Electrical Detection of Quantum Dot Hot Electrons Generated via aMn2+-Enhanced Auger ProcessCharles J. Barrows, Jeffrey D. Rinehart,† Hirokazu Nagaoka,‡ Dane W. deQuilettes, Michael Salvador,§

Jennifer I. L. Chen,∥ David S. Ginger, and Daniel R. Gamelin*

Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States

*S Supporting Information

ABSTRACT: An all-solid-state quantum-dot-based photon-to-current conversion deviceis demonstrated that selectively detects the generation of hot electrons. Photoexcitation ofMn2+-doped CdS quantum dots embedded in the device is followed by efficientpicosecond energy transfer to Mn2+ with a long-lived (millisecond) excited-state lifetime.Electrons injected into the QDs under applied bias then capture this energy via Auger de-excitation, generating hot electrons that possess sufficient energy to escape over a ZnSblocking layer, thereby producing current. This electrically detected hot-electrongeneration is correlated with a quench in the steady-state Mn2+ luminescence and theintroduction of a new nonradiative excited-state decay process, consistent with electron-dopant Auger cross-relaxation. The device’s efficiency at detecting hot-electron generationprovides a model platform for the study of hot-electron ionization relevant to thedevelopment of novel photodetectors and alternative energy-conversion devices.

Devices that convert light into current have broadapplications from sensors to solar energy conversion,and can also serve as powerful tools for investigatingfundamental material properties. Semiconductor devicesharnessing nonthermalized “hot” charge carriers are presentlytargeted because they could eventually enable solar efficienciesexceeding the Shockley-Queisser limit.1−3 Here, we explore afundamentally new photon-to-current conversion device basedon hot electrons. Photoexcitation of Mn2+-doped CdS quantumdots (QDs) deposits energy in long-lived Mn2+ d−d excitedstates. Electrons injected into the QD layer harvest this energyvia highly efficient electron-Mn2+ Auger cross-relaxation.4 Theresulting hot electrons possess sufficient energy to escape over aZnS blocking layer, producing current. Photocurrents withMn2+-doped QDs are over an order of magnitude greater thanin undoped-QD control devices and show clear rectification.These devices offer a unique electrical probe of QD Auger andhot-electron processes.Auger processes are deleterious in many contexts, for

example causing QD darkening,5−8 LED efficiency droop,9

and multiexciton recombination.10,11 Using spectroelectro-chemistry, we recently demonstrated efficient photolumines-cence (PL) quenching by conduction-band electrons in Mn2+-doped CdS QDs via an unusual electron-Mn2+ Auger cross-relaxation process.4 Here, we seek to harness this efficientAuger process to generate hot electrons within a solid-statedevice. Hot-carrier extraction from semiconductors is challeng-ing, mainly due to rapid thermalization via coupling to latticevibrations or other charge carriers.1,12−14 Carrier cooling withinsemiconductors typically occurs in femtoseconds1,15 but may beslowed to picoseconds in QDs.2,16 Recent studies have shownthat it is possible to use hot electrons generated in QDs for

photocatalysis17 and to detect transient QD hot-electronsurface trapping18 and hot-carrier transfer3,19 spectroscopically.Auger ionization of QDs is also well-known.20,21 In metals,extraction of hot electrons generated by plasmonic excitationhas been demonstrated,22 and various plasmonic hot-electrondevices have been described.23−26 During the course of thisinvestigation, hot-electron detection was reported from similarMn2+-doped CdS/ZnS core/shell QDs deposited on trans-parent conducting electrodes and placed in a photoelec-trochemical cell.27 Harvesting hot carriers generated byphotoexcited QDs in solid-state devices remains largelyunexplored.Figure 1A summarizes the electron-Mn2+ Auger process

investigated here: QD photoexcitation creates an exciton thatrapidly (ps) localizes at a Mn2+,28 generating a long-lived(∼ms) Mn2+ 4T1 d−d excited state. This excited state can decayradiatively (Figure 1A, bottom left),29 but in the presence of anexcess conduction-band electron can also decay via an electron-Mn2+ Auger-type cross-relaxation4 to yield a hot electron andthe ground-state Mn2+ (Figure 1A, bottom right). Although thisAuger process is intrinsically fast (ps), it is manifestedexperimentally as a new voltage-dependent μs Mn2+ PLnonradiative decay component because of slow electrondiffusion through the QD solid.4 Electrons are able to samplea large volume of the Mn2+-doped CdS QD film on the timescale of the Mn2+ PL, making electron-Mn2+ Auger cross-relaxation extremely efficient.4 In spectroelectrochemicalmeasurements, Mn2+ cw PL quenching and accelerated PL

Received: September 28, 2016Accepted: December 5, 2016Published: December 5, 2016

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© 2016 American Chemical Society 126 DOI: 10.1021/acs.jpclett.6b02219J. Phys. Chem. Lett. 2017, 8, 126−130

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decay times could be predicted from QD electron occupancies,4

making these experimental observables useful independentindicators for electron-Mn2+ Auger cross-relaxation.Figure 1B shows a representative cross-sectional SEM image

of a prototype device constructed for the purpose of extractinghot electrons. The device has four layers, shown from left-to-right: (i) an aluminum contact, (ii) an ∼175 nm layer of 1,7-heptanediamine cross-linked Cd1−xMnxS QDs, (iii) an ∼50 nmZnS electron-blocking layer grown by thermal evaporation, and(iv) an indium tin oxide (ITO) contact. Figure 1C illustratesthe relevant electronic-structure characteristics of this device. Inthe absence of photons, the device behaves as a capacitor:under a positive (forward) bias, electrons are injected from Alinto the QDs but cannot proceed because of the ZnS blockinglayer, which introduces an ∼1 eV conduction-band offset.Absorption of a photon by the Cd1−xMnxS QD layer forms theMn2+ 4T1 excited state within picoseconds, poising the systemfor the Auger process of Figure 1A. This Auger processtransfers the 2.1 eV energy of the Mn2+ excited state to theexcess electron. We hypothesize that under bias, the resultinghot electrons may escape over the ZnS electron-blocking layerto be collected at the ITO contact.We first test for Auger cross-relaxation in this device. Figure

2A shows room-temperature PL spectra of a deviceincorporating Cd0.9Mn0.1S QDs under different positive biases.A broad peak is observed at ∼575 nm from Mn2+ emission (4T1→ 6A1). The PL intensity is bias independent below ∼ +7 V,but decreases at higher positive bias. At +15 V, the integrated

PL intensity has decreased by 75%. The Mn2+ PL is onlyslightly sensitive to negative bias, however, decreasing by

ionization of these hot electrons within the QD or ZnS layers ofthe device. A gain of ∼175 is calculated for the data collected at+15 V bias. From these data, we conclude that the signals underforward bias arise from the generation of hot electrons via theelectron-Mn2+ Auger cross-relaxation process illustrated inFigure 1, with sensitivity enhanced by gain effects.To test the role of Mn2+, control devices were made using

undoped CdS QDs. PL spectra of a representative controldevice at different voltages are shown in Figure 2C. Excitonic(∼475 nm) and trap (∼675 nm) PL intensities are observed,and both are quenched with applied voltage but only by a smallamount (e.g., ∼ 25% quench at +15 V). Figure 2D plots PLintensity (integrated over both the excitonic and trap PL)versus bias from −15 to +15 V, relative to 0 V. The PLresponse to bias in the control device is much moresymmetrical than in the Cd1−xMnxS device, and its magnitudeis comparable to that observed in the Cd1−xMnxS device underreverse bias. Figure 2D also plots responsivities of the controldevice, measured simultaneously with the PL. In principle,electron−exciton or electron−trap Auger processes should alsobe able to generate hot electrons, but the shorter lifetimes ofthese excited states lead to correspondingly smaller proba-bilities of an electron encountering a photoexcited QD prior toexcited-state decay.4 Some small rectification is indeed observedin the control device, but the responsivity is small at allvoltages, e.g., ∼20 times smaller than that of the Cd1−xMnxSdevice at +15 V. These observations establish that Mn2+ plays acritical role in generating the large photocurrents under positivebias shown in Figure 2B. The long Mn2+ excited-state lifetime ischiefly responsible, enabling highly efficient energy storage forhot-electron generation.4 We attribute the moderate PLquenching in the control device in both directions, and of theMn2+-doped device under reverse bias, to electron−holeseparation by the electric field,31 rather than to Auger cross-relaxation.Time-resolved PL measurements were also performed on the

Cd1−xMnxS device from Figure 2A,B at various biases, and thekey result is summarized in Figure 3. At 0 V, the Mn2+ PLdecays with τ ∼ 700 μs (Figure 3A), reflecting the long lifetimeassociated with the spin-forbidden Mn2+ d−d transition. At +12V (∼57% Mn2+ PL quenching), the PL decays markedly faster,revealing a new nonradiative decay process affecting the Mn2+

excited state population. To emphasize this result, Figure 3Bplots the first 2 μs of this PL decay for 0 and +12 V, withintensities normalized at 0 μs. This representation of the datahighlights a new decay component at +12 V having acharacteristic (but voltage-dependent) time of ∼300 ns,essentially identical to that obtained from spectroelectrochem-ical measurements at similar PL quenching levels.4 Weemphasize that this time scale is associated with the transporttime of an electron to an excited Mn2+ center, not the intrinsicelectron/Mn2+ quenching rate, which has been estimated to bekAug ≈ 0.2 × 1010 s−1(e−/QD)−1.4 These data, in conjunctionwith Figure 2, provide strong evidence that the enhancedphotocurrents of the Mn2+-containing devices derive from hotelectrons generated via electron−Mn2+ Auger cross-relaxation.Figure 4 plots photocurrent excitation spectra collected for

the Cd0.9Mn0.1S device from Figures 2A,B and 3 at severalbiases, in comparison with the device’s transmission spectrum.The photocurrent onset coincides with the QD absorptiononset, supporting the mechanism of Figure 1. Through the useof chopped photoexcitation (20 Hz) and lock-in detection inthese excitation experiments, persistent gain (on the modu-

lation time scale) is eliminated. The gain effects observed inFigure 2 are vastly reduced in Figure 4, indicating a large decaytime constant associated with the gain mechanism. Forcomparison, the data in Figure 2 would suggest incidentphoton-to-current efficiency (IPCE) values of ∼17 500% at +15V and 3.06 eV excitation, compared to ∼30% shown in Figure4. It is interesting to consider that in the limit of no gain, a 30%IPCE value would correspond to an internal quantum efficiency(IQE) for hot-electron escape of ∼60%. Although the role ofgain under these modulated excitation conditions is not clear,this result does clearly emphasize that gain assists the detectionof hot-electron generation in these devices under continuous-wave excitation (Figure 2). Overall, these results demonstratesuccessful electrical detection of hot-electron generation in asolid-state QD device. The closest analogue to this deviceappears to be a photoelectrochemical cell involving aphotoanode comprising Mn2+-doped CdS/ZnS core/shellQDs on an Al2O3/ITO electrode.

27 Excitation of thisphotoanode with a cw diode laser at 3.06 eV and 1 W/cm2

Figure 3. (A) 300 K time-resolved PL decay curves of the Cd0.9Mn0.1SQD device from Figure 2, monitored at 575 nm under biases of 0(black) and +12 V (green). (B) Time-resolved PL data measuredunder the same conditions as in panel A, but at shorter times. The datawere normalized at 0 μs to emphasize the appearance of nonradiativeMn2+ Auger cross-relaxation under bias. Lines represent double-exponential best fits to the data.

Figure 4. Transmission spectrum of the Cd1−xMnxS QD layer (plottedas 1-transmittance; black dashed line, right axis), and IPCE curves ofthe Cd1−xMnxS QD multilayer device used in Figures 2 and 3 (solidlines, left axis) measured at various biases. Current is detected onlywith excitation energies exceeding the CdS QD absorption threshold.

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produced a photocurrent density of 200 nA/cm2 in the absenceof applied bias. The data were interpreted as indicatingionization of hot electrons that were generated in the QDsvia sequential two-photon excitation. These electrons wereregenerated by a Pt counter electrode using a methanolicsolution of polysulfides as the electrolyte. Although bothapproaches to hot-electron generation rely on long energystorage times at the Mn2+ dopant ions, direct comparison of thedetection efficiencies of these two devices is probably notmeaningful because of the different hot-electron generationmechanisms (two photons27 versus one photon + one electron,Figure 1) and device architectures.Finally, we comment on the role of voltage in this device

architecture. Although hot-electron generation is detected, theapplied voltages exceed the energy transferred from Mn2+ tothe electrons through the Auger process (2.1 eV). It isinteresting to consider the possibility of a similar deviceoperating at biases below 2.1 V. In this scenario, and withappropriate contacts, current would exit the device having agreater voltage than when it entered, the difference provided byphotons. This device would convert photon power intoelectrical power without ever separating electrons from holes,implying a fundamentally different operating principle fromconventional single-junction photovoltaics based on separatingelectron−hole pairs. To achieve such a hypothetical result,photocurrent onset voltages must still be diminished by a factorof at least 5 from the current work. In our measurements todate, we have observed that the onset voltage is stronglydependent on the quality of the Al-QD and ZnS-QD interfaces.With further improvements in these interfaces, the devicestructure demonstrated here may thus potentially yield aqualitatively new type of “solar voltage booster.”In summary, a new solid-state device is described that

demonstrates electrical detection of hot electrons generated inCdS QDs via an electron-Mn2+ Auger cross-relaxation. Theseresults may have potential ramifications for the development ofunconventional photodetection or solar voltage-boostingtechnologies that can capitalize on this phenomenon.

■ EXPERIMENTAL METHODSNanocrystal Preparation and General Characterization. Colloidalundoped CdS QDs and Mn2+-doped CdS QDs (Cd1−xMnxS,x = 0.1) with oleate surface ligands were synthesized by apreviously described hot-injection method.4,32 SubmonolayerZnS shells were grown on both the d = 5.4 nm (undoped) andd = 5.0 nm (doped) QDs to increase photoluminescencequantum yields. Data characterizing these colloidal QDs byelectronic absorption, photoluminescence, magnetic circulardichroism (MCD), and electron paramagnetic resonance(EPR) spectroscopies, as well as by X-ray diffraction (XRD),are provided in the Supporting Information. Mn2+ concen-trations were obtained by analysis of dried nanocrystalsdigested in ultrapure nitric acid (EMD Chemicals) usinginductively coupled plasma atomic emission spectrometry(ICP-AES; Perkin- Elmer).Device Fabrication. A thin film of ZnS (50 nm) was deposited

onto an indium tin oxide (ITO) substrate (1.5 cm × 1.5 cm) bythermal evaporation at a base pressure less than 5 × 10−7 Torr.QD films were fabricated in inert atmosphere using a layer-by-layer spin-coating method33 and cross-linked with 1,7-heptanediamine. Residual ligands on the QD film weresubsequently removed by spin coating twice with anhydrousethanol. An Al top contact (100 nm) was thermally evaporated

at a rate of 4 Å/s at a base pressure less than 5 × 10−7 Torr.Details are provided in the Supporting Information.Device Characterization. Scanning electron microscopy

(SEM) micrographs were acquired on an FEI XL30 SFEGwith through-lens detection (TLD) at the University ofWashington (UW) NanoTech User Facility. Sample crosssections were sputter-coated with ∼4 nm of Au/Pd (60:40)metal to prevent charging of nonconductive layers duringmeasurement.Room- and low-temperature PL, photocurrent, and IPCE

measurements were performed on the multilayer devices in aJanis STVP-100 optical cryostat under nitrogen atmosphere.Each device was masked to expose an area of 18 cm2. Thetemperature was controlled using a LakeShore 331 controller. AKeithley 6430 I−V source meter was used to apply potentialsfrom −15 V to +15 V across the device. For PL andphotocurrent measurements, samples were excited using a 405nm laser diode (

Present Addresses†Department of Chemistry and Biochemistry, University ofCalifornia, San Diego, California, United States.‡Ichihara Research Center, JNC Petrochemical Corporation,Chiba, Japan.§Department of Materials Science and Engineering, Friedrich-Alexander University of Erlangen-Nürnberg, Germany.∥Department of Chemistry, York University, Toronto, Canada.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Michael White, Nils Janssen, G. Michael Carroll, andLiam Bradshaw for technical assistance and helpful discussions.D.R.G. and D.S.G. gratefully acknowledge financial supportfrom the Research Corporation, Scialog program. Additionalsupport came from the U.S. National Science Foundation(CHE-1230615, DMR-1505901, DMR-1464497, and DMR-1206221 to D.R.G. and DGE-1256082 to D.W.D.) and the U.S.Department of Energy (Energy Efficiency and RenewableEnergy Fellowship to J.D.R.). D.S.G. thanks JNC Corp., Japan,for an unrestricted gift that supported part of H. Nagoka’scontribution to this project. C.J.B. acknowledges GraduateResearch Fellowship support from the University of Wash-ington Clean Energy Institute. M.S. acknowledges primarysupport from a fellowship by the Portuguese Fundaca̧õ para aCien̂cia e a Tecnologia (SFRH/BPD/71816/2010).

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