reference cell was confirmed for all these cells. The effect of inserting QD layers into emitter and base region on device performance is shown. The Jsc is reduced, while the Voc is maintained. The cell with QDs located toward the base side shows better performance, confirmed by both current-voltage and spectral response measurements.

We report high density quantum dots (QDs) formation with optimized growth temperature and V/III ratio. At lower growth temperature, QD density is increased, due to smaller surface migration length of In adatoms. With higher V/III, the QD density is higher but it results in large clusters formation and decreases the QD uniformity. The QD solar cell was fabricated and examined. An extended spectral response in contrast to the GaAs reference cell was presented but the external quantum efficiency at energies higher than GaAs band gap is reduced, resulting from the degradation for the emitter above the strained QD layers.