Material and device properties of highly birefringent ...

Material and device properties of highly birefringent nematic glasses and polymer networks for organic electroluminescence K. L. Woon A. E. A. Contoret S. R. Farrar A. Liedtke M. O’Neill P. Vlachos M. P. Aldred S. M. Kelly

Abstract — Light-emitting nematic liquid crystals are promising materials for organic light-emitting devices because their orientational anisotropy allows polarized electroluminescence and improved carrier transport. Two classes of nematics, i.e., room-temperature glasses and crosslinked polymer networks are discussed. The latter class has an additional advantage in that photolithography can be used to pixelate a full-color display. We show that the order parameter and birefringence of a new light-emitting nematic liquid crystal with an extended aromatic core both have values greater than 0.9. The performance of green light-emitting devices incorporating liquid crystals of different conjugation lengths is discussed. Efficacies up to 11.1 cd/A at 1160 cd/m2 at an operating voltage of 7 V were obtained. A spatially graded, color organic light-emitting device obtained by overlapping pixels of blue-, green-, and red-emitting liquid crystals were demonstrated. Some regions of the red pixel were only partially photopolymerized in order to obtain different hues in the overlapping region with green. We also show that the photolithographic process has micron-scale resolution. Keywords — Organic EL, birefringence, nematic, polymer networks.

1

Introduction

The polymerization of small liquid-crystalline molecules with a polymerizable group attached to the end of two aliphatic chains, the so-called reactive mesogens, is a wellknown technique to develop thermally stable, anisotropic optical films used as compensation films for liquid-crystal displays (LCDs) or for optical security products.1,2 Such reactive mesogens are deposited by solution processing to form a thin film. This is subsequently converted to a liquidcrystal (LC) polymer network by heating or photolithography. Recently, we have applied this technology to organic light-emitting diodes (OLEDs) by developing a new range of light-emitting reactive mesogens. The chemical structure of the aromatic core of the LC, which forms the light-emitting chromophore, can be tailored for hole or electron injection and transport as well as for different colored emission. A significant advantage of this novel solution-based approach to OLEDs is that mutlilayer devices are easily made, even using the same solvent for the deposition of each layer, since crosslinking renders the underlying layers insoluble. Different colored pixels can also be formed using standard photolithographic techniques,3 and the materials are also compatible with the more recently developed ink-jet printing techniques. These advantages also apply to crosslinkable main-chain polymers, which have also been developed recently.4 However, the latter materials do not benefit from the selfassembling properties of liquid crystals. The delocalized π electrons of extended nematic LCs ensures a high degree of electronic wave-function overlap.5,6 This results in relatively high mobility values. The external quantum efficiency of OLEDs, limited by total internal reflection, is improved

when the rod-like emitters are aligned in the plane compared with an isotropic orientation.7 Uniform alignment of the LCs gives linearly polarized emission. Applications include backlights for LCDs, portable displays suitable for bright daylight, and three-dimensional displays.8–10 In 2000, we first demonstrated blue-green polarized electroluminescence (EL) using an emitting reactive mesogen uniformly aligned using photoalignment techniques.9,11 Reactive mesogens have since been used by other groups in OLEDs and organic field-effect transistors.12,13 More recently, we have developed a wide range of light-emitting nematic materials,14–18 studied their physical properties,5,19 demonstrated a red, green, and blue pixellated prototype display,3 synthesized a carrier-transporting photo-aligning polymer, and shown the three-dimensional potential of the photoalignment technique.20 In this paper, we report the anisotropic properties of new extended nematic LCs and their OLED device characteristics. Both reactive mesogens and non-polymerizable light-emitting nematic liquid crystals21,22 are discussed. We also show how photolithography can be used to obtain a new type of color-graded OLED and comment on the resolution capability of the photolithographic process.

2

Experimental description

Tables 1 and 2 show the chemical structure of the nematic liquid crystals used in this work. Compounds 1 and 3–7 are reactive mesogens and compounds 2 and 8 have nonpolymerizable (octyloxy) aliphatic terminal groups. The synthesis of the compounds 1–4, an d 6 has been discussed previously.14,16,18 The synthesis of 5, 7, and 8 will be reported

K. L. Woon, A. E. A. Contoret, S. R. Farrar, A. Liedtke, and M. O’Neill are with the Department of Physics, University of Hull. P. Vlachos, M. P. Aldred, and S. M. Kelly are with the Department of Chemistry, University of Hull, Cottingham Rd., Hull, U.K. HU6 7RX; e-mail: [email protected]. © Copyright 2006 Society for Information Display 1071-0922/06/1406-0557$1.00

Journal of the SID 14/6, 2006

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TABLE 1 — Chemical structures of compounds 1–4, 6, and 7.

elsewhere. An interference method was used to measure the birefringence ∆n and extraordinary refractive index ne of LCs 1 and 2.23 Two kinds of cells were used, both consisting of glass substrates coated with rubbed polyimide alignment layers to provide uniaxial homogeneous alignment. A wedge cell was employed to find ne at 589 nm by reflection of light from a sodium lamp, using a travelling microscope. Then ne(λ) was obtained from the interference spectrum obtained by reflecting white light from both interfaces of a second cell with parallel faces. The same cell was used as a retardation plate to find ∆n(λ), assuming the ordinary refractive index of the LCs is nondispersive. This assumption is only valid for highly elongated LCs having a high degree of orientational order. The films for dichroic absorbance and PL measurements were spun-cast from chloroform and aligned on rubbed polystyrene sulphonate/ polyethylene dioxythiophene (PSS/PEDOT) (Baytron P VP CH 8000, Bayer) on a glass substrate by heating to 220°C. They were then cooled at 1°C/minute to 10°C above the glass-transition temperature and then quenched to room temperature. The PSS/PEDOT was rubbed uniaxially following annealing at 120°C for 30 minutes. Polarized absorbance spectra were recorded using a Unicam 5625 UV spectrophotometer and a Coherent sheet UV polarizer (Polacoat 105UV). On excitation with a GaN laser at 405-nm, PL was detected normally from the substrate using a fiber coupled to an Ocean Optics spectrometer. A Glan-Thompson Polarizer was used to detect the polarization ratio. OLEDs were fabricated on a glass substrate (25 × 45 × 1 mm) covered with an ITO transparent anode and PSS/PEDOT (thickness 45 nm), deposited by spin-coating. The PSS/PEDOT layer was baked at 165°C for 5 minutes in order to cure the layer and remove volatile components. Thin films of the light-emitting materials as specified below were prepared by spin-coating from a 1–1.5 wt.% solution in chloroform followed by baking at 50°C. The films of 4, 6, and 7 were crosslinked by UV irradiation using a HeCd laser

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at 325 nm of fluence 500 J/cm2 to render the materials completely insoluble. A hole-blocking layer (6 nm) of commercially available (H. W. Sands) 3-(4-biphenylyl)-4-phenyl5-tert-butylphenyl-1,2,4-triazole (TAZ) was deposited on top of the crosslinked emission layer by vapor deposition using a vacuum of 10–6 mbar or better. 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBI) was used as a hole-blocking layer for 8, which is not photopolymerizable, but has a nematic glass phase at room temperature. Layers of lithium fluoride (1 nm) and aluminium (80 nm) were sequentially deposited in the same chamber as a combined cathode. All OLED processing was carried out in a glove box. EL was measured using a Labview-controlled Agilent E3631A DC power supply with a Minolta LS100 luminance meter and Avaspec2048 fibre spectrometer. The PL quantum efficiencies were used using an integrating sphere.24 The graded-color OLED was prepared using materials 3–5.Compound 5 is a crystal at room temperature so that the red-emitting layer consisted of a blend of 20% by weight of 5 in 4. Solutions of 3 and 4 in chloroform were used to form the blue- and green-light-emitting layers, respectively. The individual layers of the device were processed similarly to the red, green, and blue pixelated OLED discussed elsewhere.3 In this case, however, the pixels had an overlapping region as discussed in detail below. The preparation of the device involved the spatial variation of the irradiation condition for photopolymerization. This was done by focusing the beam from the HeCd laser to a spot of ≈1 mm2 and translating it across the sample. The total irradiation dose was varied according to the translation speed.

3 3.1

Results and discussion Liquid-crystal transition temperatures

Table 3 shows the liquid-crystal transition temperatures of compounds 1–8. All compounds form nematic glasses on quenching from the nematic phase. Compounds 2 and 6–8 are nematic LCs with no discernible melting point. The exact transition temperatures for 7 and 8 cannot be determined exactly due to polymerization and/or thermal decomposition during DSC and optical microscopy measurements. Both compounds 3 and 4 are nematic reactive mesogens, TABLE 2 — Chemical structures of the compounds 5 and 8 with the symmetrical general formula R1–A–R2.

TABLE 3 — Transition temperatures (°C) for compounds 1–8.

which exhibit a metastable supercooled nematic phase for room-temperature processing over long periods. Compound 5 is a nematic liquid crystal, which is a crystalline solid at room temperature and therefore must be used in a guest-host configuration to obtain a room-temperature nematic phase.

3.2 Anisotropic properties of extended liquid crystals One of the potentially exciting applications of linearly polarized OLEDs is a low-cost near-to-eye three-dimensional display. The alignment direction of the liquid-crystalline emitter, and consequently the polarization direction of the emitted light, can be spatially patterned, when photoalignment techniques are employed.9,10 Illumination of a photoalignment film with polarized light generates a surface anisotropy in the alignment layer. When the LC film is deposited on top, the directors orient along the easy axis of the alignment layer, on annealing in the nematic phase. Hence, each pixel can be subdivided into two subpixels having orthogonal polarization directions of emission simply by irradiating the adjacent subpixels of the photoalignment film with orthogonally polarized light. The two sets of subpixels are separately addressed to create two separate images with orthogonal polarization. A three-dimensional effect is created when viewed through glasses having orthogonally polarized lenses; each eye sees a different image. Very high polarization ratios are required to avoid crosstalk between the images. REL(PL) gives the ratio of the measured intensity of polarized light parallel and perpendicular to the alignment direction for EL [photoluminescence (PL)]. The dichroic ratio D is the ratio of the absorbance of polarised light parallel and perpendicular to the alignment direction. An REL value of 13 was found from a photoaligned and crosslinked reactive mesogen with eight aromatic rings.3 According to the Onsager theory, the order parameter of nematics increases with molecular length,25 and Culligan et al. obtained a peak REL value of 31 using an oligofluorene with 24 aromatic rings aligned on a rubbed PSS/PEDOT substrate.22 Using a similar approach, we synthesised extended nematic LCs to increase the anisotropy. ∆n and ne of LCs 1 and 2 with six and 14 aromatic rings, respectively, are plotted as a function of wavelength in Fig. 1. The highly disper-

sive nature of ne(λ) results from the π–π* absorption resonances, which peak at 395 and 452 nm for compounds 1 and 2, respectively. The dipole moments of the transitions lie along the long axes of the molecules so that ne rather than no is affected by absorption. The measurements were made in thick homogeneously aligned cells so that the short wavelength limit of the data in Fig. 1 is determined by material absorbance. A maximum ∆n value of 0.7 and 1.1 is obtained for the six- and 14-ring compounds respectively, confirming the expected increase with molecular length. As Fig. 1 illustrates, the birefringence values of 2 are extremely high, with a maximum value equal to 0.4ne. Hence, extremely large anisotropy is also expected in absorption and emission. Figure 2(a) shows the dichroic absorption spectrum of 2 at room temperature. D has a maximum value of 35, which corresponds to an order parameter of 0.92. Figure 2(b) shows the PL spectrum of 2 aligned on rubbed PSS/PEDOT. The maximum value of RPL is 30. These extremely high values of the order parameter and R for a nematic rather then a smectic phase are equivalent to the highest obtained for LC polymers and oligofluorenes on rubbed substrates,25,22 showing the potential of the extended nematics approach. As discussed in the experimental section, the films were aligned on the rubbed PSS/PEDOT by heating to 220°C and rapid cooling to room temperature. Similar order parameters were also obtained using a low-temperature annealing method.26 The film was spin-cast from xylene, a high-boiling-point aromatic solvent, which acts as a plasticizer and thus can be aligned by annealing for 30 minutes at the relatively low temperature of 70°C. We are currently preparing polarized EL devices from compound 2 and from an aligned polymer network based on its photopolymerizable homologue 6.

FIGURE 1 — ne and ∆n of compounds 1 and 2 as a function of wavelength.


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FIGURE 3 — Current–voltage and luminance–voltage characteristics of OLED incorporating 8.

gies, and small differences in solid-state PL quantum efficiencies, which are 27% for 4 and 33% for both 6 and 7. The OLED incorporating 4 draws a much larger current than the others so that the different efficacies are tentatively attributed to differences in charge transport or charge trapping at chromophore sites. The nematic glass 8 has 18 aromatic rings and has a green/yellow emission. It has the highest efficacy of all the devices, confirming that longer chromophores improve OLED performance. Figure 3 shows the current–voltage and luminance of an OLED incorporating 8. The efficacy peaks at a luminance of 1160 cd/m2 at 7 V. FIGURE 2 — (a) Dichroic absorbance spectrum of 2 aligned on a rubbed PSS/PEDOT substrate. (b) PL spectrum from 2 aligned on a rubbed PSS/PEDOT substrate. I|| and I⊥ represent the polarized PL intensity parallel and perpendicular to the director, respectively.

3.3 OLEDs incorporating extended liquid crystals We have previously reported blue, green, and red EL from compounds 3–5.3 The green-emitting device produced using 4 showed an efficacy of 1.2 cd/A at a luminance of 100 cd/m2. We now investigate whether extended nematics provide improved performance. Table 4 shows the device characteristics of green OLEDs fabricated using compounds 4 and 6–8. The data shown is typical of the set of best devices made from the different materials. The reactive mesogen 4 has eight aromatic rings whereas the mesogens 6 and 7 each have 14. As shown in Table 4, the only difference between the latter compounds is the replacement of four thiophene rings by two fused dithiophene groups to improve the color purity of the green emission. As Table 4 shows, the efficacy of the crosslinked devices improves with increased conjugation length. Devices made from 6 and 7 have comparable values, which are more than four times greater than that from 4. All three polymerizable materials have the same value of ionization potential value, very similar band ener-

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3.4

Resolution of photolithographic process

A full-color high-information-content flat-panel OLED requires micron-sized pixels. We investigate the spatial resolution of photopatterning by irradiating a thin film of monomer 3 with light from a HeCd laser of wavelength 325 nm and a fluence of 150 J/cm2 through a shadow mask in close contact with the thin film. The irradiation conditions were chosen to give a completely insoluble film. The sample was then washed in chloroform to remove the soluble material. Figure 4(a) shows a transmission electron micrograph of the washed film. The darker regions have been irradiated with ultraviolet light and are insoluble. The lighter regions show the substrate remaining when the unirradiated film is removed. The two closest perpendicular stripes are sepaTABLE 4 — Device characteristics of single-pixel OLEDs using compounds 4 and 6–8.

FIGURE 5 — Photograph of graded-color single-pixel OLED operating at 5 V. The logo represents the Humber bridge located near Hull, England. This is one of the longest single-span bridges in the world.

is completely retained. A grating thickness of ≈85 nm is obtained.

3.5 FIGURE 4 — (a) Scanning electron micrograph of photolithographically patterned film of 3. The dark regions have been irradiated with ultraviolet light and show the insoluble film. The unirradiated regions of the film were washed away by rinsing the sample in chloroform and the substrate appears light in the image. (b) The surface profile obtained using atomic force microscopy of the washed polymer network film produced by irradiation of monomer 3 through a phase mask of 1-µm period. The period of the surface profile is equal to that of the phase mask.

rated by 20 µm and are clearly resolved. Indeed, micronscale resolution is possible with the photolithographic process, as shown by exposing a second thin film of 3 through a phase mask of 1-µm period in soft contact with the film. The phase mask produces a spatially modulated irradiance pattern with the same period as the phase mask; the exact spatial distribution of the irradiance depends on the relative intensity of the different diffraction orders of the grating.27 The sample was then washed in chloroform to remove the soluble material. Figure 4(b) shows the surface profile of the washed film measured by atomic force microscopy. A surface-relief grating is formed with a period equal to that of the phase mask. The non-crosslinked material in the minimally exposed regions is completely removed while the fully crosslinked material in the maximally exposed regions

Color-graded OLED

The photolithographic process can also give a new type of color-graded image using partial photopolymerization. Figure 5 shows a photograph of an operating single-pixel multicolor OLED, prepared by the sequential deposition of the red, green, and blue materials 3–5. The EL spectra and characteristics of single-pixel devices fabricated using the materials are given elsewhere.3 The red material was deposited first, irradiated through a mask and washed to remove the unirradiated material. The green and blue materials were then processed similarly with the crosslinked green region partially overlapping the insoluble red film and the crosslinked blue partially overlapping the green. This results in blue-green and green-red emission because there is some EL from both layers in the overlapping regions. A closer look at Fig. 5 shows that the overlapping green-red region has two distinct zones giving yellow and orange EL. This arises because the red material in the overlapping region is not uniformly thick because the irradiation conditions were spatially varied to give nonuniform and incomplete polymerization resulting in partial removal of the film on washing. The photograph was taken at an operating voltage of 5 V. The threshold voltage for EL varies between 3.5 and 4 V across the device.


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4

Conclusions

We show that light-emitting nematic liquid crystals represent an attractive alternative material choice to the morestandard small molecules and main-chain polymers for OLEDs. Extended nematic LCs, with up to 14 rings in the aromatic core, have extremely high values of birefringence. They show order parameters as large as those of main-chain polymers and give bright EL We also show that polymerizable LCs have an extra advantage in that they can be patterned photolithographically with micron-scale resolution. They also allow a new type of graded-color OLED to be fabricated.

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17 M P Aldred, S P Kitney, P Vlachos, K L Woon, M O’Neill, and S M Kelly, “Synthesis and mesomorphic behaviour of segmented light-emitting liquid crystals,” Liq Cryst 32, 1251–1264 (2005). 18 M P Aldred, A E A Contoret, P E Devine, S R Farrar, R Hudson, S M Kelly, G C Koch, M O’Neill, W C Tsoi, K L Woon, and P Vlachos, “Light-emitting polymerizable liquid crystals: Micron scale photolithographic patterning and green electroluminescence,” Mater Res Soc Symp Proc 871E, I10.7 (2005). 19 A E A Contoret, S R Farrar, S M Khan, M O’Neill, G J Richards, M P Aldred, and S M Kelly, “Photoluminescence study of crosslinked reactive mesogens for organic light-emitting devices,” J Appl Phys 93, 1465–1467 (2003). 20 M P Aldred, P Vlachos, A E A Contoret, S R Farrar, W C Tsoi, R Hudson, K L Woon, S M Kelly, and M O’Neill, “Linearly polarized organic light-emitting diodes (OLEDs): Synthesis and characterisation of a novel hole-transporting photoalignment co-polymer,” J Mater Chem 15, 3208–3213 (2005). 21 Y H Geng, S W Culligan, A Trajkovska, J U Wallace, and S H Chen, “Monodisperse oligofluorenes forming glassy-nematic films for polarized blue emission,” Chem Mater 15, 542–549 (2003). 22 S W Culligan, Y H Geng, S H Chen, K Klubeck, K M Vaeth, and C W Tang, “Strongly polarized and efficient blue organic light-emitting diodes using monodisperse glassy nematic oligo(fluorene),” Adv Mater 15, 1176–1179 (2003). 23 K L Woon, M O’Neill, G J Richards, M P Aldred, and S M Kelly, “Highly anisotropic calamatic nematic and chiral nematic liquid crystals,” Liq Cryst 32, 1191–1194 (2005). 24 J C De Mello, H F Wittmann, and R H Friend, “An improved experimental determination of external photoluminescence quantum efficiency,” Adv Mater 9, 230–233 (1997). 25 M Knaapila, R Stepanyan, M Torkkeli, B P Lyons, T P Ikonen, L Almasy, J P Foreman, R Serimaa, R Guntner, U Scherf, and A P Monkman, “Influence of molecular weight on the phase behavior and structure formation of branched side-chain hairy-rod polyfluorene in bulk phase,” Phys Rev E 71, No. 041802 (2005). 26 M J Banach, R H Friend, and H Sirringhaus, “Influence of the casting solvent on the thermotropic alignment of thin liquid crystalline polyfluorene copolymer films,” Macromolecules 37, 6079–6085 (2004). 27 P E Dyer, R J Farley, and R Giedl, “Analysis of grating formation with excimer laser irradiated phase masks,” Opt Commun 115, 327–334 (1995). Kai L. Woon received his B.S. and Ph.D. degrees in physics from the University of Hull in 2000 and 2004, respectively. His Ph.D. topic was circularly polarized photoluminescence from chiral nematic liquid crystals. As a postdoctoral researcher at Hull, he studied the light-emission and semiconducting properties of liquid crystals. He is currently a postdoctoral researcher at the University of Oxford.

Adam E. A. Contoret was awarded his M.Eng. degree in optoelectronics in 1997 and his Ph.D. in physics in 2001, both from the University of Hull. He has since worked as a researcher at the University and as a development engineer at HewlettPackard. He is currently a director of Dreamscience, Ltd.

Simon R. Farrar received his B.S. and Ph.D. degrees in physics from the University of Hull. He has since worked on laser ablation of a range of materials. More recently, he has studied charge transport and organic electroluminescence from novel light-emitting liquid crystals.

Alicia Liedtke received her Diploma degree in applied laser technology at the University of Applied Sciences in Emden in 2004 and then pursued her Ph.D. at the organophotonics group at the University of Hull.

Mary O’Neill received her Ph.D. in physics from the University of Strathclyde, Scotland. After a postdoctoral fellowship in integrated optics at the University of Glasgow and a short spell in industry, she joined the Physics department of the Univ er s it y o f Hu ll. She is jo int lead er of the interdisciplinary organophotonics group, which investigates new photonic and optoelectronic applications for liquid crystals.

Panos Vlachos completed his undergraduate degree in chemistry at the University of Hull in 1999. He obtained his Ph.D. from the University of Hull in 2003 before carrying out postdoctoral research at Hull. He is currently studying for his MBA at the University of Durham, U.K.

Matthew Aldred completed his undergraduate degree in chemistry at the University of Hull in 1999. He obtained his M.Sc. from UMIST in 2000 and his Ph.D. from the University of Hull in 2004 before carrying out postdoctoral research at Hull. He is currently a postdoctoral researcher at the Changchun Institute of Applied Chemistry in the People’s Republic of China.

Steve M. Kelly began his research career in liquid crystals in 1976 at the University of Hull with Professors G.W. Gray, FRS, and E.P. Raynes, FRS, as Ph.D. supervisors. He spent 15 years in liquidcrystal industrial research at Asea Brown Boveri with J. Nehring and T. J. Scheffer and F. Hoffmann-La Roche with M. Schadt before returning to the U.K. in 1995 as an Advanced Fellow of the EPSRC. He was appointed to the academic staff in 2000 and promoted to professor in 2004. He has over 160 scientific publications and 75 patents.


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