pubs.acs.org/NanoLett
White Light with Phosphorescent Protein Fibrils in OLEDs Aurora Rizzo,†,§ Niclas Solin,† Lars J. Lindgren,‡ Mats R. Andersson,‡ and Olle Ingana¨s*,† †
Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology (IFM), Linko¨ping University, SE-581 83 Linko¨ping, Sweden, and ‡ Department of Chemical and Biological Engineering, Polymer Technology, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden ABSTRACT Red and yellow phosphorescent insulin amyloid fibrils are used as guest-emitting species within a blue-emitting polyfluorene matrix in light-emitting diodes. The integration of the phosphorescent Ir-complex into the amyloid structures strongly improves the triplet exciton confinement and allows the fabrication of white-emitting device with a very low loading of phosphorescent complex. The overall performances of the devices are improved in comparison with the corresponding bare Ir-complexes. This approach opens a way to explore novel device architectures and to understand the exciton/charge transfer dynamics in phosphorescent light emitting diodes. KEYWORDS White OLED, phosphorescence, functionalized amyloid fibrils, polyfluorene, Dexter transfer
T
he development of white organic light-emitting diodes (OLEDs) based on conjugated polymers is still a very active research field, driven by the need for new illumination sources. The advantage of easy processing of polymers from solution makes them suitable for large area and flexible device fabrication by means of diverse printing techniques.1-3 The use of fluorescent blue emitters as host for red, green, or yellow phosphorescent emitters in white OLEDs4 seems particularly attractive due to difficulties in finding a stable deep blue phosphor, and the necessity to transfer triplets generated in the host layer into guest triplet emitters. Several reports have demonstrated that to fabricate a high efficiency polymeric OLED based on phosphorescent emitters, the triplet energy level of the polymer host should be located at higher energy than that of the guest phosphorescent complexes.5-7 Conjugated polymers have a low lying triplet energy level, meaning that the conjugated polymer host typically quenches the triplet exciton of the guest, due to energy back transfer from the guest to the dark triplet states of the conjugated polymer host.2,3,8,9 So far more promising results on phosphorescent polymeric OLEDs have been obtained by use of the nonconjugated poly(vinylcarbazole) (PVK), which has a high triplet energy state (2.5 eV)6 and do lead to triplet confinement in the guest. However, since PVK is a nonconjugated polymer with high resistivity, typically the operating voltage of these devices is relatively high.10 Moreover higher bandgap and emission in the near UV do not help the color balancing necessary to make efficient white light sources.
Several attempts to make white emission by using polyfluorene as blue emitter and triplet emitters in the green/red region of the visible spectra are reported, but none of them can avoid the use of the PVK either as interlayer11 or in the blend to improve the energy transfer.3 Recently Cao et al.11 reported the synthesis of a higher band gap polyfluorene and used it as host for a blue-emitting Ir-complex, but the efficiency of the device with a single layer of polymer/Ircomplex was low (EQE ) 0.08%). There is a need to find a solution to these problems that does not compromise blue fluorescence emission from the host while allowing green, yellow, and red phosphorescence from guests without triplet back-transfer. Preferably such a material should also be compatible with printing methods. This will open the possibility to realize new device structures with balanced white light emission and suitable for large area production. In this letter, we investigate the integration of insulin amyloid fibrils functionalized with phosphorescent Ir-complexes as active species in polymeric light-emitting diodes. We earlier developed amyloid fibrils incorporating conjugated polyelectrolytes,12 and recently reported a method to prepare amyloid fibrils incorporating iridium-complexes.13 A key aspect of the method is the ability of proteins to aggregate into fibrillar structures, known as amyloid fibrils. The fibrils typically have a diameter in the range of few nanometers and length in the micrometer range. By developing a facile preparation method to functionalize insulin with phosphorescent Ir-complexes, we are able to prepare structures where the insulin proteins act as a template for the Ir-complexes that are incorporated into these fibrils during the fibrillation process. This functionalized material gave us the possibility to investigate the effect of the isolation of phosphorescent guest molecules from the polymer matrix in OLEDs. As the diameter of the fibrils is in the 5-7 nm
* To whom correspondence should be addressed. E-mail:
[email protected]. § Current Address: National Nanotechnology Laboratory (NNL), CNR-Nanoscienze, via per Arnesano Km 5 I73100 Lecce, Italy. Received for review: 04/6/2010 Published on Web: 05/10/2010
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FIGURE 1. (a) Chemical structures of polyfluorene PPF, Ir(dhfpy)2(acac), and Ir(piq)3. (b) Absorption and photoluminescence spectra of functionalized fibrils and emission spectra of PPF. (c) AFM image of functionalized fibrils with Ir(piq)3 on a SiO2 substrate deposited by molecular combing; the scale bare represents 0.5 µm. (d,e) Fluorescence microscope images of fibril functionalized with (d) Ir(dhfpy)2(acac), and (e) Ir(piq)3; the scale bars represent 20 µm.
For the fabrication of white OLED two phosphorescent Ircomplexes emitting in the yellow (Bis(2-(9,9-dihexylfluorenyl)1-pyridine)(acetylacetonate)iridium(III), Ir(dhfpy)2(acac)) and red (Tris(1-phenylisoquinoline) iridium(III), Ir(piq)3) region of the visible spectrum were chosen (see Figure 1 for the structures of the complexes). To prepare the functionalized fibrils, the Ircomplexes were ground with bovine insulin; the resulting material, after heating in aqueous acid (25 mM HCl at 65 °C for 48 h), undergoes self-assembly into amyloid-like structures with a diameter of 5-7 nm and a typical length in the micrometer range. The detailed preparative procedure is reported elsewhere.13 Figure 1 shows the emission spectra, atomic force microscopy (AFM), and fluorescence microscopy images of the functionalized fibrils. For the AFM observation, 10 µL of a drop of the bovine insulin/Ir-complex solution was deposited onto a Si/SiO2 substrate, incubated for 1 min and then gently blown off the substrate using a nitrogen gas flow. The measurement was performed by standard procedures in tapping mode. The fluorescence images were recorded with an epifluorescence microscope using a 40× objective and a 405/30 nm filter for the yellow-emitting complex, and 470/ 40 nm filter for red-emitting complex. The blue-emitting polymer PPF (Figure 1a) was designed and synthesized with the objective to be compatible with the amyloid fibrils.
range, the incorporation of the dye molecules into the fibrils leads to a separation of the polymer host and the dye molecule in the nanometer range. This separation may have an influence on the energy transfer processes. Moreover, by inserting the dye-molecules into the electrically insulating protein structure we can affect the charge transfer and recombination mechanisms. By incorporating the Ir-complexes in the amyloid fibrils we were able to suppress the undesirable Dexter back transfer process from the phosphorescent emitters to the polyfluorene matrix. This novel approach can be helpful to improve the triplet exciton confinement in the phosphorescent guest and to increase the overall external quantum efficiency (EQE) of polyfluorene/Ir-complex-based white light-emitting diodes. For this purpose, yellow- and redemitting phosphors are incorporated into amyloid-like fibrillar structures, which are then blended in a blue-emitting polyfluorene host. Bicolor and white-emitting single layer devices show a substantial increment in the phosphorescent peak intensity and EQE, if compared to the reference devices made with the corresponding bare Ir-complexes as guests in the host polymer. In particular, for the white OLED we obtained an excellent color rendering index (CRI) of 92 and a maximum luminance of 900 cd/m2, consequently the approach provides potential for the development of whiteemitting device for lighting applications. © 2010 American Chemical Society
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FIGURE 2. (a,b) AFM images of the device active layer. (a) PPF doped with bare Ir(dhfpy)2(acac) and Ir(piq)3; (b) PPF doped with Ir(dhfpy)2(acac) and Ir(piq)3 functionalized fibrils. The scale bare represents 1 µm. (c) Fluorescence micrograph of the device active layer made by PPF doped with Ir(dhfpy)2(acac) and Ir(piq)3 functionalized fibrils; the scale bar represents 40 µm.
The amino group along the polymer chain improves the solubility in polar solvents such as THF and also facilitates the interaction between fibril and polymer. The emission of the polymer (Figure 1b) shows a large spectral overlap with the absorption band of both the Ir-complexes. Therefore, efficient Fo¨rster type energy transfer from PPF to Ir(dhfpy)2acac and Ir(piq)3 is expected. For the device fabrication, indium tin oxide (ITO) on glass was used as transparent anode. Poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS) was spin coated on top of ITO, followed by an annealing at 120 °C for 10 min in ambient atmosphere to remove any residual water. The active layer was deposited by spin-coating a blend of PPF and either bare Ir-complexes or Ir-complexfunctionalized fibrils. Finally 0.7 nm of lithium fluoride and 100 nm aluminum are thermally evaporated as cathode. To compare the emission from devices prepared utilizing either bare Ir-complexes or functionalized amyloid fibrils, it is important that similar amounts of Ir-complexes are employed in the two cases. Thus, to ensure that comparable concentrations of Ir-complexes were used for the active layer in the respective devices, the blends were prepared as follows. First, the absorbance of the Ir-complex-insulin material prior to fibril formation was measured by UV-vis absorption spectroscopy. Then a solution of the bare Ircomplex was prepared in THF, which was gradually diluted until reaching the same absorbance value as that obtained when measuring the absorbance in the corresponding Ircomplex-insulin sample. The actual concentrations used in the device preparation were 0.58 mg/mL for Ir(dhfpy)2acac and 0.17 mg/mL for Ir(piq)3. The polyfluorene was dissolved in THF (6-7 mg/mL), and the solutions of the bare Ircomplex or Ir-complex functionalized fibrils were added to the polymer solution in a 1:10 or 1:8 volume ratio for the bicolor and white emitting device, respectively. Before blending with the polymer, the fibril solutions were centrifuged for 1 min. The supernatant was then sonicated for 10 min in an ultrasonic bath, and the resulting solution was centrifuged again for 1 min. This procedure is necessary because the amyloid fibrils tend to form aggregates, spherulites, and bundles of fibrils with a size of several micrometers.13 The centrifugation results in the removal of large aggregates, which would have a detrimental © 2010 American Chemical Society
effect on the device performance. The mixture of PPF and fibrils is stirred before the deposition to prevent the fibril aggregation. Atomic force microscopy was used in tapping mode to investigate the surface of the blends spin coated on the PEDOT-PSS. The morphology of the active layer is shown in Figure 2. The reference sample with PFF doped with bare Ir(piq)3 and Ir(dhfpy)2acac is smooth with a root-mean square of 0.88 nm (Figure 2a). The film with polymer and functionalized fibril has a larger surface roughness of 5.40 nm and the fibril structures are visible in the morphology (Figure 2b). The typical height of such structures is between 11-15 nm while the fibril height is between 5-7 nm (Figure 1c), meaning that the fibrils are embedded in the polymer. Figure 2c shows a fluorescence optical micrograph of the same film. Only blue emission can be observed from the PPF confirming that the polymer decorates the fibril structures, which are visible and partially aligned along the spin coating centrifugal direction. In Figure 3 are shown the electroluminescence spectra and the characteristics of devices made with 0.25% of Ir(piq)3 or 0.97% of Ir(dhfpy)2acac in PPF (see Figure 3a,b for functionalized fibrils and Figure 3c,d for bare Ir-complexes). The EL spectrum of the device employing only bare Ircomplex is dominated by fluorescent polyfluorene emission. The low loading of phosphorescent complexes is suboptimal for the device made with bare Ir-complexes, while the same amount of triplet emitter material is enough when the Ircomplexes are inserted in the fibril structures. The behavior of the device prepared with bare Ir-complexes is typical of a system where an efficient back transfer from phosphorescent dyes to PPF takes place, which results in a quenching of the phosphorescent emission; parts of the fluorescence may also be due to triplet-triplet annihilation leading to singlet emission.9 On the other hand, the device prepared with the Ir-functionalized fibrils at the same dye concentration shows a predominant emission from the phosphorescent material. This indicates that triplets are better confined to the fibrils. Triplet excitons are transferred via diffusion and Dexter transfer. We suggest that the differences in the device performance can be explained by considering the 2227
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FIGURE 3. (a-d) Normalized electroluminescence spectra of the devices: PPF doped with (a) Ir(piq)3 and (b) Ir(dhfpy)2(acac) functionalized fibrils, (c) Ir(piq)3 and (d) Ir(dhfpy)2(acac) bare complexes. Current density versus voltage characteristics of the devices: PPF doped with (e) Ir(piq)3 bare complex (open circle) and functionalized fibrils (filled circle) (f) Ir(dhfpy)2(acac) bare complex (open circle) and functionalized fibrils (filled circle). Insets: External quantum efficiency versus current density of the same devices.
geometrical constraints imposed by incorporating the Ircomplexes into amyloid fibrils. In particular the Dexter transfer requires an overlap of the molecular orbitals of donor and acceptor, that is, a maximum distance of 1-1.5 nm between the two species.14 The insertion of the Ircomplex into the insulating amyloid structure helps the confinement of the triplet within the phosphorescent emitter and inhibits the back transfer, by means of Dexter mechanism, of triplet excitation to PPF. The long-range Fo¨rster energy transfer of singlet excitons from the blue-emitting polyfluorene to the red and yellow emitting Ir-complex singlet state is one of the possible mechanisms, followed by singlet to triplet intersystem crossing and subsequent guest phosphorescence emission (Figure 4a). The substantially lower current density at the operating range compared with the bare Ir-complex for all the devices © 2010 American Chemical Society
(see Figure 3e and f) suggests that some of the holes are blocked at the insulin fibril sites.15 The accumulated holes can then tunnel through the very thin insulating fibril barrier and can be trapped on the phosphorescent emitter guests due to the favorable highest occupied molecular orbital (HOMO) energy level alignment (Figure 4a). The hole trapped in the Ir-complex forms a cationic state of the guest, and this state will function as an electron trap. The trapping process is a mechanism present also in the devices with bare Ir-complex;1,2,9 however, in these devices the low Ir-complex concentration makes this mechanism very inefficient, that is, the probability for a hole to meet an Ir-complex molecule before recombining in the PPF sites is low. The fibrils act as a blocking and accumulation agent for holes and render the trapping more efficient. Moreover, in the device prepared from PPF and bare Ircomplexes the Dexter back transfer from Ir-complex to PPF 2228
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FIGURE 4. (a) Proposed energy level scheme and exciton energy for the emission layer.14-17 The HOMO-HOMO energy level alignment favors the holes trapping in the Ir-complex. The polyfluorene has lower triplet energy than the phosphorescence emitters. Consequently the excitons transferred by Fo¨rster transfer or trapping in the guest Ir-complex can undergo Dexter back transfer to the dark triplet state of the polyfluorene. (b) CIE chromaticity diagram. The color coordinates of the device at different voltage bias are depicted. (c) Electroluminescence of the device (PPF:0.98% Ir(dhfpy)2(acac) fibrils:0.05% Ir(piq)3 fibrils). Inset: photo of the working device. (d) Current density versus voltage characteristics of the device (filled circle) and luminance versus voltage (filled triangle). Inset: external quantum efficiency versus current density of the same device.
investigations on our system are needed in order to support this hypothesis. A white-emitting device was fabricated by blending both yellow- and red-emitting functionalized fibrils in PPF. The Commission Internationale d’E`clairage (CIE) coordinates diagram, the EL spectra of the device at 7 V, current-density and luminance versus voltage and EQE versus currentdensity characteristics, and a photo of the working device are shown in the Figure 4b-d. By accurate control of the relative concentration of the functionalized fibrils and polymer we were able to realize a device with a balanced white emission spectrum. The CIE coordinates were stable in the white region of the diagram varying from (0.37; 0.41) at 4 V to (0.33; 0.39) at 10 V. The maximum color-rendering index (CRI) value for the device is 92 at 5 V, which is one of the highest values among the reported.21 A maximum EQE of 0.2% has been obtained at 7 V that corresponds to (0.34; 0.37) CIE coordinates and a CRI of 91. A maximum luminance of about 900 Cd/m2 has been obtained at 10 V. As expected, the control device made by blending PPF with bare Ir-complexes at the same dye concentrations did not show any emission from the triplet emitters. In summary, successful integration of phosphorescent functionalized fibrils in bicolor- and white-emitting polymeric electroluminescent devices was demonstrated. The
is a very efficient process. For instance, the yellow emitting Ir(dhfpy)2(acac) has a photoluminescence peak at 559 nm that corresponds to a triplet exciton energy of about 2.2 eV, while the PF triplet exciton energy is slightly lower, about 2.1 eV14-16 (Figure 4a). The electroluminescence spectrum of the control device prepared at a concentration of 0.97% Ir(dhfpy)2(acac) in PPF shows no emission from the phosphorescent material. This is likely due to an efficient Dexter back transfer, which results in a quenching of the yellow phosphorescence. The back transfer of the excitons results also in a lowering of the device EQE. Since Dexter transfer requires the spatial overlap of the molecular orbitals of donor and acceptor, by placing the phosphor in the insulating fibril the spatial overlap requirement is not fulfilled. The fibril thus helps to inhibit the back transfer and to confine the triplets within the phosphorescent emitters. It has been demonstrated that the decoration of amyloid fibrils with a luminescent alternating polyfluorene leads to a preferential orientation of the polymer backbone along the fibrils and the formation of a more planar backbone conformation induced by the biomolecules.18,19 The interaction with amyloid fibrils and the conformational changes in the polymer chain could have an impact on the excitation and charge transfer mechanisms between the PPF and Ircomplexes. This has been already demonstrated for conjugated polyelectrolytes and amyloids.20 However, further © 2010 American Chemical Society
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overall performances of the devices were improved relative to devices made by using bare Ir-complexes. Moreover, the peak intensity of the triplet emission is considerably enhanced upon insertion into fibrils, suggesting that the insertion of the Ir-complex helps the confinement of the excitation on the triplet emitter sites and inhibits the back transfer to the low lying PPF triplet states. Extended photophysical investigation of the physical mechanisms is needed to fully understand the energy transfer mechanism in the phosphorescent functionalized fibril-PPF system. The approach can be extended to fibrils made from diverse proteins or preparation conditions, with different shapes and diameters. This could help the investigation of how the geometrical constrains can affect the energy transfer mechanisms and the device performances. Finally, we also demonstrated the possibility to fabricate white-balanced emitting and bright OLEDs by using yellow and red phosphorescent-functionalized fibrils. The improved performances and the excellent color-rendering index make this approach suitable for the development of white OLED on flexible substrate for future application in the lighting field. Nevertheless for this approach to be competitive with other polymeric LEDs there is a need to design blue fluorescent polymers compatible with amyloid fibrils with higher electroluminescence efficiency or to investigate multilayer device structures with electron and hole injection/transport layers.
REFERENCES AND NOTES
Acknowledgment. We thank the Knut and Alice Wallenberg Foundation for funding of equipment and the Swedish Science Council (VR) and the Swedish Energy Agency for project grants. N.S. gratefully acknowledges financial support from the Royal Swedish Academy of Science.
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