A thermally cured 9,9-diarylfluorene-based triaryldiamine polymer displaying
high hole mobility and remarkable ambient stability
Chi-Yen Lin,
aYu-Cheng Lin,
bWen-Yi Hung,*
bKen-Tsung Wong,*
aRaymond C. Kwong,
cSean C. Xia,
cYu-Hung Chen
dand Chih-I Wu
dReceived 16th January 2009, Accepted 27th March 2009 First published as an Advance Article on the web 21st April 2009 DOI: 10.1039/b900977a
We have synthesized and characterized a novel thermally polymerizable triaryldiamine monomer (VB-FNPD) possessing a styrene-functionalized 9,9-diarylfluorene core and have used time-of-flight transient photocurrent techniques to investigate the hole transport properties of its solution-processed and subsequently thermally cured (170C) polymer films. This novel polymeric material exhibits
non-dispersive hole transport behavior with a high hole drift mobility (up to 104cm2V1s1). The film
displayed remarkable ambient stability, even when exposed to air for one month. We tested the thermally generated polymer film as a hole transport material in organic light-emitting diodes incorporating tris(8-hydroxyquinolate) aluminium (Alq3) as the emission and electron transport layer.
The device exhibited a maximum external quantum efficiency (hex) of 1.4%, significantly better than
that of the device prepared using the corresponding model compound VB-model (hex¼ 1.1%).
Introduction
Most organic light-emitting diodes (OLEDs) are configured as multilayer structures1 in which each layer addresses a specific
function, such as charge injection, transport, and emission. Historically, it has been difficult to prepare polymer-based OLEDs in multilayer structures because solution processing has typically been used to form films; i.e., the deposition of a new polymer film can lead to mixing and/or corroding of the previ-ously deposited layers.2Much effort has been exerted to
over-come this limitation. For example, one promising approach toward forming multiple layers through solution processing is to employ alternating solvents of orthogonal polarities;3e.g., in the
deposition of conjugated polyelectrolytes collocated with neutral conjugated polymers. Another plausible approach is the use of an organic-soluble precursor that becomes insoluble upon self-assembly4 or photochemical,5 electrochemical,6 or thermal7,8
treatment; many materials possessing various functional moie-ties—such as siloxane, oxetane, styrene, and trifluorovinyl ether units—have been developed as good candidates.9
Recently, Jen et al. reported that styryl groups are promising thermal curable moieties for the preparation of cross-linkable hole transport materials (HTMs).8The precursor possessing the
styryl moiety has the advantageous property that it can undergo mild thermal polymerization in the absence of an initiator. In addition, the resulting polymer networks exhibit high solvent resistance, making them suitable materials for the deposition of subsequent layers through solution processing. Despite the
successful application of styrene-based HTMs, their intrinsic charge transporting characteristics and stability remain poorly understood. Indeed, the relationship between their molecular structures and charge carrier mobilities remains to be investi-gated.10,11In this paper, we report the synthesis, characterization,
and detailed hole mobility studies of a novel 2,7-disubstituted fluorene-based triaryldiamine (VB-FNPD) bearing two pendent thermally polymerizable styryl groups. The polymerized film exhibits not only remarkable hole transport properties (mh¼ ca.
104cm2V1s1) but also high stability, even after storage for one
month under an ambient environment. Parallel device compari-sons employing a corresponding model compound revealed the potential of using VB-FNPD as a promising hole transport material in OLEDs.
Results and discussion
SynthesisScheme 1 depicts the synthesis of the monomer VB-FNPD, which is a derivative of the well-established hole-transport material N,N0-di(1-naphthyl)-N,N0-diphenylbenzidine (a-NPD).
The diarylamino groups were attached to the coplanar rigid fluorene core to modulate the electronic properties and enhance the thermal stability.12 More importantly, we introduced two
polymerizable vinylbenzyl (styryl) ether groups onto the peripheral phenyl substituents of the central fluorene unit, so as not to perturb the structure on the active main chromophore. Our synthesis began with the amination of the dibromide 113with
N-phenyl-1-naphthylamine in the presence of a catalytic amount of Pd(OAc)2and tri-tert-butylphosphine to afford the bisphenol
2 in 82% yield. Subsequent etherification of 2 with 4-vinylbenzyl chloride under basic conditions gave VB-FNPD as a yellow solid in 54% yield. We employed VB-model12as a model compound
for VB-FNPD to compare its physical properties and device characteristics.
aDepartment of Chemistry, National Taiwan University, Taipei 106,
Taiwan. E-mail: [email protected]; Fax: +886 2 33661667; Tel: +886 2 33661665
bInstitute of Optoelectronic Sciences, National Taiwan Ocean University,
Keelung 202, Taiwan. E-mail: [email protected]
cUniversal Display Corporation, 375 Phillips Blvd, Ewing, NJ 08618, USA dDepartment of Electrical Engineering and Graduate Institute of
Electro-optical Engineering, National Taiwan University, Taipei 106, Taiwan
ª The Royal Society of Chemistry 2009
Physical properties
We used differential scanning calorimetry (DSC) to determine the thermal properties of VB-FNPD. As indicated in Fig. 1, the first scan of pristine VB-FNPD exhibited a glass transition temperature (Tg) of ca. 113C, lower than that of VB-model (129
C).12 In addition, the broad exothermic peak at ca. 166 C
corresponds to the thermal polymerization of VB-FNPD. In contrast, the second scan features no apparent peaks that can be detected at temperatures up to 250C, implying the existence of a high degree of cross-linking between the styryl groups.
Fig. 2 displays UV-Vis absorption and photoluminescence spectra of the vacuum-deposited model and dip-coated VB-FNPD thin films before and after thermal treatment. The absorption spectra of the VB-FNPD films [both as-deposited (solution-processed) and thermally polymerized] matched quite well with that of VB-model. This finding reveals that the sp3
-hybridized C9 carbon atom of the fluorene core impeded any p-orbital interactions between the chromophore and the pendent aryl groups; thus, the electronic transitions were not affected after introducing the two styrene moieties. In the emission
spectra, however, the polymerized VB-FNPD thin film exhibited a slightly red-shifted maximum signal intensity (by ca. 10 nm) and broader half-widths of the emission peak relative to that of VB-model. We attribute these differences to changes in the dielectric environment after thermal treatment; i.e., because chromophores surrounded by polystyrene chains should behave slightly different from those of model compounds in their respective excited states. Nevertheless, we cannot exclude the possibility of weak interchromophore interactions leading to the long-wavelength emission from the polymer films after high-temperature thermal polymerization.
Electrochemical characterization
We employed cyclic voltammetry (CV) to assess the electro-chemical properties of VB-FNPD and VB-model (Fig. 3). The oxidation CV traces of the VB-model and VB-FNPD monomers in solution were almost the same, with each displaying two reversible oxidation potentials at 0.64 and 1.00 V. The significant potential difference between the first and second oxidations Scheme 1 Synthetic route toward VB-FNPD, and the structure of VB-model.
Fig. 1 DSC analysis of VB-FNPD.
Fig. 2 UV-Vis absorption and photoluminescence spectra of thin films of VB-FNPD and vacuum-deposited VB-model.
clearly indicates that the removal of electrons occurred from the same chromophore. In addition, UV photoelectron spectroscopy (UPS) studies of thin films of VB-model and the non-polymer-ized and polymernon-polymer-ized VB-FNPD revealed no differences in the energy levels of their respective highest occupied molecular orbitals (HOMOs; each ca. 5.3 eV). Taken together, these results support the notion that the introduction of pendent pol-ymerizable groups as peripheral substituents on the fluorene core did not alter the electrochemical properties of the active chro-mophore to any significant extent.
Hole mobility measurements
We used time-of-flight (TOF) transient photocurrent techniques to investigate the hole-transport properties of VB-model and the thermally polymerized FNPD. The TOF transient of VB-model for holes (Fig. 4a) displays a clear plateau, which indicates non-dispersive transport behavior; exposure of the VB-model film to an ambient environment led, however, to highly disper-sive hole transport. We suspect that the penetrated oxygen and H2O acted as an extrinsic dopant and induced carrier trapping.
We observed a similar non-dispersive photocurrent transient for the polymerized VB-FNPD [Fig. 4(b)]; under ambient conditions (exposure of the sample to air for 1 month), however, the non-dispersive photocurrent transient was barely affected. After
exposing the sample to sunlight for 1 or 5 days, we found that the TOF photocurrents degraded significantly, but the hole mobil-ities remained unaffected (i.e., they possess the same transit time tT). The dispersion of a sheet of carriers migrating through the
sample can be obtained from the tail-broadening parameter, w¼ (t1/2 tT)/t1/2, where tTis the carrier transit time and t1/2is the
time when the current has decayed to half of the plateau value.14
From Fig. 4(b), we obtain almost the same w of 0.37, from a transit time tT¼ 3.2 ms and decay time t1/2 ¼ 5.1 ms. These
observations indicate that irradiation with sunlight only impeded the generation of charge carriers nearby the transparent elec-trode; it had a less pronounced effect on the carrier’s drift properties. Thus, it is clear that thermal treatment of VB-FNPD generated a polymer that could effectively suppress the pene-tration of oxygen and water. The excellent air stability of the thermally polymerized VB-FNPD suggested that it has potential for application in organic optoelectronic devices.
Fig. 5 displays the hole mobilities of the vacuum-deposited VB-model and the dip-coated and thermally cured VB-FNPD polymer films plotted as functions of the square root of the electric field. VB-model exhibited hole mobilities from 5 104
to 1 103cm2V1s1at electric fields between 1.2 105and
4 105V cm1, and from 5 105to 4 104cm2V1s1at
Fig. 3 Cyclic voltammograms of VB-model and VB-FNPD in solution.
Fig. 4 Non-dispersive transient photocurrents plotted as a function of time of (a) VB-model (thickness: 2.5 mm; E¼ 2.4 105cm V1) and (b) the
polymerized VB-FNPD (thickness: 1 mm; E¼ 3 105cm V1). Inset: double-logarithmic plots for determination of the transit times.
Fig. 5 Hole mobilities within thin films of the VB-FNPD polymer and VB-model plotted as a function of the square root of the electric field.
electric fields between 105and 106V cm1. The hole mobilities of
the VB-FNPD polymer films were about one order of magnitude lower than those of its model counterpart. We attribute this decrease in hole mobility to the polystyrene units within the VB-FNPD polymeric films hindering the hopping of the charge carriers among adjacent molecules.
Device
To assess the feasibility of using VB-FNPD polymeric thin films as hole transporting layers (HTLs), we fabricated hybrid OLEDs having the device structure ITO/PEDOT:PSS/VB-FNPD (50 nm)/Alq3 (50 nm)/LiF (0.5 nm)/Al (100 nm), in which ITO/ PEDOT:PSS and LiF/Al acted as the anode and cathode, respectively, and vacuum-deposited conventional tris(8-quinoli-nato)aluminium (Alq3) acted as the electron transport/emissive layer. For comparison, the vacuum-deposited VB-model was also used as an HTL in its corresponding device architecture.
Fig. 6 displays the current–voltage–luminance (I–V–L) char-acteristics, as well as plots of device efficiency versus luminance, for the devices incorporating the polymerized VB-FNPD and VB-model. Both devices exhibit a voltage-independent pure emission from Alq3, implying that charge recombination occurred exclusively within the Alq3 layer [see inset of Fig. 6(a)]. The hybrid OLED incorporating the polymerized VB-FNPD exhibited a turn-on voltage of 2.5 V, a maximum brightness of 52 600 cd m2at 2600 mA cm2(12 V), an external quantum
efficiency (EQE) of up to 1.4% (4.5 cd A1) at 100 mA cm2, and
a power efficiency of 2.6 lm W1. The device incorporating
vacuum-deposited VB-model exhibited a turn-on voltage of 2 V, a similar brightness of 61 000 cd m2at 2690 mA cm2(11.5 V),
a lower EQE of 1.1% (3.4 cd A1) at 70 mA cm2, and a power
efficiency of 3 lm W1. Notably, the device based on the
poly-merized VB-FNPD exhibited superior performance relative to that based on VB-model (Fig. 6b); yet, at the practical brightness of 1000 cd m2, the EQE and power efficiency of the device using
polymerized VB-FNPD remain above 1.35% and 2.1 lmW1,
respectively, which are better than those of the device using VB-model. We attribute the improved performance to the more-balanced charge flux and better exciton confinement within the emission layer that resulted from the presence of the VB-FNPD polymeric thin film as the hole transport layer. Thus, a more balanced recombination of holes and electrons can be achieved with a high EQE of 1.4%, even at a high driving current.
Conclusion
We have synthesized a fluorene-based triaryldiamine bearing thermally polymerizable pendent styryl groups. The solution-processed monomer underwent smooth polymerization upon heating at 170C to give a polymer film displaying photophysical
and electrochemical properties similar to those observed prior to thermal treatment. TOF techniques revealed the existence of non-dispersive hole transport in the VB-FNPD polymer film, with the hole mobility reaching as high as 104cm2V1s1. This
article is the first to report the intrinsic charge carrier mobility of a styrene-based thermally polymerizable material. This new VB-FNPD polymer film exhibits remarkable ambient stability and has potential for application as a hole-transporting material in OLEDs. The device fabricated using the solution-processed, thermally cured VB-FNPD polymer film exhibited a maximum EQE (hex) of 1.4%, considerably better than that of the device
incorporating the vacuum-deposited model compound VB-model (hex ¼ 1.1%). The high air stability of the VB-FNPD
polymer film suggests that it may function as an active material within robust organic optoelectronic devices.
Experimental
Synthesis2: Tri-tert-butylphosphine (6 mL, 0.05 M in toluene, 0.3 mmol) was added to a solution of the dibromide 1 (5.08 g, 10 mmol), N-phenyl-1-naphthylamine (4.39 g, 20 mmol), Pd(OAc)2 (67 mg,
0.3 mmol), and sodium tert-butoxide (7.68 g, 80 mmol) in toluene (100 mL). The mixture was heated under reflux under Ar for 72 h and then quenched with water. The solution was partitioned between ethyl acetate (300 mL) and brine (100 mL). The combined organic extracts were dried (MgSO4) and
concen-trated. The resulting solid was washed with hexane to afford a yellow product (6.41 g, 82%). M.p. 254–255C. IR (KBr) n 3544 (w), 3055 (w), 1609 (m), 1592 (s), 1573 (m), 1507 (s), 1489 (s), 1464 (s), 1434 (m), 1392 (m), 1302 (m), 1269 (s), 1172 (m), 1015 (m) cm1.1H NMR (DMSO-d 6, 400 MHz): d 9.29 (s, 2H), 7.97 (d, J¼ 8.0 Hz, 2H), 7.84 (d, J ¼ 8.4 Hz, 2H), 7.76 (d, J ¼ 8.4 Hz, 2H), 7.56–7.48 (m, 6H), 7.37 (t, J¼ 7.4 Hz, 2H), 7.28 (d, J ¼ 7.2 Hz, 2H), 7.16 (t, J¼ 7.6 Hz, 4H), 6.92–6.84 (m, 8H), 6.78 (s, 2H), 6.51 (d, J¼ 8.8 Hz, 4H), 6.41 (d, J ¼ 8.4 Hz, 4H).13C NMR (DMSO-d6, 100 MHz): d 155.5, 152.2, 147.5, 146.5, 142.6, 135.3,
Fig. 6 (a) I–V–L characteristics (inset: EL spectrum) and (b) external quantum and power efficiencies plotted with respect to the luminance for Alq3 devices incorporating the polymerized VB-FNPD and the small molecule VB-model as HTLs.
134.7, 132.9, 130.1, 129.1, 128.4, 128.2, 126.6, 126.4, 126.3, 126.1, 123.4, 121.6, 121.1, 120.4, 120.2, 118.8, 114.6, 63.0. MS (FAB+,
m/z) 785 (100), 784 (65), 154 (70), 136 (65), 57 (65). HRMS (FAB+, [M + H]+) Calcd. C
57H41N2O2785.3170, found 785.3165.
VB-FNPD: 18-Crown-6 (105 mg, 0.4 mmol), potassium carbonate (553 mg, 4 mmol), and 4-vinylbenzyl chloride (610 mg, 4 mmol) were added to a solution of the bisphenol 2 (785 mg, 1 mmol) in acetone (16 mL). After heating under reflux for 18 h, the suspension was cooled to room temperature. The salts were filtered off and washed extensively with CH2Cl2(300 mL). After
evaporation of the solvent, the residue was dissolved in CH2Cl2
(300 mL) and then extracted with brine (100 mL 2). The combined organic extracts were dried (MgSO4) and
concen-trated. The resulting residue was washed with hexane and methanol. The crude product was purified through column chromatography (SiO2, CH2Cl2–hexane 2 : 3) to afford the
product (550 mg, 54%) as a yellow solid. IR (KBr) n 2955 (w), 2922 (m), 2851 (w), 1660 (m), 1631 (m), 1592 (m), 1504 (m), 1465 (m), 1391 (m), 1271 (m), 1239 (w), 1177 (w), 1106 (w), 826 (w), 773 (w) cm1.1H NMR (DSMO-d 6, 400 MHz): d 7.95 (d, J¼ 8.4 Hz, 2H), 7.83 (d, J¼ 8.4 Hz, 2H), 7.73 (d, J ¼ 8.4 Hz, 2H), 7.57 (d, J¼ 8.4 Hz, 2H), 7.52–7.47 (m, 8H), 7.40–7.34 (m, 6H), 7.27 (d, J¼ 7.2 Hz, 2H), 7.15 (t, J ¼ 7.8 Hz, 4H), 6.92–6.84 (m, 8H), 6.80 (s, 2H), 6.79–6.68 (m, 6H), 6.63 (d, J¼ 8.8 Hz, 4H), 5.86 (d, J¼ 17.6 Hz, 2H), 5.27 (d, J ¼ 10.8 Hz, 2H), 4.99 (s, 4H).13C NMR (acetone-d6, 100 MHz): d 158.0, 153.2, 148.9, 148.0, 144.1, 138.5, 137.7, 137.2, 136.1, 134.4, 131.4, 129.7, 129.5, 129.2, 128.4, 127.4, 127.1, 127.0, 126.9, 126.8, 124.7, 122.5, 122.4, 121.8, 120.9, 120.2, 114.9, 114.2, 70.0, 64.5. MS (m/z, FAB+) 1015.7 (15), 289.0 (10), 154.0 (100), 135.9 (90). HRMS (M+, FAB+) Calcd.
C75H56N2O2 1016.4342, found 1016.4370. Anal. Calcd for
C75H56N2O2C, 88.55; H, 5.55; N, 2.75; found C, 88.71; H, 5.48;
N, 2.34.
Cyclic voltammetry
The oxidation potentials were determined by cyclic voltammetry (CV) in CH2Cl2 solution (1.0 mM) containing 0.1 M
tetra-n-butylammonium hexafluorophosphate (TBAPF6) as a
support-ing electrolyte at a scan rate of 100 mV s1. A glassy carbon
electrode and a platinum wire were used as the working and counter electrodes, respectively. The ferrocene/ferrocenium redox couples occur at values of Eo0 of +0.46 V in CH2Cl2–
TBAPF6. All potentials were recorded versus Ag/AgCl
(satu-rated) as a reference electrode.
Photophysical property measurements
Steady state spectroscopic measurements were conducted in solid films. For VB-model, a thin film was prepared by vacuum (2 106Torr) deposition on a quartz plate (1.6 1.0 cm). For
VB-FNPD, a thin film was prepared by dip-coating from a 1–2 wt% solution in THF, which was subsequently baked at 100C for 30 min, and then heated at 170 C for 30 min to expedite
poly-merization in a glove box. Absorption spectra were recorded with a U2800A spectrophotometer (Hitachi) and fluorescence spectra were acquired on a F4500 fluorescence spectrophotom-eter (Hitachi) upon exciting at the absorption maxima.
Ultraviolet photoemission spectroscopy (UPS) measurements The valence-band ultraviolet photoemission spectra were carried out with He I (21.2 eV) and He II (40.8 eV) as excitation sources. The Fermi level of the system was measured on the gold substrate before the organic deposition. The energy levels of the HOMOs of organic samples were determined by extrapolating the edges of the HOMO peak down to the background of the UPS spectra. The vacuum levels of the films were deduced from the position of the spectra cutoff at the lowest binding energy and the photon energy of excitation sources.
Time-of-flight (TOF) mobility measurements
TOF samples were prepared by dissolving appropriate weight ratios (up to 25 wt%) of the monomer VB-FNPD in THF and then dip-coating the solutions onto an ITO substrate within a glove box. The film thicknesses were controlled by varying the solution concentrations and the dip-coating conditions; the films were subsequently dried through baking at 100C for 30 min to remove
the residual solvent and then cured at 170C for 30 min to form
the polymers. A Dektak surface profilometer was used to measure the thicknesses of the VB-FNPD (ca. 1 mm) polymer films. The device samples were then completed through thermal deposition of the back electrode [Ag (100 nm)] through a shadow mask.
For the TOF measurements, the samples were mounted in a cryostat under vacuum (ca. 103Torr); the temperature was
controlled using a PID thermocontroller. A sheet of charge carriers in the organic layer was generated by radiating a short excitation pulse (nitrogen laser) through the semi-transparent ITO electrode. Under an applied DC voltage, these charge carriers sweep over the organic sample toward the counter elec-trode (Ag) and discharge to result in a transient photocurrent. When the carriers reached the counter electrode, the current dropped to zero; the time at which this event occurred corre-sponded to the transit time of the carriers. The photocurrent signal was detected using a digital storage oscilloscope. Selected carriers (holes or electrons) drifted across the sample upon switching the polarity of the applied bias. The carrier mobility (m) was calculated from the transit time (tT), the sample thickness
(d), and the applied voltage (V) using the expression m¼ d2/t
TV.
Device fabrication
ITO substrates were cleaned in an ultrasonic detergent bath and then washed sequentially in acetone and methanol. The substrates were subsequently treated in a UV–ozone cleaner to remove any residual organic contaminants. A hole-injection layer of poly(3,4-ethylenedioxythiophene)–poly(4-stylenesulfonate) (PEDOT:PSS) was first spin-coated onto the ITO substrate to a thickness of 30 nm and then dried at 120C
for 30 min to remove residual solvent. After direct dip-coating of a solution of the monomer VB-FNPD (1.2 wt% in THF) onto the PEDOT:PSS/ITO substrate, the system was baked at 100C for 30 min to remove residual solvent and then it was heated at 170
C for 30 min to expedite polymerization. A 50 nm thick layer of
Alq3 was then vacuum-deposited on top of the polymerized VB-FNPD at a deposition rate of 2–3 A˚ s1. Finally, a 0.5 nm thick
layer of LiF and a 100 nm thick Al cathode were deposited through a shadow mask (area: 0.10 cm2). OLED device
characterization was performed at 25 C under a nitrogen atmosphere using a computer-controlled Keithley 6430 source meter and a Keithley 6487 picoammeter equipped with a cali-brated silicon photodetector. EL spectra were measured using a photodiode array detector (Ocean Optics S2000) over the spectral range of 200 to 850 nm at a resolution of 2 nm.
References
1 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913. 2 D. W. Steuerman, A. Garcia, M. Dante, R. Yang, J. P. L€ofvander and
T.-Q. Nguyen, Adv. Mater., 2008, 20, 528.
3 H. B. Wu, F. Huang, Y. Q. Mo, W. Yang, D. L. Wang, J. B. Peng and Y. Cao, Adv. Mater., 2004, 16, 1826; W. L. Ma, P. K. Iyer, X. Gong, B. Liu, D. Mose, G. C. Bazan and A. J. Heeger, Adv. Mater., 2005, 17, 274; X. Gong, S. Wang, D. Moses, G. C. Bazan and A. J. Heeger, Adv. Mater., 2005, 17, 2053.
4 U. Schubert, N. H€using and A. Lorenz, Chem. Mater., 1995, 7, 2010; W. Li, Q. Wang, J. Cui, H. Chou, S. E. Shaheen, G. E. Jabbour, J. Anderson, P. Lee, B. Kippelen, N. Peyghambarian, N. R. Armstrong and T. J. Marks, Adv. Mater., 1999, 11, 730; Q. Huang, G. A. Evmenenko, P. Dutta, P. Lee, N. R. Armstrong and T. J. Marks, J. Am. Chem. Soc., 2005, 127, 10227; J. G. C. Veinot and T. J. Marks, Acc. Chem. Res., 2005, 38, 632; J. Li and T. J. Marks, Chem. Mater., 2008, 20, 4873.
5 C. D. M€uller, A. Falcou, N. Reckefuss, M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken, H. Becker and K. Meerholz, Nature, 2003, 421, 829; E. Bacher, M. Bayerl, P. Rudati, N. Reckefuss, C. D. M€uller, K. Meerholz and O. Nuyken, Macromolecules, 2005, 38, 1640; M. C. Gather, A. K€ohnen, A. Falcou, H. Becker and K. Meerholz, Adv. Funct. Mater., 2007, 17, 191; P. Zacharias, M. C. Gather, M. Rojahn, O. Nuyken and K. Meerholz, Angew. Chem., Int. Ed., 2007, 46, 4388; B. Domercq, R. D. Hreha, Y.-D. Zhang, N. Larribeau, J. N. Haddock,
C. Schultz, S. R. Marder and B. Kippelen, Chem. Mater., 2003, 15, 1491.
6 M.-Y. Chou, M.-K. Leung, Y. O. Su, C. L. Chiang, C.-C. Lin, J.-H. Liu, C.-K. Kuo and C.-Y. Mou, Chem. Mater., 2004, 16, 654; C.-C. Chang and M.-K. Leung, Chem. Mater., 2008, 20, 5816; C. Xia and R. C. Advincula, Chem. Mater., 2001, 13, 1682; S. Inaoka, D. B. Roitman and R. C. Advincula, Chem. Mater., 2005, 17, 6781.
7 S. Liu, X. Jiang, H. Ma, M. S. Liu and A. K.-Y. Jen, Macromolecules, 2000, 33, 3514; X. Jiang, S. Liu, M. S. Liu, P. Herguth, A. K.-Y. Jen, H. Fong and M. Sarikaya, Adv. Funct. Mater., 2002, 12, 745; Y. Niu, M. S. Liu, J.-W. Ka and A. K.-Y. Jen, Appl. Phys. Lett., 2006, 88, 093505; J. Zhao, J. A. Bardecker, A. M. Munro, M. S. Liu, Y. Niu, I.-K. Ding, J. Luo, B. Chen, A. K.-Y. Jen and D. S. Ginger, Nano Lett., 2006, 6, 462; B. Lim, J.-T. Hwang, J. Y. Kim, J. Ghim, D. Vak, Y.-Y. Noh, S.-H. Lee, K. Lee, A. J. Heeger and D.-Y. Kim, Org. Lett., 2006, 8, 4703; G. K. Paul, J. Mwaura, A. A. Argun, P. Taranekar and J. R. Reynolds, Macromolecules, 2006, 39, 7789.
8 Y.-H. Niu, M. S. Liu, J.-W. Ka, J. Bardeker, M. T. Zin, R. Schofiled, Y. Chi and A. K.-Y. Jen, Adv. Mater., 2007, 19, 300; Y.-H. Niu, A. M. Munro, Y.-J. Cheng, Y. Tian, J. Zhao, J. A. Bardecker, J.-L. Plante, D. S. Ginger and A. K.-Y. Jen, Adv. Mater., 2007, 19, 3371; Y.-J. Cheng, M. S. Liu, Y. Zhang, Y.-H. Niu, F. Huang, J.-W. Ka, H.-L. Yip and A. K.-Y. Jen, Chem. Mater., 2008, 20, 413.
9 F. Huang, Y.-J. Cheng, Y. Zhang, M. S. Liu and A. K.-Y. Jen, J. Mater. Chem., 2008, 18, 4495.
10 Y.-L. Liao, W.-Y. Hung, T.-H. Hou, C.-Y. Lin and K.-T. Wong, Chem. Mater., 2007, 19, 6350.
11 Y. Shirota and H. Kageyama, Chem. Rev., 2007, 107, 953.
12 K.-T. Wong, Z.-J. Wang, Y.-Y. Chien and C.-L. Wang, Org. Lett., 2001, 3, 2285.
13 C.-H. Chou and C.-F. Shu, Macromolecules, 2002, 35, 9673. 14 P. M. Borsenberger and D. S. Weiss, Organic Photoreceptor for
Imaging Systems, Marcel Dekker, New York, 1993.
