NANO LETTERS
Blue Electroluminescence from Oxadiazole Grafted Poly(phenylene-ethynylene)s
2005 Vol. 5, No. 8 1597-1601
Craig A. Breen,†,‡ Sandra Rifai,† Vladimir Bulovic´,*,‡,§ and Timothy M. Swager*,†,‡ Department of Chemistry, Laboratory of Organic Optics and Electronics, Department of Electrical Engineering and Computer Science, and Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received May 27, 2005
ABSTRACT Blue poly(phenylene-ethynylene) (PPE) electroluminescence is achieved in a single layer organic light emitting device. The polymeric system consists of an oxadiazole grafted PPE, which combines the necessary charge transport properties while maintaining the desirable efficient, narrow light-emitting properties of the PPE. Incorporation of a pentiptycene scaffold within the PPE structure prevents ground-state and excited-state interactions between the pendent oxadiazole units and the conjugated backbone.
The promise of polymer-based organic light-emitting devices (OLEDs) hinges on the cost-effective nature of their fabrication. Multiple layers of thermally evaporated materials can be eliminated when using a single polymer thin film prepared by simple spin-coating, casting, or printing techniques.1 Present methods allow for the synthetic fine-tuning of polymer emission wavelengths and materials properties. Hence, general materials solutions for improved device fabrication and performance could have broad applicability.2 We have recently demonstrated efficient, narrow, blue electroluminescence (EL) from a multilayer device utilizing energy transfer from a small molecule host to polystyrenegrafted poly(phenylene-ethynylene) (PPE).3 The modularity of the grafting process can be exploited by simply replacing the polystyrene with a material that incorporates the necessary charge transport properties into the system. This would allow for fabrication of a single-layer device exhibiting efficient PPE electroluminescence. Oxadiazole derivatives have been extensively investigated as electron-conducting and hole-blocking materials in both small molecule and polymeric systems.4 Polymers containing oxadiazole units in either the main5 or side chain6 have been widely used in LEDs. Here we report on an oxadiazole grafted PPE, which combines a charge-transporting segment with a light-emitting conjugated backbone in a single polymeric system. Chart 1 depicts the set of five PPEs we explored in this * Corresponding authors. E-mail:
[email protected] and
[email protected]. † Department of Chemistry. ‡ Institute for Soldier Nanotechnologies. § Laboratory of Organic Optics and Electronics, Department of Electrical Engineering and Computer Science. 10.1021/nl050995p CCC: $30.25 Published on Web 06/25/2005
© 2005 American Chemical Society
Chart 1
study. The first three PPEs presented are entirely dialkoxy substituted. Both of the co-monomers used to construct the polymer are para substituted in 1. To blue shift the emission, ortho-substituted monomers3,7 are used in PPEs 2 and 3. A similar blue shift may be obtained by eliminating the donating alkyloxy substituents that lower the band gap of the polymer by raising the HOMO. This is achieved by replacing one of the alkoxy-substituted monomers with a pentiptycene-containing monomer. Therefore, the solutionstate photophysics of polymers 4 and 5 should be analogous to polymers 2 and 3, respectively. However, thin films 4 and 5 should show enhanced fluorescence quantum yields, since the integrated pentiptycene functionality limits aggregation.8
Scheme 1
Table 1. Polymer Characterizationa photophysical characterization GPC analysis
solution stateb
polymer
Mn
PDI
λabs λem (nm) (nm)
1 2 3 4 5 G1 G2 G3 G4 G5
23 000 28 000 14 000 29 000 55 000 41 000 47 000 31 000 45 000 98 000
2.9 3.5 2.3 3.2 2.5 1.7 1.6 1.5 1.7 1.8
450 430 391 422 391 432 417 391 420 390
475 455 424 454 424 486 457 424 454 424
Φ 0.70 0.65 0.68 0.73 0.92 0.65 0.61 0.60 0.70 0.91
solid state λabs λem (nm) (nm) 480 460 411 441 385 432 484 385 421 386
496 504 533 462 429 506 421 461 460 430
Φ 0.10 0.09 0.62 0.35 0.43 0.18 0.30 0.54 0.67 0.85
a GPC, gel permeation chromatography; M , number average molecular n weight; PDI, polydispersity index; λabs, maximum absorption wavelength; λem, maximum emission wavelength; Φ, photoluminescence quantum yield. b Measurements done in chloroform as the solvent.
Scheme 1 schematically illustrates the oxadiazole grafting process. We have already reported on a polystyrene-grafted PPE system synthesized via an atom transfer radical polymerization (ATRP).3,9 For clarity, we show the PPE backbone with a single hydroxy-terminated alkoxy chain undergoing the ATRP reaction. The PPEs in Chart 1 undergo grafting at both hydroxyl groups. We will refer to the grafted counterparts with a G before their respective ungrafted numbers (e.g., G1, G2, ..., G5). Table 1 summarizes the characterization data for the polymers. Gel permeation chromatographic (GPC) analysis of the PPE systems reveals an increase in the molecular weight of the polymers after grafting with oxadiazole. In addition, the high polydispersity of the ungrafted PPEss typical for cross-coupling polymerizationssis lowered after the grafting process, due to the addition of low polydispersity oxadiazole grafts by controlled free-radical polymerization methods.9 Further evidence for the successful grafting process is obtained when monitoring the absorption profile of the polymer samples as they are eluted from the GPC columns. The grafted PPE absorption displays the characteristic absorption band for both the oxadiazole monomer (λmaxab ) 318 nm) and the conjugated PPE backbone at the same elution volume. This confirms that the oxadiazole grafts are indeed covalently linked to the PPE backbone. It should also be noted that despite very similar oxadiazole graft molecular weights (Mn ) 3300; PDI ) 1.1) for all of the PPE systems, 1598
Figure 1. Normalized solution-state (a) and solid-state (b) absorption spectra of the various ungrafted PPEs. The corresponding normalized photoluminescence spectra are shown in plots c and d, respectively. Solution-state measurements were conducted in chloroform solution. Solid-state measurements were obtained using polymer thin films spin-cast onto glass substrates from chloroform solution (2 mg/mL). In all cases, samples were excited at their respective absorption maxima (see Table 1).
there was not a consistent increase in the overall molecular weight, nor was the total molecular weight increase very significant considering the size of the oxadiazole grafts. This may be attributed to the more spherical shape of the grafted polymers that effectively yield a lower molecular weight by GPC as well as the limited solubility of the PPE macroinitiators8 in the oxadiazole monomer/solvent system used for the graft polymerization (see Experimental Section for details). Limited solubility will result in preferential grafting of the low molecular weight PPE chains, and thus the observed total molecular weight for the grafted PPE systems is lower than expected. This latter effect also explains the variation in molecular weight increase from the ungrafted backbone to the oxadiazole grafted PPE from one polymer system to the next (see Table 1). The solution-state absorption and fluorescence spectra for the ungrafted PPEs were recorded in chloroform and are shown in parts a and c of Figure 1, respectively. It was observed that both the peak absorption and emission (Table 1) of 2 were blue shifted compared to the corresponding maxima for 1. As expected from the polymer substitution, the spectra of 4 overlap with those of 2. By design, even wider band gaps are achieved in 3, where the dialkoxy units are entirely ortho substituted. Again, the spectra of the pentiptycene containing 5 align with those of 3. Thin films of the ungrafted conjugated polymers were spin-cast from chloroform solution (2 mg of PPE/mL). The solid-state absorption and photoluminescence (PL) results are graphed in parts b and d of Figure 1, respectively. The absorption spectra of thin films 1-3 are red-shifted and display spectroscopic features arising from interchain π-orbital interactions generally referred to as aggregation bands (Figure 1b). Aggregation phenomena cause the solid-state PL of these Nano Lett., Vol. 5, No. 8, 2005
Figure 2. Normalized solution-state (a) and solid-state (b) absorption spectra of the various oxadiazole grafted PPEs. The corresponding normalized photoluminescence spectra are shown in plots c and d, respectively. Solution-state measurements were conducted in chloroform solution. Samples were irradiated with λ ) 385 nm light in order to prevent excitation, and thus emission, from the oxadiazole grafts. Solid-state measurements were obtained using polymer thin films spin-cast onto glass substrates from chloroform solution (2 mg/mL). The films were all irradiated near the oxadiazole absorption maxima (λ ) 318 nm) with λ ) 320 nm light in order to observe energy transfer from the oxadiazole grafts to the PPEs.
polymers to be broadened and shifted significantly toward longer wavelengths compared to the emission profiles observed in solution. Similarly, the quantum efficiency of these aggregated systems is dramatically reduced in thin film. In contrast, the spectra of thin films 4 and 5 are almost identical to those in solution confirming that the pentiptycene monomer limits interchain π-orbital interactions.8 However, these systems still experience a drop in quantum efficiency going from solution to thin film, albeit not as significant as the entirely dialkoxy substituted PPEs 1-3 (Table 1). It has been shown that the grafting process can also eliminate the intrinsic aggregation of the PPE backbone, allowing PPEs without pentiptycene scaffolds to maintain their solution state characteristics in the solid state.3,8 The absorption and PL spectra for the grafted polymers are depicted in Figure 2. The solution state PL spectra were obtained by irradiating the samples with λ ) 385 nm light to avoid excitation of the oxadiazole grafts. However, in the solid-state measurements the oxadiazole units were excited with λ ) 320 nm light, to observe energy transfer to the PPE backbone. The solution PL spectra of PPEs G1-G3 are broader than their ungrafted counterparts. Furthermore, in the solid state they are also red-shifted and do not retain their solution-state quantum yield upon grafting as seen previously.3,9 We attribute this unexpected observation to ground-state and excited-state interactions between the oxadiazole and the PPE backbone. However, we note that the pentiptycene containing grafted PPEs (G4 and G5) maintain their narrow, blue emission profiles and high quantum yields in the solid state. This suggests that in the Nano Lett., Vol. 5, No. 8, 2005
Figure 3. Electroluminescence spectra (solid line) overlayed with the corresponding solid-state photoluminescence spectra (dashed line) and solution-state photoluminescence spectra (dotted line) of the various oxadiazole-grafted PPEs. Spectra for the all dialkoxy substituted PPEs G1-G3 are shown in plots a-c while plots e and f correspond to pentiptycene-containing PPEs G4 and G5, respectively. Plot d shows the EL for devices containing ungrafted PPEs 4 (dotted line) and 5 (dot-dash line) for comparison.
same way the iptycene scaffold helps to prevent PPE aggregation phenomena, it may also mitigate the interactions between the oxadiazole grafts and the PPE backbone that lead to spectral broadening. The oxadiazole grafted PPE systems were explored in electroluminescent devices; the device structure is shown in Figure 3. First, a layer of poly-3,4-ethylenedioxythiophene (PEDOT) doped with polystyrene sulfonic acid was spin cast onto indium tin oxide (ITO) coated glass substrates. Next, a layer of the grafted PPE was deposited by spin casting from CHCl3 solution (10 mg/mL (w/w)). Finally, metal electrodes were thermally evaporated onto the polymer layers using a shadow mask. The electroluminescence (EL) spectra of the oxadiazole grafted PPE devices are shown in Figure 3. In the case of polymers G1 and G2 (parts a and b of Figure 3), the EL spectra (solid line) closely match the solid-state PL spectra (dashed line). The EL for polymer G3 deviates from the solid-state PL due to what appears to be an aggregate emission centered at λ ) 490 nm. In all three cases, the EL spectra are significantly red-shifted from the solution-state PL (dotted line). This shift to longer wavelengths, combined with the broadened emission profile of these polymers, prevents these systems from realizing the narrow blue emission that we were hoping to achieve. Nevertheless, the grafted pentiptycene containing PPEs G4 and G5 (parts d and e of Figure 3) maintain their narrow emission profiles in EL. The EL closely matches the solid-state PL spectra and is only slightly shifted from the solution-state PL. The emission for these devices corresponds to CIE coordinates (x ) 0.14; y ) 0.21) and (x ) 0.16; y ) 0.13) for G4 and G5, respectively. This spectrally pure, narrow blue PPE electroluminescence is similar to the results we have recently reported.3 In this 1599
Figure 4. External quantum efficiency versus current density for the oxadiazole-grafted PPEs. Inset shows the current-voltage behavior for the various devices. The entirely dialkoxy substituted PPEs G1-G3 had peak efficiencies of η ) 0.05%, 0.07%, and 0.11%, respectively, with corresponding luminance efficiencies of 0.17, 0.21, and 0.22 cd A-1. The peak efficiency of the pentiptycene-containing PPE G4 was η ) 0.23% at 12.2 mA cm-2 and 9.9 V corresponding to a luminance efficiency of 0.40 cd A-1. The peak efficiency of PPE G5 was η ) 0.29% at 13.6 mA cm-2 and 11.4 V corresponding to a luminance efficiency of 0.34 cd A-1.
case, however, the system is a single solution cast polymer layer without the small molecule host and thermally evaporated hole-blocking and electron-transporting layers. The presence of the pentiptycene scaffold limits the interaction of the oxadiazole units with the conjugated PPE backbone allowing these systems to simultaneously maintain the excellent luminescent properties of the PPEs while harnessing the charge conduction properties of the pendent oxadiazole grafts. To further support our observations, control devices containing ungrafted PPEs 4 and 5 were fabricated (see Figure 3d). The broad EL spectra demonstrate that the pentiptycene scaffold alone is not sufficient to generate narrow PPE electroluminescence. The current-voltage-luminance characteristics of the EL devices are plotted in Figure 4. The inset shows the currentvoltage characteristics for the five oxadiazole grafted PPE devices. All of the grafted PPE systems displayed superior operational efficiencies when compared to other traditional PPE single-layer devices in the literature.10 The entirely dialkoxy substituted PPEs, G1-G3, had peak efficiencies of η ) 0.05%, 0.07%, and 0.11%, respectively. The pentiptycene-containing PPEs G4 and G5 displayed even better behavior. Devices incorporating these polymers had consistently lower leakage current, had lower turn-on voltages, and displayed higher external quantum efficiencies than G1-G3. Polymer G4 had a peak efficiency of η ) 0.23% at 12.2 mA cm-2, while G5 had a peak efficiency of η ) 0.29% at 13.6 mA cm-2. Polymers G4 and G5 also maintained higher operational efficiencies over a larger range of current densities than the entirely dialkoxy-substituted PPEs. These observations emphasize the importance of the 1600
interaction between the oxadiazole grafts and the conjugated PPE backbone. Only when the grafted PPE backbone is engineered with the pentiptycene scaffold is the system able to realize both the desired narrow, blue emission as well as the high external quantum efficiency from a single polymer layer. Such a finding is critical for future development of PPE-based LEDs. Although the oxadiazole-grafted PPE systems exhibit excellent properties for a single-layer architecture, they suffer from relatively high leakage currents and turn-on voltages. In addition, the devices achieve their maximum operational efficiency at high current densities (>10 mA cm-2). This behavior is indicative of the fact that this PPE system does not incorporate a balance in both hole and electron transport properties. Since the pendant oxadiazole units are primarily an electron transport material, a larger current density is necessary in order to introduce enough holes into the system to balance the buildup of electrons and, thus, realize efficient exciton recombination. Furthermore, the buildup of electrons may also explain the tendency for these systems to maintain their operational efficiencies over a large range of current densities. In other words, the characteristic decrease in efficiency due to an increase in the exciton-polaron interaction3,11 does not become significant until a sufficient number of holes reach the recombination region within the device. This finding suggests the overall device performance may be improved provided that another material possessing the necessary hole-transport properties is introduced into the system. To conclude, we have further demonstrated the modularity of the PPE grafting process developed in our group3,8 by simply replacing the graft monomer with a vinyl-functionalized oxadiazole moiety. Ground-state and excited-state interactions between the pendent oxadiazole grafts and the conjugated PPE backbone limit the overall device performance in the case of the entirely dialkoxy substituted PPEs. With the PPE backbone structure appropriately engineered with a pentiptycene scaffold, this excited-state interaction is removed allowing these oxadiazole-grafted systems to simultaneously incorporate the necessary charge transport properties while maintaining the narrow, wide-band gap emission characteristic of PPEs. Consequently, these systems are able to realize efficient, narrow, blue PPE electroluminescence from a single polymer layer that has not been achieved to date. Acknowledgment. This work was supported by the U.S. Army through the Institute for Soldier Nanotechnologies, under Contract DAAD-19-02-0002 with the U.S. Army Research Office and the National Research Council of Canada (NSERC). Supporting Information Available: Synthetic and experimental procedures along with polymer characterization data and electroluminescent device fabrication. This material is available free of charge via the Internet at http:// pubs.acs.org. Nano Lett., Vol. 5, No. 8, 2005
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