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Materials Physics

A photo of laser light rays going in various directions atop a corrugated metal substrate

In materials physics, NREL focuses on realizing materials that transcend the present constraints of photovoltaic (PV) and solid-state lighting technologies.

Through materials growth and characterization, coupled with theoretical modeling, we seek to understand and control fundamental electronic and optical processes in semiconductors.

Capabilities


Optimizing New Materials

An illustration showing area of sample of gallium arsenic nitride, with circular grid image indicating location of clusters. Plot to right highlights photoluminescence versus energy, with two curves: the upper curve of high magnetic field shows localized nitrogen cluster states, and the lower curve shows overlapping clusters.

We use high magnetic fields to unravel the unusual behavior of electron traps in emergent PV materials. New insight into electron trapping in dilute nitride semiconductors will help mitigate undesirable transport properties so that these semiconductors can be used for ultra-high-efficiency solar cells.

Contact: Brian Fluegel| Email | 303-384-6437

Examples of Work

These publications provide examples of our research capabilities in this area:

Magnetic field-induced delocalized to localized transformation in GaAs:N, K. Alberi, S.A. Crooker, B. Fluegel, D.A. Beaton, A.J. Ptak, A. Mascarenhas. Phys. Rev. Lett. 110, 156405 (2013).

Evolution of superclusters and delocalized states in GaAsN, B. Fluegel, K. Alberi, D.A. Beaton, S.A. Crooker, A.J. Ptak, A. Mascarenhas, Phys. Rev. B, 86, 205203 (2012).

Localization-delocalization transition of electrons at the percolation threshold of semiconductor GaAsN alloys: the appearance of a mobility edge, K. Alberi, B. Fluegel, D.A. Beaton, A.J. Ptak, A. Mascarenhas, Phys. Rev. B (Rapid Comm.) 86, 041201 (2012).


Probing Direct-Indirect Crossover

An image of an energy diagram showing concave-up conduction band above and separated from two concave-downward curves: the upper one is the heavy-hole band and the lower one is the light-hole band. Various parameters are labeled and indicate the move of a charge carrier from the low-hole band to the conduction band then to the heavy-hole band.

We also use high magnetic fields to probe the nature of the direct-indirect crossover. For example, in studying AlGaAs, we have shown that the crossover behaves as an alloy-broadened first-order phase transition.

Contact: Brian Fluegel| Email| 303-384-6437

Examples of Work

These publications provide examples of our research capabilities in this area.

Spectroscopic determination of the bandgap crossover composition in MBE-grown AlxGa1-xAs, B. Fluegel, K. Alberi, J. Reno, A. Mascarenhas, Jpn. J. Appl. Phys. 54 042402 (2015).

Magnetic field induced direct-indirect crossover in AlGaAs, K. Alberi, A.V. Mialitsin, B. Fluegel, S.A. Crooker, J.L. Reno, A. Mascarenhas, Appl. Phys. Express 7, 111201 (2014).


Strain in Semiconductors

Using our novel ultra-sensitive probe for strain in semiconductors, we have observed a new light-hole to heavy-hole interband electronic Raman process in AlGaAs that has evaded discovery for almost half a century. This effect can be used to determine epitaxial strain-induced splittings in the band structure with unprecedented resolution.

Contact: Brian Fluegel| Email | 303-384-6437

Examples of Work

These publications provide examples of our research capabilities in this area.

Electronic Raman scattering as an ultra-sensitive probe of strain effects in semiconductors. B. Fluegel, A.V. Mialitsin, D.A. Beaton, J.L. Reno, A. Mascarenhas. Nature Comm. 6, 7136 (2015).


Solid-State Lighting

A photo of a square LED creating white light

We are exploring high-bandgap III-phosphide alloys, such as AlInP, grown metamorphically on GaAs substrates, as candidate materials for amber LEDs. LED bulbs can create white light by combining discrete wavelengths from individual red, green, blue, and amber LEDsan approach known as color-mixing. These bulbs have the potential for higher efficiencies and advanced color-tuning functionality compared to current bulbs. However, the lack of efficient green and amber LEDs has impeded progress in realizing such architectures.

Contact: Kirstin Alberi| Email | 303-384-7647

Examples of Work

These publications provide examples of our research capabilities in this area.

Amber-green light-emitting diodes using order-disorder AlInP heterostructures, T.M. Christian, D.A. Beaton, K. Mukherjee, K. Alberi, E.A. Fitzgerald, A. Mascarenhas, J. Appl. Phys. 114, 074505 (2013).

Growth, microstructure, and luminescent properties of direct-bandgap InAlP on relaxed InGaAs on GaAs substrates, K. Mukherjee, D.A. Beaton, T.M. Christian, E.J. Jones, K. Alberi, A. Mascarenhas, M.T. Bulsara, E.A. Fitzgerald, J. Appl. Phys. 113, 183518 (2013).


Molecular Beam Epitaxy

Photo of depositional chamber for molecular beam epitaxy at NREL showing various ports where the sample substrate and various elemental sources are introduced into the system.

We use molecular beam epitaxy (MBE) to grow a variety of III-V and II-VI based semiconductors, including AlGaAs, GaAsBi, GaPBi, and ZnCdSe. We are also exploring the use of light stimulation of the growth surface, with the goal of overcoming limitations to the epitaxy of heavily doped semiconductors and heterovalent interfaces, as well as growth at low temperatures.

Contact: Kirstin Alberi | Email| 303-384-7647

Examples of Work

These publications provide examples of our research capabilities in this area.

Effects of incident UV light on surface morphology of MBE grown GaAs, D.A. Beaton, C.E. Sanders, K. Alberi, J. Crystal Growth 413, 76 (2015).

Fermi energy tuning with light to control doping profiles during epitaxy, C.E. Sanders, D.A. Beaton, R.C. Reedy, K. Alberi, Appl. Phys. Lett. 106, 182105 (2015).


Micro-Photoluminescence Imaging

A schematic illustration of grain boundary (irregular line) where laser injection moves a charge carrier by diffusion from the right side of the grain boundary to the left side, with an associated photoluminescence emission.

Understanding how defect states influence carrier transport in semiconductors is important for improving device performance, but most measurement techniques only provide this information indirectly. By decoupling the photon injection and collection optics, we can spatially resolve where carriers recombine in relation to where they are generated.

Carriers are photoinjected into a single location with a focused laser, while PL is simultaneously collected and imaged from a larger area. PL detected in neighboring regions of the material provides direct information about the distances and locations to which carriers have diffused. This new information can be used to guide the development of polycrystalline CdTe for improved solar cell performance.

We have also adapted this micro-PL imaging technique to include time resolution as well as spatial resolution. This approach has allowed us to determine how the carrier lifetime changes near extended defects.

Contact: Kirstin Alberi | Email| 303-384-7647

Examples of Work

These publications provide examples of our research capabilities in this area.

Measuring long-range carrier diffusion across multiple grains in polycrystalline semiconductors by photoluminescence imaging, K. Alberi, B. Fluegel, H. Moutinho, R.G. Dhere, J.V. Li, A. Mascarenhas, Nature Communications 4, 2699 (2013).

Probing carrier lifetimes at dislocations in epitaxial CdTe, K. Alberi, B. Fluegel, M.J. DiNezza, S. Liu, Y.-H. Zhang, A. Mascarenhas, Appl. Phys. Express 7, 065503 (2014).

Carrier decay and diffusion dynamics in single-crystalline CdTe as seen via microphotoluminescence, B. Fluegel, K. Alberi, M.J. DiNezza, S. Liu, Y.-H. Zhang, A. Mascarenhas, Phys. Rev. Appl. 2, 034010 (2014).


Buildings Research

Photo of an index finger and a thumb holding a small, opaque square of film.

We apply advanced materials research techniques, such as the Materials Genome Initiative approach, to address key problems related to buildings energy use. Our key focus areas relate largely to the building envelope, which includes windows technologies and opaque components such as walls and roofing. We conduct research into responsive architectural materials that allow a building to change its properties based on existing environmental conditions to maximize energy savings.

Contact: Rob Tenent| Email | 303-384-6775

Examples of Work

These publications provide examples of our research capabilities in this area.

Metal oxide films for electrochromic applications, D.T. Gillaspie, R.C. Tenent, A.C. Dillon, J. Mater. Chem. 20(43), 95859592 (2010).

Fast-switching electrochromic Li+ doped NiO films by ultrasonic spray deposition, R.C. Tenent, D.T. Gillaspie, A. Miedaner, P.A. Parilla, A.C. Dillon, J. Electrochem. Soc. 157, H318H322 (2010).

Scalable synthesis of improved nanocrystalline, mesoporous tungsten oxide films with exceptional electrochromic performance, C.-P. Li, C. Engtrakul, R.C. Tenent, et al., Solar Energy Materials and Solar Cells 132, 614 (2015).


Photovoltaic Module Soiling Mitigation and Thermal Control

Two large impacts on power generation by fielded PV modules are dust/soiling, which prevents light from getting to the absorber layer, and substantial heating of the modules when they are collecting energy. We perform systematic studies to understand the processes involved; for example, we seek to predict the effects of soiling for different environmental conditions.

We are working with the PV community to 1) develop models that predict seasonal/annualized soiling losses at new PV plant sites, 2) provide the PV industry with tools/knowledge for successful long-term deployment of modules with value-added antireflection and anti-soiling coatings, and 3) identify the most important physical and chemical adhesion mechanisms involved with dust particles adhering to PV module surfaces.

Contact: Lin Simpson| Email| 303-384-6625

Examples of Work

These publications provide examples of our research capabilities in this area.

A comprehensive review of the impact of dust on the use of solar energy, Sarver et al., Renewable and Sustainable Energy Reviews 22, 698733 (2013).

Translating outdoor CPV I-V measurements to a CSTC power rating and the associated uncertainty, M. Muller, S. Kurtz, M. Steiner, G. Siefer, Progress in Photovoltaics (April, 2015).


Transparent Window Insulation

We work on a number of technologies related to buildings, developing materials that may address the large installed windows retrofit market to substantially improve fenestration and building-envelope energy efficiency. We are developing transparent vacuum insulation films that use nano- to micrometer-sized vacuum capsules integrated with standard window plastics. They can be integrated with low-e coatings to mitigate infrared transmission and produce up to R-20 films only millimeters thick but transparent enough for window applications.

Contact: Lin Simpson| Email| 303-384-6625


Theory and Computation

The Electronic Structure Theory and Computational Materials Science activities use computational methods to study energy materials from a theoretical perspective. We collaborate closely with other experimental materials science research groups to translate physical insights gained from computations into progress in developing real materials.

Contact: Stephan Lany| Email| 303-384-6652


The Materials Science Center is part of the Materials and Chemical Science and Technology directorate, led by Associate Lab Director Bill Tumas.