Blue Light Emitting Polymers and Devices - American Chemical Society

Chapter 15

Blue Light Emitting Polymers and Devices 1

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Qibing Pei , S. Pyo , Shun-Chi Chang , and Yang Y a n g 1

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SRI International, 333 Ravenswood Avenue, Menlo Park, C A 94025-3493 Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095

Downloaded by COLUMBIA UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: August 17, 2004 | doi: 10.1021/bk-2005-0888.ch015

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Blue light-emitting polymers are critically important for the development of full-color polymer displays. A number of such polymers have been synthesized, with high photoluminescence and electroluminescence efficiencies. Both polymer light -emitting diodes (LEDs) and electrochemical cells have been fabricated, but various materials issues remain to be solved. Device lifetimes have been limited, due to factors such as (1) the instability of the polymers which usually have low glass transition temperature, and (2) accelerated failure at the polymer/electrode interfaces, due to high charge injection barriers and the blue polymers' low charge carrier mobility. Using dual functional triarylamine moieties as the side groups, we have prepared new blue light-emitting poly(paraphenylenes) exhibiting high luminescent efficiency, high glass transition temperature, good environmental stability, and enhanced carrier mobility. Blue LEDs have been demonstrated with 4.2 cd/A efficiency and 360 cd/m brightness at 8 V. 2

© 2005 American Chemical Society In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Introduction One of the most prominent conjugated polymer for blue electroluminescence is poly(paraphenylene) (i). PPP is insoluble in any solvent. It has been rendered soluble in organic solvents by the attachment of flexible side groups, such as in poly(2-octyloxy-l,4-phenylene) (2). Spin-coating was used to prepare high-quality thin films of alkoxy-PPP. Blue LEDs were demonstrated with high EL quantum efficiency. However, the LEDs' operating voltages were high. The alkoxy side groups, which are electronically passive, considerably reduce the conductivity of the polymer. They separate the PPP backbones farther from each other and further twist the phenylene rings from being coplanar. Carrier mobility is consequently hindered. The device lifetime was short. The alkoxy-PPP has a low glass transition temperature. The PPP chains tend to aggregate, due to interaction between delocalized π-electrons, causing red shift of the emission spectrum and reducing luminescent efficiency. We explored poly(fluorenes) (PFs), derivatives of PPP wherein every two neighboring phenyl rings are locked in a plane by the C-9. PFs have better semiconductivity than PPPs (3). PFs with long-chain alkyl or polyether side groups attached to the C-9 were soluble. Blue LEDs based on PFs, with high quantum efficiency and lower operating voltages, were fabricated. Recently, significant progress has been made in improving the performance of blue LEDs based on PF (4). However, device operating lifetime is still unsatisfactory. The planar aromatic rings in PFs tend to aggregate. The emission color readily shifts toward white or red, due to eximer emission (5,5). Molecular motion is the main driving force for the formation of excited-state aggregates. Several approaches have been taken to overcome this problem. An example approach was to introduce co-monomers into the PF main chain (6). Unfortunately, the resulting copolymers required high voltages to operate and still showed some eximer emission in the electroluminescent spectrum. Ladder-PPPs were also investigated which exhibited similar problems (7). Based on available results, we think that the general strategy of using flexible alkyl or alkoxy side groups or chain segments to solubilize light-emitting polymers is impractical. The preferred approach should not employ solubilizing groups that would either reduce the conductivity of the conjugated main chain, or reduce the polymer's thermal stability, or both. On the other hand, triarylamine compounds have been among the best organic/polymer materials in terms of charge injection and carrier mobility. These compounds have been widely used in organic LEDs as the hole transport

In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

203 material. We have demonstrated that triarylamine side groups are effective in rendering poly(paraphenylene vinylene) (PPV) soluble. The resulting PPV polymers have good solubility and high photoluminescent efficiency (8). In this article we report that the triarylamine-type side groups are also effective in rendering PPP soluble. Blue LEDs have been fabricated with high luminescent efficiencies and low operating voltages.


Results and discussion A representative PPP with triarylamine side groups is TA-PPP (9) with the following repeating units: TA

wherein TA stands for a triarylamine or diarylamino side group, and χ varies between 0 and 1. TA-PPP was synthesize by Yamamoto condensation polymerization using dichloro- or dibromo- monomers (10% and by Suzuki coupling polymerization using dibromo- and diborate mixed monomers (11). Gel permission chromatography (GPC) analysis (polystyrene standard) showed a moderate weight-average molecular weight of 30,000 and polydispersity of 4. TA-PPP is readily soluble in certain organic solvents. Spin-cast thin films are optically clear, with intense blue fluorescence. Figure 1 displays the absorption and PL spectra of the polymer in both solution and thin film. The spectra of the solution and thin films are almost identical, indicating little aggregation or excimer formation in the solid state, contrary to most blue light emitting polymers including poly(9,9-dioctylfluorene) (DO-PF). Figure 1 also shows the PL spectra of DO-PF in a solution and thin film. The sub-band emission, due to excited-state aggregation, becomes dominant in the soid films. Certain conjugated polymers with alkyl side groups, such as poly(3alkylthiophene) exhibit solvatochromism, that is, the absorption spectra of the polymers in solution is significant shifted to shorter wavelength compared those in solid thin films (12). The driving force is molecular motion of the flexible side groups that twists the conjugated polymer chain from being coplanar. The twisting is enhanced in solution. Similar effect was also observed when the solid thin film was heated. Polymers with higher glass transition temperature exhibit less solvatochromism. The negligible solvatochromism observed in TA-PPP is consistent with its rigid polymer chain and side groups.



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Figure 1. Top: UV-Vis absorption and PL spectra for TA-PPP in 1,4-dioxane and in the solid statefilm.Bottom: PL spectra of DO-PF in p-xylene and in the solid statefilm.The thinfilmswere spin-coated and baked at 70 °Cfor 30 min. From the absorption onset of TA-PPP, the polymer's band gap is estimated to be 3.4 eV. Through electrochemical cyclovoltametry, the oxidation potential or HOMO was determined at 5.2 eV. This puts the LUMO at 1.8 eV. Therefore, TA-PPP is hole-predominant. The absorption and PL spectra of TA-PPP do not overlap, indicative of a good luminescent material. The PL quantum efficiency, though not quantitatively measured, is comparable to that of pristine DO-PF, which is in the range of 70100%. A thin film of TA-PPP cast on glass was heated on a hot plate at 100 °C in laboratory air and normal room light for 1 month. No PL degradation


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(quantum efficiency and emission color) was observed. Even after treatment at 150 °C for 2 hr, no spectral change occurred, as shown in Figure 2. Slight color shift was observed at 200 °C. In comparison, similar treatment with DO-PF showed remarkable color shift at 100 °C. The PL of DO-PF was almost completely quenched at 200 °C, as shown in Figure 3.

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Figure 2. PL spectrum of TA-PPP thin film after thermal treatment in air

Figure 3. PL spectrum of DO-PF thin film after thermal treatment in air To verify whether the high stability of PL is from TA-PPP's rigid structure, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out. The TGA diagram shown in Figure 4 showed little weight loss until about 560 °C. Under similar condition, DO-PF decomposes at less than 400 °C. The decomposition of DO-PF likely starts with the alkyl side groups. The mass spectrum of 2,7-dichloro-9,9-dioctylfluorene, a monomer for DO-PF, are abundant of fragments attributable to broken alkyl chains.


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Figure 4. TGA diagram of TA-PPP and DO-PF with a ramping rate of 10°C/min under dry N gas flow. 2

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Figure 5. DSC thermogram of TA-PPP at a heating and cooling rate of 10°C/min.



207 DSC thermogram of TA-PPP (Figure 5) shows a glass transition at about 225 °C, confirming the high thermal stability of TA-PPP. Blue light emitting polymers and oligomers based on spiro-PPP has also been reported by Salbeck et al. with high glass transition temperature (J3). The polyquinolines also exhibit high efficiency blue luminescence and high glass transition temperature (14). However, these polymers have lower conductivity than PPP with flexible alkoxy side groups. The resulting LEDs require high voltages to operate. The TA-PPP, with its dual functional side groups, is expected to overcome the problems caused by high operating voltages. LEDs were fabricated with TA-PPP as the emissive layer. Single-layer devices of ITO/PEDOT/TA-PPP/Ca/Al were fabricated. PEDOT, poly(3,4ethylenedioxythiophene), was used to enhance hole injection from the anode. Charge injections of the single layer LEDs were clearly hole dominant. The barrier for electron injection, around 1.0 eV, is too high. Electron dominant materials such as DO-PF and 2-(4-t-butylphenyl)-5-biphenyloxadiazole (t-PBD) were used to enhance electron injection. The thin film of a TA-PPP and PF blend (95:5 weight ratio) was phase separated. Atomic force microscopy (AFM) showed PF spheres, close to 1 μπι in diameter, dispersed in the TA-PPP matrix (Figure 6). This type of phase separation is common in blends of stiff and soft polymers. The PL emission of the blend film was characteristic of TA-PPP. However, once thermally treated, the spectrum shifted bathochromically much like PF. The EL spectrum from LEDs based on the blend thin film contained much emission from PF in the 500-700 nm regime. The device efficiency was about 0.43 cd/A. TA-PPP/PF double layer LEDs were also fabricated. But the efficiency was not improved because when PF was spin coated onto TA-PPP, the PF solution washed out most of the TA-PPP layer.

Figure 6. AFM image of a thinfilmof TA-PPP.DO-PF blend (90:10, by weight). Thin films of the blend of TA-PPP with r-PBD (80:20 by weight) were also phase separated, even to the naked eyes, albeit to a lesser degree than the TAPPP and DO-PF blend. Remarkable device performance was achieved with the TA-PPP and t-PBD blend. Figure 7 shows a typical current-light-voltage


208 response of an ITO/PEDOT/(TA-PPP + *-PBD)/Ca/Al LED. Light emission turns on at 4.5 V, and reaches 100 cd/m at 7.2 V and 10 cd/m at 12 V. The highest efficiency is 4.2 cd/A at 1 mA and 360 cd/m . The emission color is nearly identical to the PL of TA-PPP spectrum, which corresponds to (0.167, 0.157) ontheCIE chart. The high efficiency is probably due to charge confinement at the TA-PPP//PBD interfaces. Figure 8 illustrates the band diagrams of TA-PPP, /-PBD, and the electrode materials. Apparently, electrons and holes injected from the electrodes are confined near the TA-PPP//-PBD interfaces formed between the TA-PPP and t-ΡΒΌ microdomains. The confined charges recombine and produce photons. The charge confinement may also be responsible for the higher-than-expected driving voltages (14). By reduction of the size of the microdomains and the thickness of the blend film, the voltages may be lowered. 2

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Figure 7. Current-light intensity-voltage response of an ITO/PEDOT/(TA-PPP + t-PBD)/Ca/Al PLED

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Figure 8. Band diagrams of varioua materials used in the efficient blue LED


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Conclusion Using the rigid, dual-functional triarylamine moieties as the side groups, we have obtained a PPP derivative with good solubility, good film-forming properties, and high P L and E L efficiencies. The rigid TA-PPP exhibit high thermal stability while retaining good semiconductivity, essential for highperforming polymer LEDs. The efficiency and color quality o f the blue LEDs based on TA-PPP compare favorably with most other blue polymer LEDs. There is much room for further improvement. We have also extended this strategy for preparing processable light-emitting polymers to other conjugated polymers including poly(paraphenylene vinylene) with lower band gaps and bathochromically-shifted emission colors (8).

Acknowledgement The work reported here was in part supported financially by the Office o f Naval Research, O N R Contract N00014-99-C-0274.

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Blue Light Emitting Polymers and Devices - American Chemical Society

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