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Selectively Controlled Orientational Order in LinearShaped Thermally Activated Delayed Fluorescent Dopants Takeshi Komino, Hiroyuki Tanaka, and Chihaya Adachi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm500802p • Publication Date (Web): 26 May 2014 Downloaded from http://pubs.acs.org on May 31, 2014
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Chemistry of Materials
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Selectively Controlled Orientational Order in Linear-Shaped Thermally Activated
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Delayed Fluorescent Dopants
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Takeshi Komino,†,‡ Hiroyuki Tanaka, ‡ and Chihaya Adachi*,†,‡,#
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†
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Motooka, Nishi, Fukuoka 819-0395
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‡
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Nishi, Fukuoka 819-0395, Japan
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#
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Motooka, Nishi, Fukuoka 819-0395, Japan
Education Center for Global Leaders in Molecular System for Devices, Kyushu University, 744
Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka,
International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744
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AUTHOR E-MAIL ADDRESS:
[email protected] 12
RECEIVED DATE:
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ABSTRACT: The orientational order of a linear-shaped thermally activated delayed fluorescent dopant
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10-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-10H-phenoxazine (PXZ-TRZ) was selectively controlled
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in a randomly oriented host matrix composed of 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) by varying
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the temperature during deposition of the thin films. While the molecular orientation of mCBP was
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random at deposition temperatures ranging from 200 to 300 K (orientational disorder), PXZ-TRZ
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molecules oriented horizontal to the substrate in this temperature range (high orientational order). This
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indicates that the orientational order was dominated by the kinetic behavior of PXZ-TRZ at the film
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surface rather than by randomization caused by aggregation of PXZ-TRZ and mCBP molecules. Using
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an orientation-controlled 6 wt% PXZ-TRZ:mCBP film as an emitting layer, organic light-emitting
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diodes (OLEDs) were fabricated. The horizontal orientation of the dopants enhanced the external
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electroluminescence quantum efficiency of the OLED by 24% compared with that of the corresponding
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OLED with slightly vertical orientation. An optical simulation found that the enhancement originates
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mainly from the improvement of the light out-coupling efficiency.
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Chemistry of Materials
INTRODUCTION
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A special feature of a thin film surface is that it markedly influences successive film growth
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during deposition. Various deposition conditions affect the molecular and/or atomic configurations and
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their dynamics at surfaces, and indirectly define the resulting configurations of entire films.
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Temperature control is generally used to tune deposition conditions because molecular and atomic
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dynamics are influenced by internal energy. For example, well-defined crystals can be formed under
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suitable conditions by adjusting the deposition temperature (Tdeposition) during epitaxial crystal growth.1
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Methodology using temperature control is effective to determine molecular configurations not only in
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systems with long-range order but also in disordered systems consisting of small molecules. Recently,
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Ediger and co-workers reported that indomethacin glass formed by vapor deposition is one of three
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types depending on distinct Tdeposition regimes: equilibrium, quasi-equilibrium, and kinetically
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controlled.2 The kinetically controlled regime, which is the lowest temperature regime and much lower
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than the glass transition temperature (Tg), generates anisotropic dipole orientations as a result of the
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slower rate of molecular configurational sampling at the film surface than deposition rate, so the
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dynamic behavior of the molecules on the surface is kinetically controlled. To date, Lin et al.,3-5
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Yokoyama et al.,6-12 our group,9-17 and others12,18-23 have reported a large number of organic
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noncrystalline films showing preferential orientations. These experimental results led us to consider that
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anisotropic materials could possibly be categorized as glasses formed under kinetically controlled
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regimes. Our previous studies have regarded the threshold temperature to produce kinetically controlled
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and isotropic ordinary glasses as Tg at the surface through analogy with that in the field of polymers.24-26
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This is because the distinctive feature of this temperature is very similar to that in polymers,27-30 i.e., Tg
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at the surface is much lower than bulk Tg. In fact, thickness-dependent elastic moduli, which are
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analogous to the behavior of thin polymer films being influenced by free surface effects, were also
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reported for thin films of small molecules frequently used in organic light-emitting diodes (OLEDs).31,32
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We express this temperature as Tg(surface) hereafter.
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The kinetically controlled regime below Tg(surface) is of potential interest in molecular science
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because it can homogeneously and continuously control orientational order over a wide range, thus
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determining the optical and electric properties of organic thin films. For example, preferential
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orientations have been revealed to affect the probabilities of charge transfer (CT) at metal/organic15 and
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organic/organic33,34 interfaces, charge carrier mobilities,7,9,10,14,15,25,26 densities of states,25 and light
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propagation.4,5,13,16 In addition, the wide variety of the dependence of optoelectronic characteristics on
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molecular orientation should allow us to further advance organic devices, because such dependence
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means we can greatly expand their advantages compared with other competing inorganic devices with
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optical and electronic anisotropy from which an anisotropic molecular shape originates. In particular,
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linear-shaped molecules have been revealed to form highly oriented films with an orientational order
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parameter of S < −0.45,12,13,26 so they are promising compounds for investigating the effects of
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molecular orientation in organic devices. In the field of OLEDs, recent progress has realized an internal
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electroluminescence quantum efficiency (φint) of nearly 100% even for a nonmetallic compound.35
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Thermally activated delayed fluorescent (TADF) materials35-43 have a small energy gap between their
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singlet and triplet excited states that is comparable to the thermal energy at room temperature. As a
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result, TADF materials can generate singlet excitons by thermal up-conversion from triplet excitons,39,40
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so their OLEDs can show an external electroluminescence quantum efficiency (φext) as high as those of
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phosphorescence-based OLEDs. Thus, to further improve φext, other factors governing φext now need to
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be improved; namely, the carrier balance (γ) and out-coupling efficiency (ηout). While ηout is difficult to
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improve by modifying the molecular structure of isolated molecules, molecular configuration can
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overcome this issue. Specifically, horizontal dipole orientation is advantageous because of the optimal
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distribution of luminous intensity in the direction normal to the substrate and the suppression of surface
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plasmon loss; surface plasmons readily couple with vertical dipole oscillations.12,17-19,41 Accordingly, we
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investigated the molecular orientational order in thin films fabricated at low Tdeposition to kinetically
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control the dynamic behavior of the deposited molecules at the surface. We prepared thin films of
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Chemistry of Materials
10-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-10H-phenoxazine (PXZ-TRZ),37 which has the simplest linear-shaped structure of reported TADF compounds. To suppress a decrease of γ, a randomly oriented host
matrix
is
also
essential
according
to
our
previous
study.26
Therefore,
we
used
3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) as a host matrix that has very stable random orientation over a wide range of Tdeposition. In this paper, we demonstrate that the orientational order of PXZ-TRZ can be selectively controlled while maintaining the random orientation of the host matrix, and that φext can be enhanced by over 20% because of the improvement of ηout associated with the horizontal orientation of PXZ-TRZ.
EXPERIMENTAL SECTION The structures of the device and compounds used in this study are depicted in Fig. 1. Phenoxazine-triphenyltriazine derivative PXZ-TRZ was synthesized as described elsewhere37 and then sublimed before use. mCBP (Nard Institute Co.),42,43 N,N’-diphenyl-N,N’-di(1-naphthyl)benzidine (α-NPD, Tosoh Organic Chemical Co.), and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi, Jilin OLED Material Tech. Co.) were used as received. Patterned 100-nm-thick indium tin oxide (ITO)-coated glass substrates with a sheet resistance of 25 Ω square-1 (Atsugi Micro Co.) were carefully cleaned as described elsewhere.26 Organic layers were formed on the precleaned substrates by vapor deposition at an evaporation rate of 0.1 nm/s under a vacuum of
