plasmonic photovoltalics

8/3/2019 Plasmonic Photovoltalics

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Atwater(10) Pub. No.:US 2007/0289623 At(43) Pub. Date: Dec. 20, 2007

(54) PLASMONIC PHOTOVOLTAICS Related U.S. Application Data

(75) Inventor: Harry A. Atwater, South Pasadena, CA(US)

(60) Provisional application No. 60/811,668, filed on Jun.7,2006.

Publication Classification

Correspondence Address:HISCOCK & BARCLAY, LLP2000 HSBC PLAZA100 Chestnut StreetROCHESTER, NY 14604-2404 (US)

(51) Int. Cl.HOlL 31100 (2006.01)

(52) U.S. Cl. 136/252(57) ABSTRACT

(21) Appl. No.:(22) Filed:

111759,752

A surface plasmon polariton photovoltaic absorber. A plas-monic photovoltaic device is provided that has a periodicsubwavelength aperture array, for example a thin metal filmcoated with an array of semiconductor quantum dots. Theplasmonic photovoltaic device generates an electrical poten-tial when illuminated by electromagnetic radiation. In someembodiments, the absorber can contain both quantum dotsof semiconductors and metal nanoparticles.

(73) Assignee: California Institute of Technology,Pasadena, CA

Jun. 7, 2007

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PLASMONIC PHOTOVOLTAICSCROSS-REFERENCE TO RELATED

APPLICATIONS[0001] This application claims priority to and the benefitof co-pending U.S. provisional patent application Ser. No.60/811,668, filed Jun. 7, 2006, which application is incor-porated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLYFUNDED RESEARCH OR DEVELOPMENT

[0002] The U.S. Govermnent has certain rights in thisinvention pursuant to Grant No. FA9550-04-I-0434 awardedby the Air Force Office of Scientific Research (AFOSR).

FIELD OF THE INVENTION[0003] The invention relates to photovoltaic devices ingeneral and particularly to photovoltaic devices that employplasmons.

BACKGROUND OF THE INVENTION[0004] Since 2001, there has been an explosive growth ofscientific interest in the role of surface plasmons in opticalphenomena including guided-wave propagation and imag-ing at the subwavelength scale, nonlinear spectroscopy and'negative index' metamaterials. The unusual dispersionproperties of metals enable excitation of propagating surfaceplasmon modes away from the plasmon resonance and nearthe plasmon resonance enables excitation of localized reso-nant modes in nanostructures that access a very large rangeof wavevectors over the visible and near infrared frequencyrange. Both resonant and nonresonant plasmon excitationallows for light localization in ultrasmall volumes in met-allodielectric structures.[0005] To date, little systematic, comprehensive thoughthas been given to the question of how plasmon excitationand light localization might be exploited to advantage inphotovoltaics. Conventionally, photovoltaic absorbers mustbe optically 'thick' to enable nearly complete light absorp-tion and photocarrier current collection. They are usuallysemiconductors whose thickness is typically several timesthe optical absorption length. For silicon, this thickness isgreater than 50 microns, and it is several microns for directbandgap compound semiconductors. High efficiency cellsmust have minority carrier diffusion lengths several timesthe material thickness. Thus conventional solar cell designand material synthesis considerations are strongly dictatedby this simple optical thickness requirement.[0006] Thus there is a need for systems and methods thatboth enhance photovoltaic performance and reduce cost byusing reduced amounts of inexpensive material.

SUMMARY OF THE INVENTION[0007] In one aspect, the invention relates to a surfaceplasmon polariton photovoltaic absorber. The surface plas-mon polariton photovoltaic absorber comprises a substrate;at least one absorber layer disposed on the substrate, theabsorber layer having a surface; a layer of conductivematerial comprising a surface plasmon polariton guidinglayer disposed on the surface of the at least one absorberlayer; and at least two electrodes, a first of which electrodes

Dec. 20, 20071

is in electrical cormnunication with a first charge collectionregion of the photovoltaic absorber in which electricalcharges of a first polarity are concentrated, and a second ofwhich electrodes is in electrical cormnunication with asecond charge collection region of the photovoltaic absorberin which electrical charges of a second polarity are concen-trated; the surface plasmon polariton photovoltaic absorberconfigured to generate an electrical potential between thefirst and the second electrodes when the surface plasmonpolariton photovoltaic absorber is illuminated with electro-magnetic radiation.[0008] In one embodiment, the at least one absorber layeris a polycrystalline semiconductor thin film. In one embodi-ment, the at least one absorber layer is an epitaxial semi-conductor thin film. In one embodiment, the at least oneabsorber layer is a thin film of absorbing molecules. In oneembodiment, the at least one absorber layer comprises asemiconductor. In one embodiment, the semiconductor isselected from the group consisting of silicon, GaAs, Cd'Ie,CulnGaSe (CIGS) CdSe, PbS, and PbSe. In one embodi-ment, the semiconductor comprises an element from one ormore of Groups II, II, IV, V , and VI of the periodic table.[0009] In one embodiment, the surface plasmon polaritonphotovoltaic absorber further comprises metallic nanopar-ticles. In one embodiment, the metallic nanoparticles com-prise a selected one of silver, gold, copper and aluminum. Inone embodiment, the conductive layer is a metallic structure.In one embodiment, the metallic structure is a thin filmcomprising a metal selected from one of silver, gold, copperand aluminum.[0010] In one embodiment, the first photovoltaic absorberlayer is configured to provide a first refractive index nl at thefirst surface and the second photovoltaic absorber layer isconfigured to provide a second refractive index n2 at thesecond surface. In one embodiment, the first refractive indexn, and the second refractive index n2 are equal. In oneembodiment, at least one of a first photovoltaic absorberlayer and a second photovoltaic absorber layer is configuredas a periodic subwavelength array of apertures, grooves orasperities. In one embodiment, the photovoltaic absorberlayer comprises a dense array of quantum dots, In oneembodiment, the photovoltaic absorber layer comprises adense array of quantum wires or nanorods. In one embodi-ment, the photovoltaic absorber layer comprises a thin layerof absorbing organic or inorganic molecules. In one embodi-ment' the surface plasmon polariton photovoltaic absorberfurther comprises a continuous metallic thin film decoratedwith apertures, grooves or asperities.[0011] In another aspect, the invention features a surfaceplasmon polariton photovoltaic absorber. The surface plas-mon polariton photovoltaic absorber comprises a layer ofconductive material having a first surface disposed on a firstside thereof and a second surface disposed on a second sidethereof, a first layer of a photovoltaic absorber disposed onthe first surface of the conductive material; a second layer ofa photovoltaic absorber disposed on the second surface ofthe conductive material; and at least two electrodes, a first ofwhich electrodes is in electrical cormnunication with a firstcharge collection region of the photovoltaic absorber inwhich electrical charges of a first polarity are concentrated,and a second of which electrodes is in electrical cormnuni-cation with a second charge collection region of the photo-


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voltaic absorber in which electrical charges of a secondpolarity are concentrated. The surface plasmon polaritonphotovoltaic absorber is configured to generate an electricalpotential between the first and the second electrodes whenthe surface plasmon polariton photovoltaic absorber is illu-minated with electromagnetic radiation.[0012] The foregoing and other objects, aspects, features,and advantages of the invention will become more apparentfrom the following description and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS[0013] The objects and features of the invention can bebetter understood with reference to the drawings describedbelow, and the claims. The drawings are not necessarily toscale, emphasis instead generally being placed upon illus-trating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout thevanous views.[0014] FIG. 1is a diagram that illustrates an exemplaryembodiment of a plasmonic photovoltaic structure coatedwith a semiconductor absorber, according to principles ofthe invention.[0015] FIG. 2(a) is a diagram showing an embodiment ofa plasmonic photovoltaic structure comprising a quantumwell active region, according to principles of the invention.[0016] FIG. 2(b) is a diagram showing an embodiment ofa plasmonic photovoltaic structure comprising a quantumdot active layer, according to principles of the invention.[0017] FIG. 2(e) is a diagram showing an embodiment ofa plasmonic photovoltaic structure comprising a metallicnanoparticle plasmon resonant scattering layer, according toprinciples of the invention.[0018] FIG. 3 is a diagram showing an embodiment for amultifunction plasmonic photovoltaic cell, according toprinciples of the invention.

DETAILED DESCRIPTION OF THEINVENTION

[0019] Dramatically reducing the absorber layer thicknesscould significantly expand the range and quality of absorbermaterials that are suitable for photovoltaic devices by, e.g.,enabling efficient photocarrier collection across short dis-tances in low dimensional structures such as quantum dotsor quantum wells, and also in polycrystalline thin semicon-ductor films with very low minority carrier diffusion lengths.Beyond enhancing carrier collection in low cost, low qualityabsorber layers, plasmonic enhanced light absorption mayincrease solar cell efficiency for cells high quality photo-voltaic absorber layers, because enhanced absorption allowsone to reduce the solar cell base semiconductor volume, andin tum the dark recombination current, leading to an increaseopen circuit voltage.[0020] We describe systems and methods derived from therapid developing plasmonics field to dramatically modifythe light absorption and transmission characteristics of pho-tovoltaic materials and devices. A general discussion ofplasmonic devices is included hereinbelow for the informa-tion of the reader. In particular, the ability of plasmonicstructures to localize light at subwavelength dimensions is

Dec. 20, 20072

synergistic with use of ultrathin quantum dot and quantumwell absorber materials, as well as inexpensive polycrystal-line thin films.[0021] Conventionally, photovoltaic absorbers must beoptically 'thick' to enable nearly complete light absorptionand photocarrier current collection. They are usually semi-conductors whose thickness is typically several times theoptical absorption length. For silicon, this thickness isgreater than 100 microns, and it is several microns for directbandgap compound semiconductors. Solar cell design andmaterial synthesis considerations are strongly dictated bythis simple requirement for optical thickness. A dramaticreduction in the required absorber layer thickness withoutloss of photon collection efficiency and generation of elec-tron-hole pairs could significantly expand the range andquality of absorber materials that are suitable for photovol-taic devices by, e.g., enabling efficient photo carrier collec-tion in low dimensional structures such as quantum dots andalso in polycrystalline thin semiconductor films with verypoor minority carrier diffusion lengths.[0022] The consequences of plasmonic structure designfor photovoltaics are potentially complex and far-reaching.Here, we focus on modifying optical absorption in photo-voltaic materials, including the application of plasmonicsystems and methods to modify light absorption in photo-voltaic structures comprising ultrathin planar surface plas-mon polariton photovoltaic absorbers, and to provide spec-tral tuning of enhanced absorption and emission in coupledquantum dot/metal nanoparticle absorbers. As is conven-tionally done in a photovoltaic absorber in order to extractelectrical current across a potential difference (e.g., to obtainpower) there are provided at least two electrodes, a first ofwhich electrodes is in electrical communication with a firstcharge collection region of the photovoltaic absorber inwhich electrical charges of a first polarity are concentrated,and a second of which electrodes is in electrical communi-cation with a second charge collection region of the photo-voltaic absorber in which electrical charges of a secondpolarity are concentrated. The surface plasmon polaritonphotovoltaic absorber is configured to generate an electricalpotential between the first and the second electrodes whenthe surface plasmon polariton photovoltaic absorber is illu-minated with electromagnetic radiation. A load placedacross the surface plasmon polariton photovoltaic absorberexperiences a flow of current proportional to the generatedelectrical potential and inversely proportional to the imped-ance of the load when the surface plasmon polariton pho-tovoltaic absorber is illuminated with electromagnetic radia-tion.

ALTERNATIVE EMBODIMENTS OF THEINVENTION

[0023] We now describe several alternative embodimentsof surface plasmon polariton photovoltaic absorbers that areexpected to operate according to the principals of theinvention as described herein.[0024] FIG. 1is a perspective cross-sectional diagram thatillustrates an exemplary embodiment of a plasmonic pho-tovoltaic structure coated with a semiconductor absorber. Inthe description of FIG. 1and the other figures, the structurewill be described from the bottom layer of the figure to thetop layer in succession. In FIG. 1a substrate (the lowest


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layer of the figure) is provided upon which a p+ layer, forexample comprising a semiconductor heavily doped with asuitable dopant that behaves as an electron deficient sub-stance (such as boron or another Colunm II element insilicon) is provided as an electrical contact layer. Adjacentthe p+ contact layer is a p-type semiconductor absorber, suchas more lightly boron-doped silicon. Adjacent the p-typesemiconductor asbsorber are one or more layers that com-prise a p-i-n (p-type-intrinsic-n-type) junction regionwhere generated charge carriers (electrons and holes) areseparated. In some embodiments the i or intrinsic layer isoptional. Adjacent the p-i-n-junction region is a layer ofn-type semiconductor absorber, for example phosphorus-doped silicon, or silicon doped with another colunm Velement. Adjacent the n-type semiconductor absorber is asurface plasmon polariton guiding layer, which can com-prise a metal such as silver (Ag), copper (Cu), gold (Au), oraluminum (AI). The surface plasmon polariton guiding layercomprises plasmon incoupling structures, which are periodicstructures in communication with the guiding layer. Theoperation of a surface plasmon polariton guiding layer andthe incoupling structures is described elsewhere herein.Electrical contacts, shown in FIG. 1 and in the other figuresas lines from specified regions of the structure that terminatein circles (e.g., external terminals for making electricalconnection to the plasmonic photovoltaic structure) areprovided to allow the connection of the device to an externalcircuit so as to obtain power when the device is subjected toincident solar irradiation, or alternatively is subjected tolight or electromagnetic radiation generally. In FIG. 1, thesurface plasmon waves are shown by the curved lines thatare indicated to flow in the direction of the horizontalarrows. The dotted portions of the curves are intended toillustrate the penetration of the surface Plasmon waves intothe underlying semiconductor material.[0025] FIG. 2(a) is a perspective cross-sectional diagramshowing an embodiment of a plasmonic photovoltaic struc-ture comprising a quantum well active region. The structureof FIG. 2(a) is similar to that of FIG. 1,with the substitutionof a p-type quantum well (QW) cladding layer, an activequantum well region, and an n-type quantum well (QW)cladding layer for the p-type semiconductor absorber layer,the pn junction depletion region, and the n-type semicon-ductor absorber layer of FIG. 1, respectively. It is wellknown that quantum well structures are commonly fabri-cated using various materials in the III-V class of semicon-ductors, such as GaAs. InP,AlSb and alloys or combinationsof such semiconductors, including materials having compo-sitions such as Al(1_x_y)GaxIn"P(1_z)As"where x, y and z arenumbers selected between zero and one. Other compoundsemiconductors, or semiconductor alloys, can be used tofabricate quantum well structures.[0026] FIG. 2(b) is a perspective cross-sectional diagramshowing an embodiment of a plasmonic photovoltaic struc-ture comprising a quantum dot active layer. In the structureof FIG. 2(b) a substrate is provided, upon which a surfaceplasmon polariton guiding layer is deposited. As in FIG. 1,this layer can comprise a metal. One or more plasmonincoupling structures are provided on the surface plasmonpolariton guiding layer. Additionally, an array of quantumdots of an absorber material, such as a semiconductor, areprovided on the surface plasmon polariton guiding layer asan active absorber layer. Electrical contact can be made to

Dec. 20, 20073

the metallic surface plasmon polariton guiding layer and toa contact layer that is in electrical communication with thequantum dot active layer.[0027] FIG. 2(e) is a perspective cross-sectional diagramshowing an embodiment of a plasmonic photovoltaic struc-ture comprising a metallic nanoparticle plasmon resonantscattering layer. In FIG. 2(e) a glass substrate is provided,upon which a semiconductor absorber is provided. Thesemiconductor absorber can be deposited by any convenientmeans, such as CVD, MBE, or other procedures known todeposit semiconductor materials. In the embodiment of FIG.2(e), the semiconductor absorber material is doped with ann-type dopant in one area to form an n-type region, and thesemiconductor absorber material is doped with a p-typedopant in another area to form a p-type region. Electricalcontacts are attached to each region, with the use of wellknown electrical contact technology, for example usingcontact technology commonly used in the semiconductorindustry. An intrinsic region of effectively undoped (orcompensated) semiconductor absorber may be providedbetween successive n-type and p-type regions. In someembodiments, successive regions of alternating polarity, in asequence such as -n-i-p-i-n-i-p-i- can be provided with eachn-type region and each p-type region having c a contactapplied thereto. A thin glass layer is provided above thesemiconductor absorber layer, within which or on top ofwhich is provided a layer comprising a plurality of metalnanoparticles that form a plasmon resonant scattering layer.In the embodiment illustrated, the metal nanoparticles com-prise one or more of aluminum or copper. The metalnanoparticles can be provided by in situ growth of nano-particles, by deposition of nanoparticles from a vapor, suchas in a PVD or MOCVD reactor, by sputtering, by evapo-ration, or by any other convenient method.[0028] FIG. 3 is a perspective cross-sectional diagramshowing an embodiment for a multijunction plasmonicphotovoltaic cell. In FIG. 3 a substrate is provided uponwhich a plurality of absorber materials are deposited in aselected order.[0029] The energy of a photon is defined by the relationE=hv=hc/A, where E represents energy, h representsPlanck's constant, v represents frequency, c represents thespeed of light, and A represents wavelength. Accordingly, itis understood that photons having longer wavelengths orlower frequency carry less energy that do photons havingshorter wavelength and higher frequency. The bandgapenergy (or "bandgap") of a semiconductor is the mininimumenergy required to excite (or stimulate) a charge carrier fromone of the valence band and the conduction band to cross thebandgap to the other band. If one has two semiconductors,one with a larger bandgap and one with a smaller bandgap,light having a high enough frequency to be absorbed by thematerial with the larger bandgap will be absorbed in bothmaterials (but with a waste of energy in the smaller bandgapmaterial) and light with a frequency just too small to beabsorbed by the larger bandgap material will still beabsorbed by the material having the smaller bandgap, butwill pass unabsorbed through the larger bandgap material(ignoring reflective and scattering effects). Accordingly, it isunderstood that to extract the maximal energy from apolychromatic radiation beam one should cause the radiationto fall on absorbers in the order of their bandgaps, beginningwith the largest bandgap. In addition, selecting bandgaps


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with a relatively small difference in bandgap energy willensure that not too many photons are absorbed with a wasteof energy (e.g., are absorbed by a material having a consid-erably smaller bandgap energy than the energy of thephoton).[0030] In the multijunction plasmonic photovoltaic cell ofFIG. 3, a sequence of materials having bandgaps of the orderof (or approximately) 0.7 eV (for example, FeSi2 or FeS2),1.0 eV (silicon), 1.4 eV (for example BaSi2, Zn3P4, orsilicon dots or wires), and 1.95 eV (for example Cu20, GaP,or silicon dots or wires) are provided successively upon asubstrate. In one embodiment, the total thickness of themultiple bandgap junction structure is of the order of 200nm. As is well known, it may be useful in some embodi-ments to provide intermediate layers between successivematerials having different bandgaps in order to provideelectrical contacts or to provide grading layers to minimizechanges in crystallographic dimensions between successivelayers (e.g., lattice matching layers). From the descriptionalready given, it will be understood that such contact orlattice matching layers will need to have bandgaps largerthat the layers that they overlay so as not to absorb photonsthat are intended to be absorbed in lower layers of thestructure. A surface plasmon polariton guiding layer isprovided on top of the uppermost subcell layer (e.g., theabsorber layer having the largest bandgap). As describedwith regard to FIG. 1 the surface plasmon polariton guidinglayer can comprise copper or aluminum, and has adjacentthereto one or more plasmon incoupling structures. Theoperation of the embodiment of FIG. 3 is similar to that ofthe embodiment of FIG. 1, with the recognition that thepresence of multiple bandgaps can permit the extraction ofmore energy from the same illumination that would beapplied to the structure of FIG. 1.Ultrathin Planar Surface Plasmon Polariton PhotovoltaicAbsorbers[0031] It is expected that the conversion of incident lightinto propagating surface plasmon polaritons can enableefficient light absorption in extremely thin (10's-100's ofnanometers thick) photovoltaic absorber layers.[0032] The extraordinary transmission properties of peri-odic subwavelength apertures and hole arrays in thin metalfilms have received wide scientific attention. The transmis-sion properties of subwavelength apertures and hole arraysin thin metal films are related to coupling of the incident andtransmitted beam to surface plasmons and also to the peri-odicity of the entrance and exit aperture arrays. A subwave-length aperture functions as a plasmonic absorber structurewith a coated semiconductor photovoltaic absorber. Specifi-cally, the aperture array can couple incident light in surfacewaves (surface plasmon polaritons and evanescent surfacewaves) that propagate normal to (or at an angle to) the lightincidence direction. The propagating surface waves areabsorbed in the photovoltaic absorber. If these media areinstead semiconductor absorbing layers such that the refrac-tive indices nl and n2 are complex, very strong absorptioncan occur since the propagating surface plasmon polaritonmode is strongly localized at the metal-semiconductor inter-face. We have demonstrated experimentally plasmonicabsorber structures consisting of subwavelength aperturearrays in Ag thin films, which are subsequently coated withthin (-20 nm or approximately 1-3 layers dot layers) of

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CdSe quantum dots whose absorption edge is at 600 nm. Wefind that the in-plane absorbance decay length is 1.2 urn forthese 20 nm thick CdSe quantum dot layers on periodicsubwavelength aperture arrays at an incident wavelengthabove the absorption edge of 514.5 nm, indicating verystrong surface wave absorption by the thin quantum dotlayer.[0033] Comprehensive exploration of the coupling of theincident solar spectrum to surface plasmon polariton modeson periodic metallodielectric arrays coated with semicon-ductor absorbers (e.g, CdSe, GaAs and/or Si thin films) canyield i) optimal conditions for enhanced integrated spectralplasmonic absorption above the semiconductor absorberbandgap, and ii) conditions that balance the integratedabsorption in the thin absorber layer on the aperture arraywith transmission through to underlying absorbers, as wouldbe required in a multifunction solar cell.Spectral Tuning of Enhanced Absorption and Emission inCoupled Quantum Dot/Metal Nanoparticle Absorbers[0034] Beginning in the 1980's, it was recognized that theenhanced local electric field in the vicinity of a metalstructure can enhance the absorption and emission rates ofactive dipole emitters, such as molecular chromophores,near the metal surface. In the last three years, it has becomeevident that field enhancement can be employed to dramati-cally alter the emission rates and intensities of semiconduc-tor quantum dots and quantum wells. Silicon nanocrystalshave tunable optical gaps, high internal quantum efficiencyand can be fabricated in dense arrays suitable for tunnelinjection and collection of photocarriers. However Sinanoc-rystals, like bulk Si, suffer low optical absorbance due to theindirect energy bandgap, even for Si nanocrystals thatexhibit strong quantum excitonic confinement.[0035] Recently, it has been demonstrated that lumines-cence emission from silicon quantum dot arrays can beenhanced by -TOx by coupling to localized surface plasmonmodes inAu nanostructures. Itis believed that the enhancedemission is due to an enhanced radiative emission rate of thecoupled Au nanostructure/Si nanocrystal system. In sampleswith less than unity quantum efficiency, enhancement of theradiative emission rate also increases the quantum efficiency.At high pump powers (high carrier injection currents forelectrical pumping), the emission intensity is independent ofthe quantum efficiency, the emission cross section, thephoton flux (carrier current), and the non-radiative decayrate. In this regime, the emission intensity therefore scalessolely with the radiative decay rate. With precise control ofthe metal-semiconductor separation distance and carefultuning of the metal particle plasmon resonance frequency,we anticipate a >100-fold enhancement in radiative rate, andtherefore absorption and emission intensity, in Si nanocrys-tals. Both analytic modeling and full field electromagneticsimulation suggest that this potential for the plasmon-en-hanced radiative rate enhancement to be >1OOxthe emissionor absorption rate that can be achieved relative to Si nanoc-rystals in purely dielectric matrices. It is believed thatachieving this goal experimentally will require carefulnanoscale engineering of the coupling between plasmonicmetal and Si nanocrystal structures.[0036] The tuning plasmon-enhanced absorption andemission can be realized by careful control of the Si nanoc-rystal/Ag nanoparticles relative separation, which optimizes


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the local field enhancement and radiative rate enhancementat the position of the Si nanocrystals. Itis expected that thiscan be done by designing coplanar arrays of aerosol andcolloidally-synthesized Si nanocrystals in spin-on glasshosts, and also by sequentially layering of deposited SiO andSi02 followed by annealing to yield coplanar arrays of Sinanocrystals by SiO decomposition. Full-field electromag-netic simulations can be used to quantity the relationshipbetween radiative rate and local field enhancement for morecomplex nanoparticles array structures.[0037] For some time, the inventor has been active inplasmonics and semiconductor nanocrystal research,focused on development of materials and electromagneticdesigns for plasmonic devices at the subwavelength-to-wavelength scale. The effort includes experimental researchon use of near field interactions to enable optical guiding andswitching below the diffraction limit and is complementedby theoretical work on near field interactions in and collec-tive modes of subwavelength scale metallodielectric struc-tures. Some of the inventor's contributions to the plasmonicsfield include the first experimental and theoretical demon-stration of light guiding below the diffraction limit in nano-particle plasmon waveguides, and the theoretical investiga-tion of optical pulse propagation in subwavelength scaleplasmon waveguides.[0038] Achieving control oflight-material interactions forphotonic device applications at nanoscale dimensions willrequire structures that guide electromagnetic energy with alateral mode confinement below the diffraction limit oflight.This cannot be achieved by using conventional waveguidesor photonic crystals. Ithas been suggested that electromag-netic energy can be guided below the diffraction limit alongchains of closely spaced metal nanoparticles that convert theoptical mode into non-radiating surface plasmons. A varietyof methods such as electron beam lithography and self-assembly have been used to construct metal nanoparticleplasmon waveguides. However, all investigations of theoptical properties of these waveguides have so far beenconfined to collective excitations, and direct experimentalevidence for energy transport along plasmon waveguideshas proved elusive. Here we present observations of elec-tromagnetic energy transport from a localized subwave-length source to a localized detector over distances of about0.5 urn in plasmon waveguides consisting of closely spacedsilver rods. The waveguides are excited by the tip of anear-field scanning optical microscope, and energy transportis probed by using fluorescent nanospheres. This has beendescribed in the article "Local detection of electromagneticenergy transport below the diffraction limit in metal nano-particle plasmon waveguides," Nature Materials vol. 2,229-232 (April, 2003).[0039] We have also provided experimental and theoreti-cal demonstration of resonant plasmon printing of 40 nmlithographic features in conventional photoresist using vis-ible light, and a theoretical demonstration of enhancedsubwavelength near field optical resolution by use of a 30nm Ag film as a lens.[0040] We have previously completed an experimentaland theoretical demonstration of strongly-coupled nanopar-ticle chain arrays in the 'sub-lithographic' size regime, i.e.,particle size of -10 nm and interparticle separations of 1-4nm. We have demonstrated plasmon-enhanced emission

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from Si quantum dots. We have provided the first experi-mental demonstration of plasmon slot waveguides.[0041] It is expected that it will be demonstrated that onecan couple dense arrays of semiconductor nanocrystals(particularly Si, CdSe, PbS and PbSe) to metallic nanostruc-tures to form active plasmonic structures. Our previousnanocrystal work has included identification of excitonicand defect luminescence mechanisms for Si and Ge nanoc-rystals; measurement of exchange energy in Si nanocrystals;tuning emission wavelength and depth profi les of Si nanoc-rystals fabrication by ion implantation; synthesis and char-acterization of GaAs nanocrystals by ion implantation andorganometallic vapor phase growth, and charge injectioninto single Si nanocrystals observed by electrostatic forcemicroscopy. In general, the surface plasmon polariton pho-tovoltaic absorbers described herein can uti lize a semicon-ductor that comprises an element from one or more ofGroups II, II, IV, V, and VI of the periodic tableGeneral Comments on Plasmonic Materials[0042] There is currently worldwide interest in developingsilicon-based active optical components in order to leveragethe infrastructure of silicon microelectronics technology forthe fabrication of optoelectronic devices. Light emission inbulk silicon-based devices is constrained in wavelength toinfrared emission, and in efficiency by the indirect bandgapof silicon. One promising strategy for overcoming thesechallenges is to make use of quantum-confined excitonicemission in silicon nanocrystals. A challenge for siliconnanocrystal devices based on nanocrystals embedded insilicon dioxide has been the development of a method forefficient electrical carrier injection. We have demonstrated ascheme for electrically pnmping dense silicon nanocrystalarrays by a field-effect electro luminescence mechanism. Inthis excitation process, electrons and holes are both injectedfrom the same semiconductor channel across a tunnellingbarrier in a sequential programming process, in contrast tosimultaneous carrier injection in conventional pn-junctionlight-emitting-diode structures. Light emission is stronglycorrelated with the injection of a second carrier into ananocrystal that has been previously programmed with acharge of the opposite sign. This work has been described inthe article "Field-effect electrolnminescence in silicon nano-cystals," Nature Materials, vol 4, 143-146 (February, 2005).We have observed field-effect electroluminescence emissionin sil icon nanocrystals. We have also quantified the internalquantum efficiency and the absolute radiative emission rateof Si nanocrystal dense arrays by variation of local densityof optical states.[0043] The following discussion appeared in an article bythe inventor entitled "The Promise of Plasmonics", Scien-tific American, April 2007. The size and performance ofphotonic devices are constrained by the diffraction limit;because of interference between closely spaced light wavesthe width of an optical fiber carrying them must be at leasthalf the light's wavelength inside the material. For chip-based optical signals, which will most likely employ near-infrared wavelengths of about 1,500 nanometers (billionthsof a meter), the minimum width is much larger than thesmallest electronic devices currently in use; some transistorsin silicon integrated circuits, for instance, have featuressmaller than 100 nanometers.[0044] Recently scientists have been working on a newtechnique for transmitting optical signals through minuscule


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nanoscale structures. In the 1980s researchers experimen-tally confirmed that directing light waves at the interfacebetween a metal and a dielectric (a nonconductive materialsuch as air or glass) can, under the right circumstances,induce a resonant interaction between the waves and themobile electrons at the surface of the metal. (In a conductivemetal, the electrons are not strongly attached to individualatoms or molecules.) In other words, the oscillations ofelectrons at the surface match those of the electromagneticfield outside the metal. The result is the generation of surfaceplasmons-density waves of electrons that propagate alongthe interface like the ripples that spread across the surface ofa pond after you throw a stone into the water.[0045] Over the past decade investigators have found thatby creatively designing the metal-dielectric interface theycan generate surface plasmons with the same frequency asthe outside electromagnetic waves but with a much shorterwavelength. This phenomenon could allow the plasmons totravel along nanoscale wires called interconnects, carryinginformation from one part of a microprocessor to another.Plasmonic interconnects would be a great boon for chipdesigners, who have been able to develop ever smaller andfaster transistors but have had a harder time building minuteelectronic circuits that can move data quickly across thechip.[0046] In 2000 the inventor's group at the CaliforniaInstitute of Technology gave the name "plasmonics" to thisemerging discipline, sensing that research in this area couldlead to an entirely new class of devices. Ultimately it maybe possible to employ plasmonic components in a widevariety of instruments, using them to improve the resolutionof microscopes, the efficiency of light-emitting diodes(LEDs) and the sensitivity of chemical and biological detec-tors. Scientists are also considering medical applications,designing tiny particles that could use plasmon resonanceabsorption to kill cancerous tissues, for example. And someresearchers have even theorized that certain plasmonic mate-rials could alter the electromagnetic field around an object tosuch an extent that it would become invisible. Although notall these potential applications may prove feasible, investi-gators are eagerly studying plasmonics because the new fieldpromises to literally shine a light on the mysteries of thenanoworld.[0047] Research into surface plasmons began in earnest inthe 1980s, as chemists studied the phenomenon usingRaman spectroscopy, which involves observing the scatter-ing of laser light off a sample to determine its structure frommolecular vibrations. In 1989 Thomas Ebbesen, then at theNEC Research Institute in Japan, found that when he illu-minated a thin gold film imprinted with millions of micro-scopic holes, the foil somehow transmitted more light thanwas expected from the number and size of the holes. Nineyears later Ebbesen and his colleagues concluded that sur-face plasmons on the film were intensifying the transmissionof electromagnetic energy.[0048] Two new classes of tools have also acceleratedprogress in plasmonics: recent increases in computationalpower have enabled investigators to accurately simulate thecomplex electromagnetic fields generated by plasmoniceffects, and novel methods for constructing nanoscale struc-tures have made it possible to build and test ultrasmallplasmonic devices and circuits.

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[0049] At first glance, the use of metallic structures totransmit light signals seems impractical, because metals areknown for high optical losses. The electrons oscillating inthe electromagnetic field collide with the surrounding latticeof atoms, rapidly dissipating the field's energy. But theplasmon losses are lower at the interface between a thinmetal film and a dielectric than inside the bulk of a metalbecause the field spreads into the nonconductive material,where there are no free electrons to oscillate and hence noenergy-dissipating collisions. This property naturally con-fines plasmons to the metallic surface abutting the dielectric;in a sandwich with dielectric and metal layers, for example,the surface plasmons propagate only in the thin plane at theinterface.[0050] Because these planar plasmonic structures act aswaveguides, shepherding the electromagnetic waves alongthe metal-dielectric boundary, they could be useful in rout-ing signals on a chip. Although an optical signal suffers moreloss in a metal than in a dielectric such as glass, a plasmoncan travel in a thin-film metal waveguide for several centi-meters before dying out. The propagation length can bemaximized if the waveguide employs an asymmetric mode,which pushes a greater portion of the electromagneticenergy away from the guiding metal film and into thesurrounding dielectric, thereby lowering loss. Because theelectromagnetic fields at the top and bottom surfaces of themetal film interact with each other, the frequencies andwavelengths of the plasmons can be adjusted by changingthe thickness of the film. In the 1990s research groups led bySergey Bozhevolnyi of Aalborg University in Denmark andPierre Berini of the University of Ottawa developed planarplasmonic components that could perform many of the samefunctions-such as splitting guided waves-usually done byall-dielectric devices. These structures could prove useful intransmitting data from one part of a chip to another, but theelectromagnetic fields accompanying the plasmons are toolarge to convey signals through the nanoscale innards of aprocessor.[0051] To generate plasmons that can propagate throughnanoscale wires, researchers have explored more complexwaveguide geometries that can shrink the wavelength of thesignal by squeezing it into a narrow space. In the late 1990sthe inventor's lab group and a team led by Joachim Krennof the University of Graz inAustria launched parallel effortsto produce these "subwavelength" surface-plasmonwaveguides. Working with the inventor at Caltech, StefanMaier built a structure consisting of linear chains of golddots, each less than 100 nanometers across. A visible beamwith a wavelength of 570 nanometers triggered resonantoscillations in the dots, generating surface plasmons thatmoved along the chains, confined to a flattened path only 75nanometers high. The Graz group achieved similar resultsand imaged the patterns of the plasmons carried along thechains. The absorption losses of these nanowires wererelatively high, however, causing the signal to die out afterit traveled a few hnndred nanometers to a few microns(millionths of a meter). Thus, these waveguides would besuitable only for very short-range interconnections.[0052] Fortunately, the absorption losses can be mini-mized by turning the plasmonic waveguides inside out,putting the dielectric at the core and surrounding it withmetal. In this device, called a plasmon slot waveguide,adjusting the thickness of the dielectric core changes the


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wavelength of the plasmons. The inventor's lab at Caltechand Mark Brongersma's Stanford University group haveshown that plasmon slot waveguides are capable of trans-mitting a signal as far as tens of microns. Hideki Miyazakiof the National Institute for Materials Science in Japanobtained a striking result by squeezing red light (with awavelength of 651 nanometers in free space) into a plasmonslot waveguide that was only three nanometers thick and 55nanometers wide. The researchers found that the wavelengthof the surface plasmon propagating through the device was51 nanometers, or about 8 percent of the free-space wave-length.[0053] Plasmonics can thus generate signals in the softx-ray range of wavelengths (between 10 and 100 nanom-eters) by exciting materials with visible light. The wave-length can be reduced by more than a factor of 10 relativeto its free-space value, and yet the frequency of the signalremains the same. (The fundamental relation between thetwo-frequency times wavelength equals the speed oflight-is preserved because the electromagnetic waves slowas they travel along the metal-dielectric interface.) Thisstriking ability to shrink the wavelength opens the path tonanoscale plasmonic structures that could replace purelyelectronic circuits containing wires and transistors.[0054] Just as lithography is now used to imprint circuitpatterns on silicon chips, a similar process could mass-produce minuscule plasmonic devices with arrays of narrowdielectric stripes and gaps. These arrays would guide thewaves of positive and negative charge on the metal surface;the alternating charge densities would be very much akin tothe alternating current traveling along an ordinary wire. Butbecause the frequency of an optical signal is so much higherthan that of an electrical one-more than 400,000 gigahertzversus 60 hertz-the plasmonic circuit would be able tocarry much more data. Moreover, because electrical chargedoes not travel from one end of a plasmonic circuit toanother-the electrons bunch together and spread apartrather than streaming in a single direction-the device is notsubject to resistance and capacitance effects that limit thedata-carrying capacity of integrated circuits with electricalinterconnects.[0055] Plasmonic circuits would be even faster and moreuseful if researchers could devise a "plasmonster" switch-athree-terminal plasmonic device with transistor like proper-ties. The inventor's lab at Caltech and other research groupshave recently developed low-power versions of such aswitch. If scientists can produce plasmonsters with betterperformance, the devices could serve as the core of anultrafast signal-processing system, an advance that couldrevolutionize computing 10 to 20 years from now.[0056] Plasmonic materials may also revolutionize thefighting industry by making LEDs bright enough to competewith incandescent bulbs. Beginning in the 1980s, research-ers recognized that the plasmonic enhancement of the elec-tric field at the metal-dielectric boundary could increase theemission rate of luminescent dyes placed near the metal'ssurface. More recently, it has become evident that this typeof field enhancement can also dramatically raise the emis-sion rates of quantum dots and quantum wells-tiny semi-conductor structures that absorb and emit light-thusincreasing the efficiency and brightness of solid-state LEDs.In 2004 Axel Scherer of Caltech, together with co-workers

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at Japan's Nichia Corporation, demonstrated that coating thesurface of a gallium nitride LED with dense arrays ofplasmonic nanoparticles (made of silver, gold or aluminum)could increase the intensity of the emitted light 14-fold.[0057] Furthermore, plasmonic nanoparticles may enableresearchers to develop LEDs made of silicon. Such devices,which would be much cheaper than conventional LEDscomposed of gallium nitride or gallium arsenide, are cur-rently held back by their low rates of light emission. Theinventor's group at Caltech, working with a team led byAlbert Polman of the FOM Institute for Atomic and Molecu-lar Physics in the Netherlands, has shown that couplingsilver or gold plasmonic nanostructures to silicon quantum-dot arrays could boost their light emission by about 10times.Moreover, it is possible to tune the frequency of theenhanced emissions by adjusting the dimensions of thenanoparticles. Our calculations indicate that careful tuningof the plasmonic resonance frequency and precise control ofthe separation between the metallic particles and the semi-conductor materials may enable us to increase radiative ratesmore than 100-fold, allowing silicon LEDs to shine just asbrightly as traditional devices.Theoretical Discussion[0058] Although the theoretical description given herein isthought to be correct, the operation of the devices describedand claimed herein does not depend upon the accuracy orvalidity of the theoretical description. That is, later theoreti-cal developments that may explain the observed results on abasis different from the theory presented herein will notdetract from the inventions described herein.[0059] While the present invention has been particularlyshown and described with reference to the structure andmethods disclosed herein and as illustrated in the drawings,it is not confined to the details set forth and this invention isintended to cover any modifications and changes as maycome within the scope and spirit of the following claims.

What is claimed is:1. A surface plasmon polariton photovoltaic absorber,

comprising:a substrate;at least one absorber layer disposed on said substrate, said

absorber layer having a surface;a layer of conductive material comprising a surface plas-

mon polariton guiding layer disposed on said surface ofsaid at least one absorber layer; and

at least two electrodes, a first of which electrodes is inelectrical communication with a first charge collectionregion of said photovoltaic absorber in which electricalcharges of a first polarity are concentrated, and asecond of which electrodes is in electrical communi-cation with a second charge collection region of saidphotovoltaic absorber in which electrical charges of asecond polarity are concentrated;

said surface plasmon polariton photovoltaic absorber con-figured to generate an electrical potential between saidfirst and said second electrodes when said surfaceplasmon polariton photovoltaic absorber is illuminatedwith electromagnetic radiation.


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2. The surface plasmon polariton photovoltaic absorber ofclaim 1, wherein said at least one absorber layer is apolycrystalline semiconductor thin film.3. The surface plasmon polariton photovoltaic absorber of

claim 1, wherein said at least one absorber layer is anepitaxial semiconductor thin film.4. The surface plasmon polariton photovoltaic absorber of

claim 1, wherein said at least one absorber layer is a thin filmof absorbing molecules.5. The surface plasmon polariton photovoltaic absorber of

claim 1, wherein said at least one absorber layer comprisesa semiconductor.6.The surface plasmon polariton photovoltaic absorber of

claim 5, wherein said semiconductor is selected from thegroup consisting of silicon, GaAs, CdTe, CuInGaSe (CIGS),CdSe, PbS, and PbSe.7. The surface plasmon polariton photovoltaic absorber of

claim 5, wherein said semiconductor comprises an elementfrom one or more of Groups II, II, IV, V, and VI of theperiodic table.S. The surface plasmon polariton photovoltaic absorber of

claim 1, further comprising metallic nanoparticles.9. The surface plasmon polariton photovoltaic absorber of

claim 8, wherein said metallic nanoparticles comprise aselected one of silver, gold, copper and aluminum.10. The surface plasmon polari ton photovoltaic absorber

of claim 1, wherein said conductive layer is a metallicstructure.11. The surface plasmon polari ton photovoltaic absorber

of claim 10, wherein said metallic structure is a thin filmcomprising a metal selected from one of silver, gold, copperand aluminum.

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12. The surface plasmon polari ton photovoltaic absorberof claim 1, wherein said absorber layer comprises a densearray of quantum dots.13. The surface plasmon polari ton photovoltaic absorber

of claim 1, wherein said absorber layer comprises a densearray of quantum wires or nanorods.14. A surface plasmon polariton photovoltaic absorber,

comprising:a layer of conductive material having a first surfacedisposed on a first side thereof and a second surfacedisposed on a second side thereof;

a first layer of semiconductor absorber disposed on saidfirst surface of said conductive material;

a second layer of semiconductor absorber disposed onsaid second surface of said conductive material; and

at least two electrodes, a first of which electrodes is inelectrical communication with a first charge collectionregion of said photovoltaic absorber in which electricalcharges of a first polarity are concentrated, and asecond of which electrodes is in electrical communi-cation with a second charge collection region of saidphotovoltaic absorber in which electrical charges of asecond polarity are concentrated;

said surface plasmon polariton photovoltaic absorber con-figured to generate an electrical potential between saidfirst and said second electrodes when said surfaceplasmon polariton photovoltaic absorber is illuminatedwith electromagnetic radiation.

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plasmonic photovoltalics

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