Hole-transporting side-chain polystyrenes based on TCTA with tuned glass transition and optimized electronic properties

Article abstract of DOI:10.1039/c5ra12963j

The development of crosslinkable materials for the fabrication of solution processable OLEDs presents challenges, especially regarding the adjustment of the glass transition (T_g), which has a significant influence on crosslinking kinetics and d

Full text of DOI:10.1039/c5ra12963j

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Hole-transporting side-chain polystyrenes based
on TCTA with tuned glass transition and optimized
electronic properties†
Cite this: RSC Adv., 2015, 5, 83122
Felix R. P. Limberg,a Arunas Miasojedovas,b Patrick Pingel,a Felix Reisbeck,a
a
b
a
*
Silvia Janietz, Andrew P. Monkman and Hartmut Kruger
¨
The development of crosslinkable materials for the fabrication of solution processable OLEDs presents
challenges, especially regarding the adjustment of the glass transition (Tg), which has a significant
influence on crosslinking kinetics and device life-time. Crosslinkable hole transport materials based on
poly(N,N-bis(4-(9H-carbazol-9-yl)phenyl)-4-vinylaniline)
(poly-TCTA)
with
covalently
attached
plasticizers for Tg control and azide functionalities for azide-alkyne crosslinking are presented. These
polymers have an optimal Tg of around 150 ꢀC and show superior crosslinking performances and
solution resistibilities. Incorporation of electron-pushing alkoxides to the pendant groups combines the
Tg adjustment approach with a systematic tuning of the HOMO level from ꢁ5.7 to ꢁ5.3 eV. All presented
polymers have good charge transport and injection properties and are ideal for applications in
phosphorescent OLEDs due to their high triplet energies (>2.8 eV). The new crosslinkable poly-TCTA-
based materials are applied as hole-transport layers (HTLs) in fully solution-processed OLEDs. An
improvement of the device performance is demonstrated for OLEDs with additional crosslinked HTL.
Received 3rd July 2015
Accepted 24th September 2015
DOI: 10.1039/c5ra12963j
www.rsc.org/advances
Crosslinking of organic layers generally requires exposure of
the sample to UV light, to elevated temperatures, or to both in
order to activate and promote the crosslinking reaction.5
Thereby, completion of the reaction can only be reached, if the
sample is heated above the glass transition or melting
temperature, at least once in a nal curing step.6 Below the glass
transition, the crosslinker groups are “frozen” in the glassy state
and hence their reaction is restricted. For an economical and
gentle processing, the glass transition temperature of a cross-
linkable material should be as low as possible; however, a
Introduction
Modern organic light-emitting diodes (OLEDs) consist of
several different functional materials and layers with well-
dened thicknesses and morphologies. In order to realize
such multi-layer structures, most OLEDs are still based on small
molecules which oen need to be processed under high-cost
vacuum conditions. Therefore, their use is still limited to
expensive high-resolution display applications, in, e.g., cell-
phones and televisions, and to rare examples of expensive
lighting applications.1,2 As an alternative, solution-based
processes enable high throughput and low-cost fabrication of
OLEDs on large size. However, solvent-processing of organic
multi-layers is still difficult; e.g., controlling de-wetting, crys-
tallization, or re-dissolving of deposited layers is a challenge.
Orthogonal solvent strategies3 or stamping techniques4 can
circumvent these problems but put severe constraints on the
choice of compatible materials, number of layers, or solvents.
Crosslinking techniques are a feasible alternative towards fully
solution-processed multi-layer OLEDs and may enable low-cost
lighting applications.5
strong correlation between
a high glass transition (Tg),
morphological stability of organic layers, and long OLED life-
times is apparent.7 In order to meet both processing and life-
time demands, we believe that a moderate glass transition, i.e.,
ꢀ
a moderate crosslinking temperature, between 150 C and 170
ꢀC would be ideal.
Recently, we introduced poly(N,N-bis(4-(9H-carbazol-9-yl)
phenyl)-4-vinylaniline) (poly-TCTA) as host material in green
phosphorescent OLEDs.8 In this polymer, a tris(4-(9H-carbazol-
9-yl)phenyl)amine (TCTA) analogue is bound as pendant group
to a polystyrene backbone. Polystyrene-based side chain func-
tionalized polymers are known for excellent lm formation and
the morphological stability of such layers.9,10 Good hole-
transporting properties and the high triplet energy (2.8 eV) of
poly-TCTA led to good overall electroluminescent perfor-
mances. Further device optimization might be possible by
introducing a hole transport layer (HTL) between anode and the
emission layer. The HTL material should possess a HOMO
aFraunhofer Institute for Applied Polymer Research, Wissenschaspark Golm,
Geiselbergstr. 69, D-14476 Potsdam, Germany. E-mail: hartmut.krueger@iap.
fraunhofer.de
bPhysics Department, University of Durham, South Road, Durham DH1 3LE, UK
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra12963j
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energy that compensates for the energy offset of the anode's the plasticizer is not conjugated and is therefore expected to
work function (PEDOT:PSS z 5.0 eV)11 and the polymer's have no inuence on the optical properties which are deter-
HOMO-derived hole transport states (e.g., poly-TCTA, ꢁ5.6 eV) mined by the TCTA units.
in order to improve charge carrier injection and to reduce the
1-Decyl-4-vinylbenzene (P) was copolymerized with mono-
driving voltage. The HTL may also serve as an electron blocking TCTA (2a); the P feed ratios are 25 M% (12 wt%) for polymer
layer. 3a25, 50 M% (29 wt%) for 3a50, and 75 M% (55 wt%) for 3a75
Liaptsis and Meerholz introduced electron-pushing or (Scheme 2). With increasing amount of plasticizer the number-
-pulling groups to the aromatic core of a crosslinkable derivative average molecular weight (Mn) is only slightly decreased,
of the hole-transporting small molecule 1-bis[4-[N,N-di(4-tolyl) whereas the weight-average molecular weight (Mw) decreases
amino]phenyl]cyclohexane (TAPC).12 This allowed for
a
signicantly accompanied by lower polydispersities (PDI) of the
systematic variation of the HOMO energy of these compounds copolymers. This can be explained by the reduced average
and, in turn, led to tunability of the hole injection barrier in monomer weight at higher plasticizer content. The amount of P
their OLED devices. In our work, we follow similar approaches in the copolymers, determined with 1H-NMR, is in good
to modify the chemical structure, electrooptical, and physical agreement with the feed ratios. Selected properties of the
properties of poly-TCTA towards its applicability as crosslinked prepared polymer samples are summarized in Tables 1 and 2.
HTL.
A strong Tg decrease was observed with increasing amounts
Here, we present strategies to reduce the relatively high glass of plasticizer P as expected. This decrease shows a linear
transition (247 ꢀC) of side-chain poly-TCTA by either intro- dependence between the measured glass transition and the
ducing plasticizers as co-monomers into the polymer chain or weight percentage of P (Fig. 1), wherein 3a50 (Tg ¼ 162 ꢀC)
by attaching exible substituents directly at the functional side- is in the desired Tg-range for OLED fabrication, while 3a25 (Tg ¼
chain units. The glass transition (Tg) can be tuned in a broad 203 ꢀC) and 3a75 (Tg ¼ 87.2 ꢀC) are outside of it. Additionally, the
range. The latter approach has been extended towards the use of plasticizer leads to an improved solubility in common solvents.
electron pushing substituents which provides a systematical
Note that the introduction of the plasticizer leads to no
tunability of the HOMO level. The crosslinkable new HTL signicant changes of the HOMO and LUMO levels of the
polymers are applied in OLED stacks.
polymers (see Table 2). However, the electrical hole current in
unipolar diodes (not shown) is considerably impaired for those
polymers as compared to the currents in polymer layers without
plasticizer. This may be due to the insulating nature of the
decylbenzene moieties.
Results and discussion
In general, the Tg of polymers can be reduced by a variety of
commercially available plasticizers. However, their application
in OLEDs is strongly limited, because they must meet the
following criteria: (1) miscibility with the host; (2) chemical and
optical inertness; (3) no diffusion into adjacent layers (there-
fore, a covalently bond plasticizer would be favorable); (4) no
detrimental interference with charge transport or light emissive
materials.
Crosslinking of low- and high-Tg polymers
In order to demonstrate that the glass transition affects the
crosslinking kinetics strongly, 1-(azidomethyl)-4-vinylbenzene
and monomer 2a were copolymerized either with or without
additional plasticizer P to give a crosslinkable low- (3a43AZ15
)
and high-Tg (3aAZ15) polymer with 15 M% azide content,
respectively (Scheme 3). Crosslinking can be performed ther-
mally aer adding 4,40-(propane-2,2-diyl)bis((prop-2-yn-1-yloxy)-
benzene) (XL).
Crosslinking of copolymerized azides and alkynes in poly-
styrenes via triazole formation has initially been described by
Spruell et al.14 The introduction of bis-alkyne (XL) as separate
crosslinker component has the advantage of an improved
bench-stability of the resulting materials. The thermal range of
crosslinking can be between 150 ꢀC, where the cycloaddition of
azides and alkynes starts, and 200 ꢀC, where unwanted thermal
Tg control by covalently attached plasticizers
The rst straight forward approach is the covalent attachment
of a plasticizer as a co-monomer into the polymer (Scheme 1).
The homo-polymꢀer of 1-decyl-4-vinylbenzene (P) has a very deep
Tg of about ꢁ65 C, the lowest reported Tg of p-alkylstyrenes,13
which makes this compound a strong plasticizer. In addition,
Scheme 1 Copolymeric Tg reduction approach using 1-decyl-4- Scheme 2 Synthetic route towards Tg adapted poly-TCTA copoly-
vinylbenzene as plasticizer (P).
mers with 1-decyl-4-vinylbenzene (P) as comonomer.
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Table 1 Polymer synthesis yields, molecular weight distributions, and glass transitions
Mn [103 g molꢁ1
]
Mwa [103 g molꢁ1
]
PDI
DPn
a
b
c
Tg [ꢀC]
Polymer
Yield [%]
3a
92
76
67
58
81
61
56
81
60
72
64
71
27.6
33.3
29.4
20.8
20.6
21.8
9.05
18.5
20.9
29.2
24.3
16.9
112
111
4.04
3.34
2.41
2.27
2.95
2.14
2.51
3.98
2.44
2.09
2.38
1.79
46
65
70
62
39
57
14
22
23
38
27
22
247
203
162
87.2
ꢁ(251)
172 (173)
220
144
146
3a25
3a50
3a75
3aAZ15
3a43AZ15
3b
3c
3d
3e
3f
70.9
47.3
60.8
46.6
22.7
73.7
51.1
60.9
57.8
30.3
184
150
144 (150)
3fAZ15
a
b
c
Determined with size exclusion chromatography (SEC) using polystyrene standards. Degree of polymerization: Mn ꢂ Mmonomer ꢁ 1. Glass
transition determined with differential scanning calorimetry (DSC), in brackets: self-crosslinked.
decomposition of the azides and nitrene formation set in.
Formed nitrenes can then easily insert into aliphatic and
aromatic bonds with detrimental effects on the charge transport
in conjugated materials.15
Azide decomposition is visible as an exothermic reaction in
differential scanning calorimetry (DSC) spectra of neat 3aAZ15
and 3a43AZ15 at around 250 ꢀC (Fig. S1 and S2†). The glass
transitions of both polymers are visible in the subsequent DSC
cycle at 250 ꢀC and 172 ꢀC, respectively. The reduced Tg of
3a43AZ15 is clearly due to the incorporation of the plasticizer.
Addition of the crosslinker XL to 3aAZ15 and 3a43AZ15 leads to
an exothermic DSC peak above 150 ꢀC which can be ascribed to
the desired azide-alkyne cycloaddition. Aer the cycloaddition,
Fig. 1 Tg dependence on the weight and molar fractions of P in the
polymers 3a, 3a25, 3a50, and 3a75
the Tgs of the crosslinked polymers are further reduced to
240 ꢀC and 158 ꢀC, respectively. This decrease can be explained
by a further plasticizing effect of XL.
.
Performing the crosslinking reaction at 170 ꢀC in the
presence of an equal amount of azide and alkyne moieties
leads, however, to vastly different azide conversions for
3aAZ15 and 3a43AZ15. The azide-alkyne reaction can be fol-
lowed with isothermal ATR-FTIR spectroscopy (Fig. S3 and
S4†) by recording the corresponding symmetrical-stretching
mode bands (N3, 2095 cmꢁ1; C^C–H, 3300 cmꢁ1). Fig. 2
shows the azide-related peak areas over time during constant
heating at 170 ꢀC. While the crosslinking reaction is complete
(i.e., 100% azide consumption) aer 35 min for low-Tg
3a43AZ15, the azide conversion in 3aAZ15 stops aer only a few
minutes and ca. 75% of the azide groups remain unreacted.
The latter is due to the high glass transition of 3aAZ15 which
Table 2 Electronic and optical properties of the synthesized polymers
Polymer Absorption max.a [eV] Fluorescence max.b [eV] Stokes shi [eV] LUMOc [eV] HOMOd [eV] HOMOe [eV] EG [eV] ETg [eV]
f
3a
3.79
3.79
3.79
3.78
4.01
4.01
3.83
3.94
3.94
2.91
2.91
2.96
3.06
3.05
3.09
3.02
3.02
3.02
0.88
0.88
0.83
0.72
0.96
0.96
0.81
0.92
0.92
ꢁ2.27
ꢁ2.24
ꢁ2.19
ꢁ2.19
ꢁ2.18
ꢁ2.19
ꢁ2.16
ꢁ2.19
ꢁ2.17
ꢁ5.62
ꢁ5.60
ꢁ5.55
ꢁ5.55
ꢁ5.49
ꢁ5.58
ꢁ5.53
ꢁ5.29
ꢁ5.29
ꢁ5.48
ꢁ5.45
ꢁ5.52
ꢁ5.49
ꢁ5.47
ꢁ5.45
ꢁ5.39
ꢁ5.35
ꢁ5.24
3.35
3.36
3.36
3.36
3.31
3.39
3.37
3.11
3.12
2.8
3a25
3a50
3a75
3b
3c
3d
2.88
2.77
2.9
2.9
2.9
2.87
2.8
2.81
3e
3f
a
As solid lm. b Photoluminescence spectra as solid lm. c LUMO values were calculated by using the optical bandgap and the HOMO-values from
PESA measurements. d Photoelectron spectroscopy in air (PESA). e Cyclovoltammetry peak potential against ferrocene standard. f Onset from UV/
Vis-spectra. g For triplet energy levels the onsets of phosphorescence spectra were taken.
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Scheme 3 Crosslinker incorporation into low and high Tg polymers.
Scheme 4 The plasticizing and electronic influence of alkoxy groups
on poly-TCTA.
For the alkoxy-substituted TCTA polymers (3b–f) an
increasing number of alkylic carbons per pendant group were
introduced. The alkoxy substitution of carbazoles (b/d/e/f,
Scheme 5) was performed following a procedure by M. A.
Keegstra et al. for the copper(I)halide catalyzed synthesis of alkyl
aryl ethers16 which was applied to carbazoles before.17,18 2-((2-
Ethylhexyl)oxy)-9H-carbazole (c) was obtained by simple ether-
ication of the aryl ether and the corresponding 2-hydroxy-9H-
carbazole. The following reactions towards the polymers 3b–f
(top of Scheme 5) were performed according to a procedure
published by Thesen et al.8 The incorporation of the alkoxy
groups leads to a notable decrease of Tg (see Table 1). As
compared to unsubstituted poly-TCTA, there is a minor Tg
reduction of 26 K for polymer 3b with two methoxy groups, a 62
K decrease for polymer 3e with four ethoxy groups, and a
decrease of 100 K for polymers 3c, 3d, and 3f with four butoxy or
two ethylhexyloxy substituents. As a rule of thumb, each addi-
tional alkylic carbon per pendant group reduces Tg by about 7–
13 K. Polymers 3c, 3d, and 3f have glass transitions at around
150 ꢀC which we prefer for the use in OLEDs as outlined earlier.
Fig.
2
Peak area isotherm of the 2095 cmꢁ1 symmetric azide
stretching at 170 ꢀC of polymers 3aAZ15 and 3a43AZ15 in the presence
of the crosslinker XL.
hinders the segmental movement of the crosslinker groups
and therefore prevents the encounter of the reaction partners.
It is thus clear that an effective crosslinking procedure must
be conducted above the glass transition of the polymer, but
well below the azide decomposition temperature. For 3aAZ15
(i.e., the high-Tg polymer without plasticizers) both condi-
tions cannot be fullled at the same time, which highlights
exemplary the importance of glass transition tuning pre-
sented in this work.
Notice that we observe a signicant (ꢃ20%) wash-off of the
(partially) crosslinked 3aAZ15 layers aer rinsing with toluene
(see UV-Vis absorption spectra Fig. S5†). Crosslinked 3a43AZ15
layers, on the contrary, exhibit excellent solvent resistance
(Fig. S6†).
Simultaneous HOMO level and Tg tuning by alkoxy groups
with varied alkyl lengths
An alternative way to achieve a Tg adaption is the direct
substitution of the pendant carbazole groups (Scheme 4). We
decided to introduce alkoxy groups at different positions on the Scheme 5 Synthetic route towards Tg-adapted poly-TCTA copoly-
mers and alkoxy-substituted poly-TCTA derivatives with improved
carbazole groups of the poly-TCTA (3a) to reduce the Tg while
additionally increasing the electron density at the carbazole.
The latter approach gives a viable ability to ne-tune the HOMO
level of the polymer, as has been shown previously in a study on
substituted molecular TAPC.12
hole injection properties. (i) NaOAlk, bromo-9H-carbazole, CuI, DMF,
120 ꢀC, 1–24 h; (ii) 9H-carbazol-2-ol, 3-(bromomethyl)heptane,
K2CO3, DMF, 100 ꢀC, 20 h; (iii) a–f, Cu-bronze, K2CO3, 18-crown-6, o-
DCB, 200 ꢀC, 1 h to 24 h; (iv) 1a–f, MePPh3Br, KOtBu, 0 ꢀC, 2 h; (v), AIBN
(2 M%), THF (toluene for 3f), 50 ꢀC, 2a–f.
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Polymers 3a, 3a25, 3a50, and 3a75 have degrees of polymeri-
zation (DP) of around 50–70 and polymers 3b–f of around 15–40
(Table 1). From our experience, the Tg values of polystyrenes
substituted with bulky side groups are essentially independent
of molecular weight, if DP $ 15. This is supported by Claudy
et al.19 who show that the Tg of polystyrene remains almost
unchanged for polymer chains with more than 25 repeat units.
The azide functionalized polymers 3aAZ15, 3a43AZ15 and 3fAZ15
have similar weight distributions compared to the non-
crosslinkable counterparts (Table 1).
In addition to the adaption of the glass transition, the
introduction of alkoxy groups at the functional p conjugated
carbazoles allows for a tuning of the energetic states. Here, we
investigate this concept by introducing substituents with varied
alkyl chain lengths and varied substitution positions. HOMO
level values were determined with photoelectron spectroscopy
in air (PESA) and cyclovoltammetry (CV); these results are in
good agreement (with a maximum deviation of 0.15 meV for
polymer 3a25) and are summarized in Table 2. All polymers were
reversible for several CV-measurement cycles, indicating a good
electrochemical stability. The unsubstituted polymer 3a has
the deepest HOMO level, followed by the polymers bearing
alkoxy groups in the meta-position of the carbazole nitrogen
(3b/3c/3d). Unexpectedly, the work function change for 3b with
one methoxy group (D ¼ 0.13 eV, relative to 3a and according to
PESA measurements) is slightly bigger than that for 3c (having
one ethoxyhexyl group, D ¼ 0.04 eV) and 3d (having two butoxy
groups, D ¼ 0.09 eV). Generally, however, the inuence of meta-
substituted alkoxy groups on the HOMO level is relatively small
and the differences are close to the uncertainty of the PESA and
CV measurements. A signicant change of D ¼ 0.3 eV is
observed for the HOMO levels of polymers 3e and 3f which have
their alkoxy substituents in para-position to the carbazole-
nitrogen. This can be explained by para-ring substituent effects.
Optical properties of the polymers. The absorption, normal-
ized uorescence and phosphorescence spectra of neat lms of the
investigated polymers 3a–f are shown in Fig. 3. The details of the
photophysical properties of these compounds are summarized in
Table 2. The absorption spectra of the 3a/3a25–75 copolymers are
identical for the most part except for higher oscillator strength at
wavelengths smaller than 240 nm, which is contributed to by the 1-
decyl-4-vinylbenzene absorption. Compounds without alkyl chains
demonstrated absorption bands with peak positions at 297 nm
and 328 nm. In the polymers with alkyl chains attached to the
carbazole at 2- and 2,7-position (3b–d) these two bands are slightly
redshied. Attaching butoxy groups to carbazole at 3,6-position
(3e and 3f) redshis those two bands to approx. 315 nm and
341 nm. This effect is due to the increased p-conjugated pendent
systems arising from the electron donating ability of methoxy/
butoxy groups.20,21 A similar situation was observed in uores-
cence spectra. Polymers 3a–3a75 exhibited broad emission with a
peak at 400 nm. Substituents at 2- and 2,7-position do not change
the peak position of the spectra, but the shape of spectra become
more narrow. The uorescence spectra of 3e and 3f show a redshi
of about 10 nm, similar as in their absorption spectra. Polymer 3f
did however exhibit a uorescence spectrum with a more
pronounced shoulder at longer wavelengths.
Fig. 3 UV/Vis (dotted lines) and photoluminescence (PL) spectra
(straight lines) and phosphorescence (dashed lines) of the polymers
3a–f. The top-graph shows additionally the spectra of 3a25 (triangle),
3a50 (circle) and 3a75 (star) for comparison.
The phosphorescence spectra of each material act in a similar
fashion to uorescence. 3a–d demonstrate small changes in
phosphorescence spectra, the spectra of polymers 3e and 3f are
redshied by 10 nm. Polymer 3e is the only material showing a
small delayed uorescence signal which might indicate enhanced
triplet mobility or higher ISC yield. Most importantly, all poly-
mers have a high triplet energy (2.8–2.9 eV, see Table 2) which is
compatible with their use as host or hole-transporting material in
OLEDs with green-to-blue phosphorescent emitters.
The various substituents on the polymers have very little
effect on the triplet energy, as desired. However, polymer 3a50
containing 50 M% of 1-decyl-4-vinylbenzene spacer demon-
strated very broad, structureless and redshied, by 50 nm,
phosphorescence. This we ascribe to emission from triplet
states dimers, formed between interacting neighbouring
carbazole units, as was previously observed in PVK.22
Electrical characteristics of the polymers. The electrical
characteristics of the substituted TCTA-based polymers have
been tested in a hole-only current diode geometry (see inset of
Fig. 4) with similar polymer layer thicknesses. In these devices,
positive charge carriers can be injected from the ITO/PEDOT:PSS
anode into the polymer layer. The Al cathode provides an elec-
tron- and hole-injection blocking contact. In Fig. 4, clear diode
behaviors with a forward (at positive voltages) and a blocking (at
negative voltages) direction are evident. The characteristics show
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Fig. 4 Electrical characteristics of hole-only diodes prepared from
polymers 3a–f. Positive voltages refer to the injection of holes from
the ITO contact into the polymer layer. The polymer layers have similar
thicknesses of 80–100 nm. Inset: sample structure of the hole-only
diodes.
a steep current rise above a certain onset voltage (2–2.5 V), which
shows that the polymers allow for hole transport. The onset
voltages clearly correlate with the HOMO level of the polymers;
i.e., the deeper the HOMO level (see Table 1), the larger the onset
voltage. This shows that all hole-only characteristics in Fig. 4
suffer from an injection barrier which is largely determined by
the HOMO level of the respective polymer. Consequently, it is
not possible to extract charge carrier mobilities from these
measurements. Note, however, that the current rise beyond the
onset voltage is much steeper for the polymers with four alkoxy
substituents in para-position (polymers 3e and 3f) than for the
respective polymer with these groups in meta-position, which
points to a larger mobility of the former set of polymers.
Fig. 5 (a) j–L–V characteristics and (b) current efficacies of OLEDs
with and without hole-transport layers based on the crosslinkable
polymers 3a or 3c with incorporated azide (AZ) moieties.
The OLED characteristics are shown in Fig. 5a. Clear diode
behaviors and high rectication ratios are observed. Comparing
the devices with 3fAZ15 and with 3a43AZ15 HTL, the current and
luminance onset voltage is increased for the latter sample due
to the deep HOMO level of 3a43AZ15, as expected.
Fig. 5b shows the current efficacies of the devices with and
without HTL. It can be clearly seen that the use of a HTL,
separating the emission layer from the anode, leads to an effi-
cacy increase as compared to devices with no HTL. The highest
current efficacy is exhibited by the device with 3fAZ15 HTL,
which is probably due to the proper adaption of its HOMO level
to the energetic landscape of the emissive layer.24 We cannot
determine the detailed physical role that the HTL plays in our
devices within the scope of this article; however, it seems
plausible that the high triplet energies and the high-lying
LUMO-derived states of the TCTA-based polymers serve for a
blocking of triplets and electrons, such that those species
become conned to the emission layer. Most importantly, we
have shown that both crosslinkable TCTA-based polymers are
suitable and advantageous to be used as HTL materials in
OLEDs as they enable the subsequent solution-processing of the
emission layer without any re-dissolution.
In order to represent the full span of the HOMO level range
which is accessible with the new TCTA-based polymers, we
tested the crosslinkable versions of 3a (unsubstituted poly-TCTA
with 15 M% azide content, 3a43AZ15) and 3f (poly-3,6-(BuO)4TCTA
with 15 M% azide content, 3fAZ15) as hole-transport layers
in fully solution-processed OLEDs (i.e., solution-processing
except for the electrodes). Notice that in the case of unsub-
stituted poly-TCTA, we have used the low-Tg variant with decyl
styrene plasticizer in order to allow for crosslinking well below
ꢀ
200 C.
15–20 nm thick hole-transport layers were fabricated by spin-
coating from toluene solutions on top of ITO/PEDOT:PSS
patterned electrodes. Crosslinking of these layers was per-
formed by heating at 170 ꢀC for 40 min. The emission layer is a
ternary blend consisting of an iridium-based phosphorescent
green emitter, a co-host, and an electron transporting polymer
as is described in our previous work, ref. 23 and 24 The emis-
sion layer was annealed at 180 ꢀC for 30 min. The Ba/Ag cathode
was deposited by subsequent evaporation. Further details of the
sample preparation can be found in the ESI.†
Conclusions
We have demonstrated two approaches to adapt functional
side-chain polystyrenes towards the needs of a solution-
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Paper
¨
8 M. W. Thesen, B. Hofer, M. Debeaux, S. Janietz, A. Wedel,
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units of the prototypical hole-transporting TCTA side-chain
polystyrene, the glass transition can be tuned in a wide range
between 87 and 245 ꢀC. This is of particular importance for
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83128 | RSC Adv., 2015, 5, 83122–83128
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