Design and Demonstration of High-Efficiency Quantum Well Solar Cells Employing Thin Strained Superlattices.

Nanostructured quantum well and quantum dot III-V solar cells provide a pathway to implement advanced single-junction photovoltaic device designs that can capture energy typically lost in traditional solar cells. To realize such high-efficiency single-junction devices, nanostructured device designs must be developed that maximize the open circuit voltage by minimizing both non-radiative and radiative components of the diode dark current. In this work, a study of the impact of barrier thickness in strained multiple quantum well solar cell structures suggests that apparent radiative efficiency is suppressed, and the collection efficiency is enhanced, at a quantum well barrier thickness of 4 nm or less. The observed changes in measured infrared external quantum efficiency and relative luminescence intensity in these thin barrier structures is attributed to increased wavefunction coupling and enhanced carrier transport across the quantum well region typically associated with the formation of a superlattice under a built-in field. In describing these effects, a high efficiency (>26% AM1.5) single-junction quantum well solar cell is demonstrated in a device structure employing both a strained superlattice and a heterojunction emitter.

Experimental and theoretical study of the optical and electrical properties of nanostructured indium tin oxide fabricated by oblique-angle deposition.

Oblique-angle deposition of indium tin oxide (ITO) is used to fabricate optical thin-film coatings with a porous, columnar nanostructure. Indium tin oxide is a material that is widely used in industrial applications because it is both optically transparent and electrically conductive. The ITO coatings are fabricated, using electron-beam evaporation, with a range of deposition angles between 0 degrees (normal incidence) and 80 degrees. As the deposition angle increases, we find that the porosity of the ITO film increases and the refractive index decreases. We measure the resistivity of the ITO film at each deposition angle, and find that as the porosity increases, the resistivity increases superlinearly. A new theoretical model is presented to describe the relationship between the ITO film's resistivity and its porosity. The model takes into account the columnar structure of the film, and agrees very well with the experimental data.

Growth of highly oriented ZnO nanowires on GaN substrates for electronic and optical sensor applications.

In this Paper we present growth and characterization results of highly oriented ZnO nanowires grown on wide bandgap GaN substrates. Experimental results on the ZnO nanowires grown on p-GaN are presented with growth morphology and dimensionality control. We also present experimental results on these nanowire arrays such as I-V measurements and UV sensitivity measurements. The ZnO nanowires can be used for a variety of nanoscale optical and electronics applications.

Gigantic enhancement in response and reset time of ZnO UV nanosensor by utilizing Schottky contact and surface functionalization.

UV response of ZnO nanowire nanosensor has been studied under ambient condition. By utilizing Schottky contact instead of Ohmic contact in device fabrication, the UV sensitivity of the nanosensor has been improved by four orders of magnitude, and the reset time has been drastically reduced from approximately 417 to approximately 0.8 s. By further surface functionalization with function polymers, the reset time has been reduced to approximately 20 ms even without correcting the electronic response of the measurement system. These results demonstrate an effective approach for building high response and fast reset UV detectors.

Patterned growth of horizontal ZnO nanowire arrays.

We report an approach to fabricating patterned horizontal ZnO nanowire arrays with a high degree of control over their dimensionality, orientation, and uniformity. Our method combines electron beam lithography and a low temperature hydrothermal decomposition. This approach opens up possibilities to fabricate ZnO NW array based strain and force sensors, two-dimensional photonic crystals, integrated circuit interconnects, and alternative current nanogenerators.