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Review of Near-field Radiation for

Thermophotovoltaics

Majid al-Dosari

12/14/2011

Thermophotovoltaics (TPVs) are solid state devices that convert ther-

mally generated radiation into electrical current. Any TPV device consists

of a thermal emitter and a photovoltaic cell separated by a gap. TPVs were

conceived in the 1950s but suffered from very low efficiency. The advent

of low band-gap photovoltaics in the 1990’s renewed interest in TPVs in

hope of increasing their efficiency. In the 2000s, near-field radiation effects

have been explored to further increase efficiency and power output. Much

of the research on near-field effects has been focused on surface phonon po-

laritons since they can be excited thermally and can transfer energy in a

narrow spectral band. The radiation transfer between (bulk) surfaces, thin

films, photonic crystals, and metamaterials have been analyzed. The re-

search seems to be headed towards analyzing arbitrary geometries and more

realistic modeling.

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Introduction

Thermophotovoltaics (TPVs) are solid state devices that convert thermally

generated radiation to heat. All TPV devices have a heated emitter and a

photovoltaic cell to recieve the radiation and convert the radiation to elec-

trical current. Conceptualized in the 1950’s, thermophotovoltaic develop-

ment was slow and suffered from very low efficiency until the 1990’s with the

emergence of low-bandgap photovoltaics [21]. This allowed the characteristic

wavelength of blackbody radiation to be within that of the band gap energy

corresponding to temperatures from about 1000 K to 1500 K (.5 to .7 eV)

thereby increasing efficiency and power output. In 2001, DiMatteo [5] was

first to exploit near-field effects in a TPV device although near-field effects

had been known prior to 2001. This was a break from research efforts that

focus on tuning far-field emission through complicated optics, use of exotic

materials, and patterning of surfaces.

Near-field effects are due to the wave nature of electromagnetic radiation

that manifest at distances around the wavelength of the radiation. Maxwell’s

equations predict an exponentially decaying electric field at surfaces. So, the

proximity of the emitter and receiver allow for the coupling of evanescent

waves thereby enhancing the heat transfer over that predicted by Planck’s

law. The evanescent waves appear from total internal reflection or surface

polaritons. Surface plasmon plasmon polaritons (SPPs) are the hybrid mode

of collective oscillations of free electrons and an electromagnetic field ap-

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pearing mainly in metals. Surface phonon polaritons (SPhPs) are the hybrid

mode of collective oscillations of lattice vibrations and an electromagnetic

field appearing mainly in polar materials. Research efforts focus on SPhPs

since they can be excited thermally in the infrared, while SPPs appear gen-

erally in the ultraviolet. Also, interference effects can arise from reflections

within the gap which are sensitive to the separation distance.

So, understanding near-field effects is crucial to optimizing near-field ther-

mophotovoltaic devices. Efforts to understand near-field radiation are thefocus of the following literature review.

Literature Review

Near-field radiation study is a nascent field with most progress made only in

the previous decade. The following literature review attempts to highlight the

progress of understanding near-field radiation as it pertains to thermophoto-

voltaics. Except where mentioned, all the studies here are theoretical and are

exhibited roughly chronologically. Parallel plates are the general geometric

configuration although a TPV device may be cylindrical.

Rytov [24] first described thermally-generated radiation in terms of fluc-

tuational electrodynamics. A random current is added to Ampere’s law to

represent thermally agitated charges. Green’s function is found to solve

Maxwell’s equations particular to the geometry of the problem. The Green’s

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function may be interpreted as a transfer function that relates the contribu-

tion of as each current source contributes to the electric field. The average

strength of emission from electromagnetic sources is related to its tempera-

ture through the fluctuation-dissipation theorem. Except for Ref. [23], all

theoretical work shown relies on the work of Rytov.

Mulet [18] used fluctuational electrodynamics to study the radiative heat

transfer between two surfaces that support surface modes: SiC and glass.

The radiative heat transfer is enhanced by several orders of magnitude whenthe surfaces support resonant surface waves. Mulet is often cited for show-

ing that the transfer is nearly monochromatic which is advantageous for

thermophotovoltaics. It is also shown that the density of photonic states

increases as inverse distance cubed from the surface. The procedure laid out

is used subsequent studies.

Similarly, Narayanaswamy [19] analyzed heat transfer between media that

support surface plasmon polaritons: BN, SiC, and BC. They also show that

even when one side was a photovoltaic, the transfer was still enhanced (al-

though less).

Fu [12] analyzed the effect of doping silicon on near-field radiation. A

fairly comprehensive model for the complex permittivity of silicon was used

taking into consideration effects of temperature and dopant concentration

on electron-lattice scattering and photon absorption, and the properties of

holes (in addition to electrons). The increased flux was observed at lower

frequencies where black body exchange becomes insignificant.

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Laroche [17] provided a quantitative model of a TPV device in the near-

field. The power and efficiency of a (broadband) tungsten and a fictitious

quasimonochromatic source matched to a GaSb cell were compared. The

power enhancement was by a factor of 25 and 35 respectively. The efficiency

was 27% and 35% respectively representing an enhancement as well due to the

concentration of frequencies around the bandgap. However, the enhancement

is not strictly monotonic with decreasing distance. Furthermore, although

respectable, the near-field efficiency for the quasimonochromatic source waslower than expected because the radiative transfer was broadened by the

presence of the GaSb cell in the near-field.

Biehs [4] showed that in thin metal films, the radiative intensity is maxi-

mized for a certain film thicknesses due to Fabry-Perot-like reflections inside

the film.

Francoeur [9] studied the emission from a 10 nm thick SiC layer on a

dielectric transmitting to bulk SiC. The radiative flux was 3.3 times larger

than that of a bulk emitter due to a splitting of the resonant frequency

into symmetric and anti-symmetric modes thereby increasing the channels

available for heat transfer.

Hu [15] provides one of few experimental verifications of near-field radia-

tion enhancement. The heat exchanged between two glass plates separated

by a micron-sized gap showed reasonable agreement with theory. Before that,

Hanamura [14] reported near-field electrical enhancement in a TPV device

with gaps in the micron range using a GaSb photocell.

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Park [22] calculated the performance of a TPV system rigorously by con-

sidering the thermal radiation absorption in different regions of a In.18Ga.82Sb

photovoltaic. Stronger absorption of thermal radiation near the surface is

calculated as the gap decreases decreasing device efficiency to around 20%

(in contrast to assuming 100% quantum efficiency as in Ref. [17]). The

component of the wavevector parallel to the plates at which maximum heat

transfer occurs changes with separation distance which changes the penetra-

tion depth. This suggests an optimal separation distance between the emitterand the PV cell. Furthermore, a back-reflector is suggested to recycle unused

photons back to the emitter.

Ben-Abdallah [3] calculated the heat transfer between isolated thin films

supporting SPhPs.

Fu [13] showed that enhancement by adding thin films of SiC or SiO2

on a substrate does not apply when the receiver is a metal. The near-field

enhancement between metals relies on TE-polarization while SPhP relies on

TM-polarization.

Francoeur [11] provides a comprehensive and general procedure for cal-

culating near-field thermal radiation in layered media. It is shown that a

SiC film emitter of about 1 m m gives the same spectral distribution as a

bulk emitter. In a following paper [10], more explanation for the radiative

transfer in thin films is provided by analyzing the local density of photonic

states (LDoS) within the gap between the thin films. Multiple SPhP cou-

pling was observed which explains enhanced radiation transfer. However, it

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was observed that the LDoS at the surface of the emitter is dominated by

SPhPs of large wavevectors parallel to the surface the are not affected by

the receiving medium. There is also a note on the limit of the macroscopic

electrodynamics given the nanometric scale of the films and gaps.

Basu [1] improved on the dielectric function used by Fu [12] by using

ionization and mobility models better suited for heavily doped silicon. The

lateral shift in the direction of energy transfer was calculated in order find a

plate size that is effectively infinite laterally. Further elaboration on energytransfer direction is given in Ref. [2].

Francoeur [8] ogranizes near-field heat transfer between thin films into

dimensionless (film thickeness divided by gap width) regimes. Each regime

has a different inverse gap width dependence.

Furthermore, Francoeur [25] modeled a tungsten and In.18Ga.82Sb TPV

system rigorously (in a way similar to Park [22]) but evaluated the effect

of higher temperature on the photocell. It was found that the the power

output and the conversion efficiency of the system are respectively 5.83 ×

105 Wm − 2 and 24.8% at 300 K, whereas these values drop to 8.09 × 104

Wm − 2 and 3.2% at 500 K. A ~105 Wm − 2 convective cooling coefficient

is required at the photocell in order to keep it at room temperature. The

heating comes from the broadband nature emission of tungsten. The point

of the paper is to comment on the feasibility of implementing such cooling

on a device.

Very recent research efforts are exploring near-field effects in metama-

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terials. Joulain [16] was first to consider near-field heat transfer between

metamaterials. It was found that the magnetic response of the materials

give additional channels for energy transfer. Zheng [26] added that the en-

hancement may not hold for if one side is different. Francoeur [7] also found

enhanced transfer in metameterials consisting of a pottasium bromide host

with SiC spherical inclusions. It was found that SP from TM-polarizations

are not affected by the size of the inclusions while SP from TE polarization

require at least a one micron radius to be excited. Nefedov [20] suggested thatby adding a metamaterial in the gap, enhanced transfer can occur in relatively

larger gaps. The metamaterial converts evanescent waves into propagating

waves.

The paper by Rodriguez [23] models near field heat transfer directly be-

tween photonic crystals by introducing a random current in finite-difference

time-domain simulation. They find a trade-off between spectral selectivity

and near-field enhancement. Also, they found that the heat transfer can be

enhanced in certain symmetric configurations.

An attempt has been made to exhibit the literature exhaustively but only

20 to 30 papers have been produced in the last decade that concern near-field

radiation for TPV’s. Many of the papers are authored by G. Chen, M. Pinar

Menguc, Z. Zhang, and their students. Perhaps the field is best introduced

in Francour’s PhD thesis [6]. There is a progression of sophistication of

analyzed systems starting with bulk materials, then on to thin-films, and

more recently photonic crystals and metamaterials.

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Research Direction

The analysis techniques used in all papers (except for Ref. [23]) are math-

ematically involved while only applicable to simple planar geometries. Fur-

thermore, this technique requires the specification of temperature of the re-

ceiver to determine the heat flux. Ideally the calculation should only involve

input heat flux or the temperature of the emitter as an input to the problem.

So it is expected that there will be modeling techniques that address these

problems. Also, the engineering problem of optimizing a TPV device will be

addressed.

References

[1] S. Basu, B. J. Lee, and Z. M. Zhang. Near-Field Radiation Calculated

With an Improved Dielectric Function Model for Doped Silicon. Journal

of Heat Transfer , 132(2):023302, 2010.

[2] S. Basu, L.P. Wang, and Z.M. Zhang. Direct calculation of energy

streamlines in near-field thermal radiation. Journal of Quantitative Spec-

troscopy and Radiative Transfer , 112(7):1149–1155, May 2011.

[3] Philippe Ben-Abdallah, Karl Joulain, Jeremie Drevillon, and Gilberto

Domingues. Near-field heat transfer mediated by surface wave hybridiza-

tion between two films. Journal of Applied Physics , 106(4):044306, 2009.

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[4] S.-a. Biehs, D. Reddig, and M. Holthaus. Thermal radiation and near-

field energy density of thin metallic films. The European Physical Journal

B , 55(3):237–251, February 2007.

[5] R. S. DiMatteo, P. Greiff, S. L. Finberg, K. a. Young-Waithe, H. K. H.

Choy, M. M. Masaki, and C. G. Fonstad. Enhanced photogenera-

tion of carriers in a semiconductor via coupling across a nonisothermal

nanoscale vacuum gap. Applied Physics Letters , 79(12):1894, 2001.

[6] Mathieu Francoeur. Near-field radiative transfer : thermal radiation ,

generation and optical characterization . PhD thesis, University of Ken-

tucky, 2010.

[7] Mathieu Francoeur, Soumyadipta Basu, and Spencer J Petersen. Elec-

tric and magnetic surface polariton mediated near-field radiative heat

transfer between metamaterials made of silicon carbide particles. Optics

express , 19(20):18774–88, September 2011.

[8] Mathieu Francoeur, M. Mengüç, and Rodolphe Vaillon. Coexistence

of multiple regimes for near-field thermal radiation between two layers

supporting surface phonon polaritons in the infrared. Physical Review

B , 84(7):1–9, August 2011.

[9] Mathieu Francoeur, M. Pinar Mengüç, and Rodolphe Vaillon. Near-

field radiative heat transfer enhancement via surface phonon polaritons

coupling in thin films. Applied Physics Letters , 93(4):043109, 2008.

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[10] Mathieu Francoeur, M. Pinar Mengüç, and Rodolphe Vaillon. Local

density of electromagnetic states within a nanometric gap formed be-

tween two thin films supporting surface phonon polaritons. Journal of

Applied Physics , 107(3):034313, 2010.

[11] Mathieu Francoeur, M. Pinar Mengüç, and Rodolphe Vaillon. Solution

of near-field thermal radiation in one-dimensional layered media using

dyadic Green’s functions and the scattering matrix method. Journal of

Quantitative Spectroscopy and Radiative Transfer , 110(18):2002–2018,

December 2009.

[12] C Fu and Z Zhang. Nanoscale radiation heat transfer for silicon at

different doping levels. International Journal of Heat and Mass Transfer ,

49(9-10):1703–1718, May 2006.

[13] C.J. Fu and W.C. Tan. Near-field radiative heat transfer between two

plane surfaces with one having a dielectric coating. Journal of Quanti-

tative Spectroscopy and Radiative Transfer , 110(12):1027–1036, August

2009.

[14] Katsunori Hanamura and Kazuhiko Mori. Nano-gap TPV Generation

of Electricity through Evanescent Wave in Near-field Above Emitter

Surface. In AIP Conference Proceedings , volume 890, pages 291–296.

Aip, 2007.

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[15] Lu Hu, Arvind Narayanaswamy, Xiaoyuan Chen, and Gang Chen.

Near-field thermal radiation between two closely spaced glass plates

exceeding Planck’s blackbody radiation law. Applied Physics Letters ,

92(13):133106, 2008.

[16] Karl Joulain and Jérémie Drevillon. Noncontact heat transfer between

two metamaterials. Physical Review B , 81(16):1–7, April 2010.

[17] M. Laroche, R. Carminati, and J.-J. Greffet. Near-field thermophoto-

voltaic energy conversion. Journal of Applied Physics , 100(6):063704,

2006.

[18] J.P. Mulet, K. Joulain, R. Carminati, and J.J. Greffet. Enhanced radia-

tive heat transfer at nanometric distances. Microscale thermophysical

engineering , 6(3):209–222, 2002.

[19] Arvind Narayanaswamy and Gang Chen. Surface modes for near field

thermophotovoltaics. Applied Physics Letters , 82(20):3544, 2003.

[20] I Nefedov. Giant radiation heat transfer through the micron gaps. Arxiv

preprint arXiv:1103.0407 , pages 1–4, 2011.

[21] Robert E Nelson. A brief history of thermophotovoltaic development.

Semiconductor Science and Technology , 18(5):S141–S143, May 2003.

[22] K Park, S Basu, W King, and Z Zhang. Performance analysis of

near-field thermophotovoltaic devices considering absorption distribu-

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tion. Journal of Quantitative Spectroscopy and Radiative Transfer ,

109(2):305–316, January 2008.

[23] Alejandro W Rodriguez, Ognjen Ilic, Peter Bermel, Ivan Celanovic,

John D Joannopoulos, Marin Soljačić, and Steven G Johnson.

Frequency-Selective Near-Field Radiative Heat Transfer between Pho-

tonic Crystal Slabs: A Computational Approach for Arbitrary Geome-

tries and Materials., September 2011.

[24] SM Rytov. Theory of Electric Fluctuations and Thermal Radiation (Air

Force Cambridge Research Center, Bedford, Mass., 1959).

[25] Nanoscale-gap Thermophotovoltaic. Thermal Impacts on the Perfor-

mance of Nanoscale-Gap Thermophotovoltaic Power Generators. En-

ergy , 26(2):686–698, 2011.

[26] ZhiHeng Zheng and YiMin Xuan. Near-field radiative heat transfer

between general materials and metamaterials. Chinese Science Bulletin ,

56(22):2312–2319, July 2011.

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