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Solar Energy Materials & Solar Cells 91 (2007) 1599–1610

www.elsevier.com/locate/solmat

Improving solar cell efficiency using photonic band-gap materials

Marian Florescua,b,�, Hwang Leea, Irina Puscasuc, Martin Prallec, Lucia Florescua,b,David Z. Tingb, Jonathan P. Dowlinga

aDepartment of Physics and Astronomy, Hearne Institute for Theoretical Physics, Louisiana State University, 202 Nicholson Hall,

Baton Rouge, LA 70803, USAbJet Propulsion Laboratory, California Institute of Technology, Mail Stop T1714 106, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

cIon Optics Inc., 411 Waverley Oaks Rd. Suite 144, Waltham, MA 02452, USA

Received 31 October 2006; received in revised form 2 May 2007; accepted 2 May 2007

Available online 29 June 2007

Abstract

The potential of using photonic crystal structures for realizing highly efficient and reliable solar-cell devices is presented. We show that

due their ability to modify the spectral and angular characteristics of thermal radiation, photonic crystals emerge as one of the leading

candidates for frequency- and angular-selective radiating elements in thermophotovoltaic devices. We show that employing photonic

crystal-based angle- and frequency-selective absorbers facilitates a strong enhancement of the conversion efficiency of solar cell devices

without using concentrators.

r 2007 Elsevier B.V. All rights reserved.

Keywords: Photonic band-gap materials; Thermophotovoltaics; Solar cells

1. Introduction

Photovoltaic (PV) solar energy conversion systems (orsolar cells) are the most widely used power systems.However, these devices suffer of very low conversionefficiency. This is due to the wavelength mismatch betweenthe narrow wavelength band associated with the semicon-ductor energy gap and the broad band of the (blackbody)emission curve of the Sun. The power loss is associatedwith both long-wavelength photons that do not haveenough energy to excite electron–hole pairs across theenergy gap (leading to a 24% loss in silicon, for instance)and short-wavelength photons that excite pairs with energyabove the gap, which thereby waste the extra kinetic energyas heat (giving a 32% loss in silicon). The efficiency of thethermophotovoltaic (TPV) system may be increased byrecycling the photons with frequency larger than the solarcell band-gap frequency, by using a spectrally dependent

e front matter r 2007 Elsevier B.V. All rights reserved.

lmat.2007.05.001

ing author. Jet Propulsion Laboratory, California Institute

Mail Stop T1714 106, 4800 Oak Grove Drive, Pasadena,

.

ess: Marian.Florescu@jpl.nasa.gov (M. Florescu).

coupling between the absorber and the cell (Fig. 1).However, any approach to solar-cell efficiency improve-ment that does not address this fundamental wavelength-band mismatch, can achieve at most around 30% efficiency[1]. Moreover, this can be achieved only for concentratedradiation, which requires an additional optical device,which is not desirable in applications where the mass is acritical concern.This article outlines novel approaches to the design of

highly efficient solar cells using photonic band-gap (PBG)materials [2,3]. These are a new class of periodic materialsthat allow precise control of all electromagnetic waveproperties [4–6]. A PBG occurs in a periodic dielectric ormetallic media, similarly to the electronic band gap insemiconductor crystals. In the spectral range of the PBG,the electromagnetic radiation light cannot propagate. Theability to tailor the properties of the electromagneticradiation in a prescribed manner through the engineeringof the photonic dispersion relation enables the design ofsystems that accurately control the emission and absorp-tion of light. This gives rises to new phenomena includingthe inhibition and enhancement of the spontaneousemission [3], strong localization of light [2], formation of

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Fig. 1. Schematic of a TPV energy conversion scheme. An intermediate

absorber is heated by the Sun’s thermal radiation. The photovoltaic (PV)

cell is illuminated by radiation from emitter transmitted by a filter.

Fig. 2. Spectral funneling of the thermal radiation by photonic crystals.

By designing a photonic band gap in prescribed frequency region of the

photonic crystal emission spectrum, the structure becomes unable to

radiate at these frequencies and the corresponding energy is re-radiated in

the allowed spectral range. As a consequence, the intensity of the

blackbody emission at these frequencies increases, and the photonic

crystal emitter radiates the same power as it would a blackbody

maintained at a higher temperature.

M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–16101600

atom–photon bound states [7], quantum interferenceeffects in spontaneous emission [8], single atom andcollective atomic switching behavior by coherent resonantpumping, and atomic inversion without fluctuations [9].These remarkable phenomena have attracted a consider-able interest for important technological applications, suchas low-threshold micro-lasers [10,11], ultra-fast all-opticalswitches, and micro-transistors [12–14].

The modifications of the spontaneous emission rate ofatoms inside the photonic crystal structure determine,in turn, important alterations of thermal radiative pro-cesses. Thermal radiation is just spontaneous emissionthermally driven and in thermal equilibrium with itsmaterial surroundings. In 1999, Cornelius and Dowlingsuggested the use of PBG materials for the modification ofthermal emission [15]. They explored two alternativeapproaches: a method based on a passive lossless PBGthin-film coating over the absorber, and an approach whichuses an active PBG material made out of an absorptivemedium. Thermal emission modification has been experi-mentally demonstrated in 2000, using a thin slab of 3Dphotonic crystal on a silicon substrate [16]. Pralle et al.demonstrated a thermally excited, narrow-band, mid-infrared source using a PBG technique [17]. Recently,researchers at Sandia Labs demonstrated a high-efficiencyTPV system using tungsten photonic crystals [18–20].These studies suggest that by optimizing the coupling ofthe multi-mode radiation field of a PBG material and aspatially extended collection of atomic or electronicemitters, it is possible to achieve dramatic modificationsof Planck’s blackbody radiation spectrum [15,21]. Inthe PBG spectral range the thermal emission of radiationis strongly suppressed, whereas for specific frequencies inthe allowed photonic bands, that correspond to transmis-sion resonances of the photonic crystal, the thermal

emission of radiation is resonantly enhanced up to theblack-body limit.The ability of the photonic crystals to funnel the thermal

radiation into a prescribed spectral range is illustrated inFig. 2, which shows a comparison between the intensityemitted by a photonic crystal sample when electricallyheated, which reaches a temperature of 420� when theelectrically heated with an input power of 135mW (blackcurve), and two blackbody systems, one kept at the sametemperature as the photonic crystal at the expense of usinga higher input power (315mW) and a second one exposedat the same input power as the photonic crystal sample, buthaving a lower temperature ð273:4�Þ. We notice in the caseof the photonic crystal sample that by eliminating theemission in certain frequency bands (corresponding to thespectral range of the PBG), the emission is enhanced in thespectral region corresponding to the allowed bands and,with the same input power, the photonic crystal reaches ahigher temperature than a blackbody. This is solely due tothe funneling of the thermal radiation from the forbiddenspectral range (the orange area in Fig. 2) into the allowedspectral range (the brown area in Fig. 2). Therefore, theheated photonic crystal emitter achieves thermal equili-brium at a higher temperature than would otherwise bepossible. These facts suggests the possibility to leverage thefunneling properties of photonic crystals to improve thespectral coupling of an emitter into the acceptance band ofa PV cell.

ARTICLE IN PRESSM. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 1601

We present a design of highly efficient solar cells usingPBG materials as intermediary between the Sun and the PVcells. We predict limiting conversion efficiency of around60%. We propose two approaches to achieve this. The firstapproach is to couple broadband solar radiation into aPBG material, engineered to re-emit the solar radiationinto a narrow frequency band corresponding to thesemiconductor energy gap. In this way, power loss due tophotons of wavelength too much above or below the gap iseliminated. Another approach is intended to eliminate theroadblocks in the design of TPV systems based on non-concentrated radiation, and makes use of a photoniccrystal-based angle-selective absorber. The selective absor-ber has the property of absorbing only certain parts of thewhole solar spectrum. If the absorber can absorb solarradiation whose frequency is above the solar cell band-gapfrequency, the TPV efficiency of 45% can be achieved byusing non-concentrated radiation (maximum of the dashedcurve in Fig. 4). In this case, additional spectral filters areneeded in front of the absorber. Here we show a speciallydesigned photonic crystal that exhibits both angular andspectral selectivity in absorption and emission. Also,experimental studies show that the photonic crystal-enhanced (PCE) infrared emitters enhance the wall plugconversion efficiency of MWIR solar cells relative toblackbody broad band sources.

1.1. Thermal emission control

From the foundations of quantum mechanics, it isknown that atomic oscillators in thermal equilibrium withphoton heat bath at temperature T have an average energy� at frequency o given by

eðo;TÞ ¼_o

expð_o=kBTÞ � 1, (1)

where _ is the Dirac constant and kB is the Boltzmannconstant. The energy density per unit frequency, then, canbe written as

uðo;TÞ ¼ rðoÞeðo;TÞ, (2)

where rðoÞ is the electromagnetic density of modes. Forfree space, the density of modes has the form

rFSðoÞ ¼2o2

pc3, (3)

such that the radiant power then takes he usual form ofPlanck’s law

pBBðo;TÞ ¼1

4cuðo;TÞ ¼

o2

2pc3_o

e_o=kBT�1. (4)

This suggests that since the density of electromagneticmodes is altered in a photonic crystal, the radiant powercan also be altered.

The ability of the photonic crystal to change the spectralproperties of the emitted and absorbed electromagneticradiation can be illustrated considering a photonic crystal

coating over a many-wavelength-thick substrate. Theradiation is emitted from the substrate and passes throughthe passive photonic crystal filter and then is emitted intovacuum. The absorbance A is given by energy conserva-tion, namely, AþTþR ¼ 1, where R and T arereflectance and transmittance, respectively. The absorbanceis unity if the source is a perfect blackbody. Finding theabsorbance is equivalent to finding the thermal emittanceE, since using Kirchhoff’s second law, the ratio of thethermal emittance to the absorbance is the same, indepen-dent of the nature of the material. Consequently, it ispossible to then compute E by matrix transfer techniques[15,22]. Once E is obtained, multiplication by the Planckpower spectrum gives the power spectrum of the PBGemitter pTHðo;TÞ in terms of the emittance EðoÞ andblackbody spectrum pBBðo;TÞ, as given by

pTHðo;TÞ ¼ EðoÞpBBðo;TÞ. (5)

Therefore, the thermal radiant power in a photonic crystalcan be controlled by altering the thermal emittance.

2. Photonic crystal-based solar TPV: concepts and designs

2.1. TPV conversion efficiency

The conversion efficiency of a TPV solar system isdetermined by both the absorption efficiency of theintermediate absorber and the cell conversion efficiency.Let us first examine the absorption efficiency of theintermediate absorber. The incident power density isrelated to the spectral power density defined as

PS ¼

Zdo_obSðoÞ, (6)

where

bSðoÞ ¼FS

4p3c2o2

e_o=kBTS � 1(7)

is the spectral photon flux. Here, TS is the temperature ofthe Sun and FS is a geometric factor, equal to 2:16� 10�5pfor non-concentrated light (determined by the radius of theSun and the distance between the Sun and the Earth), andp for the full concentration. This leads to the Stefan–Boltzmann’s law

PS ¼FA

p

� �sT4

S. (8)

The intermediate absorber loses its energy by emittingradiation with the rate ½FA=p�sT4

A, where the geometricfactor FA is equal to p since the absorber emits in alldirections. Hence the net gain of the absorber is

Pnet ¼FS

p

� �sT4

S �FA

p

� �sT4

A. (9)

In what regards the cell conversion efficiency, assumingthat the spectral filter allows only the radiation withfrequencies bigger than the gap frequency oG, and the

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recombination loss is all radiative, the open-circuitassumption may be used for estimation of the maximumconversion efficiency. Under this assumption, we have

_okBTA

¼_o� Dm

kBTC, (10)

from the generalized Planck’s law [23]. Here, TC is thetemperature of the cell and Dm is the chemical potential.The efficiency of an electron–hole pair to generate electricalenergy is then given by the chemical potential divided bythe photon energy as Dm=_o ¼ 1� ½TC=TA�, which is theCarnot efficiency. The actual working situation is a slightdeviation from the open-circuit condition, such that thisexpression of efficiency still holds (Fig. 3).

Combining the two contributions, we have the efficiencyof the TPV conversion system as

ZPV ¼ 1�FA

FS

T4A

T4S

!1�

TC

TA

� �. (11)

2.2. Non-concentrated radiation: frequency- and angle-

selective absorber

An increased efficiency of a TPV system may be obtainedmainly by recycling of photons of frequency larger than thesolar cell band-gap frequency by using a spectral filterbetween the absorber and the cell. The combined system ofthe absorber and the filter can be called a selective emitter.However, such a high efficiency can be achieved only forconcentrated radiation, which requires additional opticaldevices, not desirable for instance for space applications,where mass is of critical concern.

In order to design a high-efficiency TPV system usingnon-concentrated radiation, we have introduced an

Fig. 3. Schematic of the photonic crystal-based TPV energy conversion.

An intermediate absorber is heated by absorbing thermal radiation. The

photovoltaic (PV) cell is illuminated by radiation from emitter transmitted

by a filter.

angle- and frequency-selective absorber. The selectiveabsorber has the property of absorbing only certain partsof the whole solar spectrum. If the absorber can absorbsolar radiation of frequency above the solar cell band-gapfrequency, a TPV efficiency of 45% can be achieved [1] (themaximum of the dashed curve in Fig. 4). Again, additionalspectral filters are needed in front of the absorber. We showthat a suitably designed photonic crystal can be used as aselective emitter as well as a selective absorber. If, forexample, we match the band-edge frequency of thephotonic crystal to the semiconductor band-gap frequency,it is possible to suppress both emission and absorption ofphotons of frequency below that of the semiconductorband-gap. Consequently, the photonic crystal sample playssimultaneously the role of a selective emitter (with respectwith the cell) and a selective absorber (with respect tothe Sun).In addition to the frequency-selectivity, thermal emission

of the photonic crystal has angular selectivity as well. Thecontrol over the angular distribution of the emittedradiation can be extremely for the overall efficiency ofthe TPV system. If the solid angle of the emission at theSun side can be made very small, it is possible to achievethe same enhancement of the solar cell efficiency as indevices using concentrators. In other words, just by

engineering the emission solid angle, the energy conversion

efficiency can be increased without using concentrators.The radiation concentration in Eq. (11) is mathemati-

cally described by the increase of the Sun’s geometric factorFS. However, a decrease of the absorber’s geometricalfactor FA leads to the same effect. Physically, the decreaseof the absorber’s FA implies that the emission andabsorption of radiation is confined to a certain range ofdirections. Fig. 4 shows the TPV efficiency as a function ofthe absorber temperature assuming FA=FS ¼ 100 (solidcurve) and FA=FS ¼ 1000 (dashed curve). The TPVefficiency for 100 reaches up to 68% at about 727 �C and44% at 427 �C.Such a narrowing of absorption angle can be realized by

exploiting the absorption anisotropy of the photoniccrystal. As an illustrative example we consider an invertedopal photonic crystal consisting of FCC structure of airspheres in a solid background of silicon. Inverted opalphotonic crystals are ideal for high-quality, large-scalefabrication of PBG materials with band gaps at micron andsub-micron wavelengths [24]. In an optimal configuration,such as the one presented in Fig. 5, the PBG can be as largeas almost 10% of the central frequency. Experimentally, anartificial inverted opal can be created starting with mono-disperse silica spheres with a diameter around 870 nm.These spheres form a closed-packed FCC lattice by aprocess of sedimentation in an aqueous solution ofethylene glycol. In the second stage, silicon is grown insidethe voids of the opal template by means of chemical vapordeposition (CVD) using disilane (Si2H6) gas as a precursor.After disilane is deposited uniformly in the voids, thecrystal is heated to 600 �C in order to improve the silicon

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Fig. 4. TPV conversion efficiency as a function of the absorber temperature. The cell temperature is assumed to be 27 �C. Solid line is for FA=FS ¼ 100,

and the dashed line is for FA=FS ¼ 1000.

Fig. 5. A close-packed inverted opal structure ðr=a ¼ffiffið

p2Þ=4Þ viewed as

sequence of yABCABC yplanes grown along the ½1 1 1� direction. In

each plane, the low-dielectric constant spheres (here, for simplicity, we

assume air spheres) are embedded in a high-dielectric constant host

medium and are sitting on a triangular lattice of lattice constant

axy ¼ a=ffiffiffi2p

.

M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 1603

crystallization and allow the diffusion of silicon through-out the sample. Finally, the silica template is removed usingcontrolled fluoride-based etching designed to avoid affect-ing the silicon backbone, and leaving behind a closed-packed FCC lattice of air spheres in a silicon background[24]. Photonic crystals are usually characterized by thedispersion relation (or band structure, representing therelationship between the frequency and the wave-vector)and the photonic density of states (the number of availableelectromagnetic modes at a specific frequency and at a

location within the photonic crystal) (see Fig. 6), which canbe employed to infer their radiative response.In Fig. 7 we plot the angular dependence of the

absorption for a fully infiltrated inverted opal structureshown in Fig. 5. Each plot corresponds to different incidentangles for a fixed azimuthal angle. Figs. 8 and 9 are theenlarged portions of Fig. 7 at the lower band-edge of thefirst stop band around oa=2pc ¼ 0:5 and at the higherband-edge of the second stop band around oa=2pc ¼ 0:8,respectively. The absorption is enhanced at the band-edgelocation. However, in the presence of absorption, as theincident angle changes, the band edge is not so well definedanymore. For some directions there are some tails thatenter the gap and the position of the band edge frequencydepends on the specific direction. As a result the absorptionis enhanced considerably, for a given spectral range, forspecific incident directions. This fact opens a novel way tomake an angular-selective PBG absorber, which, in turn,enables high-efficiency solar energy conversion withoutusing concentrators.The increase in the energy conversion efficiency is

determined by the small angle of thermal emission of theintermediate absorber on the Sun side. For the intermedi-ate absorber, the gain comes from the absorption of solarradiation and the loss is determined by the emission by theabsorber. As we decrease the solid angle of the emission bythe absorber, we can decrease the loss due to the emissionby the absorber, and effectively increase the net gain. Theso-called minimal emission refers to a situation where thesolid angle of the emission (FA) is the same as the solidangle extended by the Sun (FS). Such an angular selective

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Fig. 6. Band structure and corresponding DOS for a close-packed FCC lattice of air spheres in silicon ð�Si ¼ 11:9Þ [22].

Fig. 7. Absorption spectra for the photonic crystal of a fully infiltrated inverted opal structure with different incident angles. The position of the band edge

moves as the direction changes. More detailed view is depicted in Figs. 8 and 9 for the lower band-edge of the first stop band around oa=ð2pcÞ ¼ 0:5 and

the higher band-edge of the second stop band around oa=ð2pcÞ ¼ 0:8, respectively.

M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–16101604

absorber, due to the decrease of the radiation loss, getsmuch hotter than in the conventional case. A larger valueof temperature difference between the absorber and the PVcell becomes available, leading to higher Carnot efficiency.For a small solid angle for absorption such thatFA=FS ¼ 100, the theoretical limit of TPV conversionefficiency becomes 68% at about 727 1C (see Fig. 4).However, we can see from Fig. 8 that there are absorptionpeaks at different frequencies. Hence, absorption isenhanced at a different angle of incidence for a differentfrequency. In other words, the angular selectivity is

restricted to a small frequency range. This effect might beuseful when applied in the tandem cell configuration. Inrealizing an angular selective absorber, the angularselectivity should cover a wide range of frequencies.Otherwise, the absorber emits outside of the desiredemission cone with different frequencies.

2.3. Wide-band angular-selective absorber

The best possible absorber would absorb radiation at allfrequencies, but only inside the solid angle subtended by

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Fig. 8. Absorption spectra at the lower band-edge of the first stop band around from Fig. 7.

Fig. 9. Absorption spectra at the higher band-edge of the first stop band around from Fig. 7.

M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 1605

the Sun. Wide-band angular-selectivity can be realized bydesigning a photonic crystal absorber such that theabsorption is suppressed at all frequencies for all anglesexcept inside the desired cone. In other words, all thefrequencies are lying inside the PBG except for onepreferred direction. The effect of such an absorber isequivalent to using fully concentrated radiation. Since theangular selectivity requires a sharp cutoff in the emissionsolid angle, a one-dimensional defect photonic crystal

structure embedded in the above discussed photonic crystalis an excellent candidate for a wide-band angular-selectivePBG absorber. We considered a structure with a completethree-dimensional band gap with one-dimensional absorp-tion characteristics. Such a property can be realized with a3D–2D–3D [25,26] photonic crystal heterostructure as theone depicted in Fig. 10, where a waveguide channel is builtin a 2D photonic crystal as a defect mode. The 2Dphotonic crystal is, in turn, embedded in a 3D photonic

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Fig. 10. Design of a PBG wide-band angular selective absorber. The

micro-structure consists of a waveguide channel in a 2D photonic crystal,

which is embedded in a 3D photonic crystal. The 1D waveguide is

generated by removing one row of rods in the longitudinal direction. The

3D photonic crystal is assumed to be a woodpile structure [28] that

presents a photonic band gap of about 20% of the mid-gap frequency. In

this example, the 2D photonic crystal consists of square rods of width

a2D=a ¼ 0:3. The width and the height of the stacking rods in the woodpile

structure are a3D=a ¼ 0:25 and h3D=a ¼ 0:3, respectively, where a is the

dielectric lattice constant of the embedding 3D photonic crystal [25,26].

Fig. 11. Schematic dispersion relation of the PBG hetero-structure

described in Fig. 10 for propagation along the waveguide direction

(w ¼ o=2pc). By removing one row of rods, the linear defect supports a

single waveguide mode. By appropriately choosing unit cell size, the mode

will experience a sharp cutoff in the spectral region around the desired

frequency [25].

M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–16101606

crystal. The electromagnetic field is confined vertically bythe 3D structure and in-plane by the stop gap of the 2Dphotonic crystal. By tuning the characteristics of the micro-structure (geometry and index of refraction contrast), the

1D defect in the 3D PBG can support a single waveguidemode, which experiences a sharp cutoff in the gap of a 3Dphotonic crystal, as shown in Fig. 11. In this case, the sub-gap generated by the waveguide channel has a true one-dimensional character, since there is only one directionavailable for wave propagation. The sharp cut-off of theguided mode at the Brillouin zone boundary gives rise to alow-group velocity do=dk! 0, which combined with theone-dimensional character of the system leads to adivergent density of states (DOS): rðoÞ / dk=do!1.For an infinite structure, there is a physical square-rootsingularity in the photonic DOS near the cutoff of thewaveguide modes [27]. For a finite structure, the divergenceis removed by the finite-size effects, but the strong variationwith frequency of the photonic DOS remains.The dispersion relation for the PBG hetero-structure

described in Fig. 10 presented in Fig. 11 indicates thatunidirectional light absorption can be achieved for arelatively broad spectral range. Thus, the photonic crystalheterostructure will operate as a frequency- and angle-selective absorber.Furthermore, we envisage a hybrid scheme for the

intermediate absorber as depicted in Fig. 12. It consists of a3D–2D–3D photonic crystal architecture acting as afrequency and angle-selective absorber on the Sun side,and a 3D photonic crystal structure acting as a frequency-selective emitter on the cell side. The absorber and theemitter systems are in thermal contact and reach thermalequilibrium. We note that the absorber and emitter systemsare macroscopic objects that exchange energy not only byradiative means, but also through direct thermal contact

(exchange of vibrational excitations or phonons). As aresult of the thermal contact, the photonic-crystal emitterreaches the same temperature as the absorber, and thethermal energy absorbed is subsequently funneled into anarrow spectral range by the PBG emitter. We also pointout, that due to the angle-selective character of theabsorber system, the thermal equilibrium temperature ofthe whole device it will be much higher than the one of aconventional absorber system.

3. Experimental demonstration of PCE infrared emitters for

efficient TPV applications

The development of a solar-cell device based on thefrequency and angular control of the emission andabsorption of thermal radiation in a photonic crystal is acomplex and laborious process. Such a device willincorporate all the advances in current solar-cell technol-ogy, and then selectively replace components in the solar-cell architecture by their photonic crystal engineeredcounterparts. Similar to the conventional solar cell design,the incident solar power will be absorbed by an absorbersystem. As we have shown in the previous section, theabsorber system may consist of a photonic crystalarchitecture, engineered such that it has an enhancedabsorption coefficient along the direction of the incident

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Fig. 12. A hybrid scheme for the intermediate absorber in TPV solar energy conversion. The inverted opal structure is used for the frequency-selective

emitter on the PV cell side. On the other hand, the 1D waveguide in a 3D woodpile structure provides the angular-selective absorber.

M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 1607

solar radiation. The emitter system consists of a differentphotonic crystal architecture, engineered such that itpresents an enhanced emission coefficient for a spectralrange that matches the photocell semiconductor band gap.

In our experimental study, we have focused only on aspecific problem: the improvement of the emitter efficiencyusing a photonic crystal-based emitter system. In order tosimplify the analysis, we have used a low-temperature TPVenergy conversion experimental set-up. In TPV, anincandescent radiator illuminates a PV cell that convertsradiant heat to electrical energy. These systems are anattractive long-term power source since, in principle, theycan achieve conversion efficiencies considerably higherthan the 6–8% capabilities of current thermoelectricgenerators. The crucial problem for this technology ismatching the emission spectrum of the radiator to the bandgap of the photocell. Approaches to this problem includeradiators with strong (ionic) emission lines and reflectivefilters between the radiator and photocell. In this work weexplore an alternate radiator concept a PCE narrow bandincandescent emitter tuned for peak emission near the bandgap of the photocell. Our results show the feasibility offabricating rugged emitters with tunable wavelengthsthrough control of surface geometry. Furthermore, whenradiation from this photonic crystal was shown onto a mid-IR PV device, we observe significant wall-plug efficiencyimprovements (45%) relative to a broad blackbody source.

We have developed a photonic crystal structure that actsas a selective emitter, preferentially emitting light in anarrow band when heated. With a narrow emission line,yet broader than current technology, these materials canemit greater energy in the selective band at lowertemperatures. In Fig. 13 we show a scanning electronmicro-graph of the PCE emitter surface. The initialdevelopment of PCE emitter surfaces is presented in

Ref. [29]. The PCE technology was then integrated withMEMS technology, yielding discrete silicon-based narrow-band infrared light sources as shown in Fig. 13. Thefabrication of this device uses traditional photo-lithogra-phy processing. More specifically the photonic crystal isfabricated by depositing metal onto an oxide-coated siliconwafer. Using photo-lithography, the photonic crystal holesare patterned onto the surface, and then using dry etchingtechniques (reactive ion etching), the holes are drilled intothe substrate. When heated the surface emits a narrow peakof infrared light with a center wavelength commensuratewith photonic crystal lattice spacing (in this case 4:2mm), asshown in Fig. 14. Peak wavelength and width do notchange with temperature variation. The peak of theemission curve will lie on the blackbody emission curvefor the measured sample temperature. The most importantpoint for application to TPV is that infrared emission atshort and long wavelengths relative to the central peak aredramatically suppressed. Out-of-band emission is emissiv-ity limited to 10% of the blackbody curve at the sametemperature. As a result, these emitters have demonstratedunprecedented infrared emission efficiency with total wallplug efficiencies approaching 20% in band. This shouldtranslate to improved TPV efficiency and it was thisexperiment that was the focus of this feasibility study.In order to develop an efficient TPV system, it is

necessary to first select the most efficient solar cell deviceand then tune the emission spectrum of the hot source tothe optimum conversion efficiency wavelength of that solarcell. Here, we optimized the solar-cell device to alreadyavailable PCE emitters. We selected a PV device based onHgCdTe (MCT) manufactured by Vigo Inc. This devicewas doped with Zn to maximize efficiency from 4 to 5mm inthe infrared. As a solar cell, this device would be veryinefficient, but it was chosen to demonstrate the potential

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Fig. 13. PCE MEMS infrared emitter vacuum packaged in a standard lead less chip carrier. The device shows unparalleled wall plug efficiency (larger

10%) in the mid infrared spectrum (MWIR 3–5mm). This is two orders of magnitude more efficient than IR light emitting diodes highlighting the

advantages of photonic crystal emitter enhancement.

Fig. 14. Infrared emission spectrum of the MEMS PCE emitter driven at

0.130mW of input power. Inband the emission reaches the blackbody

emission, but out of band the emission is suppressed dramatically. The

blackbody spectrum (red) is at the same temperature as the PCE. The

spikes in emission below 2mm are noise in the measurement.

Fig. 15. Spectral responsivity of the photovoltaic detector. Above 6mmthe solar cell is not longer effective. The wavelength at peak responsivity is

between 4–4:5mm, well aligned with the peak emission of the PCE emitter

shown in Fig. 14.

M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–16101608

of PCE TPV. Initial characterization of the photoniccrystal emitter device was carried out using a calibratedblackbody reference on a Nicolet Nexus 670 FTIR,equipped with an external emission port, followingprocedures outlined elsewhere [30]. The spectral character-istics are shown in Fig. 14. The integrated power in the2–6mm band is 30mW of infrared light, resulting in a wallplug efficiency of 23.5%. The spectral characteristics of theVigo middle wavelength infrared (MWIR) PV detectorwere provided by the manufacture and shown in Fig. 15.When the emission spectrum is multiplied by the respon-sivity we get the response curve for both the blackbody andthe PCE emitter shown in Fig. 16. Integrating andmultiplying by detector area we get 9.8 and 13.4mV forthe PCE emitter and the blackbody, respectively. Thisagrees well with the measured value of output voltage fromthe detector of 9.6 and 12.8mV for the PCE emitter and a

blackbody (with equal aperture), respectively. Solar celloutput power was then measured by varying the resistiveload and measuring the current and voltage. This is shownin Fig. 17. Converting to output power, we find that for theblackbody we measure 0:56mW and the PCE emitter yields0:32mW (Fig. 18) . The absolute magnitude of thesenumbers is very low, which is expected because this solarcell device is very inefficient. However, we can compare therelative efficiencies of the PCE emitter and the blackbodyreference. Interestingly, the PCE emitter only requires130mW of input power verses the blackbody (normalizedfor area), which requires 315mW. Therefore, the total wallplug conversion efficiency of the PCE emitter is 2:46� 10�6

and only 1:68� 10�6for the blackbody. The PCE emitter

resulted in a net improvement of 46% over the broadband

blackbody source. The improved performance of the PCEemitter is derived from the narrow emission band. Instead

ARTICLE IN PRESS

Fig. 16. The spectral response curve for the system. The red curve is the

blackbody and the black is the PCE emitter.

Fig. 17. Current–Voltage characteristics for emitter-solar cell system. The

total output power is higher for the blackbody (in red) versus the PCE

emitter (black) as evidenced by the higher red curve. However, the PCE

emitter required significantly less input power.

Fig. 18. Power versus voltage for the TPV solar cell setup. Peak power of

0.32 and 0.56mW are observed the PCE emitter (black) and blackbody

(red), respectively.

M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 1609

of emitting photons at all wavelengths, thereby wastingoptical power in spectral bands where the solar cell cannotconvert them, the PCE emitter concentrates all of theoptical power into a narrow band commensurate with theabsorption wavelength of the solar cell. As equal power isdumped into the PCE emitter, it achieves a higher

temperature than the blackbody and yields more outputin the spectral band of interest. This effect will be greatlyenhanced at higher emitter temperatures where radiativepower dominates.

The efficiency enhancement of photonic crystal emitterscan be compounded with a narrow spectral absorptionsolar cell. The Vigo MCT PV device has a very broadspectral responsivity (Fig. 15) from 0.8 to 5:5mm wavelength.

Therefore, much of the blackbody emission profile can beconverted to electricity. In contrast, higher efficiency solarcells, like polysilicon, have much narrower spectral band-width. For these detectors, the narrow emission fromphotonic crystal surfaces will be dramatically enhanced.We anticipate two to three times improvements are possiblewith photonic crystal enhancement.

4. Conclusions

We have shown that the ability to control the thermalemission and absorption of radiation in a photonic crystalenables the realization of high-efficiency solar cells. Wehave combined predictive modeling, micro-fabrication, andoptical measurements to provide a basis for understandingand controlling the thermal emission and absorption ofradiation in complex photonic structures and to designnovel solar cell devices. We have demonstrated that thethermal emission in photonic crystal is characterized byspectral- and angular selectivity. The spectral selectivityplays an important role in eliminating wavelength-bandmismatch between the semiconductor energy gap andblackbody emission, affecting the efficiency of solar cells,and may lead to a significant increase in the solar cellefficiency. On the other hand, the use of angle-selectiveabsorbers based on suitably designed photonic crystalstructures opens new avenues for realizing high-efficiencyTPV systems without concentrators.The current state-of-the-art thermal shields use multi-

layer devices or textured surfaces to reduce the impact ofthe thermal radiation on thermal sensitive devices. Theyoffer little control over frequency, and, typically, requiremechanical operations to achieve a limited control over theangular distribution of the absorbed radiation. The hybridphotonic crystal architecture we have proposed arefrequency- and angle-selective and allows a precise control

ARTICLE IN PRESSM. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–16101610

of the absorption and emission of thermal radiation. Byinfiltration such structures with liquid crystals and by theapplication of an external electric field, the resulting devicecan be tuned without the use of any mechanical operation.On the other hand, the capability to control the thermalemission and absorption of radiation in a photonic crystalmay also find many technological applications in the fieldof thermally pumped optical devices such as lasers andtunable infrared emitters.

Acknowledgments

Part of this work was performed at the Jet PropulsionLaboratory, California Institute of Technology, under agrant from the National Aeronautics and Space Adminis-tration. MF, HL, and JPD would like to acknowledge theHearne Institute for Theoretical Physics, the DisruptiveTechnologies Office, the Army Research Office, and theLouisiana State University Board of Regents LINKProgram for support.

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