We experimentally demonstrate the III-V nitrides as a high-performance photovoltaic material with open-circuit voltages up to 2.4V and internal quantum efficiencies as high as 60%. GaN and high-band gap InGaN solar cells are designed by modifying PC1D software, grown by standard commercial metal-organic chemical vapor deposition, fabricated into devices of variable sizes and contact configurations, and characterized for material quality and performance. The material is primarily characterized by x-ray diffraction and photoluminescence to understand the implications of crystalline imperfections on photovoltaic performance. Two major challenges facing the III-V nitride photovoltaic technology are phase separation within the material and high-contact resistances.

The Very High Efficiency Solar Cell (VHESC) program is developing integrated optical system-PV modules for portable applications that operate at greater than 50% efficiency. We are integrating the optical design with the solar cell design, and have entered previously unoccupied design space. Our approach is driven by proven quantitative models for the solar cell design, the optical design, and the integration of these designs. Optical systems efficiency with an optical efficiency of 93% and solar cell device results under ideal dichroic splitting optics summing to 42Á7 W 2Á5% are described.

No abstract

We report structural studies of InGaN epilayers of various thicknesses by x-ray diffraction, showing a strong dependence of the type and spatial distribution of extended crystalline defects on layer thickness. The photoluminescence intensity for the samples was observed to increase with thickness up to 200 nm and decrease for higher thicknesses, a result attributed to creation of dislocation loops within the epilayer. Correlation of physical properties with crystalline perfection open the way for optimized designs of InGaN solar cells, with controlled types and dislocation densities in the InGaN epilayers, a key requirement for realizing high photocurrent generation in InGaN.

No abstract

No abstract

No abstract

Field emission from flat graphene sheets is a challenge due to fewer emission sites. Here, we have synthesized free‐standing vertically oriented few‐layer graphene (FLG) films directly on dielectric substrates by hot‐filament chemical vapor deposition (HFCVD) without any catalyst or special substrate treatment. The fabricated FLGs with a large smooth surface topography, standing roughly vertical to the substrate are found to grow according to the Stranski–Krastanov growth mechanism. The ease of large‐area preparation and the low turn‐on field of 22 V µm−1 in addition to the large field enhancement factor of ≈6520 for electron field emission suggest that the vertically oriented FLGs could be used as a potential edge emitter. Scanning electron microscopy image of vertically oriented few‐layer graphene on a SiO2/Si substrate with the schematic of field electron emission.