Ryne P. Raffaelle

Carbon nanotubes for lithium ion batteries

Lithium ion batteries are receiving considerable attention in applications, ranging from portable electronics to electric vehicles, due to their superior energy density over other rechargeable battery technologies. However, the societal demands for lighter, thinner, and higher capacity lithium ion batteries necessitate ongoing research for novel materials with improved properties over that of state-of-the-art. Such an effort requires a concerted development of both electrodes and electrolyte to improve battery capacity, cycle life, and charge–discharge rates while maintaining the highest degree of safety available. Carbon nanotubes (CNTs) are a candidate material for use in lithium ion batteries due to their unique set of electrochemical and mechanical properties. The incorporation of CNTs as a conductive additive at a lower weight loading than conventional carbons, like carbon black and graphite, presents a more effective strategy to establish an electrical percolation network. In addition, CNTs have the capability to be assembled into free-standing electrodes (absent of any binder or current collector) as an active lithium ion storage material or as a physical support for ultra high capacity anode materials like silicon or germanium. The measured reversible lithium ion capacities for CNT-based anodes can exceed 1000 mAh g−1 depending on experimental factors, which is a 3× improvement over conventional graphite anodes. The major advantage from utilizing free-standing CNT anodes is the removal of the copper current collectors which can translate into an increase in specific energy density by more than 50% for the overall battery design. However, a developmental effort needs to overcome current research challenges including the first cycle charge loss and paper crystallinity for free-standing CNT electrodes. Efforts to utilize pre-lithiation methods and modification of the single wall carbon nanotube bundling are expected to increase the energy density of future CNT batteries. Other progress may be achieved using open-ended structures and enriched chiral fractions of semiconducting or metallic chiralities that are potentially able to improve capacity and electrical transport in CNT-based lithium ion batteries.

Effect of strain compensation on quantum dot enhanced GaAs solar cells

GaP tensile strain compensation (SC) layers were introduced into GaAs solar cells enhanced with a five layer stack of InAs quantum dots (QDs). One sun air mass zero illuminated current-voltage curves show that SC results in improved conversion efficiency and reduced dark current. The strain compensated QD solar cell shows a slight increase in short circuit current compared to a baseline GaAs cell due to sub-GaAs bandgap absorption by the InAs QD. Quantum efficiency and electroluminescence were also measured and provide further insight to the improvements due to SC.

Purity assessment of multiwalled carbon nanotubes by Raman spectroscopy

Carbonaceous purity assessment for chemical vapor deposition multiwalled carbon nanotubes (MWNTs) using Raman spectroscopy was investigated. Raman spectroscopy was performed on a reference sample set containing predetermined ratios of MWNTs and representative synthesis by-products. Changes in the characteristic Raman peak ratios (i.e., ID∕IG, IG′∕IG, and IG′∕ID) as a function of MWNT content were measured. Calibration curves were generated from the reference samples and used to evaluate MWNTs synthesized under different conditions with varying purity. The efficacy of using Raman spectroscopy in conjunction with thermogravimetric analysis for quantitative MWNT purity assessment is discussed.

Near 1 V open circuit voltage InAs/GaAs quantum dot solar cells

Ten-layer InAs/GaAs quantum dot (QD) solar cells exhibiting enhanced short circuit current (Jsc) and open circuit voltage (Voc) comparable to a control GaAs p-i-n solar cell are reported. 1 sun Jsc is enhanced by 3.5% compared to that of the GaAs control, while the Voc is maintained at 994 mV. Results were achieved using optimized InAs QD coverage and a modified strain balancing technique, resulting in a high QD density (3.6×1010 cm−2), uniform QD size (4×16 nm2), and low residual strain (103 ppm). This enhanced Voc is a promising result for the future of InAs QD-enhanced GaAs solar cells.

Material and energy intensity of fullerene production.

Fullerenes are increasingly being used in medical, environmental, and electronic applications due to their unique structural and electronic properties. However, the energy and environmental impacts associated with their commercial-scale production have not yet been fully investigated. In this work, the life cycle embodied energy of C(60) and C(70) fullerenes has been quantified from cradle-to-gate, including the relative contributions from synthesis, separation, purification, and functionalization processes, representing a more comprehensive scope than used in previous fullerene life cycle studies. Comparison of two prevalent production methods (plasma and pyrolysis) has shown that pyrolysis of 1,4-tetrahydronaphthalene emerges as the method with the lowest embodied energy (12.7 GJ/kg of C(60)). In comparison, plasma methods require a large amount of electricity, resulting in a factor of 7-10× higher embodied energy in the fullerene product. In many practical applications, fullerenes are required at a purity >98% by weight, which necessitates multiple purification steps and increases embodied energy by at least a factor of 5, depending on the desired purity. For applications such as organic solar cells, the purified fullerenes need to be chemically modified to [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM), thus increasing the embodied energy to 64.7 GJ/kg C(60)-PCBM for the specified pyrolysis, purification, and functionalization conditions. Such synthesis and processing effects are even more significant for the embodied energy of larger fullerenes, such as C(70), which are produced in smaller quantities and are more difficult to purify. Overall, the inventory analysis shows that the embodied energy of all fullerenes are an order of magnitude higher than most bulk chemicals, and, therefore, traditional cutoff rules by weight during life cycle assessment of fullerene-based products should be avoided.

High conductivity carbon nanotube wires from radial densification and ionic doping

Application of drawing dies to radially densify sheets of carbon nanotubes (CNTs) into bulk wires has shown the ability to control electrical conductivity and wire density. Simultaneous use of KAuBr4 doping solution, during wire drawing, has led to an electrical conductivity in the CNT wire of 1.3×106 S/m. Temperature-dependent electrical measurements show that conduction is dominated by fluctuation-assisted tunneling, and introduction of KAuBr4 significantly reduces the tunneling barrier between individual nanotubes. Ultimately, the concomitant doping and densification process leads to closer packed CNTs and a reduced charge transfer barrier, resulting in enhanced bulk electrical conductivity.

Nanostructured photovoltaics for space power

Quantum dot enhanced solar cells have been evaluated both theoretically and experimentally. A detailed balance simulation of InAs quantum dot (QD) enhanced solar cells has been performed. A 14% (absolute) efficiency improvement has been predicted if the middle junction of a state-of-the-art space multi-junction III-V solar cell can be bandgap engineered using QDs. Experimental results for a GaAs middle junction enhanced with InAs QDs have shown an 8% increase in short circuit current compared to a baseline device. The current enhancement per layer of QD was extracted from device spectral response (0.017 mA per QD layer). This value was used to estimate the efficiency of multi-junction solar cells with up to 200 layers of QDs added to the middle current-limiting junction. In addition, the radiation tolerance of QD cells, key to operation of these cells in space environments, shows improved characteristics. Open circuit voltage (VOC) in QD devices was more resilient to both alpha and proton displacement damage, resulting in a 10X reduction in the rate of VOC degradation.

Electrodeposited CdS on CIS pn junctions

We have been investigating the electrochemical deposition of thin films and junctions of cadmium sulfide (CdS) and copper indium diselenide (CIS). We show that it is possible to fabricate pn junctions based on n-type CdS and p-type CIS entirely by electrodeposition. CIS is considered to be one of the best absorber materials for use in polycrystalline thin-film photovoltaic solar cells. CdS provides a closely lattice-matched window layer for CIS. Electrodeposition is a simple and inexpensive method for producing thin-film CdS and CIS. We have produced both p- and n-type CIS thin films, as well as a CdS on CIS pn junction via electrodeposition. Elemental analysis of the CdS and CIS thin films was performed using X-ray photoelectron spectroscopy and energy dispersive spectroscopy. Optical band gaps were determined for these films using optical transmission spectroscopy. Carrier densities of the CIS films as a function of their deposition voltage were determined from capacitance vs. voltage measurements using Al Schottky barriers. Current vs. voltage characteristics were measured for the Al on CIS Schottky barriers and for the CdS on CIS pn junction.

Quantum dot solar cell tolerance to alpha-particle irradiation

The effects of alpha-particle irradiation on an InAs quantum dot (QD) array and GaAs-based InAs QD solar cells were investigated. Using photoluminescence (PL) mapping, the PL intensity at 872 and 1120nm, corresponding to bulk GaAs and InAs QD emissions, respectively, were measured for a five-layer InAs QD array which had a spatially varying total alpha-particle dose. The spectral response and normalized current-voltage parameters of the solar cells, measured as a function of alpha-particle fluence, were used to investigate the change in device performance between GaAs solar cells with and without InAs QDs.

Evaluation of strain balancing layer thickness for InAs/GaAs quantum dot arrays using high resolution x-ray diffraction and photoluminescence

The impact of strain-balancing quantum dot superlattice arrays is critical to device performance. InAs/GaAs/GaP strain-balanced quantum dot arrays embedded in p-i-n diodes were investigated via high resolution x-ray diffraction (HRXRD) and photoluminescence (PL) as a function of the GaP thickness. A three-dimensional modification of the continuum elasticity theory was proposed and an optimal thickness was determined to be 3.8 ML. HRXRD-determined in-plane strain in superlattices with this range of GaP thickness gave an empirical value for the GaP thickness to be 4.5 ML. Optical characterization indicated the highest integrated PL intensity for the sample at the optimal strain balanced condition.