Omkar Jani

Design and Characterization of GaN/InGaN Solar Cells

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.

Characterization and analysis of InGaN photovoltaic devices

The InGaN material system is investigated to achieve high efficiency solar cells, using tandem and quantum-well structures to implement high efficiency concepts. Here InGaN p-i-n and quantum-well solar cells are designed, grown by MOCVD and fabricated into mesa devices. They are electrically characterized by I-V response under dark, white light and UV illumination and internal quantum efficiency (IQE). Material characterization is done by X-ray diffraction, photoluminescence and photoemission. InGaN solar cells with high In compositions are grown in two configurations, one incorporating it into the i-region of a p-i-n solar cells, and the other incorporating as the well-region of a quantum-well device. A QE of 8% was measured from these quantum-wells. Solar cells with In lean In/sub 0.07/Ga/sub 0.93/N p-i-n device structures show an IQE of 19% as well as photoemission at 500 nm, confirming the suitability of the material for photovoltaic applications.

Correlation of crystalline defects with photoluminescence of InGaN layers

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.

Design, Growth, Fabrication and Characterization of High-Band Gap InGaN/GaN Solar Cells

One of the key requirements to achieve solar conversion efficiencies greater than 50% is a photovoltaic device with a band gap of 2.4 eV or greater. lnxGa1-xN is one of a few alloys that can meet this key requirement. InGaN with indium compositions varying from 0 to 40% is grown by both metal-organic, chemical-vapor deposition (MOCVD) and molecular beam epitaxy (MBE), and studied for suitability in photovoltaic applications. Structural characterization is done using X-ray diffraction, while optical properties are measured using photoluminescence and absorption-transmission measurements. These material properties are used to design various configurations of solar cells in PC1D. Solar cells are grown and fabricated using methods derived from the III-N LED and photodetector technologies. The fabricated solar cells have open-circuit voltages around 2.4 V and internal quantum efficiencies as high as 60%. Major loss mechanisms in these devices are identified and methods to further improve efficiencies are discussed

Influence of Growth Conditions on Phase Separation of InGaN Bulk Material Grown by MOCVD

Different types of phase separation in thick InGaN layers were studied using photoluminescence (PL) and x-ray diffraction (XRD). InGaN films of 100 nm in thickness were grown on 2 μm GaN templates with an In molar fraction ranging from 0% to 20% by metal organic chemical phase deposition (MOCVD). It is shown that suppression of the phase separation in InGaN can be made possible by increasing the TMIn flow rate, decreasing the layer thickness and decreasing the growth rate. Based on the results, two types of phase separation, microscopic quantum dots and macroscopic domains, are proposed accordingly. The influence of the growth conditions on each type is summarized respectively in this paper.