Dr. Hubbard’s research focuses on Photovoltaic and Optoelectronic Devices, Radiation Hardened Space Power, III-V Semiconductors, and Vapor Phase Epitaxy. Our activities encompass materials synthesis, device fabrication, material and device modeling, as well as characterization both at the electrical and materials level. Specific expertise lies in vapor phase epitaxy (VPE) of III-V photovoltaic devices and nanostructures, novel photovoltaic structure growth and design and all forms of photovoltaic characterization and simulation.
- Epitaxial Crystal Growth by Metalorganic Chemical Vapor Deposition (MOCVD)
- Semiconductor Device Design and Fabrication
- Thin-Film Characterization Techniques
- MOCVD System Design
- Nano-structures (quantum wires/dots) for enhanced efficiency photovoltaic cells.
- High Efficiency Nanostructured III-V Photovoltaics for Solar Concentrator Application
- Growth of semiconductor nanostructures using MOVPE growth techniques
- Novel Approaches to Power Conversion (alphavoltaics, thin-film III-Vs)
- Nanostructured gas/chemical sensors
This project aims to demonstrate uses of the earth-abundant and widely available metal Al for III-V solar cells. Pure Al may potentially serve as substrates or buffer layers, as well as sacrificial etching layers for thin-film release. Al metal is lattice matched to GaInAs, which can offer hihg efficiency and is readily partnered with GaInP passivating alloys.
The immediate goal of this project is to demonstrate heteroepitaxy of GaInAs on GaAs using Al buffer layers. If successful, metamorphic grades terminated with Al buffers could be reduced in thickness, or entirely removed, thus reducing deposition time and materials requirements. The long term goal would be to replace the expensive III-V substrate with a low cost Al alloy for direct growth of high efficiency solar cells.
III-V nanostructures (quantum wells and dots) continue to be investigated as a means of implementing advanced solar cell designs (intermediate band, hot carrier extraction, and/or restricted luminescent emissions). These device concepts require the inhibition of radiative recombination from the nanostructured materials. In this project, RIT has partnered with a private company to assess the role of radiative recombination in quantum well (QW) enhanced solar cells.
This goal of this grant is to increase the efficiency of tandem solar cells using highly mismatched, multi-junction (MJ) Sb-based solar cell with low defect density and optimal bandgap subcells. The payoff for developing this technology is 4-6 junction III-V solar cells, with high efficiency (greater than 50% under concentration) and significantly lower manufacturing costs and capital intensity compared to current inverted metamorphic (IMM) technology. We propose to do the above by gaining access to near optimal band gaps for a multijunction (MJ) solar cell by integrating Sb-based material into the InGaP2/GaAs technology using the interfacial misfit (IMF) growth technique. Our approach, based on IMF, offers an improved growth methodology that can achieve more optimum band gaps and thus much higher efficiencies. In addition, as the strain is relieved by IMF across a few monolayers (ML) at the interface, the need for a complex and growth intensive step-grade buffer is eliminated, thus reducing manufacturing costs
The purpose of this grant is to radically reduce the cost of high-efficiency III-V solar cells by developing single-junction (SJ), polycrystalline (PX) GaAs and InP thin film solar cells on low cost metal foils. The goal of this program is to demonstrate thin-film PX-GaAs solar cells on low-cost substrates with efficiencies near 18%-20% grown by low cost, large area deposition techniques. These goals will be achieved by leveraging RIT's experience in GaAs growth on poly-Ge and expertise in applying nano-technology approaches to enhancing GaAs solar cell performance, our demonstrated experience in GaAs grain boundary passivation and demonstrated high-performance GaAs cells on large-grain Ge templates.
Thus there is a compelling need for solar cell technologies that are lighter, higher efficiency, more radiation tolerant and significantly lower in cost than the conventional multi-junction cells based on germanium substrates. The focus of this project is to produce low cost high efficiency single junction based GaAs solar cells using nanotechnology and light trapping. The low cost aspect will be achieved by using high volume MOCVD production of thin GaAs solar cells and large area (6-inch) epitaxial lift off (ELO). This work combines the expertise of MicroLink Devices in high volume manufacturing of GaAs ELO solar cells and Rochester Institute of Technology’s (RIT) expertise in quantum wells, quantum dots and light trapping.
In this project, our lab has partnered with Micorlink Devices to incorporate quantum wells (QWs) and dots (QDs) in the GaAs and InGaAs subcells of an InGaP/GaAs/InGaAs IMM solar cell to increase the radiation tolerance and thereby improve the end-of-life performance. The quantum dot solar cell will be grown in an inverted metamorphic (IMM) format on GaAs and will be compatible with MicroLink’s epitaxial lift-off (ELO) process. The resulting solar cells will be lightweight, flexible, and radiation tolerant. Mechanically, they will resemble a sheet of thin metal foil. Innovative light management techniques such as reflective metal back contact and silver nanoparticle-enhanced reflectivity will be employed to increase absorption in the solar cell.
This project was a collaboration between MicroLink Devices, RIT and NRL to develop a novel, high-efficiency, all-lattice-matched solar cell that can achieve much higher power conversion efficiency and thereby enable a far lower levelized cost of energy (LCOE) than is possible with current concentrator photovoltaic (CPV) technologies. This was accomplished with a triple-junction solar cell lattice-matched to InP. The wide band-gap InAlAsSb top-junction is a novel solar cell material and was the key enabling technology. RIT has for the first time demonstrated MOCVD based growth of lattice matched InAlAsSb with Sb content over 30%.
This material had low background doping and n and p doping control using either Si or Zn. Bandgap was determined using photoreflectance to be near 1.75 eV, which is appropriate for top cell applications. PN junctions of InAlAsSb showed photovoltaic response, albeit with a high series resistance. Work is continuing under funding from NASA and the US Government to further develop this novel top cell material for application in the multi-junction device.
This initiative consists of the development of solid state photodiodes optimized for low-light levels to serve as the converter in an indirect conversion radioisotope battery. Our approach makes use of a radio-luminescent phosphor as an indirect conversion mechanism. Indirect radioisotope batteries down-convert the wide spectrum of energies from the radioisotope to a narrow range and place a photovoltaic cell away from any potential radiation damage.
The light generated by this phosphor is then converted to electric current and power in much the same way as in a solar cell. The second benefit of this method is that the photovoltaic need only be optimized to collect the narrow band of wavelengths produced by the intermediate phosphor, leading to very high efficiencies in excess of 30%. These batteries have long lifetimes due to the extended half-life of the radioisotope, making them ideal for ideal for power generation in extreme space environments, such as the outer solar system or the night side of planets as well as remote terrestrial locations.
This $1.75M capital equipment project was for the design and implementation of a III-V material growth facility using Metal Organic Chemical Vapor Deposition (MOCVD). The equipment, facility upgrades and personnel support for this project was funded in part by the National Science Foundation (NSF), Empire State Development (NYS), the Office of Naval Research and the Rochester Institute of Technology (RIT), VP of Research and the RIT-Kate Gleason College of Engineering. The system is installed in the Semiconductor and Mircrosystems Fabrication Laboratory (SMFL).
The MOCVD is devoted to the growth of III-V materials and devices containing As, P and Sb. The MOCVD has proven to provide the variety of materials, thickness, composition, and doping control necessary for the various nanomaterials and nanostructures used in our research.
Current programs that utilize the MOCVD include the development of quantum wells, wires and dots for high efficiency solar cells, III-V materials integrated with silicon based nanophotonics, next generation imaging array detectors and nanostructured III-V devices for radioisotope micro-batteries for health and security related microsystems. Adding this on-site III-V growth capability complements RIT’s outstanding processing and characterization facilities and provides our students with state-of-the-art tools to excel in their chosen fields of study.
This project sheds light on the technology and device physics of next generation quantum dot solar cells, leading to an intermediate band solar cell. One facet of the research was focused on QD materials systems with improved bandgap and little valence band offset for IBSC application. The other focus was on a doping superlattice nipi devices, which allow for longer carrier lifetime, improved absorption coefficients and high QD doping levels. Project results led to significant advances in epitaxial growth of QD, QW and strain balancing systems for both bandgap engineering of tandem devices as well as intermediate band solar cell designs. Specific advances include demonstration of current enhancement in an InAs QD enhanced GaAs solar cell, demonstration of InGaP solar cells with InAs QDs, radiation hardening of quantum enhanced multijunction solar cells, understanding of the role of the device electric field on carrier escape from QDs and development of multiple simulation tools for QW and QD enhanced solar cells.
As well this project resulted in the first experimental demonstration of a doping superlattice solar cell. These devices were shown by both simulation and experimentally to have enhanced carrier collection under extreme radiation conditions due to drift dominated collection. This project resulted in 8 journal articles, multiple conference proceedings and invited talks and 2 PhD theses
The Strain Balanced Quantum Dots for High Concentration Solar Photovoltaics projects provided insight into the fundamental material aspects of using nanomaterials for bandgap engineering multi-junction solar cells or for advanced concepts such as the intermediate band solar cell (IBSC). The overarching goal of this project is to addresses the need for future high-efficiency solar cells for either space photovoltaics (PV) or high concentration grid-tied solar farms. Specifically, the project has made advances in regards to strain balancing of InAs quantum dots using GaAsP, developing a fundamental theory for strain balanced QD superlattices. This resulted in a nanoHUB application to predict strain balancing thickness and maximum number of QD layers that can be stain balanced before onset of plastic relaxation. Improvements to understanding of QD strain balancing resulted in two record solar cell achievements: the highest reported open circuit voltage for an InAs QD solar cell and demonstration of a record 0.5% absolute increase in conversion efficiency for QD based solar cells.
As well, in collaboration with a multi-junction solar cell manufacture, we were able to demonstrate for the first time that InAs QDs can be used for bandgap engineering of the middle GaAs cell in an upright triple junction solar cell configuration on Ge substrates. In addition to strain balancing, this project has also enhanced our understanding of QD growth mechanisms by metal organic vapor phase epitaxy using on-axis and vicinal substrates, provided theory to explain the escape mechanisms in QD solar cell and QD interaction with device electric field, and demonstrated experimentally how delta doping can be used with QDs for IBSC application. Specific to IBSC, we have used numerical k.p simulation map potential IBSC materials that are more suitable than standard InAs on GaAs systems. Options identified include GaSb QDs using GaAsSb barriers and InAs QDs using AlAsSb, AlGaAs or InGaP barriers.
InAs QDs have been experimentally demonstrated on both AlGaAs and InGaP, with AlGaAs showing the most promise, best material quality and highest electron confinement. Future work under separate funding is now concentrated on experimental demonstration of AlGaAs and GaAsSb based IBSC. Our results have been disseminated though both conference talks and proceedings, journal publications, invited talks by the PI, book chapters, software programs developed, websites and outreach activities. We have published 8 journal articles, 23 conference proceedings, 1 book chapter and a software product on Nanohub. The project supported 2 faculty members, 1 post-doctoral fellow, 2 PhD, 4 MS, 6 undergraduates and 1 high school student. One of the PhD students is now an Assistant Professor and 2 of the MS students have taken positions at US Government research laboratories. Multiple outreach activities have been conducted under this project, including solar day activities though the Imagine RIT festival, public demonstrations of solar energy at the Rochester Museum and Science Center and many ½-day solar cell camps and demonstrations to high school students in grades 9-12.
In order to achieve a high efficiency using the IBSC concept, it is vital to study a suitable material system using a systematic approach. This proposal studied a solution using a QD/barrier system of antimony-based ternary materials. III-Sb materials have bandgaps from 0.3 to 2.1 eV. Moreover these systems are predicted to have a favorable IB position in energy alignment and to satisfy most if not all of the requirements for IBSCs. Our theoretical calculations predicted that InAs(Sb) QDs in AlAsSb barriers produce close to ideal band gaps required for IBSC. Though this material system is challenging to grow and not well studied, we have made excellent progress in achieving high quality QDs array that form an intermediate band. This included a demonstration of the first ever report of InAs quantum dots on AlAsSb, successful demonstration of two-photon absorption in InAs/AlAsSb QDs and the longest wavelength (~1.8 microns) photoresponse to date reported in QD-IBSCs. This project resulted in over 20 journal and conference publications and 1 PhD thesis.
Space photovoltaics degrade under the radiation environment in space, depending on the materials and design of the photovoltaic (PV) cells. In this project we have demonstrated novel quantum well enhanced multi-junction solar cells can deliver more efficient, radiation hardened and lightweight solar cells and arrays. Standard upright triple junction solar cells with QW enhanced regions were grown and investigated under various space radiation environments, with improved end of life performance. We have also develop and provided reliable, validated computational tool for assessment and optimization of the QW enhanced technologies for space PV applications.
III-V based nanowires (NW) have promising potential in next-generation electronic, photonic, and photovoltaic device applications. However, considerable effort remains at the fundamental materials level in order to fully exploit the benefits afforded at the nanoscale. In particular, the use of core-shell NWs or hybrid organic-NW approaches for photovoltaic applications, in which the junction is radial while the optical absorption is axial, can enable high photo-conversion efficiency in low diffusion length materials (such as mismatched materials) by decoupling the optical absorption from the carrier collection. However, in all NW based devices, surface recombination and thus passivation becomes a critical issue. This projected investigated the use of InAs nanowires as the photon collection mechanism in a photovoltaic device. InAs nanowires were grown by MOCVD using multiple distinct nucleation approaches: in‐situ nucleation, Au‐nanoparticle nucleation and catalyst free nucleation using a di-block copolymer masking scheme. The nanowire density and uniformity were characterized as a function of nucleation method and substrate orientation. A novel approach to NW epitaxy was demonstrated that eliminates the need for an Au-based catalyst nanoparticle using an in-situ generated metallic nano-droplet, In addition, the PI’s also collaborated with University of Wisconsin to investigating NW epitaxy mechanisms using a diblock copolymer (DBC) nanolithography template. NWs were observed to nucleate in the DBC pattern with no misfit or threading dislocations along the length of the NW. Initial photovoltaic response was demonstrated for both nucleation methods, with promise for applications in both space and terrestrial use.