Photovoltaic solar concentrators aim to increase the electrical power obtained from solar cells.  Conventional solar concentrators track the sun to generate high optical intensities, often by using large mobile mirrors that are expensive to deploy and maintain.  High optical concentrations without excess heating in a stationary system can be achieved with a luminescent solar concentrator (LSC) ^[1] . The LSC consists of a dye dispersed in a transparent waveguide.  Incident light is absorbed by the dye and then reemitted into a waveguide mode. The energy difference between absorption and emission prevents reabsorption of light by the dye, isolating the photon population in the waveguide. The performance of LSCs has been limited by two factors: self-absorption losses and a scarcity of dyes that emit efficiently in the infrared region. We have previously made significant progress on the problem of self-absorption losses ^[2] . Now we address operation in the infrared region.

Neodymium (Nd³⁺) and ytterbium (Yb³⁺) are nearly the optimal infrared LSC materials: inexpensive, abundant, efficient, and spectrally well-matched to high-performance silicon solar cells. These rare earth ions are natural three or four-level systems, therefore reasonably transparent to their own radiation and capable of generating high optical concentrations. Neodymium’s and ytterbium’s disadvantage is their relatively poor absorption overlap with the visible spectrum, meaning that the rare earth ions will require sensitization in the visible spectrum. Transition metals (TMs) can efficiently transfer energy to Nd and Yb ions, a process depicted in Figure 1. With a selection of TMs, broad sensitization of the both the visible and near infrared regions of the solar spectrum is possible, as shown in Figure 2. Finally, these ions can be combined with a high-throughput and chemically robust glass-making process for low cost and stable LSCs. Sensitized neodymium and ytterbium should enable LSCs matched to silicon.

1. W. H. Weber and J. Lambe, “Luminescent greenhouse collector for solar radiation,” Applied Optics, vol. 15, no. 10, pp. 2299-2300, Oct. 1976.
2. M. J. Currie, J. K. Mapel, T. D. Heidel, S. Goffri, and M. A. Baldo, “High-efficiency organic solar concentrators for photovoltaics,” Science, vol. 321, no. 5886, pp. 226 -228, July 2008.

Collaborators

• W. Higgins, RMD, MA
• I. D. Kim, KIST, Korea
• M. Martin & R. DeSouza, Aachen University, Germany
• D. Nocera, MIT
• R. Moos, Univ. Bayreuth Germany
• B. Yildiz, MIT
• K. Van Vliet, MIT

Postdoctoral Associates

• S. Bishop
• D.J. Yang
• P.S. Cho
• Y. Kuru

Graduate Students

• W. Jung, Res. Asst., MSE
• G. Whitfield, Res. Asst., MSE
• D. Chen, Res. Asst., MSE
• J. Engel, Res. Asst., MSE
• J.J. Kim, Res. Asst., MSE
• N. Thompson, Res. Asst., MSE

Support Staff

• A. Rothwell, Admin. Asst. II

Publications

C. Solís, W.C. Jung, H. L. Tuller and J. Santiso, Defect Structure, Charge Transport Mechanisms and Strain Effects in Sr₄Fe₆O_12+d Epitaxial Thin Films, Chem. Mater., 22, 1452–1461 (2010)

N. Yamamoto, D. J. Quinn, N. Wicks, J. L. Hertz, J. Cui, H. L. Tuller and B.L. Wardle, Nonlinear Thermomechanical Design of Microfabricated Thin Plate Devices in the Post-buckling Regime, J. Micromech. Microeng. 20, 035027-035036 (2010);

H. L. Tuller, S. J. Litzelman, and G. C. Whitfield, Electrical Conduction in Nanostructured Ceramics, in Ceramics Science and Technology Volume 2: Properties, Edited by R. Riedel and I-W. Chen, Wiley-VCH, Weinheim, Germany, 2010, pp. 697-727.

I.-D. Kim, E.-K. Jeon, S.-H. Choi,  D.-K.Choi, and H. L. Tuller, Electrospun SnO₂ Nanofiber Mats with  Thermo-compression Step for Gas Sensing Applications, J. Electroceramics, 25, 159-167 (2010)

K. Sahner, H.L. Tuller, Novel Deposition Techniques for Metal Oxides – Prospects for Gas Sensing, J. Electroceram., 24, 1385-3449 (2010) published online September 25, 2008. Feature article.

S. R. Bishop and H. L. Tuller, W. Higgins, A. Churilov, G. Ciampi, L. Cirignano, H. Kim, F. Olschner, J. Tower, K. Shah, Defects and Ionic Conductivity in Single Crystal TlBr, ECS Transactions, 28 – Ionic and Mixed Conducting Ceramics 7, Vancouver, Canada. Accepted for publication.

P. Somasundaran, M. Chin, U. T. Latosiewicz, H. L. Tuller, B. Barbiellini, V. Renugopalakrishnan. Nanoscience and Engineering for Robust Biosolar Cells, in Bionanotechnology: Global Prospects, Ed. David Reisner, CRC Press, Boca Raton, FL, USA, 2011. Accepted for publication.

H.L. Tuller, S.R. Bishop, Tailoring Material Properties through Defect Engineering, Chem. Lett.  39, 1226-1231 (2010). Invited Highlight Review.

N. G. Cho, G.C. Whitfield, D. J. Yang, H.-G. Kim, H. L. Tuller, and I.-D. Kim, Facile Synthesis of Pt-Functionalized SnO₂ Hollow Hemispheres and Their Gas Sensing Properties, J. Electrochem. Soc., Accepted

K.S. Brinkman, H. Takamura, H.L. Tuller and T. Iijima, The Oxygen Permeation Properties of Nano-Crystalline CeO₂ Thin Films, J. Electrochem. Soc., 157, B1852-B1857 (2010).

S. J. Litzelman, R. A. De Souza, B. Butz, and H. L. Tuller, M. Martin, and D. Gerthsen, Heterogeneously Doped Nanocrystalline Ceria Films by Grain Boundary Diffusion: Impact on Transport Properties, J. Electroceramics 22, 405-415 (2009)

J. Hiltunin, D. Seneviratne, R. Sun, M. Stolfi, H.L. Tuller, J. Lappalaninen and V. Lanto, Crystallographic and Dielectric Properties of Highly Oriented BaTiO₃ Thin Films: Influence of Oxygen Pressure Utilized During Pulsed Laser Deposition, J. Electroceramics, 22, 395-404 (2009)

J. Hiltunin, D. Seneviratne, H.L. Tuller, J. Lappalaninen and V. Lanto, Optical Properties of BaTiO₃ Thin Films: Influence of Oxygen Pressure Utilized During Pulsed Laser Deposition, J. Electroceramics, 22, 416-420 (2009)

J. L. Hertz, A. Rothschild, and H. L. Tuller, Highly Enhanced Electrochemical Performance of Silicon-Free Platinum – Yttria Stabilized Zirconia Interfaces, J. Electroceramics. 22, 428-435 (2009)

Figure 1: SEM, TEM, HR-TEM and SAED images of microsphere template InGaZnO3, illustrating a short range order of the spheres and amorphous phase of the sensor film.

Gas sensors are essential in the monitoring, control, and reduction of harmful emissions in the environment ^[1] .  Conductometric gas sensors based on semiconducting metal oxides are advantageous in many applications due to high sensitivity, manufacturability, and small size.  However, there are a number of drawbacks, including difficulty in control over the semiconductor/substrate interface, high power consumption, and reduced selectivity at high temperatures (300-400˚C) required for operation ^{[2] [3]} .  To address these challenges, chemical sensors comprising a wide array of material composition and morphology have been fabricated and investigated via high-throughput combinatorial test procedures.  A microsphere templating technique is employed in all device structures; it reduces the area of contact with underlying substrate and enhances interaction with the surrounding gases ^[4] .  Sensor performance has been characterized and optimized through controlled variation in the volume fraction of Pt nanoparticles that are co-deposited on the surface of SnO₂ and ZnO thin films.  In addition, novel sensors based on amorphous InGaZnO₄ have been investigated under a wide range of operating conditions and show promise for heightened sensitivity at reduced operating temperatures.  With a combination of rapid testing procedures and physical models of chemical and electronic processes involved in gas sensing, further advancements are anticipated in device sensitivity, selectivity, and response time.

1. F. Rock, N. Barsan, and U. Weimar ., “Electronic nose: Current status and future trends,” Chemical Reviews, vol. 108, no. 2, pp. 705-725, Jan. 2008.
2. K. J. Albert, N. S. Lewis, C.L. Schauer, G. A. Sotzing, S. E. Stitzel, T. P. Vaid, and D. R. Walt., “Cross-reactive chemical sensor arrays,” Chemical Reviews, vol. 100, no. 7, pp. 2595-2626, June 2000.
3. K. Wiesner, H. Knozinger,   M. Fleischer, H. Meixner, “Working mechanism of an ethanol filter for selective high-temperature methane gas sensors,” IEEE Sensors Journal, vol. 2, no. 4, pp. 354-359, Aug. 2002.
4. I. D. Kim, A. Rothschild, T.Hyodo, and H. L. Tuller,, “Microsphere templating as means of enhancing surface activity and gas sensitivity of CaCu₃Ti₄O₁₂ thin films,” Nano Letters, vol. 6, no. 2, pp. 193-198, Jan. 2006.

With continuously growing energy demands, new alternative energy solutions become essential. In order to achieve sustainability, efficient conversion and storage of solar energy are imperative ^{[1] [2]} . Photoelectrolysis utilizes solar energy to evolve hydrogen and oxygen from water, thereby enabling energy storage via chemical means. In this work, photoelectrodes are being investigated; they offer high conversion efficiency, long-term stability, and low cost. The focus is initially on semiconducting metal oxides in which the energy band, defect, and micro-structure are tuned to optimize optical absorption, charge transport, and reduced overpotentials. For high efficiency, a cobalt-based oxidation catalyst ^[3] is implemented at the photoelectrode. The electro-deposition kinetics of this catalyst are studied as part of this project to allow further insights into the catalysis mechanism.

1. N. S. Lewis and D. G. Nocera, “Powering the planet: Chemical challenges in solar energy utilization,” Proc. Natl. Acad. Sci. USA., vol. 103, pp. 15729-15735, 2006.
2. R. van de Krol, Y. Liang, and J. Schoonman, “Solar hydrogen production with nanostructured metal oxides,” J. Mater. Chem., vol. 18, pp. 2311-2320, 2008.
3. M. W. Kanan and D. G. Nocera, “In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co²⁺,” Science, vol. 321, pp 1072-1075, 2008.

Detection of high-energy radiation (e.g., γ-rays) is key in nuclear non-proliferation strategies.  When a wide-band gap semiconductor detector intercepts a γ-ray, electron–hole pairs are formed, resulting in an increase in electrical conductivity. This change in conductivity, or sensitivity, is maximized if the conductivity in the non-illuminated (dark) state is very low.  In order to achieve high sensitivity, current semiconductor technologies require device cooling to very low temperature, which adds to cost and reduces portability.  TlBr is an attractive detector material because of its low dark conductivity at room temperature as well as its high mass density, leading to higher radiation absorption.

In this project, we have characterized the dominant ionic conduction properties in TlBr using impedance spectroscopy.  Through doping techniques, we have determined that TlBr is primarily a Schottky-type ionic conductor, meaning that Tl and Br move through the material by ionic vacancy motion.  These measurements have led us to predict a doping strategy to minimize the dark conductivity, and we are collaborating with a local company (RMD) to implement this technology as well as developing it further by studying new TlBr-based material systems.  In addition, our newfound understanding of TlBr has led us to investigate novel device designs never before used in ionic conducting systems.

Solid oxide fuel cells (SOFCs) directly convert chemical to electrical energy with high efficiency and can operate using a wide variety of fuels from hydrogen to kerosene and gasified coal.  Many of the more advanced materials in SOFCs experience significant changes in oxygen content, or oxygen stoichiometry, during operation, resulting in changes in volume and elastic properties termed chemomechanics.  This chemical expansion, analogous to temperature-induced thermal expansion, is oxygen nonstoichiometry-induced and can have a negative impact on SOFC performance. In this project, we are studying the chemical expansion coefficient, elastic properties, and oxygen stoichiometry of thin film and bulk SOFC materials. Thin films are of particular interest since they allow for control of strain and increase the surface-to-volume ratio, particularly important for electrode performance.  The chemomechanical properties are being investigated using high temperature, atmosphere controlled nanoindentation, high resolution x-ray diffraction, dilatometry, impedance spectroscopy, and thermo-gravimetry techniques.

Combustion of fossil fuels, essential for electricity generation and vehicular propulsion, is generally incomplete, leading to harmful NOx, CO, and unburned hydrocarbons emissions.  Great progress in minimizing such emissions has relied on the operation of “three-way catalysts” (TWCs), which utilize a combination of precious metals and metal oxides with the ability to take up or release oxygen for reduction/oxidation of pollutants (NOx to N₂ plus CO and HC to CO₂ and H₂O, respectively).  In this project, we are investigating the rate at which oxygen storage materials (OSM) exchange oxygen with the atmosphere and the magnitude of oxygen they store with the aid of geometrically well-defined thin film structures. Impedance spectroscopy, Kelvin probe, and thermogravimetric methods are used to determine electrochemical performance and oxygen storage capabilities.  These properties, when correlated to actual TWC performance using a differential flow reactor, will allow for a more detailed understanding of performance criteria.