Ultrafast Dynamics of Multiple Exciton Harvesting in the CdSe−ZnOSystem: Electron Injection versus Auger RecombinationKarel Zídek,* Kaibo Zheng, Mohamed Abdellah, Nils Lenngren, Pavel Chabera, and Tonu Pullerits

Department of Chemical Physics, Lund University, Box 124, 22100, Lund, Sweden

*S Supporting Information

ABSTRACT: We study multiple electron transfer from a CdSe quantum dot (QD) to ZnO, which is a prerequisite forsuccessful utilization of multiple exciton generation for photovoltaics. By using ultrafast time-resolved spectroscopy we observecompetition between electron injection into ZnO and quenching of multiexcitons via Auger recombination. We show that fastelectron injection dominates over biexcitonic Auger recombination and multiple electrons can be transferred into ZnO. A kineticcomponent with time constant of a few tens of picoseconds was identified as the competition between injection of the secondelectron from a doubly excited QD and a trion Auger recombination. Moreover, we demonstrate that the multiexciton harvestingefficiency changes significantly with QD size. Within a narrow QD diameter range from 2 to 4 nm, the efficiency of electroninjection from a doubly excited QD can vary from 30% to 70% in our system.

KEYWORDS: Quantum dots, solar cell, electron transport, ultrafast transient absorption, multiple exciton generation

Demand for new, sustainable sources of energy has been adriving force for the immense development of solar cells

(SCs) during the last decades. The crucial issue of theexpansion of SCs is to find a balanced combination of theirefficiency and price. For instance, the efficiency of single-junction SCs is inherently restricted by the Shockley−Queisserlimit of about 32%.1 It originates from the fact that onlyphotons with energy close to the band gap (or HOMO−LUMO) can be fully utilized. In other cases photons are eithernot absorbed at all (the photon energy is lower), or energy ispartly lost due to thermal relaxation of excited carriers (thephoton energy is higher). Moreover, the Shockley−Queisserlimit represents an ideal case, whereas the efficiency of a realsolar cell is further diminished by various losses in the system.2

There are several approaches to overcome this limitation. Forinstance, multijunction SCs can reach high conversionefficiencies up to 43.5%:3 however, only on expenses ofextremely high price. Quantum dots (QDs) offer an alternativeto break the Shockley−Queisser limit.3 High-energy photonsabsorbed in QDs excite carriers, which can relax via the so-called “multiple exciton generation” (MEG) process. Instead ofthermal relaxation, energy is converted into the usable form oftwo or more electron−hole (e−h) pairs. A number of studies

report on efficient MEG in QDs of various materials.4−9 Severalstudies also prove successful utilization of MEG enhancementin QD-sensitized SCs based on the QD−metal oxide (MO)system.10,11

In QD-sensitized SCs, electron injection from QD to MO isresponsible for rapid charge separation and, hence, allows lightenergy harvesting.12−15 However, utilizing several excitons outof one QD is always limited by Auger recombination (AugR)inside the QD.16 During AugR, one e−h pair recombines, andits energy is transferred to the electron or the hole in thesecond e−h pair, which undergoes a rapid thermal relaxation.As a result, the energy of one e−h pair is lost for photovoltaicconversion. AugR is an inverse phenomenon to MEG, and itinevitably follows the MEG process, because multiple e−h pairsare created.5,16 Since AugR is a subnanosecond process, theMEG benefits can be used only with fast electron injectionfrom QD to MO.Recent studies, including our work employing ultrafast time-

resolved absorption and terahertz spectroscopy, show that

Received: October 8, 2012Revised: November 16, 2012Published: November 19, 2012

Letter

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electron injection in a QD-MO system can take place on a veryfast time scale of a few picoseconds.12,17 However, the reportswere directed toward the study of electron transfer in theregime where only one e−h pair is excited per QD. This isnever the case of MEG, where two and more e−h pairs coexistin one QD. To exploit MEG fully, we need information aboutelectron injection to MO under multiexciton conditions.In this Letter, we mimic MEG by exciting several e−h pairs

per QD with a high-intensity excitation. Through directobservation of multiple exciton dynamics in a CdSe QD−ZnO system we establish a solid foundation for distinguishingcompetition between AugR and electron injection. Wedemonstrate the possibility of multiple electron injectionfrom QD to ZnO, which is essential for efficient utilization ofMEG. Moreover, we study competition between electroninjection to ZnO and AugR for different QD sizes. We showthat there is a relatively narrow range of QD sizes for which theMEG effect can be successfully exploited in our system.QDs were prepared by following a method described in ref

18 (core CdSe QDs) and ref 19 (core−shell CdSe−ZnS QDs).Details about sample preparation procedures can be found inthe Supporting Information.Briefly, the core CdSe QDs capped by oleic acid were

prepared using hot injection of the trioctylphosphine-Se (TOP-Se) precursor to the oleic acid-Cd precursor. The desired sizeof QDs (2.5−3.1 nm) was obtained by cooling the mixturefrom the reaction temperature (180−240 °C) suddenly toroom temperature by using an ice bath.Core−shell CdSe−ZnS QDs were prepared by using a single-

step hot injection method. Both the Cd2+ and the Zn2+ oleate in1-octadecene solution were heated to 325 °C. Then Se2− andS2− in 3 mL of TOP solution were swiftly injected into thecation precursor. Cooling in an ice bath was used to stop theshell growth (at 5 s for thin shellsample “I”and at 5 minfor thick shellsample “S”). The core−shell QDs feature agradual change from core material (CdSe) to pure shellmaterial (ZnS), as it is described in detail in ref 19.As-prepared QDs were purified twice before the sensitization.

For the sensitization, the surface capping of the QDs wasexchanged from oleic acid to a bifunctional linker molecule, 2-mercaptopropionic acid (2MPA). Then the ZnO nanoparticlefilms were immersed into the QD solution for 2 h in the dark.For our study, we chose three principal examples of QDs

with fast, intermediate, and slow electron injection to ZnO. Wedenote the samples as “F” (fast electron injection, CdSe coreQDs, 2.9 nm size), “I” (intermediate case, CdSe core−ZnS thinshell), and “S” (slow electron injection, CdSe core−ZnS thickshell). Different injection rates are a consequence of differentshell thickness of the QDs for each sample, as the rate isexpected to decreases exponentially with increasing shellthickness.20 For each sample, pure QDs and QDs attached toZnO were studied to clearly separate the effect of electroninjection. Pure QDs prepared as colloidal samples were alsodeposited on glass to verify that the process of deposition itselfdoes not induce a fast TA decay due to surface trapping or QDagglomeration (see the Supporting Information for details).Both types of samples featured the same optical density of QDsat the excitation wavelength (∼0.2), so that excitation intensitydependences can be compared.Transient absorption (TA) dynamics were measured by

using a pump−probe setup described in detail elsewhere.17 Inbrief, laser pulses at 800 nm (80 fs pulse duration, 1 kHzrepetition rate) were generated by a Ti:sapphire femtosecond

laser system with a regenerative amplifier and converted by anoptical parametric amplifier to 470 nm (excitation) and 520−540 nm (probe) pulses. To construct the TA signal, everysecond excitation pulse is blocked; the excitation repetition rateis thereby 500 Hz. TA spectra were recorded by using asupercontinuum generated in a thin sapphire plate as the probebeam, which was detected by a diode array coupled to aspectrograph. During the measurements the samples wereplaced in N2 atmosphere to avoid oxygen-related photo-degradation.21

The absorption spectra of the three studied types of QDsconsist of overlapping excitonic bands as a consequence ofcarrier confinement (see Figure 1A).22 The samples were

selected so that the lowest absorption band is centered near540 nm. Hence the effective QD radius for all samples issimilar, and the difference in electron injection between thesamples is predominantly given by different shell thicknesseswith a marginal contribution from QD size dependence. Thebroader absorption band of the “S” sample (thick shell layer,see solid blue line in Figure 1A) is a consequence ofinhomogeneity introduced by the long shell growth.The acquired TA spectra reflect the band structure of the

QD absorption (see Figure 1B). The dominant feature is thelowest absorption band bleach induced by electron statefilling.16 The amplitude of the bleach can therefore be used todetermine the mean population of excited electrons per QD.The shape of the lowest absorption band bleach is not changingsignificantly during the decay. This allows us to describe thesystem dynamics by focusing to TA kinetics in the bleachspectral maximum (see Figure 1C). A minor shift in the lowestabsorption band position can be observed for the sensitizedsamples (see Supporting Information for all spectra), which canbe ascribed to changes in the QD environment due to theattachment to ZnO together with a possible QD−ZnOcoupling effect.23

Figure 1. (A) Absorption spectra of studied CdSe QDs (solid lines)and example of CdSe−ZnO film absorption (dashed line). (B) TAspectra (delay of 1 ps) of CdSe QDs (solid line) and sensitized NPfilm (dashed line). For all spectra see Supporting Information. (C)Comparison of TA kinetics for pure QDs (dotted lines) and the QD−ZnO system (solid lines) for the three studied samples. Note that x-axes over 0.5 ps are logarithmic. Probe wavelengths: samples “F”: 526nm, “I”: 540 nm, “S”: 540 nm; exc. wavelength: 470 nm; exc. intensity:40 μJ/cm2 (∼1014 photons/cm2).

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For pure QDs, all three samples feature a long-lived TAsignal, which can be well-described by a two-exponential decaywith a long nanosecond component (τ = 13−18 ns, A ∼ 80%)and a minor subnanosecond component (τ = 0.2−0.7 ns, A ∼20%)see Figure 1C, dotted lines. Such multiexponentialdecay is typical for CdSe QDs due to the dynamics ofquenching mechanisms and has been described in detail inprevious reports.24,25

After attachment of QDs to ZnO NPs, the TA decaybecomes significantly fastersee Figure 1C, solid lines. This isa consequence of introducing an additional quenchingpathwayelectron injection to ZnO.12,17 As anticipated, thecore sample (“F” sample) features very fast electron injection.On the contrary, core−shell QDs with a thick shell layer (“S”sample) inject electrons to ZnO only on the nanosecond timescale, as the electron has to tunnel through a thick shell barrier.The intermediate case (thin-shell sample) stands between thosetwo cases.The electron injection from core−shell QDs to MO is a

complex process, which will likely feature a broad distributionof lifetimes due to variation in QD shell properties.14,19 Theexact calculation of the lifetime distribution is beyond the scopeof this article; however, we can estimate the mean electroninjection rate ⟨kinj⟩ from the half-life of the TA signal tH, forwhich holds ⟨k⟩ = ln 2/tH. By comparing the mean lifetimes ofQDs attached and unattached to ZnO, we obtain an estimate ofthe injection rate:15

τ τ⟨ ⟩ = ⟨ ⟩ − ⟨ ⟩−k 1/ 1/inj QD ZnO QD (1)

which is, in our case, 0.18 ps−1 (⟨τinj⟩ ∼ 6 ps) for the “F”sample, 0.018 ps−1 (⟨τinj⟩ ∼ 60 ps) for the “I” sample, and 2.5 ×10−4 ps−1 (⟨τinj⟩ ∼ 4 ns) for the “S” sample.So far we have concentrated on a low excitation intensity

(∼0.2 e−h pairs per QD), where the number of multiexcitons issmall (∼1% of all QDs, < 10% of the excited QDs) andmeasurements reflect transport of one electron from QD toMO. To study the extraction of multiple electrons from a QD,we need to excite several e−h pairs per QD by increasing theexcitation intensity.A typical sign of multiexcitons in QDs is a rapid Auger

recombination (AugR) decay, which takes place in our samplesduring the first 300 pssee Figure 2A.16,26 AugR in QDscannot be described via carrier concentration, which iscommonly used for bulk materials. It is well-established thatQD systems have to be represented instead by a distribution ofsingly, doubly, up to n-excited QDs, because only an integernumber of e−h pairs can be present in one QD. AugR or anyother recombination in QD systems corresponds to a step-liketransition from a QD with n excitations to n−1 excitation.Therefore, unlike in the case of bulk material, for QDs it ispossible to assign a distinct biexciton, triexciton, or n-excitonAugR lifetime.26 The features of AugR in QDs are discussed indetail in Supporting Information.After excitation, the multiexcitons recombine step by step

until a single e−h pair remains in the QD and decays on thenanosecond time scale (see Figure 2A). It is possible tonormalize the TA kinetics for different intensities on the long-lived single exciton decay (see Figure 2A) and extract thecontribution of AugR to the TA kinetics. It can be obtained asthe difference between normalized TA kinetics for high and lowexcitation power (gray area, Figure 2A).26 The AugR dynamics(Figure 2B) has a single-exponential character (straight line insemilog scale) given by the biexcitonic AugR decay. Besides,

there is a minor fast decay in the initial stages (triexciton andhigher order AugR).By fitting the AugR dynamics with a single-exponential

function, we obtain the biexciton AugR lifetime of 18 ps for the“F” sample (τAR /⟨τinj⟩ = 4.5), 73 ps for the “I” sample (τAR/⟨τinj⟩ = 1.2), and 52 ps for the “S” sample (τAR/⟨τinj⟩ = 0.01).All of the data are presented in Supporting Information. TheAugR rate differs from sample to sample, as it highly dependson various parameters, such as the effective size of the QD, orits character (core, core−shell).16,27,28 The gradual change ofpotential from core to shell can significantly alter the AugR rate.The three selected samples clearly represent three principalcases of the AugR−electron injection lifetime balance, whereτAR/⟨τinj⟩ is ranging from 0.01 to 4.5.AugR in QDs leads to a universal intensity dependence of the

long-lived TA signal, as we will show in the followingparagraph.29−31 This is a consequence of rapid AugR of allmultiexcitons, after which only a single e−h pair is present ineach excited QD. As we described before, the initial e−h pairpopulation in QDs can be represented by a distribution of n-excited QDs. A short excitation pulse which excites in averageN0 e−h pairs per QD leads to a Poisson distribution of QDswith k excitations as pk = N0

k/k! exp(−N0).26 The long-lived

signal IL consists of the single-exciton contribution from anyQD which was excited, because all multiexcitons are quenchedby AugR:

∝ − = − −I p N1 1 exp( )L 0 0 (2)

This dependence has been reported in a number of differentQD materials, including CdSe QDs.29−31 We observe thedependence for our QDs, as it is illustrated in Figure 2C. Theimportant feature of eq 2 is the possibility to determine thescaling factor between the number of excited e−h pairs per QDand the used excitation power, that is, to obtain the QDabsorption cross section. For our samples the resultingabsorption cross section at 470 nm ranges from 7 × 10−16

cm2 to 16 × 10−16 cm2 in good agreement with the previouslyreported values.32

Figure 2. (A) TA kinetics at different excitation intensities forunattached QDs (sample “S”) normalized on the long-lived decay. (B)Difference between TA kinetics acquired for high-power and low-power excitation of pure QDs (gray area) fitted by single-exponentialdecay (dashed black line). (C) Long-lived TA signal (IL) dependenceon excitation fluence (squares) fitted by eq 2 (solid lines). IL valueswere rescaled for a better comparison. The probe wavelength is 540nm.

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We will now compare how the multiexcitonic dynamicschange after QD attachment to ZnO. Figure 3A,B displays

dependence of TA decay on excitation intensity for the QD−ZnO samples. In case of the “S” sample (where AugR isconsiderably faster than the electron injection) the change inTA dynamics with excitation intensity is the same for attachedand unattached QDs. This can be illustrated by comparing aratio between long-lived TA signal IL and TA signal amplitudeI0 for attached and unattached QDs (see Figure 3D). As theintensity dependence is the same for both cases, we canunambiguously assign the fast appearing component to AugR.The situation partly changes for the “I” sample, where

already the low-intensity kinetics feature a fast TA decay

connected to electron injection. With increasing intensity, theTA decay becomes faster as expected for AugR. The TAdynamics are then a combination of decay induced by electroninjection and intensity-dependent AugR. Ratios between long-lived TA signals and TA amplitudes can be found in theSupporting Information.We can use the measured kinetics to estimate the AugR rate

for the “I” sample and compare the rates for the cases ofattached and unattached QDs. By following the procedure usedto determine the mean injection time, we can find the mean TAkinetics lifetime for the QD-ZnO system under variousexcitation intensities. The mean lifetime changes from 60 psat low excitation intensities (electron injection only, rate kinj) to35 ps at high excitation intensities (electron injection incombination with AugR, total rate kT = kinj + kAR). Thereforewe can calculate the AugR rate kAR for sample “I” (QDsattached to ZnO) as kAR = kT − kinj, which corresponds to anAugR lifetime of ∼80 ps. This is in good agreement with thevalue observed for the unattached “I” sample QDs (73 ps).Therefore we can conclude that the AugR rate is notsignificantly altered by the attachment of the QD to ZnOand the presence of electron injection.The role of a fast electron injection becomes clearly

dominant only in the case of the “F” sample (see Figure 3B).Unlike in the previous cases, with increasing excitation powerthe TA dynamics slightly slow down for excitation intensitiesup to ∼700 μJ/cm2 (corresponds to N0 ∼ 1.5). To obtain moreinformation about the changes, we calculate the differencebetween normalized TA kinetics (320 μJ/cm2 and 780 μJ/cm2).We observe that the slow dynamics feature about 10% of theamplitude and decays on the scale of tens of picoseconds (fittedlifetime value of ∼50 ps).For the unattached QDs, the changes in the TA dynamics

with excitation intensity are due to the biexcitonic AugR. Incase of the above slowdown of the dynamics in the “F” sample(QD−ZnO), this origin can be excluded, as it would lead tofaster TA signal decay. Therefore we conclude that the 50 pscomponent is a signature of electron injection to MO.We will now shortly review the sequence of processes

present for doubly excited QDs (see Scheme 1). First, two e−hpairs are excited in a QD and electron injection to ZnOcompetes with biexcitonic AugR. After the first electron isinjected to ZnO, an electron and two holes are present in QDforming the so-called positive trion, which can also undergoAugR. Only after the second electron injection, AugR is notpossible anymore.

Figure 3. (A, B) Normalized TA kinetics of CdSe−ZnO films fordifferent excitation intensities (arrow and darker color signalizesincreasing excitation power). (C) Difference between normalized TAkinetics of the “F” sample (solid squares) fitted by a single-exponentialdecay (red solid line)see text for details. (D, E) Dependence of thelong-lived signal (IL) to amplitude (I0) ratio on number of excited e−hpairs (N0) for unattached QDs (open squares) and QD−ZnO systems(solid squares). “Long-lived signal” refers to the TA signal at 600 ps(“S” sample) and 200 ps (“F” sample)both more than 10τAR. Exc.powers used: “F”: 90, 320, 780, 1440 μJ/cm2; “I”: 30, 150, 470, 980μJ/cm2; “S”: 20, 360, 1800 μJ/cm2. Exc. wavelength: 470 nm; probewavelengths: “F”: 526 nm, “I”: 540 nm, “S”: 540 nm.

Scheme 1. Electron Injection and Auger Recombination Processes Present in Doubly-Excited QDs Attached to ZnO

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A slow decay can be assigned to the competition between thesecond electron injection and trion decay. It provides us withimportant information about electron injection, because theobserved total rate (lifetime ∼50 ps) is a sum of the rates of thetwo involved processes. Therefore, the second electroninjection can take place only on the scale of tens of picosecondsor longer.The second electron injection is, indeed, expected to be

slower than the first (or single) electron injection. Whensecond and other electrons are injected into ZnO, they areaffected by interaction with the already charged QD.Furthermore, the injection can be also slowed down byelectrons already injected into ZnO, which can slightly modifythe ZnO conduction band properties due to band gaprenormalization and state filling.33 However, due to the nativen-type character of ZnO NPs, the change in ZnO bands shouldonly be minor.A better estimate of the second electron injection lifetime

would be possible only by determining the trion lifetime.Although trions have been extensively studied in CdSeQDs,28,34−36 their lifetime is highly dependent on QDproperties, and the reported values cannot be straightforwardlyused for our QDs. An estimate can be based on the observationthat the lifetime of a negative trion (two electrons and a hole)was reported to be approximately 8× longer than the biexcitonAugR lifetime.34 From this crude estimate we obtain for thesample “F” the trion lifetime of ∼140 ps, which would facilitatean efficient injection of the second electron in line with thepreviously reported multiple electron collection.10 Never-theless, our results suggest that the second electron injectiontakes place on the time scale of tens of picoseconds, which issignificantly longer compared to ⟨τinj⟩ ∼ 6 ps for the singleelectron.Under excitation fluences above approximately 700 μJ/cm2

(N0 ∼ 1.5) even for the “F” sample the TA dynamics becomefaster with increasing excitation power (see Figure 3B). For N0> 2, the portion of triply and more exited QDs pn>2 calculatedfrom the Poisson distribution rapidly rises with excitationintensity. Namely, for the measured values of N0 ∼ 0.2, 0.6, 1.5,2.5, and 4, we obtain pn>2 = 0.001, 0.02, 0.19, 0.46, and 0.76,respectively. Very fast AugR from more than doubly excitedQDs overrides the electron injection, and it is responsible forthe observed faster dynamics of the “F” sample for highexcitation fluences.To summarize the obtained picture, in the case of sample “S”

AugR is fast enough to quench practically all multiple excitonsin QD before electron injection; sample “F” features rapidelectron injection to MO, which dominates over biexcitonAugR, and injection of the second electron can be observed.This is a very favorable situation for MEG exploitation, becauseMEG typically generates 2 e−h pairs per QD, which can beefficiently harvested in this case.We can estimate the probability of the first electron injection

to ZnO from a doubly excited QD by comparing the rate ofelectron injection kinj, the rate of e−h pair recombination kR =1/⟨τQD⟩, and the rate of biexcitonic AugR (rate kAR). Electronscan undergo one of the processes, and their number N willchange as dN/dt = −kinjN − kRN − kARN. The efficiency of thefirst electron injection (injection efficiency η1) can be thencalculated as:

η = + +k k k k/( )1 inj inj AR R (3)

For the samples “F”, “I”, and “S” the η1 values reachesapproximately 80%, 50%, and 1%, respectively.If the system dynamics is simpler (purely core CdSe QDs on

ordered ZnO nanowires) and better explored (for instance bycombination of TA and THz spectroscopy), it is possible todetermine the electron injection rate precisely, as we haveshown in our previous work.17 We will use this case to answerthe important question of QD size dependence of multiexcitonharvesting.The change in electron injection with QD size for the CdSe

QD−2MPA−ZnO NW system has been determined in ref 17.for four different QD sizes (see Figure 4B). The injection rates

can be well-described by Marcus theory,37 whose calculationand fit to the data has been discussed in detail in ref 17 and theSupporting Information. The calculation of free energydifference ΔG before and after the electron transfer is basedon experimental measurements of the band alignment of theCdSe−ZnO system38 together with the Coulomb e−hinteraction calculation proposed by Tvrdy et al.12

To calculate the AugR-limited injection efficiency, it is alsoessential to know the size dependence of the AugR rate,namely, dependence on the QD diameter D, as the used QDshave a spherical shape. Therefore we have repeated theprocedure illustrated in Figure 2A and B to extract the AugRdecay from TA kinetics for each of the QD sizes studied in ref17 (the same QDs were used). The change in TA dynamics ofunattached QDs under high and low intensity excitation (seeSupporting Information for details) allows us to evaluate thebiexciton AugR lifetime.In accordance with previous reports,23,28 the AugR lifetime

becomes significantly shorter with decreasing QD size (from 5ps for 2.5 nm size to 20 ps for 3.1 nm sizesee Figure 4A andB, circles). Our results are in line with the study of Klimov etal.26 (Figure 4B, triangles), where the same method of biexciton

Figure 4. (A) Normalized biexciton AugR contribution to TA kineticsfor various QD diameters (open symbols) fitted by single-exponentialdecay (solid lines). (B) Electron injection rate dependence on QD size(orange squares, adopted from ref 17) fitted by the Marcus theoryprediction (solid orange line, adopted from ref 17); AugR biexcitonrecombination rates from the fit on panel A (dark cyan circles) and ref26. (dark cyan triangles) fitted by D−p

fit (p = 4.9, dark cyan line). (C)First electron injection efficiency from doubly excited QDs for variousQD sizes calculated from experimental data (open circles) and fitspresented in panel B (line). Probe wavelengths used in panel (A): 2.5nm QDs: 506 nm; 2.7 nm QDs: 516 nm; 2.9 nm QDs: 526 nm; 3.1nm QDs: 540 nm.

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AugR lifetime extraction of CdSe QDs was used. The obtainedQD size dependence of AugR rates can be well fitted by a D−p

function for p = 4.9, which is in agreement with previouslyreported dependences.39 It is worth stressing that this fit is anapproximation valid for the studied range of QD sizes. The QDsize dependence of the AugR rate can be very complicated, ashas been suggested by theoretical calculations.27,40 Thereforethe overall AugR rate scaling for a broad range of QD sizes,which is usually described as R−3, may not be the best fit for thisparticular span of QD sizes.16

The AugR and electron injection rates feature considerablydifferent dependences on QD size. Therefore the AugR-limitedelectron harvesting is also strongly size-dependent (see Figure4C). Already within the narrow range of studied QD sizes, theefficiency of the first electron injection can vary from 30% up to70%. For small QDs (below 2 nm), AugR is very fast, and it isefficiently quenching all multiple excitations. Analogously, forlarge QDs (over 3.5 nm), electron injection becomes inefficientbecause of unfavorable electron states alignment, and again theAugR is dominating the system dynamics.12,14

A relatively narrow range of CdSe QD sizes (2−3.5 nm)offers a good electron injection efficiency with maximum valueof approximately 70% at about 3 nm for this particular system(2MPA linker molecule, ZnO nanowire acceptor). Naturally,the resulting efficiency dependence will change for differentlinker molecules, QD materials, and electron acceptors.However, the general rule of existence of an optimum QDsize for MEG exploitation will remain, because of different QD-size dependence of the electron injection rate in QD−MOsystems and AugR in QDs.12,26

In conclusion, our results clearly demonstrate that a fastelectron injection from QD to ZnO can inhibit the unwantedAuger recombination in doubly excited QDs. Efficient energyharvesting from multiple excitons created by MEG is thereforea feasible scenario. Moreover, we demonstrate that MEGharvesting efficiency changes significantly with QD size. Alreadywithin the narrow range of QD sizes (2−4 nm), double-excitonharvesting efficiency can vary from 30% to 70%. Therefore forfull utilization of the MEG effect, a combination of band gaptuning (various alloys, such as PbSxSe1−x) and QD size tuningshould be used to maximize the benefits of MEG.

■ ASSOCIATED CONTENT

*S Supporting InformationDetailed information about sample preparation; samplecharacterization; all transient absorption kinetics; and biexcitonAuger recombination lifetime extraction. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: Karel.Zidek@chemphys.lu.se.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by the Knut and Alice WallenbergFoundation, the Crafoord Foundation, and the Swedish EnergyAgency. We thank K. J. Karki for valuable discussions and T.Pascher for his help with experiments. Collaboration withinnmC@LU is acknowledged.

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