Solution-processed thermally stable amorphous films of small molecular hole injection/transport bi-functional materials and their application in high efficiency OLEDs

Article abstract of DOI:10.1039/c5tc02559a

A series of novel triphenylamine-based small molecular hole transport materials (HTMs) are reported for solution processed organic light-emitting devices (OLEDs). The character of this series of HTMs, denoted as TPD(BTPA)_n (n = 1, 2, 4), is connecting the flexible moieties of butadiene bridged triphenylamine (BTPA) to N,N,N′,N′-tetraphenyl-[1,1′-biphenyl]-4,4′-diamine (TPD). The glass transition temperature and crystallization temperature (T_g and T_c) showed a proportional relationship with the number of BTPA moieties. The T_g value of TPD(BTPA)₄ can be up to 125.5 °C, which is higher than most of the reported small molecular HTMs (T_g: 54-116 °C). The TPD(BTPA)₄ spincoated film showed an outstanding thermal stability which remained amorphous even when annealed at 110 °C, for 48 h. This indicated that the breaking of the planar molecular structure with BTPA moieties can suppress intermolecular stacking. The solution processed OLEDs with 8-hydroxyquinoline aluminum (Alq₃) as emission and electron transport layers showed high stability at high operation current (>400 mA cm^-2). The OLED with TPD(BTPA)₄ achieved a maximum current efficiency of 5.83 cd A^-1 (at the operation current density > 400 mA cm^-2), which is higher than the maximum current efficiency of most evaporation and solution processed OLEDs in identical structures.

Full text of DOI:10.1039/c5tc02559a

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Solution-Processed Thermal-Stable Amorphous Films of Small Molecular Hole
Injection/Transport Bi-functional Materials and its Application in High Efficient
OLEDs
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoming Zhaoa,b, Shirong Wang*a,b, Jing You *a,b, Yuteng Zhang a,b and Xianggao Li a,b
DOI: 10.1039/x0xx00000x
A series of novel triphenylamine-based small molecular hole transport materials (HTMs) are reported for solution
processed organic light-emitting devices (OLEDs). The character of this series of HTMs, denoted as TPD(BTPA)n (n=1,2,4) is
connecting the flexible moieties of butadiene bridged triphenylamine (BTPA) to the N,N,N',N'-tetraphenyl-[1,1'-biphenyl]-
4,4'-diamine(TPD). The glass transition temperature and crystallization temperature (Tg and Tc) showed an proportional
relationship with the number of BTPA moieties. The Tg of TPD(BTPA)4 can be up to 125.5℃, which is the higher than most
of reported small molecular HTMs (Tg : 54℃-116℃). The TPD(BTPA)4 spincoated film showed an outstanding thermal
stability which maintained amorphous even annealed with 110℃, for 48 h. This indicated the break of planar molecular
structure with BTPA moieties can suppress the intermolecular stacking. The solution processed OLEDs with 8-
Hydroxyquinoline aluminum (Alq3) as emission and electron transport layer showed high stability at high operation current
(> 400 mA cm-2). The OLED with TPD(BTPA)4 achieved a maximum current efficiency of 5.83 cd A-1 (at the operation current
density > 400 mA cm-2), which is higher than the maximum current efficiency of most evaporation and solution processing
OLEDs in identical structure.
www.rsc.org/
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Introduction
21 as energy level of the polymers depends on not only the unit
22 structure, but also the length of polymer chain which is not a
Organic light-emitting devices (OLEDs) are regarded as the 23 constant.8-10 It is believed that the design, synthesis and
next generation flat-panel displays because of advantages, 24 purification of small molecular materials are more
such as portable, full colored, glare free, flexible, short 25 convenient.
response time, wider viewing angles etc.1-3
26
For the second problem, the crystallization of small
To date, there mainly are two issues need to be solved 27 molecular materials during film-forming in the solution
about small molecular OLEDs. First, the ultra-thin films in 28 process and operating at high temperature are main
OLEDs are typically fabricated by vacuum evaporation (VE), 29 obstacles for its application in OLEDs.11-14 The low glass
10 which has demerits of high energy consuming, waste of 30 transition and crystallization temperatures (Tg and Tc) lead to
11 materials and unfriendly to environment.4 Secondly, the 31 formation of polycrystalline in films resulting from thermal
12 charge transfer and injection require homogeneous interface 32 stress.15 In the research reported by Douglas E Loy et. al., the
13 and layer, which means that the organic semiconductor film 33 OLED with N,N,N',N'-tetraphenyl-[1,1'-biphenyl]-4,4'-diamine
14 should be amorphous form.5
34 (TPD) and N,N'-diphenyl-N,N'-bis-(1-naphthyl)-1,1'-biphenyl-
The first problem can be solved through solution 35 4,4'-diamine (NPD) as HTL showed irreversible failure when
due to crystallization.16 As reported,
15
16 processing techniques such as layer-by-layer spin coating 36 heated at 100 and 110
℃
17 and ink jet.6 While, those techniques are mainly applied in 37 the Tg and Tc of organic semiconductor are determined by
18 the polymer materials. However, the research on structure- 38 the molecular structure. Larger molecular volume resulted in
19 functional relationship of small molecular materials is more 39 higher Tg.17-19
20 mature than the polymers.7 Functional characteristics such
a.Department of Applied Chemistry,School of Chemical Engineering and
Technology,Tianjin University, Tianjin 300072, China. E-mail:
Youj1983@tju.edu.cn
b.Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, China. E-mail: wangshirong@tju.edu.cn
†Electronic Supplementary
DOI: 10.1039/x0xx00000x
Informaꢀon
(ESI)
available.
See
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Journal of Materials Chemistry C
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9
In this work, we designed and synthesized three new 58 Synthesis and Characterizations
HTMs based on the TPD, which has a HOMO level of -5.30 eV 59
20, 21
and a Tg of 69
℃
.
The TPD is modified with different 60 The TPD(BTPA)n (n=1,2,4) are synthesized by formylation and
numbers of 4-(penta-1,3-dien-1-yl)-N,N-diphenylaniline 61 Wittig-Honor reactions starting with TPD (Scheme 1).45
(BTPA) moieties (Fig. 1) to suppress the formation of crystal 62 The BTPA moieties were synthesized from 4-(di-p-
in film through reducing the molecular rigidity and breaking 63 tolylamino)benzaldehyde (Me-TPA-CHO) by formylation and
the molecular planarity. Comparing with triphenylamine, 64 Wittig-Honor reactions. The total yield of 3-(4-(di-p-
BTPA moieties can be introduced simply with witting-honor 65 tolylamino)phenyl)prop-2-en-1-ol (Me-TPA-CH=CH-CH2OH)
reaction, which reaction condition is much milder than 66 was 92.0%. The aldehybe groups were introduced on the
10 Ullmann reaction or carbon-carbon coupling reaction. 22, 23 In 67 TPD with different numbers. Then the BTPA moieties were
11 addition, the BTPA moiety has larger volume than 68 introduced on the TPD(CHO)n (n=1,2,4) by Witting-Honor
12 triphenylamine, which are benefit for suppressing 69 reaction. The yields were 72.3% (TPD(BTPA)1), 68.2%
13 intermolecular stacking in solid states. With considering 70 (TPD(BTPA)2) and 63.4% (TPD(BTPA)3), respectively. The
14 above reasons, we chose BTPA moieties to modify the TPD 71 TPD(BTPA)n(n=1,2,4)
compounds
are
soluble
in
15 The molecular structures are shown in Fig. 1 and denoted as 72 methylbenzene, chlorobenzene, tetrahydrofuran and
16 TPD(BTPA)n (n=1,2,4). The crystallization during the spin- 73 chloroform, etc. The well-defined structures of all novel
17 coating film forming process and the thermal stability of 74 compounds are adequately verified by 1H NMR, mass
18 these films are investigated by X-ray diffraction (XRD), 75 spectrometry and IR spectroscopy (see details in the ESI).
19 attenuated total reflectance fourier transform infrared 76
20 spectroscopy (ATR-FTIR) and differential scanning 77 to 5% weight loss) of TPD(BTPA)n (n=0-4), recorded by
21 calorimetry (DSC) measurements. The thermal stability 78 thermal gravity analysis (TGA), are 411 (TPD(BTPA)1), 425
22 increases gradually with the increasing number of BTPA 79 (TPD(BTPA)2), and 448 (TPD(BTPA)4), respectively (Fig. S10
23 moieties connected to the TPD. Notably, the spin-coating 80 Supporting Information). With the increase of
The decomposition temperatures (Td, which correspond
℃
℃
℃
,
n
in
24 film of TPD(BTPA)4 can maintain amorphous even after 81 TPD(BTPA)n, the Td of as-synthesized HTMs increase
o
25 annealed at 110 C for 48 h. At present, the reported HTM 82 gradually.
o
26 films crystallized with annealing at < 95 C for < 2 h, such as 83
27 N,N,N′,N′-tetraphenylbenzidine (TPB), N,N,N′,N′-tetra-p- 84 Scheme 1 Synthetic routes of TPD(BTPA)n (n=1,2,4).
28 tolyl-benzidine
(TTB),
N,N′-di(naphthalene-1-yl)-N,N′-
N
N
N
POCl3
DMF
t-BuOK
NaBH4
29 diphenyl-benzidine (NPB), 9,10-(2-naphthyl)anthracene
30 (AND), 1,4-bis(phenyl-m-tolylamino) biphenyl (TPD), 1,4-
31 bis(benzothiazole-vinyl) benzene (BT) et. al.. 24-30
32
N
CHO
CH2OH
CHO
R1
R2
CH3
Me-TPA-CH=CH-CH 2OH
33
34
35
36
37
38
39
40
N
N
N
N
N
N
BTPA=
R3
R4
CH3
DMF
CH2OH
Ph3P+CH3Br-
POCl3
R1'
R2'
TPD, R1=R2=R3=R4=H
TPD(BTPA)1, R1=R, R2=R3=R4=H
TPD(BTPA)2, R1=R2=R, R3=R4=H
TPD(BTPA)4, R1=R2=R3=R4=R
N
N
N
41 Fig. 1 Molecular structures of TPD(BTPA)n compounds
42
R3'
R4'
R1'=R2'=R3'=R4'=CHO
CH2Ph3P+CH3Br-
43
The hole transport properties of as-synthesized HTMs
R1'=R2'=CHO R3'=R4'=H
R1'=CHO R2'=R3'=R4'=H
44 are evaluated in the solution processed OLEDs with the
45 structure of ITO/HTMs/Alq3/LiF:Al. This structure with small
46 molecular HTMs (molecular weight < 6000 g mol-1) has been
47 reported by many groups with maximum current efficiency
48 (CEmax) smaller than 5.7 cd A-1, which are listed in Table 5.6, 11,
31-44
49
t-BuOK
50
In this work, the Device-TPD(BTPA)4 can exhibit the
51 CEmax of 5.83 cd A-1 at the operation current intensity larger
52 than 400 mA cm-2. This CEmax is highest among the reported
53 small molecular OLED (molecular weight < 6000 g mol-1) in
54 identical structure no matter fabricated with evaporation or
55 solution processing.6, 11, 31-44
R1
R2
N
N
R3
R4
56
TPD(BTPA)1, R1=R, R2=R3=R4=H
TPD(BTPA)2, R1=R2=R, R3=R4=H
TPD(BTPA)4, R1=R2=R3=R4=R
57 Results and Discussions
2 | J. Mater.Chem.C, 20XX, XX, X-X
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Photophysical and Electrochemical Properties
1
2
58 Thermal Properties of TPD(BTPA)n
59
3
UV-vis absorption spectra of TPD(BTPA)n(n=0,1,2,4) are 60 The thermal properties of TPD(BTPA)n (n=0,1,2,4) are
4
5
6
shown in Fig. 2 and relative data are summarized in Table 1
Cyclic voltammograms of TPD(BTPA)n (n=0,1,2,4) are shown 62
in Fig. S11 ESI).
.
61 investigated by the differential scanning calorimetry(DSC)
As shown in Fig. 3, the Tg, Tc and Tm of TPD(BTPA)n
63 (n=0,1,2,4) increase gradually with the increasing number of
(
7
8
9
10
11
12
13
14
15
64 BTPA moieties (n). The Tg and Tc of TPD(BTPA)4 are higher
65 than most of common used small molecular HTMs.29-34 The
66 corresponding enthalpies of crystallization are 41.51 Jg-
67 1(TPD), 44.26 Jg-1(TPD(BTPA)1), 31.5 Jg-1(TPD(BTPA)2) and
68 28.26 Jg-1(TPD(BTPA)4), respectively. This indicates that the
69 TPD and TPD(BTPA)1 have higher crystallization tendency
70 than TPD(BTPA)2 and TPD(BTPA)4.6 Apparently, the BTPA
71 moieties showed a capability for improving thermal stability,
72 while only one BTPA moiety was not enough for changing
73 crystallization property noticeably.
16 Fig. 2 Normalized absorption spectra of TPD(BTPA)n
17 (n=0,1,2,4) in chlorobenzene solution with concentration of
18 1.0
19
74
×
10-5 M (a) and spincoating films (b).
20
As shown in the Fig. 2a, the absorption peaks in the
21 wavelength region 280-310 nm can be assigned to the n-π
22 transition of the TPA moiety. The absorption peaks in the
23 region 375-450 nm can be assigned to the intramolecular
24 charge transfer (ICT) of π-π* transition. There is an obvious
25 red shift of ICT peaks and decrease of energy gap (Eg) with
26 the increasing number of BTPA moiety. The absorption
27 spectrum of each TPD(BTPA)n compound in solid state film
28 reveals no red-shift compared to that in chlorobenzene
29 solution, indicating that no significant aggregation or
30 crystallization occurs in the films of TPD(BTPA)n(n=1,2,4).46
75
31
As shown in Table 1, with the increase of number
n
in
32 TPD(BTPA)n, the HOMO level increases from
-
76
33 5.30eV(TPD(BTPA)1) to -5.12eV(TPD(BTPA)4), which exhibits
34 the HOMO level can be modified through changing the
35 number of BTPA moiety. The HOMO level of TPD(BTPA)4
36 matches the work function of ITO (-4.80 eV) quite well,
37 which could benefit the charge injection without the
38 assistant of the normally-used hole injection layer (HIL)
39 poly(3,4-ethylene-dioxythiophene) poly(styrene sulfonate)
40 (PEDOT:PSS).47
77 Fig. 3 DSC curves of TPD(BTPA)n (n=0,1,2,4) selected from
78 the heating of second loop.
79
80
81
82
83
84
85
41
42
Table 1. The optical, thermal properties and energy levels of TPD(BTPA)n (n=0,1,2,4)
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
86
a)
b)
b)
b)
c)
d)
Td
Tg
Tc
Tm
λITC
Eg
HOMO e)
(eV)
LUMO e)
(eV)
87
88
(°C)
375
411
425
448
(°C)
70.1
(°C)
(°C)
(nm)
391
399
407
415
(eV)
3.10
2.94
2.82
2.73
89
90
TPD
146.7
17.4
205.7
-5.30
-5.21
-5.16
-5.12
-2.20
-2.27
-2.34
-2.39
91
230.5
92
TPD(BTPA)1
TPD(BTPA)2
TPD(BTPA)4
95.9
112.2
125.5
188.1
201.8
247.8
93
94
260.7
95
a) Measured by TGA at a heating rate of 10 °C min-1 under nitrogen atmosphere. Tg glass transition temperature, Tc
b)
96
97
crystallization temperature, and Tm melt point measured by DSC according to the heat-cool-heat procedure (Fig. 3). c)
Intramolecular charge transfer peak of thin films. d) Optical energy gaps calculated from the absorption thresholds from UV-
98
Vis absorption spectra of chlorobezene solutions (1.0×10-5 M). e) Measured by cyclic voltammetry.
99
100
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Journal of Materials Chemistry C
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Crystallization of Spincoated Films
28 TPD and TPD(BTPA)1, a broad band in the wavenumber
29 region of 900-1200 cm-1, and
small band in the
a
The amorphous films can improve charge injection and 30 wavenumber region of 700-800 cm-1 generated after
transfer between different layers.48-50 It is important to find 31 annealing. The broad band can be assigned to the in-plane
out when the crystallization occurs. Therefore, we measured 32 vibration of benzene.52 This phenomenon generate from the
the crystallinity of spincoated films before and after 33 change of optical property after crystallization. The change
annealing though X-Ray Diffraction(XRD) and Attenuated 34 of ATR-FTIR spectra for TPD(BTPA)2 and TPD(BTPA)4 films
6
7
8
9
10
Total
Spectroscopy (ATR-FTIR).
The XRD spectra of TPD(BTPA)n films are shown in Fig. 4
Reflectance-Fourier
Transformed
Infrared 35 before and after annealing at 100℃ are unobvious, which
36 mean the TPD(BTPA)2 and TPD(BTPA)4 films maintained
. 37 amorphous. For most of reported small molecular HTMs,
11 When the TPD(BTPA)n (n=0,1,2,4) were spincoated on the 38 such as TPB, TTB, NPB, AND, TPD and BT, these films cannot
12 ITO substrate, the XRD patterns in the region of 0<2θ<5.0° 39 maintain amorphous form with annealing > 80
℃ less than 2
13 were same as that of ITO substrate, which indicating that all 40 h. 29-34
14 these spincoated films were amorphous. The diffraction
15 peaks (2θ
16 when these films were annealed at 70
17 diffraction peaks (2θ
1.5o) in TPD(BTPA)2 and TPD(BTPA)4
18 films appeared at annealing temperature of 100 and 120℃,
≈
1.5o) can be observed in TPD and TPD(BTPA)1
℃
for 24 h. The
≈
℃
19 respectively. As shown in Fig. 5(B), for the TPD(BTPA)4 film,
20 the XRD pattern maintained unchanged even annealed at
21 110℃ for 48 h.
41
42 Fig.5 ATR-FTIR spectra of TPD compounds before(a) and
43 after annealing(b).
44
45 Fig. 6 Crystallization process in spincoated films.
46
47
All the results above indicate that the crystallization
48 capability is suppressed with BTPA moieties. The
49 crystallization process does not happen during spincoating
50 process. Instead, it occurs after the film-forming process. The
51 possible process is shown in Fig. 6
.
52
All the spincoated films are amorphous before
53 annealing. As shown in Fig. 6, we discuss the two extreme
54 situations: TPD (crystalline) and TPD(BTPA)4 (amorphous). In
55 the case of TPD, after spincoating the TPD molecules deposit
56 on the substrate randomly and the TPD film is amorphous
57 accordingly. While, the TPD molecule tends to stack because
58 of its planarity molecular structure, low Tg and Tc, as well as
59 the large change of enthalpies after crystallization. Then, the
60 orientated aggregation (crystallization) occurs, which will
61 break the homogenous TPD films.
22 Fig. 4 X-ray diffraction patterns of films on ITO glass
23 substrate annealed from 70-140 for 24 h (a); annealed at
℃
24 110℃ for 0-48 h(b)
25
26 The crystallinity were confirmed directly by ATR-FTIR
27 measurement.51 As shown in Fig. 5, for spincoated films of
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37 Normalized with minimum value of L,W and H; all the units are
38 a.u..
1
2
3
4
5
6
7
8
9
In the case of TPD(BTPA)4, the film maintains
amorphous after annealing. Compared with TPD, the
molecular structure of TPD(BTPA)4 is nonplanar. In addition,
the TPD(BTPA)4 has higher Tg, Tc and smaller change of
enthalpies after crystallization. These prevent the orientated
aggregation (crystallization) in TPD(BTPA)4 after annealing.
The crystallization property of TPD(BTPA)1 and TPD(BTPA)2
should be between that of TPD and TPD(BTPA)4.
39
40 Application in OLEDs
41
42 One role of BTPA moiety is increasing the molecular volume
43 to suppress crystallization in films and improve the thermal
44 stability. Another role is increasing the molecular
45 conjugation by introducing the butadiene units. As discussed
46 above, with the increasing number of BTPA moieties, the
47 HOMO levels of as-synthesized HTMs slightly increases.
10 Molecular Geometry
11
12 One of the most important roles of BTPA moiety is breaking
13 the planarity of TPD. In this work, the molecular geometry
14 was applied to estimate the change of planarity. First, the
15 molecular geometries of TPD(BTPA)n (n=0,1,2,4) were
16 optimized with Gaussian 03 program.53 Then the cartesian
17 coordinates of all the atoms in each molecule were obtained
18 from above step. We put the molecule into a regular
48
As the HOMO levels of TPD(BTPA)n (n=0-4) are in the
49 range of -5.12 -5.40 eV which are very close to the HOMO
50 level of commonly-used hole injection material PEDOT:PSS (-
～
54
51 5.00 eV),
we fabricate double-layer OLEDs using
52 TPD(BTPA)n as hole injection/transport double-functional
53 layers, Alq3 as electron transport and emission layers, with
54 the device structure: ITO/TPD(BTPA)n (200 nm)/Alq3 (70
55 nm)/LiF (0.5 nm)/Al (120 nm). The current-voltage-
56 luminance (J-V-L) characteristic curves and LE-J of these
57 OLEDs are shown in Fig. 8, the parameters are listed in Table
19 hexahedron as shown in Fig. 7
20
.
The L, W, H and volume of regular hexahedron are listed
21 in Table 2. Apparently, the TPD molecule is much close to a
22 plate. With the increasing BTPA moieties, the molecules of
23 TPD(BTPA)n (n=1,2,4) are approach to cubes. This indicated
24 that, beside increasing the molecular volume, the BTPA
25 moieties can break the molecular planar effectively. This
26 change of molecular geometry greatly affect the thermal
27 stability of TPD(BTPA)n, which leads to the amorphous films
28 of TPD(BTPA)4 even after annealing at 110 oC for 48 h.
29
58
3.
30 Fig. 7 Molecular geometry of TPD(BTPA)n (n=0,1,2,4) in a
31 regular hexahedron with length, width and height of L, W
32 and H, respectively.
33
34 Table 2. Geometry Parameters of TPD(BTPA)n (n=0, 1, 2, 4)
La)
W.a) Ha)
Volumeb)
1440
L:W:Hc)
TPD
15
6
16
25
36
36
2.5:1.0:2.7
TPD(BTPA)1 14
TPD(BTPA)2 14
TPD(BTPA)4 38
16
21
25
5600
10584
34200
1.0:1.1:1.8
1.0:1.5:2.6
1.52:1.0:1.44
35 a) Estimated by the cartesian coordinates of all the atoms after
59
36 geometry optimized with Gaussian 03. b) Volume = L×W×H. c)
60 Fig. 8 OLED structures(a), J-V-L curves (b) and J-η curves(c)
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As shown in Fig. 8(b) and Table
Journal of Materials Chemistry C
1
2
3
4
5
3, the ture-on 47 (100nm)/TPD(BTPA)4 (200 nm)/Alq3 (70 nm)/LiF (0.5 nm)/Al
voltage(Von) of Device-TPD and Device-TPD(BTPA)1 are the 48 (120 nm)and Device-TPD(BTPA)4 (device structure:
same, despite the HOMO level of TPD(BTPA)1 (-5.21eV) is 49 ITO/TPD(BTPA)4 (200 nm)/Alq3 (70 nm)/LiF (0.5 nm)/Al (120
much closer to the work function of ITO (-4.80 eV) than TPD 50 nm)), respectively. The device structures, current-voltage-
(-5.40 eV). However, compared to Device-TPD, the Von 51 luminance (J-V-L) characteristic curves and current-efficiency
6
reduces 28% (Device-TPD(BTPA)2) and 38% (Device- 52 (J-η) curves are shown in Fig. 9, parameters are listed in
7
TPD(BTPA)4). This indicated the smaller difference between 53 Table 4
.
8
ITO work function and HOMO level of TPD(BTPA)n are 54
As reported, the PEDOT:PSS normally applied to
9
10
beneficial to hole injection.
55 improve hole injection because of its HOMO level closing to
56 the work function of ITO. However, the Device-PP-
57 TPD(BTPA)4 shows a higher Von (3.1 V) and a lower CEmax
58 (4.48 cd A-1) than Device-TPD(BTPA)4. From Fig. 9 (c), it can
59 be found that with the increasing current intensity, the
11 Table 3. Electroluminescence characteristics of OLEDs
12
a)
b)
d)
Device
Von
Lmax
CDb)
(mA cm-2)
272
CEc)
(cd A-1)
2.16
μh
(V)
5.0
(cd m-2)
(×10-5 cm V-1S
6
)
-10 efficiency of the Device-PP-TPD(BTPA)4 reduces obviously.
61 On the contrary, the Device- TPD(BTPA)4 remains stable.
TPD
8152
140
(17.0 V)
9590
TPD(BTPA)1
TPD(BTPA)2
TPD(BTPA)4
5.0
3.6
3.1
334
327
455
3.17
4.83
5.83
24
15
9
(13.9 V)
12281
(13.1 V)
14446
(15.2 V)
13 a) Turn-on voltage:the voltage at the brightness of 1 cd m-2;
b)
14
Maximum brightness and corresponding voltage in the
15 bracket;
16 c) Maximum current efficiency;
17 d) Measured in hole-only devices (structure: ITO/HTMs (~1
18 μm)/Al (120 nm) ). Hole mobilities are determined by time-
19 of-fight (TOF) method29, details see ESI Fig. S12
.
20
21
From the Table 3, compared with Device-TPD, the
22 maximum brightness (Lmax
)
increase 20% (Device-
23 TPD(BTPA)1), 50% (Device-TPD(BTPA)2) and 80% (Device-
24 TPD(BTPA)4); the maximum current efficiency (CEmax
)
25 increase 50%, 120% and 170% , respectively. The LUMO
26 levels of TPD(BTPA)n (n=0-4) are in the range of -2.10~-2.39
27 eV, which are higher than the LUMO level of Alq3 and it
28 means that the TPD(BTPA)n (n=0-4) layer can play the role of
29 electron blocking.
30
Hole mobilities of TPD(BTPA)n (n=0,1,2,4) decreased
62
31 with increasing number of BTPA moieties n. This mainly
32 result from the suppression of crystallization by BTPA
33 moieties. The electron mobility (
63 Fig. 9 OLED structures(a), J-V-L curves(b) and J-
64
65 Table 4. Electroluminscence characteristics of OLEDs
η curves(c)
μe) of Alq3 was reported to
34 be 1.1-2.3
~
×
10-5 cm V-1S-1. 29 The equilibrium of electron and
a)
b)
35 hole transport in device-TPD(BTPA)4 resulted higher CE than
36 other OLEDs.
Von
Lmax
CDc)
CEd)
Device
(V)
(cd m-2)
14446
(15.2)
12280
(16.1)
(mA cm-2)
(cd A-1)
37
The most interesting phenomenon is that the CE
e)
38 increases with higher operation current in the Device-
39 TPD(BTPA)1, Device-TPD(BTPA)2, and Device-TPD(BTPA)4.
40 This should result from the outstanding thermal stability of
41 the TPD(BTPA)n (n=2,4) films which can maintain amorphous
42 even at high operation current.
TPD(BTPA) 4
3.1
455
5.83
4.48
f)
PP- TPD(BTPA)4
3.8
348
66 a) Turn-on voltage:the voltage at the brightness of 1 cd m-2; b)
67 Maximum brightness; c) Current efficiency at the maximum
68 brightness; d) Maximum current efficiency;
43
To further investigate the hole injection/transport
44 properties of TPD(BTPA)4, we compare the performance of
45 OLEDs with and without PEDOT:PSS layer, which are
46 denoted as Device-PP-TPD(BTPA)4 (structure: ITO/PEDOT:PSS
6 | J. Mater.Chem.C, 20XX, XX, X-X
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Journal of Materials Chemistry C PAPER
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4
5
6
7
8
9
It has been reported that during spin-coating the acidity 58 Conclusions
of PEDOT:PSS colloidal solution can corrode ITO to dissolve 59
some In3+. The In3+ will make the PEDOT:PSS interface 60 In this work, the novel HTMs TPD(BTPA)n (n=1,2,4) have been
unstable
61 designed and synthesized. The BTPA moieties are introduced
and obstruct the hole injection, which will diffuse to emitting 62 into TPD to improve thermal stability and suppress
layer to form a quenching center when devices operating at 63 crystallization in spincoated films. The thermal stability of
high current. This should be the reason for decrease of CEmax 64 TPD(BTPA)n (n=0,1,2,4) have been investigated by DSC,
at higher current density in Device-PP-TPD(BTPA)4.55-59
65 which shows gradual increases of Tg, Tc and Tm with the
In Device-TPD(BTPA)4, both the performance and the 66 increasing number of TBPA moieties. The spincoated film of
10 stability at high current density of OLEDs have been 67 TPD(BTPA)4 maintained amorphous form even with 110
℃,
11 improved. The improvement of OLED performance at high 68 48 h annealing which confirmed by XRD and ATR-FTIR. The
12 operation currents should be result from the combine effects 69 OLEDs with TPD(BTPA)n (n=0,1,2,4) as HIL and HTL showed
13 of outstanding thermal stability of hole transport layer and 70 an improvement of performance with increasing the number
14 balance of electron-hole transporting in device. This confirms 71 of TBPA moieties. The BTPA moieties can improve the
15 that the TPD(BTPA)n (n>1) can be applied as hole 72 stability of OLED operated with high current. The OLED with
16 injection/transport di-functional material to improve the 73 TPD(BTPA)4 showed the highest CEmax compared with
17 performance and stability of OLEDs, as well as simplify the 74 reported small molecular (Mw<6000) HTMs with identical
18 fabrication process of solution-processing OLEDs.
75 structure no mater solution processing or vacuum deposition.
19
The HTMs applied in solution-processing OLEDs with 76
6, 11, 31-44
20 identical structure are reported.
The molecular 77
21 formula (MF), molecular weight (MW), HOMO level, LUMO 78
22 level, energy gap of reported HTMs and corresponding 79
23 parameters of OLEDs are listed in Table 5. All the molecular 80
24 structure are listed in ESI Fig. S13
.
81
82
83
84
85
86
87
88
89
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
Table 5. Reported HTMs for solution-processing OLEDs with identical structure
HOMO LUMO
Eg
(eV)
Von
(V)
2.9
CE
J90%
(mA cm-2)
5
Componds
MF
MW
Device Structure
(eV)
-5.22
-5.4
(eV)
(cd A-1)
2.36
a)DT2 [6]
a)G3F2[34]
C432H300N22 5799
C242H212N14 3316
-2.60 2.62
-2.19 3.21
ITO/PEDOT:PSS/DT2/Alq3/LiF:Al
ITO/PEDOT:PSS/G3F2/Alq3/LiF:Al
2.6
5.63
123
ITO/TPD(BTPA)4/Alq3/LiF:Al
3.1
5.83
Unobservable
c)
a)TPD(BTPA)4
C123H112N6 1782
-5.12
-2.39 2.73
ITO/PEDOT:PSS/TPD(BTPA)4/Alq3/LiF:Al 3.8
90
4.48
5.11
5.07
3.2
168
103
160
155
120
\
a)G3C[36]
a)T4C[35]
C97H77N7
C96H85N5
C85H97N3
C54H45N3
C53H44N2
C36H28N2
1341
1309
1161
735
-5.37
-5.15
-5.21
-5.20
-5.80
-5.40
-2.01 3.36
-1.91 3.24
-2.39 2.82
-2.40 2.80
-2.12 3.68
-2.10 3.30
ITO/PEDOT:PSS/G3C/Alq3/LiF:Al
ITO/PEDOT:PSS/T4C/Alq3/LiF:Al
ITO/PEDOT:PSS/FC/Alq3/LiF:Al
ITO/TECEB/Alq3/Mg:Ag
3
91
92
93
94
95
96
97
98
99
2.5
5.6
3.5
6.5
2.0
a)FC[37]
b)TECEB [31]
b)FTPD7 [30]
a)TPD [11]
3.27
2.18
3.67
708
ITO/FTPD7/Alq3/LiF:Al
488
ITO/TPD/Alq3/LiF:Al
200
a) Spin-coated HTMs; b) Vacuum-deposited HTMs ; c) As-synthesized HTM in this work
As shown in Table 5, the Device-TPD(BTPA)4 shows the
44 highest CEmax which is far beyond other reported small
100 Acknowledgements
45 molecular HTMs (Mw< 6000) with identical structure no
46 matter fabircated by vacuum deposition or spin-coated. 6, 11,
101 We are grateful to the National High Technology Research
102 and Development Program of China (863 Program,
103 2015AA033402) and the Key Technologies R&D Rrogram of
104 Tianjin (13ZCZDGX00900) for financial support of this
105 research.
31-44
47
In addition, the efficiency roll-off is unobservable in
48 device
applying TPD(BTPA)n (n=1,2,4) as hole
49 transport/injection bi-functional material, which confirmed
50 the outstanding operation stability even at large operation
51 current. At present, only the 1,2,3,4,5,6-hexakis(9,9-dihexyl-
52 9H-fluoren-2-yl)benzene T3(CEmax =6.45 cd A-1) showed
53 slightly higher efficiency than Device-TPD(BTPA)4.40 However,
54 the MW of T3 is much larger than TPD(BTPA)4 (>4 times). Not
55 a mention the Von of Device-T3 (5.5 V) is much higher than
56 Device-TPD(BTPA)4 (3.1 V). 40
106 Notes and references
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TPD(BTPA)4 can improve the stability of OLED operated with high current
and can achieve the one of the highest CEmax (5.83 cd A-1) compared with
reported small molecular (Mw<6000) HTMs in OLEDs with identical structure
no mater solution processing or vacuum deposition.