Nicholas P. Sergeant

I am a Ph.D. Candidate in the group of Shanhui Fan at the electrical engineering department at Stanford University. I have a specific interest in renewable energy resources. My research focuses on nanophotonic design for improved solar energy conversion. During my PhD I have applied nanophotonic approaches to solar-selective coatings for application in Concentrated Solar Thermal (CST) and ThermoPhotoVoltaics (TPV) as well as transparent conducting electrodes in thin-film organic photovoltaics (OPV) .

Spectral tuning of absorption
This research project focuses on spectrally selective absorbers and emitters for concentrated solar thermal (CST) and thermophotovoltaic (TPV) applications. For both applications, spectral tuning of the absorber surface is important. An ideal solar absorber operating at elevated temperature has high absorptivity over the solar spectrum, while suppressing parasitic IR thermal emission from its surface. We have studied the use of aperiodic multilayer coatings both on planar as well as on nanostructured substrates and shown desired spectral selectivity. Based on our modeled results, our optimized aperiodic multilayer stacks have the potential to outperform current commercially available solar thermal coatings. Our work on solar thermal absorbers using aperiodic coatings on subwavelength V-groove gratings was featured in SPIE newsroom. This work is funded by the Global Climate and Energy Project (GCEP) at Stanford University.

(left) Spectral absorptivity of ideal solar-selective absorber for operation at 720K relative to the normalized power spectral density of the sun (AM1.5G) and a blackbody at 720K. (right) Modeled spectral absorptivity at normal incidence for aperiodic metal-dielectric stacks optimized for operation at 720K. The spectral selectivity improves as the number of layers in the coating increases from 5 to 11 layers.

Absorption enhancement in thin film photovoltaics

This project focuses on absorption enhancement in thin film organic photovoltaic cells, a technology that has an enormous potential to reduce the cost of solar electricity. In thin-film PV the use of thin photoactive layers leads to incomplete optical absorption unless light-trapping strategies are employed. Several light trapping approaches are investigated to enhance absorption efficiency without sacrificing charge carrier collection efficiency. For photovoltaics, it is important to both spectrally and spatially tune the absorption inside the photoactive region to optimize the exciton creation and to minimize parasitic losses. Here multilayer metal-dielectric stacks are investigated to enhance absorption efficiency in organic solar cells by creating a resonant cavity effect. We have experimentally shown that these multilayer stacks can at the same time serve as an transparent contact as well as enhance photon harvesting resulting in improved power conversion efficiency. This has been demonstrated both on glass as well as on flexible substrates. In addition these multilayer anodes offer attractive way towards multi-terminal multi-junction organic solar cells. This work is done in collaboration with Southwall Technologies and imec. The work is partially funded by the Center for Advanced Molecular Photovoltaics (CAMP) at Stanford University.

:: | Journal Publications

N. P. Sergeant, C.-E. Chang, P. Peumans and S. Fan, High temperature solar-selective absorption in aperiodic multilayer coatings, in preparation
N. P. Sergeant, B. Niesen, A. S. Liu, L. Boman, C. Stoessel, P. Heremans, P. Peumans, B. P. Rand and S. Fan, Roll-to-roll fabricated Zn_xSn_yO_z/Ag/In_xSn_yO_z electrodes for improved light harvesting in thin-film organic solar cells, submitted
N. P. Sergeant, A. Hadipour, B. Niesen, D. Cheyns, P. Heremans, P. Peumans and B. P. Rand, Design of transparent anodes for resonant cavity enhanced light harvesting in organic solar cells, Advanced Materials, Vol. 24, Issue 6, pp. 728–732 (2012), (.pdf)
S. B. Mallick, N. P. Sergeant, J.-Y. Lee, M. Agrawal and P. Peumans, Coherent light trapping in thin-film photovoltaics, MRS Bulletin Vol.36, Issue 6, pp. 453-460 (2011), (.pdf) [Joint First Author]
N. P. Sergeant, M. Agrawal and P. Peumans, High performance solar-selective absorbers using sub-wavelength gratings, Optics Express, Vol. 18, Issue 6, pp. 5525-5540 (2010), (.pdf)
N. P. Sergeant, O. Pincon, M. Agrawal and P. Peumans, Design of wide-angle solar-selective absorbers using aperiodic metal-dielectric stacks, Optics Express, Vol. 17, Issue 25, pp. 22800-22812 (2009), (.pdf)
B. Verreet, N. P. Sergeant, D. M. Negrete, M. Torstensson, D. Winkler and A. Yurgens, Self-consistent estimations of heating in stacks of intrinsic Josephson junctions, Superconductor Science and Technology, Vol. 20, S48-S53 (2007), (.pdf)

:: | Conference Contributions

[Oral] N. P. Sergeant, A. Hadipour, B.Niesen, D. Cheyns, P. Heremans, P. Peumans, B. P. Rand and S. Fan, The Origin of Enhanced Light Harvesting in Organic Solar Cells with ITO-Free MoO₃/Ag/MoO₃ Transparent Anodes, MRS 2011 Fall Meeting, J9.6: Organic Solar Cells, Boston, December 1, 2011
[Poster] N. P. Sergeant, C.-E. Chang, P. Peumans and S. Fan Nanophotonic Approaches for Selective Absorption Enhancement , Poster Session, GCEP Annual Symposium 2011, Stanford University, October 4-5 2011 [Outstanding Poster Award]
[Oral] N. P. Sergeant, B. P. Rand, P. Heremans and P. Peumans, MoO₃/Ag/MoO₃ Transparent Electrodes for Coherent Light Trapping in Thin Film Organic Solar Cells, MRS 2011 Spring Meeting, G7.2: Transparent Conducting Oxides II: Photovoltaics, San Francisco, April 28, 2011
[Poster] C.-E. Chang, N. P. Sergeant, O. Pincon, M. Agrawal and P. Peumans, Nanophotonic Approaches for Selective Emission and Absorption Enhancement, Poster Session, GCEP Annual Symposium 2010 , Stanford University, September 28-29 2010
[Oral] N. P. Sergeant, A. S. Liu, B. Niesen, C. Stoessel, J. Kozak, L. Boman, B. Rand, P. Heremans, M. D. McGehee, P. Peumans, Multilayer transparent electrodes for flexible thin-film organic solar cells, 8th International Conference on Electroluminescence and Organic Optoelectronics, ICEL2010 , Ann Arbor, October 17-21, 2010
[Oral] N. P. Sergeant, A. S. Liu, M. W. Rowell, W. Gaynor, C. Stoessel, J. Kozak, L. Boman, M. D. McGehee, P. Peumans, Multilayer transparent electrodes that coherently trap light in thin-film organic solar cells, SPIE Optics + Photonics 2010, San Diego, August 3 2010
P. Vereecken, G. Hautier, N. P. Sergeant, and J. D'Haen, Surface-layer Mediated Electrodeposition of Indium-based Nanostructure, 218th ECS Meeting (Las Vegas) (October 2010)
[Oral] N. P. Sergeant, M. Agrawal and P. Peumans, Design of selective coatings for solar thermal applications using sub-wavelength metal-dielectric structures, Optical Modeling and Measurements for Solar Energy Systems III, SPIE Optics + Photonics 2009, San Diego, August 2 2009
[Paper] N. P. Sergeant, M. Agrawal and Peter Peumans, Design of selective coatings for solar thermal applications using sub-wavelength metal-dielectric structures, Proc. of SPIE Vol. 7410, 74100C-1 (2009), (.pdf)
[Poster] N. P. Sergeant, M. Agrawal and P. Peumans, Solar-Selective Absorbers using Sub-Wavelength Gratings , Poster Session, GCEP Annual Symposium 2009, Stanford University, October 1 2009
[Poster] N. P. Sergeant, O. Pincon and P. Peumans, NanoPhotonic Approaches to ThermoPhotoVoltaics and Solar Thermal ,Poster Session, GCEP Annual Symposium 2008, Stanford University, October 2 2008

:: | Other

[Invited Lecture] N. P. Sergeant, Thermophotovoltaics, EE237 - Solar Energy Conversion, Stanford University, Electrical Engineering Department, March 3 2011
[Invited Lecture] N. P. Sergeant, Concentrated Solar Thermal Energy Conversion, EE237 - Solar Energy Conversion, Stanford University, Electrical Engineering Department, March 1 2011
[Invited Lecture] N. P. Sergeant, Concentrated Solar Energy Conversion, EE237 - Solar Energy Conversion, Stanford University, Electrical Engineering Department, March 9 2010
N. P. Sergeant and P. Peumans, High-performance absorbers for solar thermal applications, SPIE newsroom, Solar & Alternative Energy, 20 November 2009 . (.pdf)

:: | Fellowships

Leland F. and Margaret Perry Johnson Fellowship, SOE, Stanford University (2007)

Francqui Fellowship, Belgian American Educational Foundation (BAEF) (2007)

Fulbright Fellowship (2007) (Declined)

:: | Work Experience

McKinsey & Company, Brussels, Belgium, Summer Associate, Jun 2011 - Aug 2011.

imec, Leuven, Belgium, Visiting Scholar, Organic Photovoltaics, Aug 2010 - Sep 2010.

imec, Leuven, Belgium, Summer Intern, Electrodeposition of semiconductor nanowires, Jun 2006 - Sep 2006.