Electroluminescence from Nanoscale Materials via Field-Driven

Department of Chemical and Biological Engineering, Tufts University, 4 Colby Street, Medford, Massachusetts 02155, United States. Nano Lett. , 0, (),...
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LETTER pubs.acs.org/NanoLett

Electroluminescence from Nanoscale Materials via Field-Driven Ionization Vanessa Wood,†,‡ Matthew J. Panzer,‡,|| Deniz Bozyigit,† Yasuhiro Shirasaki,‡ Ian Rousseau,‡ Scott Geyer,§ Moungi G. Bawendi,§ and Vladimir Bulovic*,‡ †

Department of Information Technology and Electrical Engineering ETH Zurich, 35 Gloriastrasse, 8092 Zurich Switzerland Department of Electrical Engineering and §Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States Department of Chemical and Biological Engineering, Tufts University, 4 Colby Street, Medford, Massachusetts 02155, United States

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bS Supporting Information ABSTRACT: The high degree of morphological and energetic disorder inherent to many nanosized materials places limitations on charge injection into and transport rates through thin films of these materials. We demonstrate electroluminescence achieved by local generation of charge that eliminates the need for injection of charge carriers from the device electrodes. We show electroluminescence from thin films of nanoscale materials that do not support direct current excitation and suggest a mechanism for the charge generation and electroluminescence that is consistent with our time-averaged and time-resolved observations. KEYWORDS: Colloidal quantum dots, organic small molecules, charge transport, electroluminescence

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he past two decades have witnessed enormous progress in the development of techniques to synthesize and deposit nanoscale materials, but their integration into electronic devices as thin films is still limited because of the difficulties in achieving sufficient charge injection and transport. For example, colloidal quantum dots (QDs) that can be synthesized to emit across the entire visible and the near-infrared spectrum have been used to achieve thin film electroluminescent light-emitting devices (QLEDs) of high color quality and luminous efficiencies.1 8 However, poor injection and transport of charge carriers within the QD films present significant challenges for luminescence, and, most often, monolayers of QDs are used instead of thin films to reduce effects such as QD charging.6,9 19 To date, electrical excitation of QLED structures has been primarily achieved by direct charge injection into the thin film stack containing a QD emissive layer, leading to the formation of excitons on the QDs that recombine radiatively.4 16 Previous work has also shown that electroluminescence can be achieved from nanoscale materials by using alternating electric fields; however, the mechanism by which electroluminescence occurs is not yet well understood.20 24 In this Letter, we demonstrate that colloidal QDs can be a model system for understanding field-driven electroluminescence and use our findings to propose a mechanism through which charge is locally generated and recombines. We extend our work to blends of semiconducting and insulating materials that do not exhibit DC charge transport, further confirming that our proposed method of electrical excitation and electroluminescence eliminates the need for charge injection and long-range charge transport. r 2011 American Chemical Society

Here, we present one possible model of device operation that is consistent with our experimental observations. Under a sufficiently large electric field, a nanoscale semiconductor, such as a QD or a molecule within a thin film, can undergo ionization. In the ionization process, an electron can be extracted from the valence band (or highest occupied molecular orbital) of a QD (or molecule) and transferred into the conduction band (or lowest unoccupied molecular orbital) of a neighboring QD (or molecule), creating an electron and a hole that are spatially separated onto neighboring nanoscale entities. Electrons and holes from multiple ionization events that arrive at the same QD (or molecule) can form excitons, which can then radiatively recombine, contributing to the thin film luminescence. We use time-resolved measurements to study the processes by which the field-ionized electrons and holes are created, form excitons, and recombine. We demonstrate device operation using either single or alternating polarity pulses and show that through the ionization process, sustained electroluminescence can be realized even for material sets where charge injection or long-range electronic transport rates are prohibitively low. The device architecture we choose to illustrate this nanoscale ionization process does not allow for charge injection from the electrodes into the luminescent material. As shown in Figure 1a, in a typical device the luminescent layer is sandwiched between two insulating, 35 nm thick wide band gap oxide layers (such as Al2O3 or SiO2) and consists of spin-cast or inkjet-printed QDs, Received: April 26, 2011 Published: June 16, 2011 2927

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Figure 1. (a) Cross-section of device structure depicting the luminescent layer sandwiched between two insulating oxide layers. The photograph shows that using indium-doped tin oxide for both electrodes results in a transparent device. (b) Photographs showing electroluminescence of pixels containing red, green, and blue QD luminescent layers. (c) The narrow electroluminescence (solid lines) and photoluminescence (dashed lines) spectra of QDs in completed device structures. (d) Plot of the electroluminescent light power output from devices that contain QDs emitting in the blue, green, red, and nearinfrared as a function of the voltage dropped across each QD. The voltage drop per QD that results in device turn-on approximately corresponds to the QD band gap energy. The solid lines indicate devices with SiO2 insulating layers; the dashed line shows a device with red QDs and Al2O3 insulating layers.

or thermally evaporated molecular organic layers. A detailed description of device fabrication is available in Supporting Information. Because the oxide insulators possess wide band gaps and the emissive layer is thin, transparent devices can be realized using a sputter-deposited transparent conductive oxide such as indium-doped tin oxide (ITO) for both electrodes (see

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Figure 1a). Alternatively, an aluminum top electrode, which also serves as a back reflector for the electroluminescence, can be used. This device architecture is similar to that employed in thin film electroluminescent (TFEL) devices;25,26 however, the fielddriven electroluminescence we demonstrate here using both QD and organic thin films is distinctly different from the mechanism at play in TFEL structures. In a TFEL device, an approximately 1 μm thick crystalline, phosphor-doped film, such as manganesedoped zinc sulfide (ZnS/Mn), is sandwiched vertically between two insulators that are contacted by electrodes. When a sufficiently high voltage is applied across the electrodes, electrons trapped at the insulator-phosphor interface are injected into the conduction band of the host material (e.g., ZnS), where they are accelerated by the field and can excite luminescent dopant centers in the phosphor layer via impact excitation and impact ionization mechanisms.27,28 In our structures, the luminescent layer consists of a disordered QD (or molecular) film, approximately 40 nm in thickness, where band transport of hot electrons is not expected due to efficient charge scattering at QD/QD (or molecular) interfaces. Additionally, as will be shown below, the ionization process can also occur within clusters of QDs or organic semiconductor molecules with feature sizes that are of insufficient length scale for accelerating hot electrons. These insulator/luminescent layer/insulator devices do not require charge injection at electrode contacts or long-range carrier transport, thereby eliminating the energy band alignment considerations that typically dictate which emissive materials can be electrically excited.15 To our knowledge, these devices are also the first reported structures that can be successfully utilized to electrically excite both CdSe/ZnS and PbS/CdS (core/shell) QDs, luminescent materials that have different chemistries and absolute energy level positions, and whose peak emission wavelengths span the visible to near-infrared regions. Figure 1b shows photographs of electroluminescence from completed devices with red, green, and blue QD emissive layers driven by a 30 kHz sinusoidal voltage waveform at peak-to-peak applied biases of 120, 130, and 145 V, respectively. Electroluminescence (EL) and photoluminescence (PL) spectra of devices with QDs that emit in the near-infrared, red, green, and blue are presented in Figure 1c. Analogous data obtained using a variety of organic semiconductor molecules (and presented in the Supporting Information, Figure S2) shows excellent agreement with previous results for alternating current driven organic light-emitting devices and highlights the versatility of this device structure.29 35 In our proposed mechanism of device operation, when the voltage applied across each QD exceeds its band gap energy an electron can transfer from the valence band of one QD to the conduction band of a neighboring QD, creating a spatially separated electron and hole in the QD film that can subsequently recombine to yield luminescence. To validate this mechanism, we first experimentally determine the minimum voltage drop per QD that results in EL and compare this value to the band gap of the QD. For each device containing a different color-emitting QD film, we choose three different sample biases slightly above the threshold voltage at which EL is first observed and plot the collected light power versus the voltage drop per QD (Figure 1d). The voltage-axis intercepts for each device provide an approximate value for the EL threshold. The EL threshold voltages for the blue, green, red, and infrared-emitting devices correspond closely with the band gap energies of their constituent QDs, which are given by the peak positions of the PL spectra in Figure 1c. We note that the error in this approximation is larger 2928

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Figure 2. (a) Electrical circuit used to study the charge present in the active layer of the device by measuring the voltage across the device and the voltage across the sense capacitor (10 nF) in series with the device. The resistor (2.6 kΩ) is used to minimize damage to the device electrodes during abrupt voltage pulses. (b) Time-resolved measurements of voltage dropped across the device (solid gray line), voltage dropped across the sense capacitor (solid black line), and electroluminescence (orange line) in response to single polarity voltage pulses (dashed gray line). The inset shows an expanded view of the voltage across the sense capacitor. The steep rises and falls in voltage across the sense capacitor that follow the changes in the applied bias are due to displacement currents; the slow changes in the voltage across the sense capacitor (indicated by shaded regions) are due to charge fluctuations in the luminescent active layer of the device. (c f) Schematic, one-dimensional energy band diagrams showing the fundamental processes of field-driven electroluminescence in a QD film. In the presence of a sufficiently high applied electric field, an electron can transfer from the valence band of one QD to the conduction band of the neighboring entity (c). (d) The field-generated electron and hole may experience electric-field assisted transport to the excited state of the neighboring QDs during which the electrons and holes can form an exciton and recombine nonradiatively or radiatively. (e) The electrons and holes distributed within the QD film cause internal fields that screen the applied electric field and that continuously redistribute charge so that recombination and luminescence persists throughout the applied pulse. (f) If the applied electric field is removed (or decreased), the remaining charges create an internal field opposite in direction to the previously applied field, which causes electron and holes to move toward each other and a peak in the luminescent response. (g) Time-resolved measurements of voltage and electroluminescence in response to the alternating polarity pulses. The opposite polarity pulse (which results in a zero average applied voltage) reduces the screening charge in the QD layer (evidenced by a low voltage drop across the sense capacitor. (h) When the voltage of the alternating pulse is sufficient, more ionization occurs to enable sustained electroluminescence as a function of time.

than the exciton binding energy, which is typically on the order of 0.1 eV in these QDs. Because of the different dielectric constants of SiO2 (ε ∼ 3.9) and Al2O3 (ε ∼ 9), the turn-on voltage for a device with SiO2 insulating layers is greater than the turn-on voltage for a device with Al2O3 insulating layers as shown in Figure S4 of the Supporting Information. However, despite the different absolute turn-on voltages in these two structures, the data in Figure 1d show that when the voltage drop from across the insulating films is subtracted the EL threshold occurs at the

same voltage drop per QD for structures containing the same (redemitting) CdSe/ZnS QDs, independent of the choice of insulating material (Al2O3 or SiO2). This is indicative of an EL process that is entirely determined by the electronic and optical properties of the luminescent material. Figure S5 of the Supporting Information shows that although high voltages are applied to this device structure, it can operate continuously in air for long periods of time. To understand the dynamics of charge generation and recombination in the luminescent layer, we apply a series of shaped 2929

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Figure 3. (a) Plots of the photoluminescence (PL) efficiency and normalized EL intensity as a function of the fraction of luminescent material present in the active layer. Two different semiconducting nanoscale systems, QDs embedded in an insulating polymer matrix (red lines) and organic small molecules embedded in a wider band gap molecular organic host matrix (green lines), show the same trend of decreasing PL efficiency and increasing EL response with increasing emitter mass fraction (which we correlate with an increasing cluster size of QDs or luminescent molecules). (b) Plot of time-resolved data for two different devices: one with a luminescent layer consisting of only QDs (solid lines), and one with a QD-polymer blend active layer (dashed lines). The applied voltages are chosen so that comparable electric fields of approximately 3.3  105 kV/m are dropped across both the QD and QD-polymer layers. The applied waveform for the QD-only device is shown (gray dashed line). The electroluminescent intensity is shown in orange with the voltage across the sense capacitor is shown in black.

electrical pulses (V0, dashed gray lines in Figures 2 and 3) to a standard device with red-emitting CdSe/ZnS QDs and SiO2 insulating layers and record the time-resolved electroluminescence (IEL, orange lines). As shown in Figure 2a, we place a sense capacitor in series with our device and also record the timeresolved voltage that is dropped across it (Vc).26 Because our device is capacitive, changes to the amount of charge present in the QD film through ionization or recombination will be reflected as changes in the amount of charge (and therefore voltage) dropped across the sense capacitor. While these measurements cannot detect where charge is located within the QD film, they provide us with an indication of the amount of charge present in the device active layer as a function of time. Figure 2b plots time-resolved data in response to single polarity pulses, and Figure 2c f schematically illustrates the electric fields and the physical processes that may occur within the QD thin film. The voltage drops across the insulating SiO2 layers vary in response to dynamic changes in the internal electric field and the applied external field. We observe that when the voltage pulse exceeds the ionization voltage, charge is generated within the QD film (Figure 2c). The net generation of charge

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carriers can be observed in the voltage drop (Vc) across the sense capacitor, which increases during the applied voltage pulse (see first highlighted inset of Figure 2b). Some of this charge recombines radiatively, as evidenced by the onset of EL. The applied electric field simultaneously causes electrons and holes to be transported away from each other toward opposite QD/SiO2 interfaces (see Figure 2d). This redistribution of charges creates an internal electric field that screens the external applied field (Figure 2e). The EL response decreases because screening of the electric field limits carrier generation through ionization and the spatial separation of hole and electron distributions reduces the amount of recombination. When the external applied electric field is removed, the internal field, which is present due to the spatial separation of electron and hole populations (Figure 2e), causes these electrons and holes to drift toward each other and recombine, as evidenced by a second peak in the EL response (Figures 2f). Figure 2g shows the time-resolved voltage and EL data in response to alternating polarity pulses. With this biasing scheme, the internal electric field, created by screening of charges generated during one voltage pulse, enhances the effect of the externally applied electric field of opposite polarity (Figure 2h). The recombination of charges in the device has a long time constant, which is visible in the small slopes of the highlighted regions of the inset in Figure 2b. The choice of the voltage pulse scheme determines the amount of screening charge in the QD layer as a function of time, which can impact device efficiency and performance. For devices driven with the single polarity pulse train, as presented in Figure 2b, the average applied voltage is positive, and we observe an accumulation of screening charge, which leads to a constant positive background in the sense capacitor voltage (Vc ≈ 17 V). In contrast, when the devices are driven with a series of pulses of alternating polarity, the screening charge is comparatively lower (Vc ≈ 0 V between pulses). These screening charges affect device performance; the presence of screening charge results in a larger voltage drop across the insulating layers and more rapid degradation of the device (often manifested as delamination of the metal electrode from the insulator). Data in Figures 1 3 indicate that field-driven ionization locally generates charge carriers from within the luminescent film, eliminating the need for charge injection or long-range carrier transport. It therefore follows that an emissive layer need not be a continuous QD film but could consist of clusters of QDs embedded within an insulating matrix, such as a transparent polymer film. The use of such QD-polymer blends has thus far been restricted to applications that involve optical excitation of colloidal QDs in order to take advantage of the higher QD thin film PL efficiencies that are achieved when QDs are dispersed.36 38 Results displayed in Figure 3 demonstrate that an active layer consisting of clusters of QDs embedded in an insulating polymer can also be used in an electrically excited emissive device (see Supporting Information Figure S1 for morphological images of thin films of QDs embedded in a polymer matrix). The choice of a particular QD ligand and an insulating polymer matrix as well as the QD loading fraction will determine the size and spatial separation of resulting QD clusters in a QDpolymer film. Thin films with larger clusters of QDs exhibit lower PL efficiencies, which is expected since larger QD clusters are more likely to contain QDs with trap state defects that can quench luminescence on any neighboring QD.36,38 Accordingly, in Figure 3a, increasing the mass fraction of the luminescent material in the active layer corresponds to increasing the cluster 2930

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Nano Letters size in the QD-polymer blends, and a decrease in the blended film PL efficiency. To facilitate direct comparison of the EL response of films with different QD loadings (right panel of Figure 3a), the thickness of the QD-polymer films is chosen such that each device has approximately the same total number of QDs and the applied electric field is chosen such that the fields dropped across QDs in both devices are approximately the same. Under these conditions, we find that increasing the size of the QD clusters results in a larger EL signal. Indeed, dispersing QDs in a polymer such that few QDs are in contact with one another leads to thin films with the highest PL efficiencies of (70 ( 5)% but no observable EL. These observations are consistent with our proposed operating mechanism, which presumes that the QD ionization process requires at least several QDs in close proximity so that an electron can be extracted from one QD and transferred to a neighboring QD. In larger clusters of QDs, we expect more ionization events since a greater percentage of QDs have neighbors into which charge can transfer. Indeed, we see from the voltage drop across the sense capacitor (Vc) plotted in Figure 3b that more charge accumulates in the device with a neat QD film than in a device with QD clusters. These initial observations on clusters of semiconducting materials inside insulators confirm that electric field driven luminescence in QD films is a highly localized process that does not necessitate long-range transport of charge in the QD film. To corroborate our findings with another type of nanoscale emitter, we choose to build similar LED structures using luminescent organic molecular thin films. Molecular organic semiconductors offer the possibility for constructing luminescent dopant/wide-band gap host systems analogous to the QD/ polymer blends. As shown in the Supporting Information Figure S3, the degree of aggregation (i.e., clustering) of the dopant molecules can be determined by observing the red-shifting of the film PL spectrum.39,40 In analogy to QDs embedded in an insulating polymer, we choose films of fac tris(2-phenylpyridine)iridium (Irppy3) molecules embedded in the wide band gap organic host bis(triphenylsilyl)benzene (UGH2) deposited by coevaporation of the two materials. Spectral data of the films exhibiting progressive red-shifting of the photoluminescence spectra for films containing 25, 50, 75, and 100% Irppy3 (by mass) indicates increased clustering with increasing Irppy3 mass fraction. Figure 3a shows that, analogously to QDs embedded in an insulating polymer, no EL is observed from isolated Irppy3 clusters (25% Irppy3 in UGH2). The PL response of the molecular organic films decreases with increasing Irppy3 concentration due to molecular aggregation, while the EL increases with increasing Irppy3 concentration because larger clusters provide more neighboring molecules to which an ionized electron can tunnel, again consistent with the results measured for the QD-polymer structures. In summary, we demonstrate that electric field-driven ionization can locally generate the charge carrier pairs needed to achieve electroluminescence in both QD and organic thin film semiconductors, as well as QD-insulating polymer composites. Field-driven ionization removes the design constraints of charge injection and carrier transport that are typically the dominant considerations in choosing the materials, deposition methods, and architecture of nanostructured light-emitting devices. We identify the basic processes responsible for operation of the fielddriven EL devices by matching them to the time-resolved electroluminescence behavior. Results of the present work indicate that

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this new class of field-driven electroluminescent devices provides a robust platform for studies of electronic and excitonic processes in nanostructured luminescent structures.

’ ASSOCIATED CONTENT

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Supporting Information. Additional information provided. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: 617-253-7012.

’ ACKNOWLEDGMENT We thank QD Vision Inc. for providing samples of CdSe/ZnS QDs and the polymer resin. This work was supported by a National Science Foundation Graduate Student Fellowship (V.W.) and the DOE Excitonics Center, an Energy Frontiers Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under award number DE-SC0001088. This work made use MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-08-19762. ’ REFERENCES (1) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (2) Dabbousi, B. O.; et al. J. Phys. Chem. B. 1997, 101, 9463. (3) Stokes, K. L.; Chen, F.; Zhou, W.; Fang, J.; Murray, C. B. Mater. Res. Soc. Symp. Proc. 2001, 691, G10.2. (4) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (5) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (6) Coe, S.; Woo, W.; Bawendi, M. G.; Bulovic, V. Nature 2002, 420, 800. (7) Anikeeva, P. O.; Halpert, J. E.; Bawendi, M. G.; Bulovic, V. Nano Lett. 2009, 9, 2532. (8) Cho, K. S.; et al. Nat. Photonics 2009, 3, 341. (9) Kim, L.; et al. Nano Lett. 2008, 8, 4513. (10) Anikeeva, P. O.; Halpert, J. E.; Bawendi, M. G.; Bulovic, V. Nano Lett. 2007, 7, 2196. (11) Anikeeva, P. O.; Madigan, C. F.; Halpert, J. E; Bawendi, M. G.; Bulovic, V. Phys. Rev. B 2008, 78, 085434. (12) Mueller, A. H.; et al. Nano Lett. 2005, 5, 1039. (13) Caruge, J.-M.; Halpert, J. E.; Wood, V.; Bawendi, M. G.; Bulovic, V. Nat. Photonics 2008, 2, 247. (14) Achermann, M.; Petruska, M. A.; Koleske, D. D.; Crawford, M. H.; Klimov, V. I. Nano Lett. 2006, 6, 1396. (15) Wood, V.; et al. ACS Nano 2009, 3, 3581. (16) Wood, V.; et al. Nano Lett. 2010, 10, 24. (17) Drndic, M.; Jarosz, M. V.; Morgan, N. Y.; Kastner, M. A.; Bawendi, M. G. J. Appl. Phys. 2002, 92, 7498. (18) Morgan, N. Y.; et al. Phys. Rev. B 2002, 66, 075339. (19) Woo, W. K.; Shimizu, K. T.; Jarosz, M. V.; Neuhauser, R. G.; Leatherdale, C. A.; Rubner, M. A.; Bawendi., M. G. Adv. Mater. 2002, 14, 1068. (20) Pope, M.; Kallmann, H.; Magnante, P. J. Chem. Phys. 1963, 38, 2042. (21) Van Slyke, S. A.; Chen, C. H.; Tang, C. W. Appl. Phys. Lett. 1996, 69, 2160. 2931

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