Figure 1: The sample arm and probe arm on the cantilever are decoupled. The aim is to reduce the effective thermal conductance of the sample arm as much as possible by using a low conductivity material and avoid bending by using a single layer. The probe arm is attached to the sample arm and made up of a bi-material layer to enable temperature dependent deflection and allow for optical detection.

Microfabricated cantilever beams are used in microelectromechanical systems (MEMS) for a variety of sensor and actuator applications. Bimaterial cantilevers accurately measure temperature change and heat flux with resolutions several orders of magnitude higher than those of conventional sensors such as thermocouples, semiconductor diodes, as well as resistance and infrared thermometers and thus have allowed new applications to emerge where other techniques are unable to probe ^{[1] [2] [3] [4] [5]} . An important limitation in these systems, however, is the deflection of the measurement sample and sensitivity limitation due to inherent bimaterial design constraints ^{[4] [5] [6] [7] [8]} . To this end, a measurement platform based on the picowatt sensitivity of optomechanical microcantilever sensors was developed in which the probe- and sample section of the cantilever are separated ^[9] . The bending of a custom-designed bimorph cantilever accurately allows the absolute amount of transferred heat to be extracted and temperature to be determined based on the response from thermal inputs while the sample remains immobilized (Figure 1). Optimally tailoring the material properties for the different cantilever sections enhances the measurement sensitivity by over an order of magnitude with respect to current commercial systems. The rigid sample section offers measurement versatility ranging from thermal radiation and conduction measurements to the characterization of material thermal conductivities and absorptivities in nearly identical configurations. This measurement platform for fundamental heat transfer measurements will considerably improve the current understanding of nanoscale energy transport and conversion and material characterization. The platform will also lead to advanced design guidelines for energy capture and conversion devices, in particular thermophotovoltaic cells, (solar) thermoelectric generators, and waste heat recovery heat exchangers.

1. J. K. Gimzewski, C. Gerber, E. Meyer, and R. R. Schlittler, “Observation of a chemical reaction using a micromechanical sensor,” Chemical Physics Letters, vol. 217, pp. 589-594, Jan. 1994.
2. J. R. Barnes, R. J. Stephenson, M. E. Welland, C. Gerber, and J. K. Gimzewski, “Photothermal spectroscopy with femtojoule sensitivity using a micromechanical device,” Nature, vol. 372, pp. 79-81, Nov. 1994.
3. J. R. Barnes, R. J. Stephenson, C. N. Woodburn, S. J. O’Shea, M. E. Welland, T. Rayment, J. K. Gimzewski, and C. Gerber, “A femtojoule calorimeter using micromechanical sensors,” Review of Scientific Instruments, vol. 85, pp. 3793-3798, Dec. 1994.
4. J. Lai, T. Perazzo, Z. Shi, and A. Majumdar, “Optimization and performance of high-resolution micro-optomechanical thermal sensors,” Sensors and Actuators A:  Physical, vol. 58, pp. 113-119, Feb. 1997.
5. J. Varesi, J. Lai, T. Perazzo, Z. Shi, and A. Majumdar, “Photothermal measurements at picowatt resolution using uncooled micro-optomechanical sensors,” Applied Physics Letters, vol. 71, pp. 306-308, July 1997.
6. S. Shen, A. Narayanaswamy, and G. Chen, “Surface phonon polaritons mediated energy transfer between nanoscale gaps,” Nano Letters, vol. 9, pp. 2909-2913, Aug. 2009.
7. E. Rousseau, A. Siria, G. Jourdan, S. Volz, F. Comin, J. Chevrier, and J.-J. Greffet, “Radiative heat transfer at the nanoscale,” Nature Photonics, vol. 3, pp. 514-517, Sept. 2009.
8. S. Shen, A. Henry, J. Tong, R. T. Zheng, and G. Chen, “Polyethylene nanofibers with very high thermal conductivities,” Nature Nanotechnology, vol. 5, pp. 251-255, Mar. 2010.
9. B. R. Burg, J. Tong, W.-C. Hsu, P. Sambegoro, A. Mavrokefalos, and G. Chen, “Decoupled Cantilever Arms for Optomechanical Thermal Measurements,” U. S. Patent Application 61/599547, February 16, 2012.

The cost of silicon solar cells has fallen precipitously in recent years, primarily as a result of manufacturing improvements, increasing scale, and decreasing profit margin.  Further cost reductions will depend upon technical advances that increase cell efficiency and/or minimize the variable costs associated with module production.  One strategy to lower the cost of silicon photovoltaic modules is to dramatically reduce the amount of material required in a cell from the current 180-μm standard to 10 μm or less.  However, since the absorption length for longer-wavelength photons (red and infrared) is significantly larger than this thickness, thin cells must be designed to trap photons in the absorber layer very effectively to yield competitive efficiencies.  Light-trapping in conventional wafer-based photovoltaics is well understood ^[1] , and light-trapping structures are integrated into virtually all commercially available solar cells.  However, the physical basis associated with light-trapping in thin-film silicon cells is quite different than that for conventional silicon photovoltaics, both because the thinness of the material physically limits the dimensions of light-trapping features and because light-trapping based on geometric optics is less effective for very thin materials.  In this work, simulations based on the transfer matrix method were developed to identify optimal surface structures for light trapping.  As shown in Figure 1, a variety of potential geometries offer significant enhancement over a planar silicon surface.  Of the structures that are modeled in this work, the one offering the best combination of light-trapping effectiveness and manufacturability is a two-dimensional periodic array of inverted pyramids on a sub-micrometer pitch ^{[2] [3]} , as in Figure 2.  Theoretical calculations suggest that a 10 μm silicon film textured in this way can absorb as effectively as a flat film 300 μm thick.  Demonstration versions on SOI substrates are currently being fabricated in the Microsystems Technology Laboratories at MIT.

1. P. Campbell and M. A. Green, “Light-trapping of pyramidally textured surfaces,” Journal of Applied Physics, vol. 62, no. 1, pp. 243-249, July 1987.
2. S. E.  Han and G. Chen, “Toward the Lambertian limit of light-trapping in thin nanostructured silicon solar cells,” Nano Letters, vol. 10, pp. 4692-4696, Oct. 2010.
3. S. E. Han, A. Mavrokefalos, M. S. Branham, and G. Chen, “Efficient light-trapping nanostructures in thin silicon solar cells,” Proc. of  SPIE:  Micro- and Nanotechnology Sensors, Systems, and Applications III, 2011, vol. 8031, p. 80310T.

Collaborators

• T. Borca-Tasciuc, Rensselaer Polytechnic Institute
• D. Broido, Boston College
• D. Cahill, University of Illinois at Urbana Champaign
• O. Delaire, Oak Ridge National Laboratory
• M. S. Dresselhaus, MIT
• S. Fan, Stanford
• E. A. Fitzgerald, MIT
• P. Jarillo-Herrero, MIT
• S. G. Kim, MIT
• W. P. King, University of Illinois at Urbana Champaign
• K. A. Nelson, MIT
• C. Opeil, Boston College
• G. Ramanath, Rensselaer Polytechnic Institute
• Z. Ren, Boston College
• C. Schuh, MIT
• Y. Shao-Horn, MIT
• D. J. Sing, Oak Ridge National Laboratory
• M. Soljacic, MIT
• E. N. Wang, MIT

Graduate Students

• Anurag Bajpayee, Research Assistant, MECHE
• Matthew Branham, Research Assistant, MECHE
• Jacky (Chia-Chi) Chen, Research Assistant, MECHE
• Vazrik Chiloyan, Research Assistant, MECHE
• Kimberlee Collins, Research Assistant, MECHE
• Wei-Chun Hsu, Research Assistant, MECHE
• Sohae Kim, Research Assistant, MECHE
• Daniel Kraemer, Research Assistant, MECHE
• Sangyeop Lee, Research Assistant, MECHE
• Bolin Liao, Research Assistant, MECHE
• Maria Luckyanova, Research Assistant, MECHE
• Kenneth McEnaney, Research Assistant, MECHE
• Jonathan Mendoza, Research Assistant, MECHE
• George Ni, Research Assistant, MECHE
• Poetro Sambegoro, Research Assistant, MECHE
• Shuang Tang, Research Assistant, MECHE (co-supervised with Prof. M. Dresselhaus)
• Zhiting Tian, Research Assistant, MECHE
• Jonathan Tong, Research Assistant, MECHE
• James (Jinjian) Wang, Research Assistant, MECHE
• Jenny Wang, Research Assistant, MECHE
• Lee Weinstein, Research Assistant, MECHE
• Lingping Zeng, Research Assistant, MECHE

Visitors and Postdoctoral Associates

• Svetlana Boriskina, Postdoctoral Associate
• Brian Burg, Postdoctoral Associate
• Keivan Esfarjani, Research Scientist
• Sang Eon Han, Postdoctoral Associate
• Kazuki Ihara, Visiting Scholar
• Zhichun Liu, Visiting Scholar
• Lei Ma, Visiting Student
• Anastassios Mavorkefalos, Postdoctoral Associate
• Selcuk Yerci, Postdoctoral Associate
• Mona Zebarjadi, Postdoctoral Associate
• Mengyun Zhang, Visiting Student

Support Staff

• Tuyet-Mai Ha Hoang, Administrative Assistant II
• Mary Ellen Sinkus, Administrative Assistant Director of Finance and Administration

Publications

Y. Q. Zhang, M. S. Dresselhaus, Y. Shi, Z. F. Ren, and G. Chen, “High thermoelectric figure of merit in Kondo insulator nanowires at low temperatures,” Nano Letters, vol. 11, pp. 1166-1170, January 2011.

A. Bajpayee, T. F. Luo, A. Muto, and G. Chen, “Very low temperature membrane-free desalination by directional solvent extraction,” Energy and Environmental Science, vol. 4, pp. 1672-1675, March 2011.

R. T. Zheng, J. Gao, J. Wang, and G. Chen, “Reversible temperature regulation of electrical and thermal conductivity using liquid-solid phase transitions,” Nature Communications, vol. 2, pp. 289, April 2011.

D. Kraemer, B. Poudel, H. P. Feng, J. C. Caylor, B. Yu, X. Yan, Y. Ma, X. W. Wang, D. Wang, A. Muto, K. McEnaney, M. Chiesa, Z. F. Ren, and G. Chen, “High-performance flat-panel solar thermoelectric generators with high thermal concentration,” Nature Materials, vol. 10, pp. 532-538, May 2011.

M. Zebarjadi, G. Joshi, G. H. Zhu, B. Yu, A. Minnich, Y. C. Lan, X. W. Wang, M. Dresselhaus, Z. F. Ren, and G. Chen, “Power factor enhancement by modulation doping in bulk nanocomposites,” Nano Letters, vol. 11, pp. 2225-2230, May 2011.

H. P. Feng, T. Poudel, B. Yu, S. Chen, Z. F. Ren, and G. Chen, “Nanoparticle-enabled selective electrodeposition,” Advanced Materials, vol. 23, pp. 2454-2459, June 2011.

A. J. Minnich, J. A. Johnson, A. J. Schmidt, K. Esfarjani, M. S. Dresselhaus, K. A. Nelson and G. Chen, “Thermal conductivity spectroscopy technique to measure phonon mean free paths,” Physical Review Letters, vol. 107, pp. 095901, August 2011.

M. Zebarjadi, K. Esfarjani, M. S. Dresselhaus, Z. F. Ren, and G. Chen, “Perspectives on thermoelectrics: from fundamentals to device applications,” Energy and Environmental Science, vol. 5, pp. 5147-5162, January 2012.