Air-Stable Operation of Transparent, Colloidal Quantum Dot Based

Air-Stable Operation of Transparent, ColloidalQuantum Dot Based LEDs with a UnipolarDevice ArchitectureVanessa Wood,† Matthew J. Panzer,† Jean-Michel Caruge,‡ Jonathan E. Halpert,‡Moungi G. Bawendi,‡ and Vladimir Bulovic*,†

†Department of Electrical Engineering and Computer Science and ‡Department of Chemistry, MassachusettsInstitute of Technology, Cambridge, Massachusetts 02139

ABSTRACT We report a novel unipolar light-emitting device architecture that operates using direct-current, field-driven electrolu-minescence of colloidally synthesized quantum dots (QDs). This device architecture, which is based only on transparent ceramicsand QDs, enables emission from different color QDs and, for the first time, constant QD electroluminescence during extended operationin air, unpackaged.

KEYWORDS Quantum dots, electroluminescence, metal oxides, ionization

Colloidally synthesized quantum dots (QDs) haveshown promise as the active material in light-emit-ting devices (LEDs) because of their tunable, narrow

band emission and high photoluminescent efficienciesthroughout the visible wavelength region. Efficient electrolu-minescence (EL) from QDs with different emission wave-lengths and chemical compositions has been demonstratedand explained in LEDs with small molecule organic chargetransport layers.1-6 More recently, QD-LEDs incorporatingp- and n-type inorganic charge transport layers demon-strated electrical excitation of QDs in air-stable structuresthat operate via direct charge injection.7-9 In these devices,efficient EL is dependent on favorable energy band align-ment between the charge transport layers and the film ofQD emitters.9 To obviate the restriction of precise bandalignment, in this study we introduce a new unipolar devicearchitecture in which a QD multilayer is embedded in atransparent, n-type ceramic matrix and which is designedto enable only one type of charge carrier (in this case,electrons) to be injected into the device. We present resultsindicative of a novel, field-driven mechanism for QD EL anddemonstrate both green and red QD emission, with a peakluminance of 1000 Cd/m2 and a luminous efficiency of 1Cd/A from one face of our most efficient transparent devices.These LEDs exhibit long shelf lives and enable constantluminance over extended operating times in air, unpackaged.

Previously, QDs have been incorporated into thin filmLEDs that use some combination of semiconducting poly-mers, small organic molecules, III-V materials, and ceram-ics as charge transport layers.1-13 All of these devices relyon a p-i-n type architecture, which requires the simulta-

neous transport of electrons and holes toward the QDemissive layer. For example, the most efficient visible-emitting QD-LEDs to date utilize a monolayer of closelypacked QDs sandwiched between hole and electron trans-porting organic layers.4 The use of molecular organic materi-als in these QD-LED structures introduces the fabricationchallenges similar to those facing organic LEDs (OLEDs),namely, the need for packaging in order to prevent degrada-tion due to atmospheric oxygen and water vapor exposure.We demonstrated previously that radio frequency (rf) sput-ter-deposited metal oxides, which are chemically and mor-phologically stable in air, can be used as charge transportlayers to achieve robust QD-LEDs that do not require pack-aging.8 These first all-inorganic devices exhibited lowerefficiency than state of the art organic-based QD-LEDs duein part to the difficulty of balancing hole and electroninjection into the luminescent QD film.9 Furthermore, sincethe most efficient red, green, and blue QDs have differentchemical compositions,4 a series of carefully selected trans-port layers is required to achieve multicolor EL with com-parable efficiency and luminosity.9 In response to theseconcerns, in this work we demonstrate QD-LEDs that utilizea unipolar (electron-transporting) ceramic matrix.

We form an n-i-n QD-LED structure by surrounding theQD film with n-type, alloyed ZnO and SnO2 (ZTO). ZTO wasused previously as the electron transport layer in our initialQD-LEDs with metal oxide charge transport layers, resultingin stable device operation.8 Two types of device structures(structure 1 and structures 2A-2C), differentiated by thepresence of at least one ZnS layer in structures 2A-2C, aredepicted schematically in Figure 1a. As discussed below, ZnSlayers are included in order to gain a deeper understandingof operating mechanism and improve device stability. A

* Corresponding author, [email protected] for review: 07/28/2009Published on Web: 12/22/2009

pubs.acs.org/NanoLett

© 2010 American Chemical Society 24 DOI: 10.1021/nl902425g | Nano Lett. 2010, 10, 24-29

detailed description of device fabrication and material prop-erties is given in the Supporting Information.

We demonstrate that these novel, n-i-n architecturesare optically transparent and enable electroluminescence(EL) from multicolored QDs. For structure 2A, we measurean absorption of less than 4% at wavelengths above λ ) 500nm (see Figure 1b), with most of the absorption due to the50 nm thick QD layer. There is negligible absorption fromthe ceramic thin films, all of which have band gaps greaterthan 3 eV. The inset photograph of the completed deviceprovides a visual indication of device transparency. A pho-tograph of a structure 2A device biased at 18 V highlightsthe uniform EL over the entire pixel area (Figure 1c).

Figure 2 shows the energy band diagrams of structure 1and structure 2A under bias to elucidate the device operatingmechanism. The equilibrium energy bands for each materialin our device are determined using ultraviolet photoelectronspectroscopy and optical absorption measurements (seeSupporting Information). The location of the voltage drop inthe device structures is determined through conductivity andcapacitance measurements of the metal oxide and QD layersand through atomic force microscopy (AFM) measurementsof the QD layer thickness. The band diagrams show that theelectron affinity of ZnS is 0.6 ( 0.2 eV lower than that ofZTO, indicating that the ZnS shell on the QDs and the ZnSlayers in structures 2A, 2B, and 2C serve as electron blocking

layers. When a voltage is applied across the device structure,electrons are injected into the n-type ZTO and accumulate

FIGURE 1. (a) Cross-sectional schematics of the device architectures described in this Letter. Structure 1 consists of colloidal QDs sandwichedbetween two layers of ZnO:SnO2 (ZTO). Structures 2A, 2B, and 2C incorporate at least one layer of ZnS within the ZTO layers, located 15 nmaway from the QD film. The ZnS can be positioned in the ZTO above the QDs (structure 2A), below the QDs (structure 2B), or on either sideof the QDs (structure 2C). (b) Plot of structure 2A device absorption versus wavelength. The inset photograph shows the device on top of textto demonstrate the transparency of the wide band gap ceramics and thin QD layer. (c) Photograph of device structure 2A operating at 18 V,demonstrating the uniformity of pixel illumination as well as device transparency.

FIGURE 2. Schematic band diagrams of device structures 1 (a) and2A (b) under forward bias conditions. A larger applied bias is neededto generate the same voltage drop across the QD film in structure2A as in structure 1.


at the first ZnS barrier they encounter, which can be eithera ZnS shell of a QD or a sputtered ZnS layer, depending onthe device structure and bias direction (1). This causes mostof the voltage applied to the device to be dropped across theQD film (in the case of structure 1) or across both the ZnSlayer and the QD film (in the case of structures 2A, 2B, and2C). If the electric field generated across the QD film issufficiently large, an electron can be extracted from thevalence band of the QD, leaving behind a hole (2), in aprocess that we refer to as field-induced QD ionization. Wedemonstrate below that the voltage drop required acrosseach QD to achieve EL is related to the band gap energy ofthe QD, which is consistent with the QD ionization mecha-nism. An electron extracted from a neighboring QD (orinjected from ZTO) can then couple with this hole to forman exciton on the QD and lead to radiative recombination(3). To achieve steady-state electroluminescence, a suf-ficiently large electric field must be maintained across theQD film by way of charge accumulation in the device.

In the case where the first energy barrier electronsencounter is the QD layer (for example in structure 1, instructure 2A under reverse bias, or in structure 2B underforward bias), the electron accumulation occurs in the QDfilm as well as in the ZTO layer. Electrons can be injectedfrom the ZTO into QDs causing buildup of space charge inthe first layer of QDs that will limit further injection ofelectrons into the QD film and thereby enhance the ac-cumulation of electrons in the ZTO layer adjacent to the QDs(4). In this case, QD charging aids in the accumulation ofelectrons that in turn enables a sufficient voltage drop acrossthe QD layer to achieve electroluminescence via QD ioniza-tion; however, QD charging will quench luminescence andlead to a decrease in the device current14,15 that is ultimatelydetrimental to efficient device operation. Figure 2b showsthat a ZnS layer between the electron injecting electrode andthe QD layer displaces the electron accumulation away fromthe QD film (4), limiting the amount of QD charging. InFigures 5 and 6, we provide experimental confirmation ofthis reduced QD charging for devices in which the electronsencounter a sputtered ZnS layer prior to the QD film.

As the proposed device operating mechanism suggests,these unipolar devices display low-voltage, forward andreverse bias direct current (dc) as well as alternating current(ac) operation. In ac operation, luminescence is typicallyobserved for applied square wave pulses with root-mean-square voltages (Vrms) of 10-15 V at frequencies up to 50kHz. Figure 3a presents the typical current density versusvoltage (J-V) characteristics. The inset device schematicindicates our biasing convention where “reverse bias” refersto the bottom ITO layer as the source of electrons (i.e., thecathode). Device structures 2A, 2B, and 2C require highervoltages than structure 1. While the sputtered ZnS layer isbetter at blocking electrons than the QD film, the additionalvoltage dropped across the insulating ZnS layer necessitatesa larger applied bias in the case of structures 2A, 2B, and

2C to achieve QD electroluminescence. As shown in Figure3b, a peak external quantum efficiency (EQE) of 0.15% isrecorded from the glass side of a structure 2A device withadditional ZnO buffer layers, as described in the SupportingInformation. With a luminance of 1040 Cd/m2, which is 10times video brightness, recorded at a current density of 92mA/cm2, the luminous efficiency of these transparent struc-

FIGURE 3. (a) Current density versus voltage characteristics fordevice structures 1 (black line), 2A (red line), 2B (purple line), and2C (blue line) under forward and reverse dc bias conditions. Theinset schematic indicates the bias convention. (b) External quantumefficiency (EQE) versus the absolute value of current density fordevice structures 1 and 2A. The dotted red lines in panels a and bshow the current density and EQE measurements for the optimizedstructure 2A device. (c) Luminance (solid symbols) and luminousefficiency (open symbols) for device structure 1 (black) and structure2A (red). Data for the optimized structure 2A are shown in red withcircle symbols.


tures is 1.1 Cd/A. (Note that a similar amount of light isemitted through the top ITO surface, corresponding to anoverall external luminous efficiency of over 2 Cd/A.)

The EL spectra for green and red QDs in structure 1 at14 V demonstrate that a unipolar, n-i-n structure can beused to obtain multiple color emission from the same devicestructure (Figure 4a) where the EL is due entirely to QDemission. The red-shifting and broadening of the EL spectrain comparison to the dilute QD solution PL spectra (dashedblack lines) can be explained by well-characterized interac-tions in QD ensembles.16-18 We note that the PL spectrumof the concentrated solution of red QDs used for the QD layerdeposition (solid black line) matches the device PL spectrum(dashed red line). This suggests that QD proximity, througheffects such as energy transfer16 and dielectric dispersion,17

can explain the red shift and some of the broadening. At 14V applied bias, an increase in the full width half-maximum(fwhm) between the device PL and EL spectra of 7 nm forthe red device and 8 nm for the green device is observed.The red shift in peak position of the PL spectra as a functionof applied voltage across the device displays a quadraticdependence for all measured structures (1, 2A, and 2C). Thisred shift in peak position as a function of applied voltage isa reflection of the quantum confined Stark effect in QDensembles and the spectral broadening as a function ofapplied voltage is consistent with the increased LO (longi-tudinal optical)-phonon coupling expected with larger exci-ton polarization in the presence of an electric field.18

The hypothesis of field-induced QD ionization is alsosupported by our observation of red and green QD lumines-cence from the same device structure. We expect the QDionization process to be dependent on the band gap energy

of the QD. Figure 4b relates the electroluminescent (EL) turnon to the voltage drop across each layer of QDs. We find thatEL turn on in a device containing green QDs requires 0.3 (0.1 V more per QD than in a device containing red QDs.Figure 4a shows that these green-emitting QDs have a band

FIGURE 5. Plot of normalized photoluminescence (PL) as a functionof applied voltage in forward bias for device structures 1 (squares),2A (upward pointing triangles), 2B (downward pointing triangles),and 2C (diamonds). In the case of device structures 2A and 2C inforward bias, electrons injected into the ZTO encounter a ZnS layerprior to the QD film. In contrast, electrons in the ZTO in structures1 and 2B do not encounter a ZnS layer and accumulate at the QDfilm. Consequently device structures 2A and 2C show less PLquenching with increasing voltage than structures 1 and 2B, indicat-ing that accumulation of electrons at the ZnS layer instead of at theQD film reduces charging of the QD layer. The trend in PL quenchingof device structures 1 and 2B are the same but offset in voltagebecause of the presence of the insulating ZnS layer in structure 2B.This plot illustrates that the extent of electron accumulation at theQD film, not the electric field across the QDs, determines the amountof QD charging and luminescence quenching.

FIGURE 4. (a) Spectra corresponding to photoluminescence (PL) of red and green QDs in a dilute solution (dashed black lines), PL of QDs inthe device structure (dashed red and green lines), and electroluminescence (EL) from red and green QDs in device structure 1 at 14 V bias(solid red and green lines). The PL of the concentrated solution of red QDs is shown by a solid black line. (b) Plot of current density (solid line)and EQE (dashed line) as a function of voltage drop per QD. We observe a 0.3 V difference between the voltage drop per QD required toobserve EL in a device containing red QDs and a device containing green QDs. Part a shows that the difference in band gap energy betweenthese red and green QDs is 0.3 eV, suggesting that QD ionization is the mechanism governing device operation.


gap of approximately 0.3 eV more than the red-emittingQDs. Comparing Figures 4a and 4b shows that the magni-tudes of the electroluminescent turn-on voltages for the redand green QD devices are comparable to the bandgapenergies of the red and green QDs. These two observationssuggest that the band gap energy of the QDs determines thevoltage drop across each QD needed extract an electron (i.e.,ionize the QD), generate free charge, and thereby achieveEL.

The addition of at least one ZnS layer in the devicestructure improves device performance by limiting QDcharging. To demonstrate this, we measure the photolumi-nescence (PL) quenching in the QD layer due to an appliedvoltage across the device structures. This measurementprovides an indication of the number of Auger nonradiativerecombination and exciton dissociation events due to QDcharging or due to the presence of an electric field acrossthe QD layer. The data presented in Figure 3 indicate that acertain minimum electric field across the QDs is requiredfor luminescence. The addition of the 30 nm of ZnS in devicestructures 2A, 2B, and 2C shifts the voltage at which thiselectric field is established by 4 V. However, Figure 3b showsthat once this field is obtained, a device with or without ZnSexhibits similar external quantum efficiencies. If we assumethat above the turn on, device structures 1, 2A, 2B, and 2Call exhibit similar degrees of exciton dissociation, then thedifferences observed in PL quenching between the structurescan be attributed to charging of the QD layer. Figure 5 showsthat device structures 2A and 2B, which are identical exceptfor the placement of the ZnS either above or below the QDlayer, exhibit different PL quenching trends at the samevoltage. When structure 2A is under forward bias, theelectrons are injected from the top contact and are blockedby a ZnS layer barrier prior to reaching the QD film.Consequently, the electric field needed for electrolumines-cence is achieved with relatively little charging of the QDsand the PL from the QD film shows little quenching as a

function of voltage. In the case of structure 2B in forwardbias, the electrons arrive first at the QD film. Accordingly,structure 2B exhibits the same charging properties and trendin PL quenching as structure 1 but offset in voltage (by 4 V)due to the presence of the insulating ZnS. The slightly largeramount of PL quenching in structure 2C compared tostructure 2A can be explained by the fact that, in forwardbias, electrons encounter a 15 nm thick ZnS layer followinginjection in structure 2C but a 30 nm thick layer in structure2A. The thinner ZnS layer in structure 2C is a weaker electronblocking layer that allows for more QD charging. By usingZnS layers, we are thus able to validate our understandingof the device operating mechanism and controllably reduceQD charging to improve device performance.

Finally, we demonstrate stable device operation in airwith unpackaged devices, enabled by the environmentalstability of the ZTO layers19 and the device operating mech-anism based on QD ionization. All device measurementspresented here were performed in ambient laboratoryconditions. Repeated J-V and EQE measurements recordedevery week over the course of 40 days on devices stored inair showed stable luminescent response. This long shelf lifesuggests the possibility for extended operation in ambientconditions. An understanding of the device operating mech-anism enables us to choose favorable bias conditions forextended testing. A device held under constant dc biasexhibits gradual charging of the QD layer, which is mani-fested by a corresponding decrease in device current overtime.14,15 Device structure 1 will, in fact, no longer exhibitEL after approximately 5 min at constant applied bias dueto a lack of sufficient current through the device. Structure2A, in which the QD charging occurs at a slower rate due tothe presence of the ZnS layer, will exhibit EL for ap-proximately 45 min when forward biased in a constantvoltage mode. In order to circumvent eventual device turn-off with a constant applied voltage, we therefore performextended lifetime testing on structure 2A run in a constant

FIGURE 6. (a) Plot of luminosity (top, left axis) and voltage (bottom, right axis) as a function of time for device structure 2A operated inforward bias at a constant current of 30 mA/cm2. The device was unpackaged and operated in air. Voltage and luminescence data wererecorded every 5 s for 20 consecutive hours and then binned and averaged into 1 min intervals. (b) Scatter plot of luminosity versus voltageusing all of the data in panel a shows that despite fluctuations in the voltage and luminosity over time, the luminosity at a given voltage isconstant, indicating that these fluctuations do not correspond to changes in the device performance with time. For example, the voltage andluminosity at 1 h 50 min (R) and 10 h (�) are comparable.


current mode, with voltage and luminescence measured in5 s intervals. We choose to test a device with an average EQEto ensure that our results are representative. Figure 6a showsluminescence as a function of time during 20 h of continuousdevice operation. The overlap onto a single curve of the14400 luminosity and voltage data points from panel a(Figure 6b) shows that for a given voltage and current, atany point during the 20 h of testing, the device exhibits thesame luminosity. This indicates that an unpackaged devicedoes not degrade over 20 h of continuous operation in air.

In summary, we developed a unipolar, n-i-n light-emitting device architecture that operates via a new methodof QD electrical excitation: field-induced QD ionization. Thedevice structure facilitates the use of stable transport materi-als and reduces the need for complicated band alignmentengineering. We demonstrated that introduction of an ap-propriate thin film of ceramic material (in this case, ZnS) intothe charge transport layer to create a potential barrier formajority carriers limits charging of the QD layer. Thisenables the first QD-LED with constant luminance overextended operation. These unique unipolar devices can yieldmulticolor QD emission in transparent device structures thatexhibit stable operation in air, without packaging.

Acknowledgment. The authors acknowledge Lisa Mar-shall and Dr. Polina Anikeeva for their assistance andProfessor John Wager for helpful discussions. We thank QDVision, Inc., for supplying some of the QDs used in this work.This work is supported by the Institute for Soldier Nanotech-nologies (DAAD-19-02-0002), a Presidential Early CareerAward for Scientists and Engineers (V. Bulovic), and aNational Defense Science and Engineering Graduate Fellow-ship (V. Wood). This work made use of MRSEC SharedExperimental Facilities at MIT, supported by the NationalScience Foundation under award number DMR-02-13282.

Supporting Information Available. Descriptions of devicefabrication, optimized device structure, the band structure,and device characterization and a figure of AFM images andspectra showin QD photoluminescence. This material isavailable free of charge via the Internet at http://pubs.acs.org.

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Air-Stable Operation of Transparent, Colloidal Quantum Dot Based