Precise Color Tuning via Hybrid Light-Emitting Electrochemical Cells

Dec 20, 2010 - We report color-tunable light-emitting devices employing CdSe/ZnS quantum dots (QDs) blended into a polymer light-emitting electrochemi...
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LETTER pubs.acs.org/NanoLett

Precise Color Tuning via Hybrid Light-Emitting Electrochemical Cells Amanda J. Norell Bader,†,‡ Anton A. Ilkevich,† Ilya V. Kosilkin,§ and Janelle M. Leger*,†,‡ †

Department of Chemistry, ‡Department of Physics and Astronomy, Western Washington University, Bellingham, Washington 98225, United States § Department of Chemistry, University of Washington, Seattle, Washington 98195, United States

bS Supporting Information ABSTRACT: We report color-tunable light-emitting devices employing CdSe/ZnS quantum dots (QDs) blended into a polymer light-emitting electrochemical cell (LEC) architecture. This novel structure circumvents the charge-tunneling barrier of QDs to achieve bright, uniform, and highly voltageindependent electroluminescence, with nearly all emission generated by the QDs. By blending varying ratios of two QD materials that emit at different wavelengths, we demonstrate precise color control in a single layer device structure. KEYWORDS: Quantum dots, color tuning, light-emitting electrochemical cells, QD-LECs, organic semiconductors

S

emiconductor quantum dots (QDs) are attractive as active materials for light-emitting devices because they demonstrate high color tunability due to size-tunable band gaps, high photoluminescence (PL) quantum efficiency, and good stability.1-7 Colloidal QDs in particular are soluble in common organic solvents and are therefore compatible with low-cost, high throughput solution processing techniques. Initially, QDs were incorporated into polymer light-emitting diode (PLED) structures by either blending QDs into a conjugated polymer matrix8 or by building a multilayer of QDs next to a thin polymer film.9 However, due to a charge-tunneling barrier on the QDs caused by the presence of insulating surface ligands, these devices displayed significant emission from the polymer host. In addition, the relative emission from the polymer was highly voltage-dependent, causing the emission color to shift with applied voltage. Several other methods of incorporating QDs into a PLED have been explored to circumvent this issue. Successful devices have incorporated QDs as a monolayer sandwiched between electron transport and hole transport layers.10-16 In these devices, preferential QD emission can be achieved by manipulating the location of charge recombination in the active film.15,17 Although these multilayer devices have shown pure QD emission, they are limited by a relatively complex architecture and often retain a moderate amount of voltage dependence in their spectra where reported.11,15 An alternative and often overlooked architecture for polymer light-emitting devices is the light-emitting electrochemical cell (LEC). In essence, an LEC can be understood as a dynamic, selfassembled polymer p-i-n junction in which p and n type doping occur via electrochemical oxidation and reduction, respectively (Figure 1).18-21 To achieve doping, salts are incorporated into the film as counterions to allow oxidation and reduction of the emissive polymer. LECs in general have been of interest due to the reduction of charge injection barriers at the electrode/ r 2010 American Chemical Society

polymer interface made possible by electrochemical doping, allowing more air-stable metals to be used and reducing thickness dependence.18,22-24 The mechanism of operation of an LEC is in fact more complicated than this simplified description illustrates and has been a subject of debate in recent years18,25-29 Nonetheless, it is agreed that in an LEC light emission occurs over a relatively thin intrinsic (nondoped) layer, making it ideal for the incorporation of QDs using a simple blended hybrid architecture. The motivation behind this approach is similar to that behind the use of QD monolayers, specifically to separate the functions of charge transport and recombination within the device in order to optimize each process. In the case of the QD-LEC, charge transport in the nonjunction region is facilitated by the presence of a doped (and therefore highly conductive) conjugated polymer matrix, which allows charges to move easily to the emission zone without tunneling between quantum dots. The reduced thickness of the emissive (nondoped) region in a polymer/QD film results in a decrease in the number of QDs a charge must tunnel across before recombination, thereby facilitating light emission by the QDs and reducing contribution from the polymer host. However, in contrast with QD monolayer devices, a simple blended architecture is employed, therefore the concentration of QDs in the emission zone remains constant even under a shift in the location of emission within the device. Such shifts are known to occur with changing voltage and likely contribute to the strong voltage dependence of QD monolayer devices.15,17,26 We would therefore expect QD-LECs to exhibit true voltage-independent electroluminescence spectra. In this letter, we present QD-LECs fabricated with QDs blended into a Received: September 7, 2010 Revised: December 13, 2010 Published: December 20, 2010 461

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Figure 2. Device structure employed in this study, consisting of an ITO coated glass bottom electrode, gold top electrode, and a blend of electroluminescent polymer, ATOAAS, and QDs.

a fixed junction device in which the counterions are thought to be immobilized by covalent bond formation concurrent with electrochemical doping of the polymer.30 In fixed junction LECs, once the p-i-n junction is formed, it will not reverse if the electric field is reversed. This results in a diode-like I-V curve with very low current and no emission in the negative voltage regime.30,34-37 Fixed junction devices are of interest due to the reduced turn-on time and improvement in photovoltaic properties afforded by immobilizing charge carriers.37 Two different sizes of QDs were used for this experiment. The photoluminescence (PL) peak of the smaller QDs is centered at λ = 576 nm and PL peak of the larger QDs is centered at λ = 615 nm (Figure 3a). Figure 3a shows the normalized electroluminescence (EL) spectra of three LEC devices: one with no QDs, one with 576 nm QDs only, and one with 615 nm QDs only. The EL spectrum of the polymer-only device displays the characteristically broad emission of polymers. However, when QDs are added to the LEC, the spectrum is shifted to an intense narrow peak centered at the PL of the QDs, and emission from the polymer is limited to a relatively small shoulder in the spectrum. The full width at half-maximum (fwhm) of the QD peak in the PL and the EL is very similar; for 576 nm QDs the fwhms (PL and EL) are 30 and 31, nm respectively, and for 615 nm QDs the fwhms are 25 and 31 nm, respectively. This indicates that the high color purity of QDs is maintained when blended into LECs. Figure 3b displays representative currentvoltage (I-V) and radiance-voltage (R-V) curves of these devices. Before testing the devices in this study, each device was “charged” to allow the fixed p-i-n junction to form by holding the device at 8 V with ITO biased as the cathode until a steady current was achieved (∼2 min). Before taking current and radiance data, devices were held at 16 V for ∼1 min, until a stable point in current and radiance was reached. This two-step charging procedure is used because the device does not appear to be fully charged at the lower voltage, however extended charging at high voltages leads to low lifetimes. I-V and R-V curves were then taken simultaneously by sweeping voltage from 16 to -16 V in 0.33 V increments with 200 ms delay between step and measure. I-V and R-V curves behaved as expected for fixedjunction LECs, displaying a diodelike I-V relationship and no light emission in the negative voltage regime, indicative of the formation of a stable junction. Brightnesses of the devices reported here are between 40 and 50 μW/cm2, or 200-300 cd/m2, at 16 V. This is competitive with LECs employing ionic liquids (1-40 μW/ cm2)30,31,38-40 and similar to other hybrid systems (10-1000 cd/m2),12,14,16,41 where comparative data is reported at similar voltages. Similarly, the external quantum efficiency of these devices was ∼0.1% at 16 V, similar to conventional and ionic liquid based LEC efficiencies where reported.35,42,43 The turn-on voltage of LECs employing ionic liquids is highly dependent on ion concentration, with higher concentration yielding lower turn-on

Figure 1. Device schematic showing the operation of a standard LEC as it has been classically understood. The charges shown are associated with ions, which move under an applied voltage and serve as counterions in the electrochemical doping of the polymer.

single polymer layer that show nearly pure emission from the QDs, independent of applied voltage. We also demonstrate precise color-tuning through variation in the mass ratio between two peak emission QD materials blended in the polymer film. On the basis of promising initial performance, we expect that the ability to achieve high color tunability and stability in a bright, uniform device with a simple single layer architecture and airstable electrodes may be feasible using this approach. Devices in this study were fabricated using a sandwich structure, consisting of a patterned indium tin oxide (ITO) glass substrate, the light-emitting film, and gold top contacts (Figure 2). Before use, the glass substrates were sonicated in successive solutions of glass detergent, deionized water, acetone, and isopropyl alcohol. The film consisted of the PF/PPV copolymer poly[(9,9-dioctyl-2,7-divinylene-fluorenylene)-alt-co-{2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene}], CdSe/ZnS coreshell QDs, and the polymerizable ionic liquid allyltrioctylammonium allylsulfonate (ATOAAS) as the source of counterions.30 The light-emitting films were prepared by spin-casting the polymer, QDs and ATOAAS from chlorobenzene solutions onto an ITO substrate to achieve a film thickness of about 500-800 nm. Solutions were made using a 10:3.5 mass ratio of polymer to ATOAAS, and a 10:8 mass ratio of polymer to total QDs. Gold top electrodes were deposited by thermal evaporation following overnight drying under high vacuum (10-7 Torr). Devices were then tested in a glovebox under a nitrogen atmosphere. The use of the polymerizable ionic liquid, ATOAAS, is motivated by its improved compatibility with polymer films in comparison with traditional LEC salts such as lithium trifluoromethanesulfonate, eliminating the need for the ion conducting polymer poly(ethylene oxide) (PEO).30-33 This results in less phase separation and a higher quality of the blended film as discussed previously.30 This material has been shown to result in 462

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Figure 3. (a) Normalized EL spectra of three LEC devices presented in this study, as well as the PL spectra of the two QD materials used. (b) Representative current-voltage and radiance-voltage curves of QD-LECs. (c) The chemical structure of ATOAAS, the source of counterions used in these QD-LECs.

Figure 5. Atomic force micrograph of a typical QD-LEC. A 5  5 μm scan was collected in tapping mode with a height scale of 17 nm.

of device performance and stability may be achieved by optimizing the structure of the ionic liquid, a subject of current studies.30,32,39 It is worth noting that the incorporation of quantum dots in these devices does not specifically appear to cause any additional problems with device stability. EL spectra at varying voltages were taken to determine voltage dependence of device emission. After the initial ∼2 min charging stage at 8 V, spectra were taken at 8, 10, 12, 14, and 16 V. Figure 4 displays representative normalized EL spectra for QD-LECs that incorporated either 576 or 615 nm QDs. The ratio of the QD peak to the polymer shoulder is nearly the same in each spectra, therefore the device color is not affected by operating voltage. Because the emission zone in LEDs and LECs is known to change location with voltage, many multilayer systems have exhibited drastic color changes when the voltage is varied.11,15,17,26 Therefore, blending QDs into a single-layer architecture increases the color-stability of devices due to the uniform dispersion of QDs throughout the film, resulting in a constant concentration of QDs in the emission zone, regardless of emission zone location. Figure 5 shows the surface morphology of a typical QD-LEC using atomic force microscopy (AFM) in tapping mode. As

Figure 4. Normalized EL spectra of two QD-LECs at varying voltages. (a) QD-LEC with 576 nm QDs only, (b) QD-LEC with 615 nm QDs only. For both QD materials used, EL spectra exhibit a nearly constant ratio between the polymer shoulder and the QD peak, regardless of operating voltage.

voltage.31,32,40 However, high ionic liquid concentration is known to reduce device lifetime,30,32,39 therefore a trade-off exists between lifetime and turn-on voltage. The higher turn-on voltage of these devices (∼7 V) is in part due to this trade-off. Further improvement 463

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Figure 6. (a) Normalized EL spectra of five different QD-LEC devices. Mass ratio of the two different sized QDs was varied to precisely control the overall color of emitted light. (b) CIE chromaticity diagram indicating the color of each of the five multi-QD LECs.

Table 1. Characterization of Color in Multi-QD LECs

expected, films blended with QDs are rougher (rms roughness 19.0 nm) than polymer-only films (rms roughness 10.7 nm). Phase separation between the polymer and ATOAAS does not appear to be noticeable. It has been shown in QD systems made with multiple sizes of QDs that controlling the ratio of the QDs facilitates precise control over PL and EL spectra.14,44-46 This is because the LUMO level of CdSe QDs does not change significantly when their size is changed,47,48 therefore electron injection into large versus small band gap QDs in blended systems is not energetically favored. In addition, exciton transfer between QDs of different sizes has been shown to be limited even in monolayer QD films.14 These studies indicate the potential for precise color tuning across the visible spectrum possible with QD mixtures. Motivated by these results, we explored blending two different size QD materials (576 and 615 nm) into the polymer matrix of our QD-LECs, varying the mass ratio of QDs while keeping the overall polymer:QD mass ratio constant. Devices were bright and uniform and displayed the same small, voltage-independent shoulder from the polymer emission with the bulk of emission originating from the QDs. Figure 6a displays the normalized spectra of five LEC devices with varying mass ratios of the two different QD materials, specifically 8:0, 6:2, 4:4, 2:6 and 0:8. The

relationship between the mass ratio of each QD material and the relative intensities of the emission peaks appear to be directly proportional, clearly demonstrating that the relative emission from the two species can be easily controlled by controlling their mass ratio. The CIE (Commision Internationale de l0 Eclairage) coordinates and photograph of each device at 10 V are presented in Table 1. Additionally, CIE coordinates are plotted on the CIE chromaticity diagram in Figure 6b to illustrate the very finetuning of color achieved in these multi-QD devices. These results suggest that either direct carrier injection into QDs, or efficient exciton transfer between the polymer and the QDs (assuming very similar probability of transfer into either size QD), is likely to be the dominant emission mechanism in these devices. In conclusion, we have demonstrated a novel light-emitting device, the QD-LEC, that utilizes quantum dots as the active emitter in a light-emitting electrochemical cell architecture. Because of the thin emission zone present in an LEC, blending QDs into an LEC provides a facile route to preferential emission from the QDs, eliminating the need for a complex multilayer structure. These simple single-layer devices are bright and uniform with robust voltage-independent spectra. Further, their emission spectra are sharp and can be controlled through simple mass ratio variation of different-sized QD materials. Additionally, the QD-LEC combines the flexibility and low-cost processing of organics with the stability and efficiency of inorganic emitters. Overall, the results of this study indicate the potential for exquisite color control in a simple, single-layer solutionprocessable device.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental details including device fabrication and testing. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully acknowledge the Research Corporation Cottrell College Science Award, the National Science 464

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Foundation (CHE-0935920), Western Washington University, and the University of Washington for supporting this research. Additionally, the authors thank Dr. David Patrick for the use of his atomic force microscope.

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