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We propose a highly efficient hybrid light-emitting device (LED) with a single active layer where CdSe/ZnS quantum dots (QDs) are dispersed as a guest material in a conjugated polymer (co-polymer) matrix used for a host material. In our structure, the QDs act on light-emitting chromophores by trapping the migrating excitons in the co-polymer matrix via Förster energy transfer, and improve the charge balance within the co-polymer by trapping the injected electron carriers. Experimental results show that the electroluminescent properties highly depend on the doping density of the QDs within the co-polymer matrix, where the luminance as well as the external current efficiency are initially enhanced with increasing the concentration of the dispersed QDs in the co-polymer solution, and then such properties are degraded due to aggregation of the QDs. We can get the maximum brightness of 9,088 cd/m2 and the maximum external current efficiency of 7.5 cd/A in mixing ratio of the QDs by 1.0 wt%. The external current efficiency is enhanced by over 15 times and the turn-on voltage is reduced in comparison with the corresponding values for a reference device that uses only a co-polymer as an active layer.
©2010 Optical Society of America
1. Introduction
Since the introduction by Burroughes [1] in the 1990s, organic light-emitting diodes (OLEDs) have attracted considerable interest in academia and industry fields because they can be used as integrated flexible light sources with various applications from display technologies and solid-state illuminations to medical applications. Their main advantages are that they can be manufactured at low cost using simple techniques, they can cover a wide range of visible spectra, and they have the ability to operate at low voltage and low power consumption [2–5]. However, OLEDs have critical some disadvantages which are short lifetime from the degradation by water vapor and oxygen permeation.
In parallel, inorganic LED made of quantum dots (QDs) by a solution process were reported by Bawendi, Bulovic [6–9] and Sun [10]. QDs have several favorable characteristics such as a size-tunable band gap as well as high photoluminescence efficiency due to the quantum confinement effect. They can provide not only pure and saturated colors with full color expression in the visible range, and a cost-effective process but also stability for water vapor or oxygen permeation to the device. However, QDs-LED suffers from leakage current because of the poor structural stability such as particle aggregation and grain boundary of QDs in the active layer. In addition, they show low conversion efficiency due to the energy barrier at junctions by their ionization energy (IE) and electron affinity (EA) properties, which requires additional layers such as carrier injection and transporting layers.
Recently, several researchers reported QD/polymer hybrid devices with multiple stacked structures for enhancing the efficiency or obtaining color tuning capability. With a tandem structure of OLED, Ryu et al. and Jeon et al. reported that current efficiency and luminance can be highly improved by using QDs as interfacial layers [11,12] for enhancing the exciton recombination in electroluminescent polymer layers, where the interfacial QD layers does not act on an emitting layer. By using QDs as an emission layer, Tan et al. reported a white light emitting device with a poly-TPD/QDs bilayer structure [13]. In these bilayer or tandem structures, available materials are highly limited and their selection should be carefully determined with considering chemical resistance between adjacent layers. Currently, highly efficient devices made by all solution process have not reported yet.
The QD/polymer hybrid devices can be made also by blending the QDs into a host polymer matrix. In this case, the single QD/polymer hybrid layer can be made simply by the solution process when both of the QDs and the host polymer have high solubility in the same solvent. By using the QDs as emissive chromophores, Torriss et al. reported the white-emitting device using red-emitting QDs within PVK polymer matrix [14] where they used additionally thermally evaporated Alq3 layer for a green-emitting layer and an electron transporting layer (ETL). Rizzo et al. [15] reported the blue-emitting device using blue-emitting QDs within a CBP polymer matrix where a BCP layer is used as an ETL for enhancing charge balance. Electroluminescent property of the QDs in these host/guest structures can be explained by the Förster energy transfer. Lee et al. also reported that the QDs doped in the polymer matrix improved the charge balance and the external current efficiency where the QDs are phase-separated after the annealing process and the final structure and the role of the QDs are similar to those of the QD/polymer bilayer structure [16].
Here, we propose a hybrid LED that is made of QDs embedded in a co-polymer (PQD) as a single active layer. Our PQD device comprises simply five layers without both of the electron injection layer (EIL) and ETL, which can be made by all solution process except the electrodes. Without EIL and ETL, the luminance and the current efficiency are very poor due to charge balance problem during exciton formation in the active layer in general. However, our PQD device showed high luminance and current efficiency due to doped QDs. The doped QDs play a role in improving the charge balance in the host matrix as electron trapping sites as well as in increasing luminance as exciton trapping sites via Förster energy transfer [17]. The effects of the doped QDs on the luminance and external current efficiency are discussed by varying the doping concentration.
2. Device structure and principle
Figure 1 showed our five-layer structure; anode (ITO), HIL, HTL, single emissive layer of PQD, and cathode (Al). This device has merits of solvent process and simple structure with an improved charge balance because the co-polymer contributes reduction of the hole injection barrier and embedded QD trap the carriers in trap site.
During the charge transfer process, the band gap of the guest should fall within that of the host in order to favor the transport of electrons and holes from the host to the guest. The recombination of electrons and holes is shown in Fig. 1. If this energy transfer process dominates, the energy of the excited state of the host should be higher than that of the guest in order to ensure efficient energy transfer [18].
The efficiency of charge (or energy) transfer can be easily verified if there is an overlap between the PL spectrum of the host and the absorption spectrum of the guest [15]. In addition, the distance between the host and the guest should be shorter than Förster radius. This distance can be reduced by increasing the concentration of the guest within the host.
However, excess concentration of the guest produces strong guest interactions in active layer, which leads to decrease in luminance and efficiency.
3. Fabrication method
3.1 Sample preparation for active layer
In our experiment, we have investigated the hybrid material which is consists of QDs and co-polymer. We used TOPO(trioctyl phosphine oxide)-CdSe/ZnS QDs (QD Solution, Korea) with PL wavelength of 540 nm and an absorption wavelength of 532 nm (Fig. 2 (a) ). Full width of half maximum (FWHM) was 27 nm. In addition, we used poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1',3}-thiadiazole)] (American Dye Source, USA) with an PL wavelength of 538 nm as a host material (Fig. 2 (b)).
Fig. 2 The chemical structures of each material: (a) TOPO-QD, (b) poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1',3}-thiadiazole)].
The QD purification was carried out by adding methanol to the QDs in a toluene solution [19]. Then, the mixed solution was centrifuged at 8,000 rpm for 30 min in order to separate the QDs by precipitation. The supernatant liquid phase was decanted to remove excess reagents [20]. After centrifuging, we controlled QDs concentration.
For a host matrix, the yellowish-green-light-emitting co-polymer was dissolved in toluene to a concentration of 1 wt%. In order to enhance the purity of the emissive layer without particle aggregation, we used a micro-filter with pore size of 0.2µm (Whatman, USA).
In order to embed the solution of QDs in the co-polymer, we added the precipitated QDs to the co-polymer in the presence of toluene. We did not encounter any difficulties in blending the QDs with the co-polymer because both materials were dissolved in the same solvent. So, a PQD active layer can simply achieved by single step of spin casting, which does not need multi-step procedures such as tandem structures.
3.2 Fabrication of device
Indium tin oxide (ITO), a typical anode material, was deposited on a glass substrate to a thickness of 150 nm and patterned through the photolithography process. Then, the HIL was formed using poly(ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS; Baytron VP AI 4083; H. C. Starck) and dried at 150 °C for 10 min. It was used as a buffer layer on the anode mainly to increase the anode work function from 4.7 eV (ITO) to 5.0 eV, to reduce the surface roughness of the anode, and to obtain stable and pin-hole-free electrical conduction across the device. Next, the HTL was formed using poly(N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine) (poly-TPD, American Dye Source, USA) dissolved in a 2 wt% chlorobenzene solution and was spin-coated on PEDOT:PSS with a molecular weight (MW) of 108,000. After spin coating, the HTL was dried in a vacuum oven at a temperature of 110 °C for 30 min. Its highest occupied molecular orbital (HOMO) level is 5.2 eV, which is very close to the work function of PEDOT:PSS, thereby implying that HTL possesses excellent hole transport capability. Moreover, it has been shown that poly-TPD shows good resistance to non-polar organic solvents such as toluene used for PQD solution in our experiment [10]. The only co-polymer and PQD were spin-coated to form an emissive layer. Annealing was performed at 110 °C under vacuum conditions for 10 min. The Al electrode was thermally evaporated and the active area was defined as 3x3 mm2. In our hybrid LED, all the layers except for the electrodes were simply fabricated by the spin-coating process.
3.3 Evaluation of device performance
The current-voltage (I-V) characteristics of the fabricated hybrid PQD device, as well as the luminance-voltage (L-V) characteristics, were measured by a computer-controlled voltmeter (Keithley 2400 series sourcemeter, Keithley Instrument Inc., USA) and a luminance meter (CS-100A chromameter, Minolta, Japan). The LabVIEW software (National Instruments, USA) was used to record the I-V and L-V characteristics, as well as the current efficiency, while CIE 1931 color space data were transferred to the computer for in situ monitoring.
4. Result
4.1 Performance of device
We fabricated LED devices with the following different QDs concentration within co-polymer matrix: (a) only the co-polymer (serving as the reference device), (b) 0.2 wt% QDs embedded in the co-polymer, (c) 0.6 wt% QDs embedded in the co-polymer, (d) 1 wt% QDs embedded in the co-polymer, and (e) 2 wt% QDs embedded in co-polymer.
Figure 3 shows the spectral overlapping in the 540 nm region between the absorption spectrum of the QDs and the PL spectrum of the co-polymer. This spectral overlap enables efficient Förster energy transfer which highly depends on excess overlap spectrum and short distance (below 10 nm considering Förster radius) between host and guest materials.
At the beginning, we measured the electroluminescence (EL) spectra. Figure 4 shows the EL intensities of the only co-polymer device and the PQD device. The wavelengths of the peaks are nearly the same about 540 nm. In spite of the same applied voltage, the EL intensity of the PQD device is much higher than the co-polymer device. It shows that the QDs can enhance the EL intensity of the active layer by the trapping of the carriers as trap sites. Therefore, the PQD device attributed to balanced carrier injection, enhanced recombination, and reduced emission quenching.
In order to confirm the morphology of an active layer depending on doping concentration of the QDs, we performed an AFM analysis at each doping condition as shown in Fig. 5 .
Fig. 5 AFM analysis of films morphology; (a) only co-polymer layer (b) co-polymer with 0.2 wt% QDs, (c) co-polymer with 0.6 wt% QDs, (d) co-polymer with 1.0 wt% QDs, (e) co-polymer with 2.0 wt% QDs.
The reference film with only co-polymer has the roughness RMS value of 1 nm and the maximum roughness of 10 nm. The film morphology of the PQD device with the QDs doping of ~1 wt% shows the roughness RMS value of 1 ~1.5 nm but the maximum roughness increased to 20 ~30 nm due to QDs dispersion. These show that the surface of co-polymer is homogeneous on a narrow scale (Fig. 5a). As increasing the QD concentration in the co-polymer matrix, gradual morphology changes are observed in active layer (Fig. 5b ~d). The excess QDs doped in the co-polymer layer produce highly rough surface above 8 nm RMS value with the maximum roughness of 114 nm. Based on these AFM results, the PQD layers under the QDs of 1 wt% have just partially aggregations on surface, but the excess QDs over the concentration of 2.0 wt% (Fig. 5e) produces large clustering via over 20 QDs particle aggregation by the strong QDs interaction. This result shows that QDs are not uniformly dispersed in the co-polymer and has small clustered islands at the cathode/co-polymer interface. So, high roughness of active layer via excess QDs leads to performance degradation in the PQD device.
Figure 6 shows the luminance of the fabricated devices as a function of applied voltages. The maximum luminescence of 5,541 cd/m2 and maximum external quantum efficiency of ~0.5 cd/A are obtained in the reference device. The PQD devices exhibit maximum luminance of 6,901 cd/m2 at 0.2 wt% QDs, 8,030 cd/m2 at 0.6 wt% QDs, and 9,088 cd/m2 at 1 wt% QDs. The inset result shows the turn-on voltage, which is defined by the applied voltage at a luminance of 1 cd/m2. The turn-on voltages of devices are 6.5V, 5.5V, 5.0V and 4.5V, respectively. These results indicate that increased QD concentration improved the luminance as well as the reduced the turn-on voltage.
Figure 7 shows the current density at each condition. Generally, the probability of non-radiative Auger recombination increase when electrons are accumulated at the LUMO level in the co-polymer. This phenomenon results in increase of the current density in devices [7].
In our band structure as shown in Fig. 1, the injected electrons can be easily transfer from the LUMO level of the co-polymer to that of the QDs, if the QDs are doped in the co-polymer with proper concentration [12,21]. Consequently, the electron concentration in the LUMO level of the co-polymer decreases, which results in reduced non-radiative Auger recombination. The luminescence and the current density results of Fig. 6 and Fig. 7, respectively, show the enhanced energy transfer from the host to the guest in high current injection condition in our system, where the current density becomes lower and the luminescence is enhanced with increasing the QD concentration.
Figure 8 shows the external current efficiency as a function of the current density. The maximum external quantum efficiency of the 0.2 wt%, 0.6 wt%, and 1 wt% devices are ~2.5 cd/A, ~4.0 cd/A, and ~7.5 cd/A, respectively. By dispersing QDs within the co-polymer matrix, a uniformly distributed structure is formed that accelerates charge injection in the LUMO level of the QDs and induces QD illumination. Since the HOMO level of the QDs is lower than that of the co-polymer in our structure, is not easy to inject the holes in the co-polymer into the QDs because of energy barrier.
However, as the voltage increases, an increasing number of electrons accumulate at the LUMO level of the QDs, thereby facilitating QD stability. Then, it make hole injection possible from the co-polymer into QDs by tunneling, which is carrier interaction process between each material according to the Förster energy transfer. Thus, Förster energy transfer is considered to contribute to improving the current efficiency as well as the luminance of the device.
In contrast, as shown in the above Fig. 6-8, when 2 wt% QDs are dispersed in the co-polymer, the maximum luminance decreases to 1,961 cd/m2 and the maximum efficiency decreases to ~4.0 cd/A. The turn-on voltage is 6.5 V, which is higher than other PQD device. As the concentration of QDs increases, the aggregation of QDs can be severely increased. This can induce strong interactions among the QDs, consequently reducing the number of electrons accumulated at the LUMO level and the amount of holes injected. In our results, the current density is very low at the same applied voltage, as shown in Fig. 7, which means the probability of recombination is decreased. It is considered that the rapid decrease of the current density at 2 wt % QD concentration would result from the increased traps density caused by interaction between QDs, which can be ascertained also by the significant decrease of the luminance although the band structure have the same condition with other samples. For achieving efficient energy transfer in our PQD device, proper QD concentration within the Förster radius is essential.
4.2 Color coordinate
To verify fabricated device emission, we evaluated CIE (Commission Internationale de l'Eclairage 1931) color space for the reference device and the devices with QDs doped in the co-polymer (Fig. 9 ). The CIE color space for the reference device were (0.32, 0.56), i.e., within the yellowish green NTSC region. In contrast, the coordinates of the PQD device were (0.35, 0.61), i.e., outside the NTSC range [6]. So, the color coordinate shift in our PQD device originated from that of the co-polymer to that of the QDs used as guest material.
5. Conclusion
We reported the improved luminance and efficiency of LED devices that comprise the QDs embedded in the co-polymer. The proposed devices exhibit efficient Förster energy transfer because of the overlap between the PL spectrum of the co-polymer and the absorption spectrum of QDs. Such effects are enhanced with increasing the QD concentration due to the reduced Förster radius between the host and the guest. In addition, QDs embedded in the co-polymer enhance the processes of carrier injection and recombination, and reduce the quenching of emission. However, excess of QDs doped in the co-polymer matrix decrease the probability of recombination due to strong interactions among the QDs.
Our results show that the luminance as well as the efficiency of hybrid LED with the QD/co-polymer composite structure can be effectively enhanced by optimizing the QD concentration.
Acknowledgments
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010-0001882, No. 2010-0001884)
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