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March 24, 2013

C 2013 American Chemical Society

Visible to Near-Infrared Sensitization ofSilicon Substrates via Energy Transferfrom Proximal Nanocrystals: FurtherInsights for Hybrid PhotovoltaicsMichael T. Nimmo,† Louis M. Caillard,‡ William De Benedetti,‡ Hue M. Nguyen,† Oliver Seitz,‡

Yuri N. Gartstein,† Yves J. Chabal,‡ and Anton V. Malko†,*

†Department of Physics and ‡Department of Materials Science, The University of Texas at Dallas, Richardson, Texas 75080, United States

Harvesting solar energy is an impor-tant objective to fulfill the ever-growing energy demand. As a re-

sult, several new types of solar cells (SCs)have been introduced and discussed overthe last two decades based onmaterials andprinciples that are quite different from theoriginal crystalline Si p-n junction cell.1�4

The quest for higher efficiency and lowerproduction cost SCs with extended life-time and versatile applications is drivingthe research. Despite much progress, manyof these new approaches are yet to reachconceptual and operational maturity. Theactual photovoltaic (PV) module productionnowadays continues to overwhelmingly relyon Si technology. While some single- andmulti-junction devices exhibit record effi-ciencies, the efficiencies of most of themanufactured SCs remain in the 10�18%

range, well below the thermodynamic limitsfor the photovoltaic conversion.4 Hence,there is a well-recognized need and con-tinuing effort to explore more opportunitiesto improve different aspects and/or achievecertain goals of SC performance and opera-tion. According to an interesting recentviewpoint,4 such opportunities may lie notonly within the realm of the so-called third-generation photovoltaics1,2 (multiple-carriergeneration, up- and down-conversion, etc.)but also within a more familiar framework ofa single-band-gap SC when combined withmodern advances in the control of light andmaterial processing.Nanostructured materials attract consider-

able attention as candidates for PV devices.Among them, inorganic colloidal nanocrystalquantumdots (NQDs) have gained large pop-ularity as photon absorbers owing to their

* Address correspondence toanton.malko@utdallas.edu.

Received for review December 20, 2012and accepted March 24, 2013.

Published online10.1021/nn400924y

ABSTRACT We provide a unified spectroscopic evidence of efficient energy transfer

(ET) from optically excited colloidal nanocrystal quantum dots (NQDs) into Si substrates in a

broad range of wavelengths: from visible (545 nm) to near-infrared (800 nm). Chemical

grafting of nanocrystals on hydrogenated Si surfaces is achieved via amine-modified carboxy-

alkyl chain linkers, thus ensuring complete surface passivation and accurate NQD positioning.

Time-resolved photoluminescence (PL) has beenmeasured for a set of CdSe/ZnS and CdSeTe/

ZnS NQDs of various sizes and compositions grafted on Si and SiO2 substrates. The measured

acceleration of the PL decays on Si substrates is in good agreement with theoretical

expectations based on the frequency-dependent dielectric properties of Si and NQD�Si

separation distances. A comparative analysis reveals separate contributions to ET coming

from the nonradiative (NRET) and radiative (RET) channels: NRET is a dominant mechanism

for proximal NQDs in the middle of the visible range and becomes comparable with RET

toward near-infrared wavelengths. The broad range over which the ET efficiency is

estimated to be at the level of∼90% further supports the concept that hybrid nanocrystal/

silicon thin-film photovoltaic devices could efficiently harvest solar energy across the entire spectrum of wavelengths.

KEYWORDS: hybrid nanostructures . solar cells . energy transfer . silicon substrates . semiconductor nanocrystals

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large absorption cross sections, photostability, tunabil-ity of the absorption edge via size-dependent quan-tum confinement, and ease of chemical manipulationand production. We have recently demonstrated5 thatphotoexcited NQDs can also serve as efficient energydonors for adjacent ultrathin Si nanomembranes. Hy-brid nanostructures combining NQDs with crystallineSi layers thus offer a potential for development of thin-film PV devices that take advantage of beneficialproperties of both components. The excitonic sensiti-zation of Si layers via energy transfer (ET) from stronglyabsorbing NQDs eliminates the issue of the weak solarlight absorption in indirect band gap Si as a definingfactor for the device thickness. At the same time, onecan exploit the maturity of Si technologies to reliablyand inexpensively manufacture high-carrier-mobilitySi components for charge separation and transport.Achieving high efficiency in SCs demands the effec-

tive use of the full solar spectrum, that is, harvestingphotons of all energies, including those in the near-infrared (NIR) that account for ∼37% of the Sun'sphoton flux. (The NIR content becomes ∼27% for thephotonswith energies above the band gap of Si.) NQDswith the absorption onset at 1.2�1.3 eV would matchconditions for the maximum conversion efficiency in asingle-band-gap SC.1 First infrared colloidal NQD SCswere reported in 2005,6 and since then, their powerconversion efficiencies have increased to the currentlevel of 8%.7 These SCs rely on the traditional chargetransfer scheme putting extra demands on the carriertransport in the NQD network. Consequently, theysuffer from insufficient interface quality issues mani-fested in a large number of defect sites that trap chargecarriers, leading to poor transport through inhomoge-neous NQDmedia. While tremendous progress has beenachieved trying to mitigate these obstacles (variousligand exchange protocols to suppress surface statesand enhance the electron mobility, mesoscopic TiO2/ZnO substrates for more efficient charge collection),the resulting efficiency has not yet achieved techno-logically required levels.Given these issues, the utilization of infraredNQDs in

ET-based PV hybrids presents an attractive alternative.Charge carrier transport via NQDs (relying on overlapof the electronic wave functions) would no longer berequired while energy transfer processes (relying onpurely electromagnetic interactions) could be effec-tive. Sensitization of Si layers in the NIR would boostthe overall conversion efficiency. It is therefore impor-tant to understand the relevant ET processes, particu-larly ET from NQDs into Si in this region of thespectrum, to assess the viability of the concept andits practical realization. Recent experiments with PbSNQDs8,9 indicate the existence of ET into Si materials atwavelengths λ ∼ 900 nm.In our previous studies of ET from colloidal CdSe/ZnS

NQDs (middle part of the visible spectrum at the

wavelength λ ∼ 560 nm) into bulk10 and thin-layer5

crystalline Si, we identified and distinguished two ETmechanisms. The nonradiative energy transfer (NRET)corresponds to the direct production of electron�holepairs11 in Si by the oscillating electrostatic-like dipolefield of NQD excitons. In the radiative energy transfer(RET) process, the NQD excitons preferentially decayinto photonic modes that propagate only within Si (thosewould become waveguiding modes in thin layers12). Adifferent character of these processes is reflected in theirspecific dependence on the system parameters, particu-larly their distance from the substrate. NRET is estimated tobe effective over distances of a fewnanometers, while RETcan be considerably longer-ranged and effective up todistances of ∼0.1λ (λ being the vacuum wavelength). Inthe case of proximal CdSe/ZnS NQDs, we established thatNRETdominates, and the combined efficiency of ET into Siwas estimated to be ∼90%. We note that while ob-served5,10 distance dependence of transfer rates isconsistent with ET mechanisms, it is not compatiblewith a typical exponential dependence of charge trans-fer. The latter mechanism is evidently non-operative atlarge nonconductive separations of NQDs from thesubstrate in our system.Two considerations should be taken into account for

discussion of ET processes in the NIR: first, the physicalsize of NQDs emitting in the near-IR is typically larger,and NRET rates from larger NQDs decrease due tolonger effective separation distances (see refs 14 and15 for recent discussions of different aspects of size-dependent Förster NRET with NQD energy donors).Second, the dielectric properties of Si exhibit thefrequency dependence with strongly decreasing ab-sorption at longer wavelengths (see Figure 1). Just asthe dipole�dipole interaction-mediated Förster ETbetween small species depends on the overlap be-tween the emission spectrum of the energy donor andthe absorption spectrum of the acceptor,12 the rate ofNRET into Si is proportional to the imaginary part ε00(ω)of the dielectric function of Si at the NQD emissionfrequencyω. One therefore would expect a substantial

Figure 1. Frequency-dependent dielectric function ε(ω) =ε0(ω) þ iε00(ω) of crystalline Si as measured by Aspnes andStudna.13 Dots show the reported tabulated data points,and solid lines display a smoothing-spline fit that is used incalculations for this paper.

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reduction of NRET rates in the NIR. The efficiency ofRET, on the other hand, is related to the high refractiveindex of Si, that is, to the real part ε0(ω) of its dielectricfunction. As the latter changes only slightly, the role ofRET is expected5 to increase in the NIR.In this article, we present a detailed comparative

study of ET into Si substrates from a set of NQDs ofvariable sizes and compositions that emit in a widespectral range between 545 and 800 nm. Submono-layers of NQDs have been carefully grafted on Si andSiO2 substrates under the same processing conditions,thus allowing for a unified picture to be drawn fromexperimental results supplemented by a theoreticalanalysis. The determination of NRET and RET rates isbased on time-resolved measurements of the NQDphotoluminescence (PL) decay. We find that all of theresults are largely consistent with a basic theoreticalpicture of the modification of the electric dipole transi-tion decay rate in the vicinity of the dielectric interfaceas determined by the dielectric properties and thedistance to the interface.12,16 Confirming our expecta-tions, the relative role of NRET and RET processes into Sichanges for proximal NQDs as a function of the emis-sion frequency: while NRET dominates at shorter wave-lengths, it becomes comparable with RET toward theNIR. At larger distances from Si substrates, RET is aprimary mechanism. Importantly, the efficacy of ETprocesses is consistently observed over a practicallywide region of the solar spectrum with broad rangeoverall ET efficiencies estimated at the level of ∼90%.Along with our earlier demonstration5 of the robust-ness of ET into ultrathin nanomembranes, the currentobservations strongly support the viability of the con-cept of ET-based hybrid nanocrystal/silicon thin-filmPV devices for efficient harvesting of solar energyacross the spectrum of wavelengths.

RESULTS AND DISCUSSION

In our experiments, we prepare uniform submono-layer coverages of nanocrystals specifically grafted onSi and SiO2 surfaces. We used NQDs grafted on barecrystalline Si wafer samples and on samples with a d =10 nm thick interfacial, thermally grown spacer layer ofSiO2 on top of the crystalline Si substrate. Samples withNQDs grafted on a thick, 2 μm, layer of the oxide on topof the original Si substrates (referred to as “glass” in thispaper) were used as a reference. Grafting of a self-assembled monolayer (SAM) passivates the Si surfacethrough Si�C bonding and provides functional headgroups for the NQD attachment.17�19 In addition,linker-assisted grafting allows for accurate NQD posi-tioning at well-defined distances from the Si substrate,well-suited for our ET studies. A submonolayer ofnanocrystals is then attached to the surface with adensity of ∼1011 cm�2 to minimize the interdot inter-action effects. By introducing the SiO2 spacer, we are

able to obtain a larger range of distances and thereforeto affect the relationship between NRET and RET decaychannels and thus to experimentally quantify botheffects, as discussed later.Figure 2a shows the PL emission spectra of a set of

NQDs used in our experiments when grafted on glasssubstrates, covering a wide spectral region from 545 to800 nm (2.3�1.55 eV). We label NQDs here by theirpeak emission wavelength λ: “NQD-λ”. Of six NQDsamples employed, two with the smallest sizes arespherical: CdSe/ZnS core/shell dots with diameters ofD = 4 nm (NQD-545) and D = 4.6 nm (NQD-565). Theother two are CdSe/ZnS dots of ellipsoidal shapes,with main axes of ∼(4�5) � 9 nm (NQD-605) and6 � 12 nm (NQD-655). The longer wavelength NQDs

Figure 2. (a) PL emission spectra of different NQDs emittingfrom 545 to 800 nm as grafted on SiO2. NQDs on Si exhibitthe same PL profile. (b) AFM image of a sample with NQD-705 grafted on the Si surface. (c) AFM depth profilingextracted from (b) that indicates the preferential position-ing of ellipsoidal NQDs with their longer axis parallel to thesurface.

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(NQD-705 andNQD-800) aremade of CdSeTe/ZnSwithdifferent Te concentrations, and both have an ellipsoi-dal shape, 6 � 12 nm. To better determine the NQDplacement on the surface, wemeasured NQD position-ing by atomic force microscopy (AFM) for ellipsoidaldots. A representative AFM image of larger dots (NQD-705) is shown in Figure 2b. Here we used a samplewith a more dilute NQD surface coverage in order toaccurately perform the AFM depth profiling displayedin Figure 2c. The profile shows that the majority ofthe ellipsoidal dots are positioned with their longeraxis parallel to the surface, probably due to a largernumber of linkermolecules available for grafting in thisgeometry. The AFM measurements confirm that thesmallest diameters of the ellipsoidal dots are very closeto the values specified by themanufacturer, in this case∼5.5�6 nm (two larger spikes in the depth profilefigure are likely from dots facing upward or from smallclusters). The chemistry of our linkers ensures theirdirect attachment to the NQD body surface. Given thelength of the grafting molecules, ∼1.7 nm, we thusestimate NQD's center-to-surface separations as z =3.7 nm for NQD-545, z = 4 nm for NQD-565, and z ∼4�5 nm for NQD-605. The other three samples haveapproximately the same distance z ∼ 5�6 nm (NQD-655, NQD-705, and NQD-800).It is instructive to first assess the effects of the

proximity to substrates on the NQD exciton decaywithin a well-known macroscopic electrodynamicsframework.12,16,20 The NQD exciton is described hereas an effective point electric dipole emitter at the NQDcenter position, distance z from the substrate thattakes into account both the physical size of the NQDand the grafting linker. The framework then treats theelectric dipole transition decay via studies of themodification of electromagnetic fields of a classicaloscillating electric dipole in dielectric environmentscharacterized by their frequency-dependent complexdielectric functions ε(ω) = ε0(ω) þ iε00(ω). For weakexciton field coupling, the results obtained in thequantum picture of the exciton decay due to fluctua-tions of the electromagnetic field are identical to thoseobtained with classical dipoles (section 8.4 of ref 12).We used this picture in our previous work5 and willbriefly discuss some of its elements here for complete-ness and reader's convenience.When in vacuum, the lifetime τ0 and decay rate

Γ0 = 1/τ0 of the exciton due to radiative transitionsare determined from12

Γ0 ¼ k3jpj23πε0p

(1)

where k = ω/c = 2π/λ is the emission wavenumberand p the (effective) dipole transition moment. For thedipole positioned at distance z from a planar surfaceof the substrate, the electromagnetic decay rate is

modified to Γ, such that

Γ=Γ0 ¼ 1þ I(0,¥) (2)

I(a, b) ¼ ReZ b

a

sds

2ffiffiffiffiffiffiffiffiffiffiffiffiffi1 � s2

p [(2s2 � 1)r(p)(s)

þ r(s)(s)]exp(2ikzffiffiffiffiffiffiffiffiffiffiffiffiffi1 � s2

p) (3)

Here r(s) and r(p) are the reflection coefficients for s- andp-polarized waves, respectively, that contain the in-formation on the dielectric function ε(ω) of the sub-strate. The decay rate of a dipole generally depends onits orientation; eq 3 is a result of averaging eq 10.26 ofref 12 over random dipole orientations. Upon reflection/refraction at the planar interface, the in-plane (parallel tothe interface) components of the wave vector areconserved. The integration variable s in eq 3 relatesmagnitude k ) of those in-plane components to thevacuumwavenumber k as k )(s) = sk. Consequently, thenormal components kz of the wave vector in each ofthe spatial regions are determined by their dielectricfunctions:

k2z1(s) ¼ (1 � s2)k2, k2z2(s) ¼ (ε � s2)k2 (4)

in vacuum and in the substrate, respectively. Splittingthe integration range in eq 2 into appropriate partsallows one, when possible, to distinguish betweenthe “propagating” (real values of normal componentsin eq 4) and evanescent (imaginary values) character ofthe waves in different spatial regions.For our numerical illustrations in Figure 3 and Figure 5,

we directly compute integrals in eq 3with the dielectricfunction ε(ω) of Si as shown in Figure 1. Note that whilethe reported experimental data on ε0(ω) in Si areconsistent, the literature data on ε00(ω) in this spectralrange feature variations, including by different mea-surement methods (see, e.g., ref 21 for a comparison ofsome data). Panels a and b of Figure 3 show the ratioΓ/Γ0 in eq 2 calculated for distances z = 4 and z = 6 nmfrom the dipole to the Si substrate as a function of theemission wavelength. These panels show the ratio notonly for the total decay rate Γ = ΓSi but also for thecontributions to it coming from different channels:

ΓSi ¼ Γv þΓRET þΓNRET (5)

For the purposes of this paper, we define and denotethese contributions by the following splitting of theintegration range in eq 2:

Γv=Γ0 ¼ 1þ I(0, 1) (6)

ΓRET=Γ0 ¼ I(1,ffiffiffiffiε0

p) (7)

ΓNRET=Γ0 ¼ I(ffiffiffiffiε0

p,¥) (8)

Given the weak absorption in Si in this spectral range,ε00 , ε0, the separation into channels eq 6�eq 8 is“clean”, with the understanding that “propagating”

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modes also experience this weak absorption in Si. It isthen clear from eq 4 that eq 6 corresponds to the decayinto electromagnetic modes that can propagate in thewhole space (both in vacuum and in Si). The contri-bution eq 7, on the other hand, corresponds to theelectromagnetic modes that can propagate only insideSi, and the fields of thesemodes are evanescent on thevacuum side from the interface. These two contribu-tions are radiative: they exist even when ε00 = 0 andpractically do not change for small ε00. The contributioneq 8 is however very different: it would vanish for ε00 = 0and is proportional to ε00 for the weak absorption,which is clearly seen in the analytical expression forits “electrostatic limit”:16,22

ΓNRET

Γ0=

λ

2πz

� �3 ε00

2jεþ 1j2 (9)

There are no propagating electromagnetic modescorresponding to the range of variable s in eq 8; itdescribes a purely nonradiative process due to theJoule losses22,23 in Si caused by the electrostatic-likenear-field of the dipole.It should be clear that contributions eq 7 and eq 8

describe decay channels corresponding to ET into Si:

eq 8 via direct excitation of the electron�hole pairsand eq 7 via the eventual absorption of the propagat-ing modes in Si. In fact, a portion of Γv in eq 6 wouldalso be absorbed in Si contributing to radiative ET.Keeping in mind the applications for ultrathin layers,5

however, we do not add this contribution in theassessment of the efficiency and define RET here asexclusively due to exciton coupling with evanescentmodes of the substrate, eq 7.Panels a and b of Figure 3 show thatΓv is suppressed

in the vicinity of the substrate (a lower probability ofdecay into electromagnetic modes that can propagatein the whole space), while ΓRET and ΓNRET are substan-tial ensuring the overall acceleration of the decay withrespect to vacuum. The relationship betweenNRET andRET changes as a function of distance z and wave-length λ, which is explicitly illustrated in panel c. It isclear that NRET dominates only in the visible part of thespectrum at shorter distances from Si. NRET becomescomparable to RET toward the NIR and is alreadyovercome by RET at z = 6 nm over the whole spectralrange shown here. Details of this relationship, ofcourse, depend on numerical values of ε(ω) and arediscussed here in the context of the data of Figure 1. Avery different distance dependence of RET and NRETrates, however, does ensure the dominance of RET atsufficiently large distances from the substrate.The analytical expression for NRET rate, eq 9, fea-

tures a well-known 1/z3 distance dependence and isnoteworthy for understanding why the decrease ofΓNRET/Γ0 toward longer wavelengths in numericalresults of Figure 3 is milder than would be expectedjust from the behavior of ε00 in Figure 1. First, thedielectric screening factor |ε þ 1|2 in eq 9 becomessmaller toward the NIR. Second, it is not distancez itself, but the dimensionless z/λ ratio that is relevantfor eq 9 (in other words, the vacuum radiative rateΓ0 ineq 2 also becomes smaller for lower frequencies).Consistent with our considerations above, we define

the efficiency of ET into Si as the fraction

ΓRET þΓNRET

ΓSi(10)

which is shown in Figure 3d. Somewhat higher valueswould be obtained if a portion of Γv was added to RETthat corresponds to the absorption in the substrate. Inaddition to displaying remarkably high resulting ETefficiencies for proximal dipoles, panel d also illustratesthe efficacy of RET at much longer distances, such asthe example of z = 50 nm. At distances z in excess ofseveral nanometers, say zg 10 nm, NRET is a negligibleprocess, while RET still contributes in a very substantialway. Characteristically, at larger distances z, RET is evenmore efficient in the NIR, the reason being smallerdimensionless ratios z/λ in that spectral region.The described theoretical framework featured only

electromagneticdecay channels; it canbeeasilymodified

Figure 3. Theoretical results illustrating modification of thedecay time as a function of vacuum wavelength for arandomly oriented electric dipole transition in the vicinityof the Si substrate described by the dielectric function ε(ω)of Figure 1. Top panels display the total decay rate and itscontributions from different channels in terms of the vac-uum rate: (a) for distance z = 4 nm and (b) z = 6 nm from thesubstrate. (c) Ratio of contributions fromNRET and RET for aseries of indicated distances from the substrate. (d) Corre-sponding total efficiencies of ET, eq 10, into the substratefor distances shown in panel c and, in addition, for largerseparations as indicated.

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by addition of another unspecified nonradiative decaychannel that would correspond to quantum yields (QYs)of NQD emission less than 100%.16 In the absence ofthe direct experimental data on QY, its value can betreated as a theoretical fitting parameter in comparisonof experimental and theoretical results. With the samemagnitude of QY < 100% for the decay in vacuum andin the vicinity of the substrate, theoretical results forthe relative acceleration of the overall decay near thesubstrate would understandably (extra decay channel)exhibit lower values.We will now discuss our experimental observations

in the context of the theoretical framework. Time-resolved PL measurements have been conducted forvarious samples with all NQD types described above.We note here that while there is a general correspon-dence between the PL decay and PL intensity data, theabsolute PL intensity is affected not only by ET butalso by the optical generation rates as well as by thespecifics of the emission collection. The interferenceeffects in multilayered structures can modify the elec-tric field intensity at the surface, therebymodifying the

number of photogenerated NQD excitons. The radia-tive emission pattern is also well-known to dependon the geometry.12 PL lifetime changes, on the otherhand, depend only on the relevant decay rates such asET. Thus, we uniformly employ changes in PL dynamicsas a more unambiguous quantitative means for ETdetection. Six panels a�f in Figure 4 show the timedependence of PL decays for six types of NQDs emit-ting at certain wavelengths (see Figure 2a). Each paneldisplays three traces: the PL decay for a referencesample with NQDs grafted on the glass substrate

Figure 4. (a�f) PL dynamics for nanocrystals emitting atdifferent wavelengths spanning the range from (a) 545 nmto (f) 800 nm. Shown on the panels are the respective PLmeasurement wavelengths. The horizontal scale (time) foreach panel is normalized to the specific NQD lifetime (fromdouble exponential fits) as measured on glass, τG, indicatedon the panels, and extends to tmax = 3τG. Green traces (top)show measured NQD PL decays on reference glass sub-strates, red traces (bottom) on bulk Si substrates, and lightblue traces (middle) on Si substrates with a d = 10 nm thickinterfacial SiO2 spacer. Black curves on the top of the tracesshow the fits used to extract the lifetime values. A relativeslowdown of the PL decays on Si substrates toward longeremission wavelengths is clearly visible.

Figure 5. (a) Comparison of relative decay rates ΓSi/ΓG fordifferent emission wavelengths as calculated from theoryand as extracted from the PL decay data in Figure 4. Forsampleswith NQDs on bare Si substrates: theoretical resultsshown as solid red lines for a set of dipole-to-substratedistances z; experimental data points with connected solidcircles. For samples with NQDs on Si substrates with a d =10 nm oxide spacer layer: theoretical results shown as solidblack lines; experimental data points with connected solidsquares. Blue- and green-colored data points representdifferent estimates for experimentally measured decayrates as extracted from, respectively, single/single andsingle/double fits to PL decays on bare Si and on glasssubstrates discussed in the text. Error bars represent meansquare deviations for two or three independent sets ofgrafted NQD samples. (b) Total ET efficiency Etotal shownby black squares along with separate contributions ENRETfrom NRET (red circles) and ERET from RET (blue triangles).The efficiencies were estimated as described in the text.

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(top trace, green), for a sample with NQDs grafted onthe bare Si substrate (bottom trace, red), and for asample where NQDs were grafted on the top of a d =10 nm thick oxide layer separatingNQDs from the bulk Sisubstrate (middle traces, light blue). In order to visuallycompare changes in the decay rates, the horizontal axis(time) for each panel is normalized to the measureddecay time τG of the corresponding reference NQD-on-glass sample. The actual measured decay times on glassare indicated in the panels and range from τG = 21.5 nsfor small spherical (D = 4 nm) CdSe/ZnS dots to τG∼ 170ns for larger ellipsoidal (6 � 12 nm) CdSeTe/ZnS dots.Lifetimes are generally well-fitted with monoexpo-

nential curves. However, some of the PL decays forNQDs grafted on SiO2 surfaces exhibit small (10�15%)initial faster components. In a separate study, we haveobserved that such initial components can be consid-erably suppressed bypassivating already graftedNQDswith diamine molecules on top. Such a behavior couldbe resultant from the less than perfect NQDgrafting onSiO2 surfaces as compared to the bare Si substrates, orit could originate from trapping channels specific toNQDs as have also been suggested in the literature forthiol-capped NQDs.24 In instances with biexponentialdecay fits, the longer components of the decay havetherefore been considered as lifetimes indicated inFigure 4.It is transparent that the decays on bare Si substrates

are much faster in comparison with decays on glasssubstrates;consistent with the picture of new decaychannels due to ET from NQDs into Si. It is also clearlyseen that decays on Si become progressively longer(with respect to the time-normalized reference traces)toward the NIR. This trend is also consistent withtheoretical expectations of Figure 3 owing to a gradualreduction of NRET rates toward longer emission wave-lengths. Measurements on samples with a thin d =10 nm oxide spacer layer serve to assist in distinguish-ing effects due toNRET and RET decay channels. On thebasis of theoretical calculations and on our previouswork,10 we expect that a 10 nm SiO2 separation wouldmake NRET into Si a negligible contribution whileaffecting radiative channels relatively insignificantly(d/λ ∼ 0.01�0.02).For a quantitative assessment and comparison with

theoretical calculations, the PL lifetimes τ have beenextracted from the data in Figure 4 and plotted inFigure 5a in terms of the corresponding decay rates(Γ = 1/τ) relative to rates ΓG on reference glass samplesfor each of the emission wavelength, respectively.Given some uncertainty in the fitting of the PL decays,the decay rates have been evaluated in two differentways: using single exponential fits to both PL decays onSi and on glass surfaces, and using single exponentialfits on Si and longer components of the double ex-ponential fits on glass surfaces as discussed above. Fitson Si are quite monoexponential across nearly two

decades of the PL signal change. Figure 5a shows theexperimental data points evaluated in both ways foreach type of NQDs as a function of their emissionwavelength. Also plotted in Figure 5a are theoreticalcurves computed for dipole decay rates on bare Si andon Si with a d = 10 nmoxide spacer relative to decay onreference glass substrates (in computations, SiO2 hasbeen characterized by refractive index n = 1.5 acrossthe whole spectral range).Overall, there is a good agreement between salient

features of experimental and theoretical results inFigure 5a. As a function of the wavelength, the resultsfor NQDs on bare Si substrates exhibit a reduction fromΓSi/ΓG∼ 6�7 at λ= 545 nm toΓSi/ΓG∼ 4 at λ= 800 nm.For NQDs on substrates with a d = 10 nm oxide spacer,on the other hand, there is little variation across thespectral range, with ΓSi/ΓG ∼ 2. As the theoreticalcurves are shown for a set of different distances z fromthe dipole to the top surface, one can see that the datafor NQDs on bare Si substrates is indeed reflective oflarger NQD sizes toward the NIR and consistent withour estimates of z based on the sizes and linker lengths.Once again, there is very little dependence on z andNQD sizes in the data for substrates with the oxidespacer;as expected due to the suppressed NRET andrelative insensitivity of ΓRET/Γ0 to the emission wave-length at these distances.In fact, even quantitatively the agreement between

experimental data and theoretical expectations inFigure 5a appears to be quite reasonable. With this inmind, one could now use experimental data in con-junction with theoretical results to estimate the effi-ciency of ET from NQDs into the Si substrate. Figure 5bshows the estimated total energy transfer efficiencyEtotal defined in eq 10 as well as separate contributionsERET and ENRET from RET and NRET processes, as afunction of the wavelength for NQDs on bare Si sub-strates in our experiments. In these estimates of Etotalfrom experimental data, we used theoretical values ofΓv = 0.6Γ0 (see Figure 3) and ΓG = 1.6Γ0 (see ref 5).Assuming now that ΓRET does not change much forsamples with a d = 10 nm oxide spacer layer, whileNRET is practically negligible at these separations fromthe Si substrate, we directly use experimental data onsamples with the spacer as a measure of ΓRET. Theresulting assessment of separate contributions ERETand ENRET to Etotal is displayed in Figure 5b. It clearlyshows that NRET dominates at shorter emission wave-lengths in the visible but becomes comparable withRET toward longer wavelengths in the NIR. The jointcontributions from both channels result in high effi-ciencies Etotal across the whole spectral range, esti-mated at the level of ∼90%.While precise numerical values for ET efficiency are

of course subject to the accuracy of the data andadequacy of the model representation, it is importantto realize that the fact of high ET efficiencies is clearly

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borne out by the raw experimental data in Figure 5a.Even in the most “simple-minded” approach neglect-ing modifications of the reference radiative decayrates, that is assuming ΓG = Γ0 and Γv = Γ0, the ob-servation ofΓSi/ΓG= 4would alreadymean Etotal = 75%,and the observation of ΓSi/ΓG = 7 would mean Etotal =86%. Our estimates taking into account modificationsof radiative decay rates in the vicinity of dielectricinterfaces then understandably result in higher effi-ciency numbers.We note that a generally good quantitative agree-

ment between experimental and theoretical data hasbeen achieved without usage of any extra fittingtheoretical parameters, including that of QY. In fact,computational results with QYs appreciably smallerthan 100% (the same QY for NQDs on Si and on SiO2)showmore substantial deviations from the experimen-tal data. This is suggestive of high intrinsic QYs of NQDsin our submonolayers, which is consistent with otherobservations. The grafting technique passivates Si sur-face and hence eliminates trapping states that areusually the reason of decreased QY of NQDs on sub-strates. HighQY is elucidated from long-time PL decaysthat are measured when NQDs are grafted on refer-ence glass substrates;and these decays are verymuch comparable to PL lifetimes of NQDs in manu-facturer solutions, with small differences that are di-rectly attributable to somewhat different dielectricenvironments. The fact that a good agreement isachieved with high-QY theoretical calculations mayindicate that emissive NQDs (that we study) do notpossess any substantial intrinsic nonradiative channels.Experimental observations of lower QYs could then berather interpreted as due to the conversion of emissivespecies into nonemissive (such as charged or trapped)excitons.The very logic of the trends we observed in agree-

ment with theoretical expectations indicates that theinterplay of NRET and RET processes would be furthertilted in favor of RET at longer wavelengths thanstudied here. In recent publications8,9 on PbS NQDs(λ ∼ 900 nm) deposited on various Si materials, how-ever, the observed changes in PL dynamics have beenattributed purely to the effects of NRET. We note that inexperiments of ref 9, for example, NQDs were depos-ited on top of a d = 5 nm interfacial SiO2 layer, thusresulting in at least a 7�8 nm separation from the NQDcenter to the Si substrate. Our results suggest thatunder such conditions NRET would be quite limitedand ET may be mostly due to radiative coupling withelectromagnetic modes propagating in Si. The inter-pretation of experimental data as entirely due to NRETeffects would result in quite large values of the corre-sponding Förster radius;these surprisingly largevalues have already been noted23 in the context ofexperiments in ref 8.

A proper evaluation of contributions from differentchannels to overall ET is relevant for design of ultra-thin-film PV devices;it suffices to mention that whileNRET is practically independent of the thickness of Sinanomembranes, the excitation of waveguiding modesvia RET can be affected by that size.5 Further improve-ments in that assessment should probably include bothexperimental and theoretical refinements. Experimen-tally, even better quality samples are desired exhibitingmonoexponential PL decays. That would assist in modeltheoretical fits perhaps allowing one to distinguishbetween different radiating dipole orientations (theresults in this paper have been averaged over randomorientations). The theoretical framework also needs tobe extended to evaluate the influence on our observa-tions of dielectric local field effects for NQDs. It hasbeen known for a long time that local field effects cansubstantially affect the magnitude of the effectivedipole moment as seen in different uniform dielectricenvironments.25 The corresponding influence of theuniform surrounding on the radiative lifetime of NQDexcitons has become a subject of recent theoretical26

and experimental27 interest. How strongly similar ef-fects would manifest themselves for NQDs in the inter-facial region we study is not clear and requires adedicated study.As our observations here indicate an even larger role

played by RET in the NIR spectral range, it is useful tocompare this radiative coupling of NQD's PL emissionwith the coupling recently achieved in plasmonic-assistednanostructures. There, surface-propagating plasmonicmodes and in-coupling of the incident light into asemiconductor substrate have been realized with sub-micrometer-sized metal structures.28 While such ap-proaches certainly present interest for light manipula-tion and harvesting, we note that plasmonic modescan be inherently lossy, thus possibly reducing energyconversion efficiency. In our ET-based structures, thelight conversion mechanism is implemented via photonabsorption in the NQDs and coupling of the re-emittedlight into propagating modes of the substrate. Giventhe broad absorption spectrum of semiconductor NQDsthat can be tuned across visible and NIR spectral ranges,the majority of the incoming photons could be poten-tially converted into band edge excitons, which, inturn, are efficiently transferred into the Si substrate viacombined effects of NRET and RET. The significance ofRET becomes more accentuated at the NIR wave-lengths where the absorption of crystalline Si is ex-tremely weak. To our knowledge, the important role ofRET in harvesting NIR photons in ET-based hybrid nano-structures have not been properly addressed so far, andthis paper provides a first quantitative determination ofthe efficacy of this process for individual NQDs.We already discussed5 that device-oriented ET-

based nanostructures need to employ optically thick,multilayer NQD assemblies to provide for sufficient

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absorption of the incident light. These dense assem-blies can substantially affect the local and global di-electric environment leading to changes in the inter-play of NRET and RET processes, both between theNQDs and in their coupling with Si substrates. Weforesee measurements of ET in macroscopically thickNQD layers grafted on Si substrates as the next im-portant step in assessing the effectiveness of ET-basedhybrid PV structures.

CONCLUSIONS

To summarize, we have demonstrated a hybridsystem composed of colloidal nanocrystals and Sisubstrates that can effectively transfer optically createdNQD excitations into high-carrier-mobility Si materialover awidewavelength range, from visible to the near-infrared. We employed covalent grafting of NQDs oncrystalline Si substrates ensuring electronic passivationof surfaces and positioning NQDs at well-defined dis-tances from the Si substrate. Observations and aquantitative analysis indicate very high broad rangeefficiencies of ET, estimated to be at the level of∼90%.This high efficiency is achieved via combined effects ofNRET and RET whose relative roles change with the

wavelength. We assessed separate contributions fromthese ET channels experimentally by comparing PLdecays of NQDs on bare Si substrates and on substrateswith a thin SiO2 spacer layer. We found NRET to be adominant contribution for proximal NQDs in the mid-dle of the visible range but become comparable withRET toward the NIR. At larger distances from the Sisubstrate, RET serves as a major ET mechanism. As ETcan be as effective into ultrathin Si nanomembranes,5

our current observations show that the ET-based strat-egy is a viable proposition for NIR absorbing/emittingfluorophores that are optimally tailored for light har-vesting in thin-film solar cells. In thin-film PV devices,NQD excitonsmay effectively excite waveguiding elec-tromagnetic modes propagating within the Si film.Even in this range of weak Si absorption, these modeswill then eventually decay into electron�hole pairs,without demanding the film to be thick. NQD-sensi-tized thin-film Si structures thus emerge as promisingcandidates for PV devices harvesting solar energyacross the spectrum of wavelengths. Their actual po-tential and achievable conversion efficiencies shouldbe further explored in practically oriented structureswith optically thick NQD assemblies.

METHODSCrystalline Si(111) substrates were used for the experiments.

Reference “glass” samples as well as samples with thin inter-facial SiO2 spacer layers were prepared by thermal oxidation in acleanroom environment. Surface functionalization was basedon two approaches depending on the starting surfaces. Foroxide-free Si surfaces, a thermal hydrosilylation reaction (200 �C)using ethyl undecylenate molecules on a hydrogenatedsilicon surface (30 s in 10% HF solution and 150 s in 40% NH4Fsolution) was performed, leading to a carboxylic-acid-terminatedalkyl chain monolayer after transformation of the headgroups.18 Ethylene diamine was then grafted to the carboxylicacid group in a solution of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and 2-(N-morpholino)ethanesulfonic acid(MES).5,10,29 For silicon oxide (SiO2) surfaces, the freshly cleanedsamples were immersed in a preheated (70 �C) anhydroustoluene solution containing 0.2% of triethoxysilyl undecanal(C11-Ald) molecules for 12 h in a recirculation glovebox(anhydrous conditions). Organic soluble colloidal NQDs emit-ting at the range from λ = 545 nm to λ = 800 nm werepurchased from Invitrogen. Immediately before grafting, NQDsolutions were diluted to the required concentrations necessaryto obtain desired NQD surface coverage in order to avoid liganddetachment commonly observed in ageddilutions. All function-alized surfaces have an amine group as an active group used forthe NQD attachment. The latter was performed by immersingsamples in a solution of 2mL of hexanewith 7 μL of a stock NQDsolution for 1.5 h. Samples with grafted NQDs were thenimmersed in a solution of 1,6-hexanediamine for 1.5 h toimprove NQD passivation. Samples were sonicated in tolueneand rinsed at least twice with deionized water (DIW) and driedunder the N2 gas. PL spectra of NQDs grafted on all surfaceslooked similar with no “deep trap” PL emission features atlonger wavelengths. The measured ellipsometric thicknessesof SAM molecules employed in these attachments were ap-proximately 1.7 nm. As described in our previous work,10 theseconcentrations correspond to submonolayer NQD coverages inorder to minimize interdot-NRET effects that typically compli-cate data interpretation.

For spectroscopic studies, we used amicroscope-based time-resolved PL system. Samples were mounted on the microscopetable and excited at 405 nm by 50 ps laser pulses at 2.5 MHzfrom a Picoquant diode laser. Excitation was focused on thesample via NA = 0.6 objective that also ensured a high photoncollection efficiency necessary to obtain PL signatures from asubmonolayer of NQDs. The collected emission was passedthrough a spectrometer and directed either to a CCD camera forspectral analysis or to a sensitive photon detector (MicroPhotonDevices) for the wavelength-dependent PL lifetime detection.PL decays were collected via the time-correlated single photoncounting (TCSPC) performed on board the Pico300E photoncounting hardware (PicoQuant GmbH). The overall time resolu-tion was better than 70 ps.

Conflict of Interest: The authors declare no competingfinancial interest.

Acknowledgment. This work was supported by the NationalScience Foundation Grant No. DMR-1207123.

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