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red PHOLEDs
Journal Pre-proofs Chemically Doped Hole Transporting Materials with Low Cross-linking Temperature and High Mobility for Solution-Processed Green/Red PHOLEDs Jingxiang Wang, Hongli Liu, Sen Wu, Yi Jia, Hang Yu, Xianggao Li, Shirong Wang PII: DOI: Reference:
S1385-8947(19)32892-X https://doi.org/10.1016/j.cej.2019.123479 CEJ 123479
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
28 August 2019 27 October 2019 11 November 2019
Please cite this article as: J. Wang, H. Liu, S. Wu, Y. Jia, H. Yu, X. Li, S. Wang, Chemically Doped Hole Transporting Materials with Low Cross-linking Temperature and High Mobility for Solution-Processed Green/Red PHOLEDs, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123479
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© 2019 Published by Elsevier B.V.
Chemically Doped Hole Transporting Materials with Low Cross-linking Temperature and High Mobility for Solution-Processed Green/Red PHOLEDs
Jingxiang Wang a,b, Hongli Liu a,b,*, Sen Wu a,b, Yi Jia a,b, Hang Yu a,b, Xianggao Li a,b, Shirong Wang a,b,*.
a
Tianjin University, School of Chemical Engineering and Technology, Tianjin 300072, China
b
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
* Corresponding author. Email: [email protected] (Hongli Liu), Email: [email protected] (Shirong Wang). Abstract: Recently, developing insoluble cross-linkable functional layers plays a vital role for solution-processed organic light emitting diodes (OLEDs). Here, two vinyl-based cross-linkable hole transporting materials V-TPAVTPD and V-TPAVCBP are designed and synthesized. Additionally, cationic photoinitiator 4-octyloxydiphenyliodonium hexafluoroantimonate (OPPI) is first introduced to chemically induce vinyl-based photo cross-linking process, aiming at lowering cross-linking temperature and enhancing hole mobility. As a result, cross-linking can occur at expressly low temperature of 120 °C with >95% solvent resistance. Moreover, hole mobility is markedly enhanced with the value higher than 10-3 cm2 V-1 s-1. When applying hole transporting layers (HTLs) to solution-processed green and red phosphorescent OLEDs, devices exhibit excellent properties. 1
Particularly, the maximum current efficiency of 54.0 cd A−1 (green), 9.8 cd A−1 (red) and external quantum efficiency of 15.5% (green), 15.0% (red) are obtained when OPPI doped V-TPAVCBP serves as HTL. This low temperature feasible cross-linking process to prepare HTLs with preferable hole mobility promotes the development of OLEDs .
Keywords: cross-linkable; hole transporting material; low temperature; 4-octyloxydiphenyliodonium hexafluoroantimonate; solution-processed; organic light emitting diode. 1. Introduction Organic light emitting diodes (OLEDs) are widely used in display and illumination due to the advantages of self-luminescence, low energy consumption and large-area fabrication possibility[1, 2].In order to achieve high performance, most OLEDs devices adopt multilayer structures[3]. These functional layers are mainly prepared by vacuum evaporation or solution processing. Compared with vacuum evaporation, solution processing is more cost effective and exhibits promising potential in large-area fabrication, which benefits for the commercial popularization of OLEDs[4, 5]. However, during the solution processing, the prepared under-layer film can be dissolved in the solvents of following layers due to their similar solubility[6, 7]. To address this issue, two effective methods, orthogonal solvents[8, 9] and cross-linking[10], were proposed in the previous studies. Unfortunately, most of hole transporting materials (HTMs) and emitting materials (EMs) are dissolvable in solvents with similar polarity, which uglily impedes the application of orthogonal solvents[11]. What’s more, orthogonal solvent is also detrimental to the prepared films, attributing to the fact that films can be 2
partially washed away even if they cannot be dissolved in the orthogonal solvent[12]. Hence, cross-linking is the best way to resist the solvents. Not only that, the desirable thermal stability of cross-linked materials also plays an important role in improving the device stability and lifetime, especially in perovskite and organic solar cells[13-15]. Therefore, developing cross-linkable functional layers is in great desire for solution-processed semiconductor devices. Cross-linkable HTMs are usually constructed by introducing cross-linking groups to soluble molecules. Besides, insoluble and infusible films are obtained through ultraviolet (UV) irradiation or heating procedure [16]. Bromo[14, 15], oxetane[17], azide[18], cinnamate[19] and acrylate[20] are common photo cross-linking groups. Trifluorovinylether[21], organosilane[22] and vinyl are conventional thermally cross-linkable groups. To the best of our knowledge, among the cross-linkable groups, vinyl is most popular due to its merits of ease introduction and simple cross-linking procedure[23-27]. However, the majority of thermally cross-linking processes require temperature higher than 200 °C, which not only limits the choice of underlying functional layers, but also hinders its practical application in flexible plastic substrates[28]. In order to lower cross-linking temperature, much effort has been devoted to develop plenty of cross-linkable groups and cross-linking processes in the previous studies. Lee and coworkers reported that cross-linking could occur at 120 °C by forming strong hydrogen bonds between compounds, which employed uracil as the cross-linkable group[29]. Lin et al introduced a highly-efficient amine-epoxide reaction happening at 80 °C with amine based crosslinker 3
tris(2-aminoethyl)amine (TAA), and the obtained epoxy-functionalized compounds could result in highly cross-linking insoluble films[30]. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), an aliphatic crosslinker containing four thiol groups, was also employed to lower the high temperature to about 150 °C in a facile thiol-ene reaction with vinyl groups[31, 32]. Additionally, for the same cross-linking group, the reaction temperature largely depends on its flexibility. Based on this, long alkyl and ether chains are always selected to link the cross-linking groups. Ameen et al lowered the cross-linking temperature of HTMs from 207 to 157 °C through long butoxy chains[33, 34]. Although the cross-linking temperature is lowered in these themes, most of such cross-linkable groups or crosslinkers have relatively large molecular weight but no hole transporting property. Besides, long alkyl or ether chains can increase intermolecular distance, which will restrain the hole transporting ability of hole transporting layers (HTLs) in OLEDs application. To resolve this issue, doping p-type dopants such as organic salt lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), incorporating high mobility
HTMs
like
poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine)
(TFB)
or
introducing appropriate groups, such as methoxy and methyl, into molecules are commonly adopted[22, 35, 36]. In this regard, to obtain low cross-linking temperature without sacrificing hole mobility, two vinyl-based V-TPAVTPD and V-TPAVCBP were firstly designed and synthesized. Here, the intramolecular vinyl bridging groups played an important role in improving the flexibility of the whole molecules, which could lower the cross-linking temperature. Due to the small proportion of the 4
cross-linkable vinyl groups together with the large-area conjugation of the molecules, the hole mobility was hardly impaired. Furthermore, in this study, cationic photoinitiator 4-octyloxydiphenyliodonium hexafluoroantimonate (OPPI) was also employed to lower cross-linking temperature by serving as chemical dopant in V-TPAVTPD and V-TPAVCBP. OPPI produces a super acid HSbF6 during photoinitiation, which not only promotes photo cross-linking process but also acts as a p-type dopant. Hence, OPPI can also do a favor to the hole transport ability of HTLs. Accordingly, cross-linked films, possessing superior hole transporting property and solvent resistance (over 95%), were obtained at 120 °C after being initiated by 365 nm UV. When the resultant cross-linked HTLs were applied to Tris(4-carbazoyl-9-ylphenyl)amine
(TCTA)
and
Tris[2-(p-tolyl)pyridine-C2,N)]iridium(III)
(Ir(mppy)3) based solution-processed green phosphorescent OLEDs, a maximum current efficiency (CE) of 54.0 cd A−1 and an external quantum efficiency (EQE) of 15.5 % were achieved. For the red phosphorescent
OLEDs,
which
based
on
1,3-Di-9-carbazolylbenzene
(mCP)
and
Bis(1-phenyl-isoquinoline-C2,N) (acetylacetonato)iridium(III) (Ir(piq)2(acac)), an EQE of 15.0% was obtained. 2. Material and methods 2.1. Materials All the chemical materials used for synthesizing V-TPAVTPD and V-TPAVCBP were purchased from Aladdin or Energy Chemical Ltd. OPPI was purchased from Bide Pharmatech Ltd. The PEDOT:PSS(4083) solution, TCTA, Ir(mppy)3, mCP, Ir(piq)2(acac) and 2,2',2"-(1,3,5-Benzinetriyl) 5
-tris(1-phenyl-1-H-benzimidazole) (TPBi) were purchased from Xi’an Polymer Light Technology Corporation. V-P-TPD was prepared according to our previously reported method[23]. Cesium fluoride (CsF) was purchased from Energy Chemical Ltd. THF and toluene were freshly distilled using sodium /benzophenone before use. Other chemical products were used directly without further purification. 2.2. Devices fabrication For the fabrication of hole-only devices (HODs), glass substrates with patterned indium-tin oxide (ITO) were utilized. Before device fabrication, the ITO glass substrates were rinsed by distilled water, ethanol, acetone and isopropyl alcohol, then treated with oxygen plasma for 10 min. PEDOT: PSS film was produced by spin coating and dried at 130 °C for 30 min to form a hole injection layer (HIL). HTM was dissolved in THF (10 mg mL-1) and spin-coated onto the HIL. After coating the HTL, the substrate was heated or irradiated by ultraviolet light for cross-linking. After these processes, MoO3 (10 nm) and Al (100 nm) were sequentially deposited onto the substrate. For OLEDs fabrication, ITO substrates were cleaned by the above process. PEDOT: PSS was spin-coated onto ITO glass and dried at 130 °C for 30 min. The HTM was dissolved in toluene (8 mg mL-1) and spin-coated on the PEDOT: PSS film. V-P-TPD was dissolved in toluene (4 mg mL-1) and spin-coated on cross-linked HTLs. The thermal cross-linking process was carried out at 190 °C for 30 min in a glove box. The photo cross-linking process was carried out by irradiation with UV (365 nm) for 5 min and then heated at 120 °C for 30 min. The emitting layers (ELs) were obtained by mixing host and dopant materials in toluene, then spin-coated onto the HTLs. After these processes, TPBi (55 6
nm), CsF (1 nm) and Al (100 nm) were thermally deposited under a pressure of 7×10−4 Pa at a rate of 1, 0.2 and 10 Å s-1, respectively. 2.3. Instruments and measurements 1H
NMR spectra were measured using a Bruker Avance III HD 400 NMR spectrometer. Mass
spectra were obtained on a Bruker Autoflex tof/tofIII mass spectrometry. Elemental analyses were performed on Elementar vario Micro cube. Thermogravimetric analysis (TGA) and the differential scanning calorimetry (DSC) were performed with a TA Q500 thermogravimetric analyzer and a TA Q20 instrument respectively at a heating rate of 10 °C min-1 under N2 atmosphere. UV-visible (UV-vis) absorption spectra were measured on a Thermo Fisher Scientific GEN10S UV-Vis spectrometer. Ultraviolet photoelectron spectroscopy (UPS) were measured using a ThermoFisher ESCALAB 250Xi spectrometry. The highest occupied molecular orbital (HOMO) levels of cross-linked HTMs films were calculated with the equation of HOMO = -[hν - (Ecutoff - EHOMO)], where Ecutoff is the energy of secondary electron cutoff edge and EHOMO is the HOMO onset energy positions. Density functional theory (DFT) calculations were performed using the Gaussian 09 suite at the B3LYP/6-31G(d,p) level of theory. Fourier transform infrared (FT-IR) spectra were measured using Nicolet 380 spectrometer. The photoluminescence (PL) spectra were measured using a FLUOROMAX-4 fluorescence Spectrometer. Phosphorescence spectra of solutions in THF recorded at 77 K with 1 ms delay were measured using a F7000 Spectrometer. For the surface analysis, atomic force microscopy (AFM) NTEGRA Spectra was used. The electron paramagnetic resonance (EPR) spectrum was measured 7
using a Bruker A300 spectrometer. The layer thickness was measured with an Alpha-Step D300. The luminance of devices was measured with a calibrated silicon photodiode. The luminance was calibrated with a Konicaminolta CS-2000, with simultaneous acquisition of the electroluminescence (EL) spectra and CIE coordinates. The current-voltage characteristics were recorded using a Keithley 2400 SourceMeter. 3. Results and discussion 3.1. Synthesis of the HTMs The structures and synthesis routes of V-TPAVTPD and V-TPAVCBP were shown in Scheme 1. Intermediates 2 and 3 were prepared by Vilsmeier formylation reaction. Intermediates 4 and 5 were prepared by Wittig reaction. Intermediate 1, V-TPAVTPD and V-TPAVCBP were synthesized by Buchwald-Hartwig reaction. Specific synthetic procedures and structure identification were provided in the Supplementary data.
N
N
POCl3 DMF
N
2
O
BrPhPPh3Br KO(t-Bu) THF
N
N
Br Br
+
N
Br
N
H N
Pd2(dba)3 P(t-Bu)3 NaO(t-Bu)
N
Intramolecular vinyl bridging groups
Pd2(dba)3 P(t-Bu)3 NaO(t-Bu)
4
O
NH2
N
N
V-TPAVTPD
1
N
N
POCl3 DMF
O
BrPhPPh3Br KO(t-Bu) THF
N
N
3
Pd2(dba)3 P(t-Bu)3 NaO(t-Bu)
N
N
N
Intramolecular vinyl bridging groups
5
O
N
Br
Br
N
N
V-TPAVCBP
Scheme 1. Structures and synthesis routes of V-TPAVTPD and V-TPAVCBP.
8
3.2. Photophysical properties of the HTMs Photophysical properties of cross-linked V-TPAVTPD and V-TPAVCBP films were investigated by UV-vis absorption spectroscopy and UPS. As displayed in Fig. 1a, the maximum absorption peaks of V-TPAVTPD and V-TPAVCBP were observed at 398 and 369 nm, respectively, attributing to the n-π* transition of molecules. The optical band gap (Eg) of cross-linked V-TPAVTPD and V-TPAVCBP films determined by the band edge of the UV-vis spectra was respectively 2.73 and 2.85 eV. UPS spectra were carried out to determine the HOMO levels of cross-linked HTMs films, as revealed in Fig. 1b and 1c. Respectively for V-TPAVTPD and V-TPAVCBP, Ecutoff were observed at 16.67 and 16.77 eV, EHOMO were found at 0.70 and 0.92 eV. The energy of the ultraviolet excitation light was 21.22 eV, then the HOMO levels were calculated to be -5.25 and -5.37 eV, respectively, and lowest unoccupied molecular orbital (LUMO) energy levels were calculated to be -2.52 eV for both films according to HOMO levels and Eg. Phosphorescence spectra of V-TPAVTPD and V-TPAVCBP were supplied in Fig. S1. It could be observed that the highest-energy peaks of the phosphorescence spectra were 475 and 467 nm, accordingly, the triplet energy levels (ETs) were calculated to be 2.61 and 2.65 eV, respectively for V-TPAVTPD and V-TPAVCBP. The ETs obtained in this study are higher than the common green emitters (e.g. 2.40 eV for Ir(mppy)3), which is conducive to block electrons from ELs [37]. All referred data were summarized in Table 1.
9
Fig. 1. (a) UV-vis absorption spectra of V-TPAVTPD and V-TPAVCBP. (b) The whole UPS spectra, and (c) HOMO regions of the UPS spectra of V-TPAVTPD and V-TPAVCBP. Table 1 Summary of the physical properties of V-TPAVTPD and V-TPAVCBP. Compound
λmax(abs) (nm)
HOMO (eV)
LUMO (eV)
Eg (eV)
ET(eV)
V-TPAVTPD
398
-5.25
-2.52
2.73
2.61
V-TPAVCBP
369
-5.37
-2.52
2.85
2.65
To further understand the energy level properties of V-TPAVTPD and V-TPAVCBP, DFT calculations were performed to investigate the molecules’ geometry configuration and electronic cloud distribution. As depicted in Fig. 2, HOMO orbitals of both materials were mainly located on triphenylamine and intramolecular vinyl groups. The HOMO of V-TPAVTPD almost distributed uniformly in the whole molecule. On the contrary, the HOMO orbitals of V-TPAVCBP hardly located on carbazole and biphenyl groups, which was ascribed to the weak electron donating ability of carbazole groups and the non-coplanarity of biphenyl and carbazole. Besides, in terms of molecular
10
configuration, V-TPAVCBP exhibited poor planarity and conjugation compared to V-TPAVTPD. All the factors permitted V-TPAVCBP a wider band gap and deeper HOMO level compared with V-TPAVTPD.
Fig. 2. HOMO distribution of (a) V-TPAVTPD and (b) V-TPAVCBP. 3.3. Thermal properties of the HTMs DSC of the both HTMs were depicted in Fig. 3a and 3b. During first scan, V-TPAVTPD and V-TPAVCBP showed an exothermic process in the range of 150~190 °C with peaks at 165 and 155 °C, respectively, implying cross-linking process occurred in this range. No exothermic peak was observed during second scan, representing the fully accomplishment of cross-linking process. This phenomenon also indicated that the cross-linked HTMs are endowed with excellent thermal stability, which would be conducive to resist the high annealing temperature of emitting layer. The similar cross-linking 11
temperature of the two compounds arose from the same structures around cross-linkable vinyl groups. Intramolecular vinyl bridging groups afforded the molecular flexibility, which contributed to the relatively low cross-linking temperature. The thermal decomposition temperatures (Td, corresponding to 5% weight loss) of V-TPAVTPD and V-TPAVCBP were 517 and 537 °C, respectively, revealed by TGA in Fig. 3c and 3d. This further verified that cross-linked V-TPAVTPD and V-TPAVCBP possessed satisfactory thermal stability.
Fig. 3. DSC analysis of (a) V-TPAVTPD (insert: the second scan), (b) V-TPAVCBP (insert: the second scan); TGA thermograms of (c) V-TPAVTPD, (d) V-TPAVCBP. 3.4. Cross-linking temperature of HTLs characterized by solvent resistance 12
It is worth noting that both the cross-linking temperature of V-TPAVTPD and V-TPAVCBP are over 150 °C, which is beyond the tolerance of the plastic substrates used for flexible OLEDs. Consequently, pursuit for much lower cross-linking temperature is pressingly demanded. Inspired by the fact that vinyl groups can polymerize through photo-initiation, photoinitiator can be introduced to reduce the cross-linking temperature via a chemical photo cross-linking process. Here, cationic OPPI was selected as photoinitiator by virtue of its low toxicity and good solubility in low polarity solvents[38]. More importantly, OPPI can produce a super acid HSbF6 during photoinitiation, which promotes photo cross-linking process and then lowers the cross-linking temperature[39]. The molecular structure of OPPI and chemically photoinitiated polymerization mechanism were demonstrated in Fig. S2[40]. Cross-linking process is always proceeded with the aim of rendering the films with excellent solvent resistance (~100%), which is employed to characterize the cross-linking temperature. Herein, solvent resistance of undoped and OPPI-doped cross-linked films was characterized by UV-vis absorption before and after soaked in solvent (solvent: toluene). For undoped films, the cross-linking reaction was respectively carried out at 180 and 190 °C for 30 min. The films, doped with 3, 5, and 10 wt% OPPI, respectively, were irradiated by 365 nm UV for 5 min and heated at 120 °C for 30 min. Here, heating process ensured sufficient rotational diffusion of cross-linking groups, thereby increasing the degree of cross-linking[41]. Afterwards, cross-linked films treated with and without OPPI were immersed in toluene for 1 min, and their UV-vis absorption spectra before and after immersion were respectively exhibited in Fig. 4 and 5. 13
Fig. 4. UV-vis absorption spectra of cross-linked undoped V-TPAVTPD films heated at (a) 180 °C, (b) 190 °C and cross-linked undoped V-TPAVCBP films heated at (c) 180 °C, (d) 190 °C before and after toluene rinsing.
14
Fig. 5. UV-vis absorption spectra of cross-linked (a) V-TPAVTPD: 3 wt% OPPI film, (b) V-TPAVTPD: 5 wt% OPPI film, (c) V-TPAVTPD: 10 wt% OPPI film, (d) V-TPAVCBP: 3 wt% OPPI film, (e) V-TPAVCBP: 5 wt% OPPI film and (f) V-TPAVCBP: 10 wt% OPPI film heated at 120 °C before and after toluene rinsing. As revealed in Fig. 4, solvent resistance of undoped cross-linked films was 84%, 99% for V-TPAVTPD and 79%, 98% for V-TPAVCBP after heating at 180 and 190 °C, respectively. For OPPI doped films (Fig. 5), when heated at 120 °C, solvent resistance was achieved to 65%, 74%, 95% for V-TPAVTPD and 70%, 79%, 98% for V-TPAVCBP, respectively, corresponding to OPPI content at 3, 5, and 10 wt%. In addition, solvent resistance of 10 wt% OPPI doped films heated at 110 °C was also measured. As revealed in Fig. S3a and 3b, films could not resist the solvent dissolution effectively. 15
These results indicated that the cross-linking temperature could be vastly lowered from 190 to 120 °C as the films were doped with 10 wt% OPPI, where cross-linked films showed nearly 100% solvent resistance. Additionally, the HTL films also demonstrated perfect solvent resistance in chlorobenzene and o-dichlorobenzene, as revealed by Fig. S3c-f. Thus, 10 wt% OPPI and 120 °C were chosen for the stepwise research. To verify the occurrence of the cross-linking, FT-IR spectra, taking the obtained V-TPAVTPD as example, were measured before and after cross-linking procedure(Fig. S4). After thermal and photo cross-linking at 190 and 120 °C respectively, peaks at 990 and 905 cm-1 assigning to C–H out-of-plane bending vibration of terminal vinyl groups disappeared, which implies the complete cross-linking of films. 3.5. Hole transporting properties of the cross-linked HTLs The surface morphology of undoped and doped films on the ITO substrates was determined by AFM. As shown in Fig. S5, all the cross-linked films presented superior flatness with small root mean square (RMS). The RMS of the undoped films showed tiny differences before and after cross-linking, implying that the cross-linking groups were too small to affect the morphology of the films during the cross-linking process. In order to investigate the hole transporting properties of V-TPAVTPD and V-TPAVCBP, the hole mobility of undoped and doped cross-linked films was measured by space-charge-limited current method (SCLC). The HODs were prepared as follows[42-45]: 16
A: ITO/PEDOT: PSS (30 nm)/ V-TPAVTPD (300 nm)/MoO3 (10 nm)/Al (100 nm) B: ITO/PEDOT: PSS (30 nm)/ V-TPAVCBP (300 nm)/MoO3 (10 nm)/Al (100 nm) C: ITO/PEDOT: PSS (30 nm)/ V-TPAVTPD: OPPI (10 wt%, 300 nm)/MoO3 (10 nm)/Al (100 nm) D: ITO/PEDOT: PSS (30 nm)/ V-TPAVCBP: OPPI (10 wt%, 300 nm)/MoO3 (10 nm)/Al (100 nm) Fig. 6a was the J–V characteristics of fabricated HODs. The carrier mobility (μ) was calculated by the following equation (1) (2)[46]: 2
J
E 9 = μ0ε0εr exp(γ E) L 8
(1) μ(E)
= μ0exp(γ E)
(2)
Where μ0 is the zero-field mobility, 𝜀0 and 𝜀r are the vacuum dielectric constant and relative dielectric constant, respectively, L is the thickness of the organic layer, E is the electric field and 𝛾 is the Poole-Frenkel factor.
Fig. 6. (a) J-V characteristics, (b) fitting curves and (c) mobility curves for HODs. J-V characteristics of HODs, fitting curves and hole mobility were shown in Fig. 6. The zero-field hole mobility of undoped V-TPAVTPD and V-TPAVCBP were 1.1×10-4 and 5.5×10-5 cm2 V-1 s-1, 17
respectively, which were comparable to the small molecular HTMs used in the vacuum deposition process. This is because HTMs designed in this study are endowed with large π-conjugation area and little hole-blocking cross-linking groups. Furthermore, V-TPAVTPD exhibited higher hole mobility than that of V-TPAVCBP, corresponding to its better planarity and more uniform HOMO distribution. For 10% doped V-TPAVTPD and V-TPAVCBP, the hole mobilities were improved by two orders of magnitude and dramatically enhanced to 1.2×10-2 and 4.9×10-3 cm2 V-1 s-1, respectively. Other HODs, in which the organic layers are respectively doped with 3% and 5% OPPI, were also fabricated to evaluate their hole mobilities. As revealed in Fig. S6, hole mobility was in positive correlation with OPPI doping content. That is to say, the hole mobility of HTLs could be controlled by adjusting OPPI content. The super acid HSbF6 derived from OPPI after photoinitiation affords for the increase of film hole mobility. HSbF6, remained in the hole transporting layers after photo cross-linking processing, may promote triphenylamine forming triphenylamine cation radical through plundering an electron from N atom for its strong oxidizing property[47, 48]. Triphenylamine cation radicals are the fundamental unit for hole transporting, and thus the hole mobility of HTLs is enhanced. To verify the hypothesis, EPR spectrum of V-TPAVTPD film doped with 30 wt% OPPI was performed. As shown in Fig. 7a, the sample film had a paramagnetic signal with g factor of 2.0034, which was indicative of the existence of free radicals. UV-vis absorption spectra of V-TPAVTPD and V-TPAVCBP doped with 0 wt%, 10 wt%, 20 wt%, and 30 wt% OPPI were also carried out. As depicted in Fig. 7b and 7c, after doped with 18
OPPI, cross-linked V-TPAVTPD and V-TPAVCBP films showed new shoulder peaks around 650 and 610 nm, respectively, and the absorption peak intensity strengthened with the increase of OPPI, which might be assigned to oxidized V-TPAVTPD and V-TPAVCBP[49, 50]. The shorter wavelength obtained by V-TPAVCBP might be the consequence of poor planarity and small conjugation. Although an absorption band in long-wavelength area arose, it could hardly reduce the light transmission of HTLs and impact the device performance, which was ascribed to that its intensity was extremely low with 10% OPPI.
Fig. 7. (a) EPR spectrum of V-TPAVTPD: 30 wt% OPPI. UV-vis absorption spectra of (b) V-TPAVTPD: x wt% OPPI, (c) V-TPAVCBP: x wt% OPPI. (x=0, 10, 20, 30) 3.6. Green and red PHOLEDs characteristics using cross-linkable HTLs To our knowledge, photoinitiator usually leads to fluorescence quenching[28], but in this work, it can be firmly confined in the cross-linked HTLs which perform outstanding solvent resistance. Thus, the initiator will not enter the ELs and ruin the device performance. To confirm this point, samples with structures of quartz/ with or without HIL (PEDOT:PSS)/ with or without thin HTLs/ EL (TCTA: Ir(mppy)3) were prepared and the PL spectra of these samples were tested. As revealed in Fig. S7, 19
sample without HTLs showed the lowest emission peak intensity, indicating excitons quench at the interface of PEDOT:PSS/EL. On the contrary, samples with HIL and various HTLs demonstrated higher PL intensity, indicating that HTLs could reduce exciton quenching and the photoinitiator led to no fluorescence quenching. Green phosphorescent OLEDs were fabricated to evaluate the performance of the cross-linked HTLs. HTLs and ELs were prepared by solution processing. The electron transporting layers (ETLs) and the cathode were prepared by vacuum evaporation process. The chemical structures of PEDOT: PSS, TCTA, Ir(mppy)3 and TPBi were shown in Fig. S8. Devices were fabricated as follows: Device A: ITO/PEDOT: PSS (30 nm) /TCTA: Ir(mppy)3 (10 wt%, 25 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device B: ITO/PEDOT: PSS (30 nm) / V-TPAVTPD (25 nm)/TCTA: Ir(mppy)3 (10 wt%, 25 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device C: ITO/PEDOT: PSS (30 nm) / V-TPAVCBP (25 nm)/TCTA: Ir(mppy)3 (10 wt%, 25 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device D: ITO/PEDOT: PSS (30 nm) / V-TPAVTPD: OPPI (10 wt%, 25 nm)/TCTA: Ir(mppy)3 (10 wt%, 25 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device E: ITO/PEDOT: PSS (30 nm) / V-TPAVCBP: OPPI (10 wt%, 25 nm)/TCTA: Ir(mppy)3 (10 wt%, 25 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm)
20
Among them, Device A without HTLs was served for comparison. Device B and C were fabricated with thermally cross-linked HTLs only, while Device D and E were prepared using photo cross-linking HTLs. The energy level diagrams of these materials were drafted in Fig. 8a. All the device data were summarized in Table 2.
Fig. 8. (a) Energy level diagram of devices. (b) Current density (J)−voltage (V)−luminance (L) curves, (c) EQE−L curves (insert: the photo of Device B applied at 10 V) and (d) current efficiency (CE)−L−power efficiency (PE) curves of device characteristics. Fig. 8b displayed the J–V–L characteristics of fabricated green PHOLEDs. As revealed, compared with device A, devices with HTLs were endowed with higher current density and lower turn-on voltage 21
(Von). This manifested that the existence of HTLs contributed to the reduction of hole injection barrier and improved the hole injection efficiency. In addition, the current density and luminance of these devices ordered as D > E > B > C at operating voltages higher than 6 V, in accordance with the hole mobility sequence. Notably, as the operating voltage was lower than 6 V, the current density of device B and C were comparable although V-TPAVTPD possessed higher hole mobility. This mainly arose from lower hole injection barrier between V-TPAVCBP and TCTA. EQE-L, CE-L and PE-L curves of the devices below 10000 cd m-2 were depicted in Fig. 8c and 8d. The maximum CE were all above 50 cd A-1 for V-TPAVCBP based device C and E. Device E showed the highest EQE, CE and PE of 15.5%, 54.0 cd A−1 and 48 lm W−1, which attributed to the more suitable HOMO level and superior injection ability of V-TPAVCBP: OPPI. Due to the larger hole injection barrier, V-TPAVTPD based device B and D showed lower CE and PE. Device D exhibited the lowest CE among the devices with HTLs, only a little better than device A, which might attribute to the excessive injection of holes caused by the strictly high hole mobility of V-TPAVTPD: OPPI. Significantly, efficiency-off of device E, with the value of 4.4 % at 1000 cd m-2, was lower than any other devices containing HTLs. This can be ascribed to the carrier balance in EL profiting from the desirable hole mobility and HOMO level of OPPI doped V-TPAVCBP. J–V–L and EQE-L curves including the maximum brightness were demonstrated in Fig. S9a and 9b. To evaluate the device stability, operational lifetime of the devices was measured under a constant driving current with an initial luminance of 500 cd m−2. As revealed in Fig. S10, the operation lifetimes 22
(T50) for device A-E were respectively 1.0, 1.4, 3.3, 1.7 and 3.4 h. Clearly, all new HTLs based devices were endowed with preferable lifetime compared with that of the reference device. This result testified that thermally stable cross-linkable V-TPAVTPD and V-TPAVCBP could improve the stability of devices. Device C and E based on V-TPAVCBP demonstrated longer lifetime, which could be attributed to the smaller current at 500 cd m−2. Less heat was generated when the devices worked, which was responsible for the enhanced stability. Table 2 Device performances of the solution processed green PHOLEDs.
Device
Vona (V)
Maximum Luminance (cd m-2)
DeviceA: No HTL
3.9
51738
43.6/25.2/12.5
43.0/20.2/12.3
(0.34, 0.60)
Device B: V-TPAVTPD
3.2
77247
49.6/39.0/14.2
46.4/24.0/13.3
(0.34, 0.60)
Device C: V-TPAVCBP
3.0
80349
50.8/38.6/14.6
46.1/23.3/13.2
(0.34, 0.60)
2.6
59478
44.0/40.3/12.6
41.1/26.7/11.8
(0.34, 0.60)
2.9
72868
54.0/48.0/15.5
51.6/30.9/14.8
(0.34, 0.60)
Device D: V-TPAVTPD: OPPI Device E: V-TPAVCBP: OPPI aV : on
CE / PE /EQE (cd A−1 /lm W−1 /%) maximum
At 1000 cd
m-2
CIEb (x, y)
measured at 1 cd m−2; b CIE: measured at 10 V.
Since LUMO of V-TPAVTPD and V-TPAVCBP is lower than that of the emitter Ir(mppy)3, electron-blocking properties of the target HTLs are poor, which may lead to low EQEs of PhOLEDs. 23
To further evaluate the performance of the cross-linked HTLs, solution processed red PHOLEDs based on Ir(piq)2(acac) (LUMO = -2.70 eV) were fabricated. The chemical structures of mCP and Ir(piq)2(acac) were offered in Fig. S8. Devices were fabricated as follows: Device F: ITO/PEDOT: PSS (30 nm) / V-P-TPD (10 nm)/mCP: Ir(piq)2(acac) (7 wt%, 45 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device G: ITO/PEDOT: PSS (30 nm) / V-TPAVTPD (25 nm)/ V-P-TPD (10 nm)/mCP: Ir(piq)2(acac) (7 wt%, 45 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device H: ITO/PEDOT: PSS (30 nm) / V-TPAVCBP (25 nm)/ V-P-TPD (10 nm)/mCP: Ir(piq)2(acac) (7 wt%, 45 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device I: ITO/PEDOT: PSS (30 nm) / V-TPAVTPD: OPPI (10 wt%, 25 nm)/ V-P-TPD (10 nm)/mCP: Ir(piq)2(acac) (7 wt%, 45 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device J: ITO/PEDOT: PSS (30 nm) / V-TPAVCBP: OPPI (10 wt%, 25 nm)/ V-P-TPD (10 nm)/mCP: Ir(piq)2(acac) (7 wt%, 45 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Among them, Device F was selected for comparison. Device (G, H) and (I, J) were respectively prepared using thermal and photo cross-linking HTLs. The energy level diagrams and devices’ performance were drafted in Fig. 9. Here, since the HOMO levels difference between HTLs and mCP was very large (0.65 and 0.53 eV), thin cross-linkable V-p-TPD films were added to reduce the hole injection barrier. The chemical structure, UPS and UV-Vis spectra of V-P-TPD were supplied in Fig. S11, All the device data were summarized in Table 3. 24
Fig. 9. (a) Energy level diagram of devices. (b) J−V−L curves, (c) EQE−L curves (insert: the photo of Device B applied at 10 V) and (d) CE−L−PE curves of device characteristics. As revealed in Fig. 9, because of the large hole injection barrier and thicker EL, the obtained red devices exhibited higher Von than that of green PHOLEDs. Luminance, efficiency and efficiency-off of devices G-J were superior than device F. Particularly, device J was endowed with the highest EQE (15.0%), CE (9.8 cd A−1) and PE (5.0 lm W−1), which attributed to the matched HOMO level and balanced carrier injection. In general, the narrow band gap of red light-emitting molecules can greatly enhance the non-radiative transition rate and results in low efficiency [51]. In this study, the highest EQE of 15.0% achieved by red PHOLEDs was even close to that of green PHOLEDs (15.5%), 25
indicating that V-TPAVTPD and V-TPAVCBP were more suitable for red devices. J–V–L and EQE-L curves including the maximum brightness were demonstrated in Fig. S9c and 9d. Table 3 Device performances of the solution processed red PHOLEDs. Device
Vona (V)
Maximum Luminance (cd m-2)
Device F: No HTL
5.7
9325
8.2/4.1/12.6
5.9/1.3/9.1
(0.68, 0.31)
Device G: V-TPAVTPD
4.6
15134
9.3/5.9/14.3
7.4/2.0/11.4
(0.68, 0.31)
Device H: V-TPAVCBP
5.2
12764
9.2/4.9/14.1
7.2/1.7/11.1
(0.68, 0.31)
5.1
11667
8.8/4.6/13.5
6.8/1.8/10.4
(0.68, 0.31)
5.2
13956
9.8/5.0/15.0
7.6/1.9/11.7
(0.68, 0.31)
Device I: V-TPAVTPD: OPPI Device J: V-TPAVCBP: OPPI aV : on
CE / PE /EQE (cd A−1 /lm W−1 /%) maximum
At 1000 cd
m-2
CIEb (x, y)
measured at 1 cd m−2; b CIE: measured at 10 V.
4. Conclusions In this study, two novel vinyl-based cross-linkable hole transporting materials, V-TPAVTPD and V-TPAVCBP, were designed and synthesized. In cross-linking stage, profiting from the introduction of cationic photoinitiator OPPI, the cross-linking temperature drastically dropped to 120 °C and the hole mobility was dramatically enhanced by two orders to 1.2×10-2 and 4.9×10-3 cm2 V-1 s-1 respectively for V-TPAVTPD and V-TPAVCBP. When served as HTLs in green and red phosphorescent OLEDs, the devices demonstrated excellent performance with higher current density and lower Von in contract to 26
device without HTLs. Especially, OLEDs based on V-TPAVCBP: OPPI exhibited maximum CE of 54.0 cd A−1 (green), 9.8 cd A−1 (red) and EQE of 15.5% (green), 15.0% (red). This work provided a simple strategy to develop hole transporting layers with low cross-linking temperature and high mobility, which is promising for solution-processing OLEDs. Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgements This work was supported by the National Key Research and Development Program of China (2016YFB0401303), the National Science Foundation for Young Scientists of China (No. 61804106) and the National Natural Science Foundation of China (No. 21676188). The calculation in this work was supported by high performance computing center of Tianjin University, China. Appendix A. Supplementary data Synthesis of V-TPAVTPD and V-TPAVCBP, Structure of OPPI and mechanisms of photoinitiation, AFM images, PL and Phos. spectra, FT-IR spectra, structures of materials used in OLEDs, NMR and Mass spectra are supplied. References
27
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· Two vinyl-based cross-linkable hole transporting materials are synthesized.
· Cross-linking temperature is markedly lowered by doping photoinitiator.
· Hole mobility is also enhanced by doping photoinitiator.
· The highest external quantum efficiencies are 15.5% (green) and 15.0% (red).
36
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
37
S1385-8947(19)32892-X https://doi.org/10.1016/j.cej.2019.123479 CEJ 123479
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
28 August 2019 27 October 2019 11 November 2019
Please cite this article as: J. Wang, H. Liu, S. Wu, Y. Jia, H. Yu, X. Li, S. Wang, Chemically Doped Hole Transporting Materials with Low Cross-linking Temperature and High Mobility for Solution-Processed Green/Red PHOLEDs, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123479
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© 2019 Published by Elsevier B.V.
Chemically Doped Hole Transporting Materials with Low Cross-linking Temperature and High Mobility for Solution-Processed Green/Red PHOLEDs
Jingxiang Wang a,b, Hongli Liu a,b,*, Sen Wu a,b, Yi Jia a,b, Hang Yu a,b, Xianggao Li a,b, Shirong Wang a,b,*.
a
Tianjin University, School of Chemical Engineering and Technology, Tianjin 300072, China
b
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
* Corresponding author. Email: [email protected] (Hongli Liu), Email: [email protected] (Shirong Wang). Abstract: Recently, developing insoluble cross-linkable functional layers plays a vital role for solution-processed organic light emitting diodes (OLEDs). Here, two vinyl-based cross-linkable hole transporting materials V-TPAVTPD and V-TPAVCBP are designed and synthesized. Additionally, cationic photoinitiator 4-octyloxydiphenyliodonium hexafluoroantimonate (OPPI) is first introduced to chemically induce vinyl-based photo cross-linking process, aiming at lowering cross-linking temperature and enhancing hole mobility. As a result, cross-linking can occur at expressly low temperature of 120 °C with >95% solvent resistance. Moreover, hole mobility is markedly enhanced with the value higher than 10-3 cm2 V-1 s-1. When applying hole transporting layers (HTLs) to solution-processed green and red phosphorescent OLEDs, devices exhibit excellent properties. 1
Particularly, the maximum current efficiency of 54.0 cd A−1 (green), 9.8 cd A−1 (red) and external quantum efficiency of 15.5% (green), 15.0% (red) are obtained when OPPI doped V-TPAVCBP serves as HTL. This low temperature feasible cross-linking process to prepare HTLs with preferable hole mobility promotes the development of OLEDs .
Keywords: cross-linkable; hole transporting material; low temperature; 4-octyloxydiphenyliodonium hexafluoroantimonate; solution-processed; organic light emitting diode. 1. Introduction Organic light emitting diodes (OLEDs) are widely used in display and illumination due to the advantages of self-luminescence, low energy consumption and large-area fabrication possibility[1, 2].In order to achieve high performance, most OLEDs devices adopt multilayer structures[3]. These functional layers are mainly prepared by vacuum evaporation or solution processing. Compared with vacuum evaporation, solution processing is more cost effective and exhibits promising potential in large-area fabrication, which benefits for the commercial popularization of OLEDs[4, 5]. However, during the solution processing, the prepared under-layer film can be dissolved in the solvents of following layers due to their similar solubility[6, 7]. To address this issue, two effective methods, orthogonal solvents[8, 9] and cross-linking[10], were proposed in the previous studies. Unfortunately, most of hole transporting materials (HTMs) and emitting materials (EMs) are dissolvable in solvents with similar polarity, which uglily impedes the application of orthogonal solvents[11]. What’s more, orthogonal solvent is also detrimental to the prepared films, attributing to the fact that films can be 2
partially washed away even if they cannot be dissolved in the orthogonal solvent[12]. Hence, cross-linking is the best way to resist the solvents. Not only that, the desirable thermal stability of cross-linked materials also plays an important role in improving the device stability and lifetime, especially in perovskite and organic solar cells[13-15]. Therefore, developing cross-linkable functional layers is in great desire for solution-processed semiconductor devices. Cross-linkable HTMs are usually constructed by introducing cross-linking groups to soluble molecules. Besides, insoluble and infusible films are obtained through ultraviolet (UV) irradiation or heating procedure [16]. Bromo[14, 15], oxetane[17], azide[18], cinnamate[19] and acrylate[20] are common photo cross-linking groups. Trifluorovinylether[21], organosilane[22] and vinyl are conventional thermally cross-linkable groups. To the best of our knowledge, among the cross-linkable groups, vinyl is most popular due to its merits of ease introduction and simple cross-linking procedure[23-27]. However, the majority of thermally cross-linking processes require temperature higher than 200 °C, which not only limits the choice of underlying functional layers, but also hinders its practical application in flexible plastic substrates[28]. In order to lower cross-linking temperature, much effort has been devoted to develop plenty of cross-linkable groups and cross-linking processes in the previous studies. Lee and coworkers reported that cross-linking could occur at 120 °C by forming strong hydrogen bonds between compounds, which employed uracil as the cross-linkable group[29]. Lin et al introduced a highly-efficient amine-epoxide reaction happening at 80 °C with amine based crosslinker 3
tris(2-aminoethyl)amine (TAA), and the obtained epoxy-functionalized compounds could result in highly cross-linking insoluble films[30]. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), an aliphatic crosslinker containing four thiol groups, was also employed to lower the high temperature to about 150 °C in a facile thiol-ene reaction with vinyl groups[31, 32]. Additionally, for the same cross-linking group, the reaction temperature largely depends on its flexibility. Based on this, long alkyl and ether chains are always selected to link the cross-linking groups. Ameen et al lowered the cross-linking temperature of HTMs from 207 to 157 °C through long butoxy chains[33, 34]. Although the cross-linking temperature is lowered in these themes, most of such cross-linkable groups or crosslinkers have relatively large molecular weight but no hole transporting property. Besides, long alkyl or ether chains can increase intermolecular distance, which will restrain the hole transporting ability of hole transporting layers (HTLs) in OLEDs application. To resolve this issue, doping p-type dopants such as organic salt lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), incorporating high mobility
HTMs
like
poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine)
(TFB)
or
introducing appropriate groups, such as methoxy and methyl, into molecules are commonly adopted[22, 35, 36]. In this regard, to obtain low cross-linking temperature without sacrificing hole mobility, two vinyl-based V-TPAVTPD and V-TPAVCBP were firstly designed and synthesized. Here, the intramolecular vinyl bridging groups played an important role in improving the flexibility of the whole molecules, which could lower the cross-linking temperature. Due to the small proportion of the 4
cross-linkable vinyl groups together with the large-area conjugation of the molecules, the hole mobility was hardly impaired. Furthermore, in this study, cationic photoinitiator 4-octyloxydiphenyliodonium hexafluoroantimonate (OPPI) was also employed to lower cross-linking temperature by serving as chemical dopant in V-TPAVTPD and V-TPAVCBP. OPPI produces a super acid HSbF6 during photoinitiation, which not only promotes photo cross-linking process but also acts as a p-type dopant. Hence, OPPI can also do a favor to the hole transport ability of HTLs. Accordingly, cross-linked films, possessing superior hole transporting property and solvent resistance (over 95%), were obtained at 120 °C after being initiated by 365 nm UV. When the resultant cross-linked HTLs were applied to Tris(4-carbazoyl-9-ylphenyl)amine
(TCTA)
and
Tris[2-(p-tolyl)pyridine-C2,N)]iridium(III)
(Ir(mppy)3) based solution-processed green phosphorescent OLEDs, a maximum current efficiency (CE) of 54.0 cd A−1 and an external quantum efficiency (EQE) of 15.5 % were achieved. For the red phosphorescent
OLEDs,
which
based
on
1,3-Di-9-carbazolylbenzene
(mCP)
and
Bis(1-phenyl-isoquinoline-C2,N) (acetylacetonato)iridium(III) (Ir(piq)2(acac)), an EQE of 15.0% was obtained. 2. Material and methods 2.1. Materials All the chemical materials used for synthesizing V-TPAVTPD and V-TPAVCBP were purchased from Aladdin or Energy Chemical Ltd. OPPI was purchased from Bide Pharmatech Ltd. The PEDOT:PSS(4083) solution, TCTA, Ir(mppy)3, mCP, Ir(piq)2(acac) and 2,2',2"-(1,3,5-Benzinetriyl) 5
-tris(1-phenyl-1-H-benzimidazole) (TPBi) were purchased from Xi’an Polymer Light Technology Corporation. V-P-TPD was prepared according to our previously reported method[23]. Cesium fluoride (CsF) was purchased from Energy Chemical Ltd. THF and toluene were freshly distilled using sodium /benzophenone before use. Other chemical products were used directly without further purification. 2.2. Devices fabrication For the fabrication of hole-only devices (HODs), glass substrates with patterned indium-tin oxide (ITO) were utilized. Before device fabrication, the ITO glass substrates were rinsed by distilled water, ethanol, acetone and isopropyl alcohol, then treated with oxygen plasma for 10 min. PEDOT: PSS film was produced by spin coating and dried at 130 °C for 30 min to form a hole injection layer (HIL). HTM was dissolved in THF (10 mg mL-1) and spin-coated onto the HIL. After coating the HTL, the substrate was heated or irradiated by ultraviolet light for cross-linking. After these processes, MoO3 (10 nm) and Al (100 nm) were sequentially deposited onto the substrate. For OLEDs fabrication, ITO substrates were cleaned by the above process. PEDOT: PSS was spin-coated onto ITO glass and dried at 130 °C for 30 min. The HTM was dissolved in toluene (8 mg mL-1) and spin-coated on the PEDOT: PSS film. V-P-TPD was dissolved in toluene (4 mg mL-1) and spin-coated on cross-linked HTLs. The thermal cross-linking process was carried out at 190 °C for 30 min in a glove box. The photo cross-linking process was carried out by irradiation with UV (365 nm) for 5 min and then heated at 120 °C for 30 min. The emitting layers (ELs) were obtained by mixing host and dopant materials in toluene, then spin-coated onto the HTLs. After these processes, TPBi (55 6
nm), CsF (1 nm) and Al (100 nm) were thermally deposited under a pressure of 7×10−4 Pa at a rate of 1, 0.2 and 10 Å s-1, respectively. 2.3. Instruments and measurements 1H
NMR spectra were measured using a Bruker Avance III HD 400 NMR spectrometer. Mass
spectra were obtained on a Bruker Autoflex tof/tofIII mass spectrometry. Elemental analyses were performed on Elementar vario Micro cube. Thermogravimetric analysis (TGA) and the differential scanning calorimetry (DSC) were performed with a TA Q500 thermogravimetric analyzer and a TA Q20 instrument respectively at a heating rate of 10 °C min-1 under N2 atmosphere. UV-visible (UV-vis) absorption spectra were measured on a Thermo Fisher Scientific GEN10S UV-Vis spectrometer. Ultraviolet photoelectron spectroscopy (UPS) were measured using a ThermoFisher ESCALAB 250Xi spectrometry. The highest occupied molecular orbital (HOMO) levels of cross-linked HTMs films were calculated with the equation of HOMO = -[hν - (Ecutoff - EHOMO)], where Ecutoff is the energy of secondary electron cutoff edge and EHOMO is the HOMO onset energy positions. Density functional theory (DFT) calculations were performed using the Gaussian 09 suite at the B3LYP/6-31G(d,p) level of theory. Fourier transform infrared (FT-IR) spectra were measured using Nicolet 380 spectrometer. The photoluminescence (PL) spectra were measured using a FLUOROMAX-4 fluorescence Spectrometer. Phosphorescence spectra of solutions in THF recorded at 77 K with 1 ms delay were measured using a F7000 Spectrometer. For the surface analysis, atomic force microscopy (AFM) NTEGRA Spectra was used. The electron paramagnetic resonance (EPR) spectrum was measured 7
using a Bruker A300 spectrometer. The layer thickness was measured with an Alpha-Step D300. The luminance of devices was measured with a calibrated silicon photodiode. The luminance was calibrated with a Konicaminolta CS-2000, with simultaneous acquisition of the electroluminescence (EL) spectra and CIE coordinates. The current-voltage characteristics were recorded using a Keithley 2400 SourceMeter. 3. Results and discussion 3.1. Synthesis of the HTMs The structures and synthesis routes of V-TPAVTPD and V-TPAVCBP were shown in Scheme 1. Intermediates 2 and 3 were prepared by Vilsmeier formylation reaction. Intermediates 4 and 5 were prepared by Wittig reaction. Intermediate 1, V-TPAVTPD and V-TPAVCBP were synthesized by Buchwald-Hartwig reaction. Specific synthetic procedures and structure identification were provided in the Supplementary data.
N
N
POCl3 DMF
N
2
O
BrPhPPh3Br KO(t-Bu) THF
N
N
Br Br
+
N
Br
N
H N
Pd2(dba)3 P(t-Bu)3 NaO(t-Bu)
N
Intramolecular vinyl bridging groups
Pd2(dba)3 P(t-Bu)3 NaO(t-Bu)
4
O
NH2
N
N
V-TPAVTPD
1
N
N
POCl3 DMF
O
BrPhPPh3Br KO(t-Bu) THF
N
N
3
Pd2(dba)3 P(t-Bu)3 NaO(t-Bu)
N
N
N
Intramolecular vinyl bridging groups
5
O
N
Br
Br
N
N
V-TPAVCBP
Scheme 1. Structures and synthesis routes of V-TPAVTPD and V-TPAVCBP.
8
3.2. Photophysical properties of the HTMs Photophysical properties of cross-linked V-TPAVTPD and V-TPAVCBP films were investigated by UV-vis absorption spectroscopy and UPS. As displayed in Fig. 1a, the maximum absorption peaks of V-TPAVTPD and V-TPAVCBP were observed at 398 and 369 nm, respectively, attributing to the n-π* transition of molecules. The optical band gap (Eg) of cross-linked V-TPAVTPD and V-TPAVCBP films determined by the band edge of the UV-vis spectra was respectively 2.73 and 2.85 eV. UPS spectra were carried out to determine the HOMO levels of cross-linked HTMs films, as revealed in Fig. 1b and 1c. Respectively for V-TPAVTPD and V-TPAVCBP, Ecutoff were observed at 16.67 and 16.77 eV, EHOMO were found at 0.70 and 0.92 eV. The energy of the ultraviolet excitation light was 21.22 eV, then the HOMO levels were calculated to be -5.25 and -5.37 eV, respectively, and lowest unoccupied molecular orbital (LUMO) energy levels were calculated to be -2.52 eV for both films according to HOMO levels and Eg. Phosphorescence spectra of V-TPAVTPD and V-TPAVCBP were supplied in Fig. S1. It could be observed that the highest-energy peaks of the phosphorescence spectra were 475 and 467 nm, accordingly, the triplet energy levels (ETs) were calculated to be 2.61 and 2.65 eV, respectively for V-TPAVTPD and V-TPAVCBP. The ETs obtained in this study are higher than the common green emitters (e.g. 2.40 eV for Ir(mppy)3), which is conducive to block electrons from ELs [37]. All referred data were summarized in Table 1.
9
Fig. 1. (a) UV-vis absorption spectra of V-TPAVTPD and V-TPAVCBP. (b) The whole UPS spectra, and (c) HOMO regions of the UPS spectra of V-TPAVTPD and V-TPAVCBP. Table 1 Summary of the physical properties of V-TPAVTPD and V-TPAVCBP. Compound
λmax(abs) (nm)
HOMO (eV)
LUMO (eV)
Eg (eV)
ET(eV)
V-TPAVTPD
398
-5.25
-2.52
2.73
2.61
V-TPAVCBP
369
-5.37
-2.52
2.85
2.65
To further understand the energy level properties of V-TPAVTPD and V-TPAVCBP, DFT calculations were performed to investigate the molecules’ geometry configuration and electronic cloud distribution. As depicted in Fig. 2, HOMO orbitals of both materials were mainly located on triphenylamine and intramolecular vinyl groups. The HOMO of V-TPAVTPD almost distributed uniformly in the whole molecule. On the contrary, the HOMO orbitals of V-TPAVCBP hardly located on carbazole and biphenyl groups, which was ascribed to the weak electron donating ability of carbazole groups and the non-coplanarity of biphenyl and carbazole. Besides, in terms of molecular
10
configuration, V-TPAVCBP exhibited poor planarity and conjugation compared to V-TPAVTPD. All the factors permitted V-TPAVCBP a wider band gap and deeper HOMO level compared with V-TPAVTPD.
Fig. 2. HOMO distribution of (a) V-TPAVTPD and (b) V-TPAVCBP. 3.3. Thermal properties of the HTMs DSC of the both HTMs were depicted in Fig. 3a and 3b. During first scan, V-TPAVTPD and V-TPAVCBP showed an exothermic process in the range of 150~190 °C with peaks at 165 and 155 °C, respectively, implying cross-linking process occurred in this range. No exothermic peak was observed during second scan, representing the fully accomplishment of cross-linking process. This phenomenon also indicated that the cross-linked HTMs are endowed with excellent thermal stability, which would be conducive to resist the high annealing temperature of emitting layer. The similar cross-linking 11
temperature of the two compounds arose from the same structures around cross-linkable vinyl groups. Intramolecular vinyl bridging groups afforded the molecular flexibility, which contributed to the relatively low cross-linking temperature. The thermal decomposition temperatures (Td, corresponding to 5% weight loss) of V-TPAVTPD and V-TPAVCBP were 517 and 537 °C, respectively, revealed by TGA in Fig. 3c and 3d. This further verified that cross-linked V-TPAVTPD and V-TPAVCBP possessed satisfactory thermal stability.
Fig. 3. DSC analysis of (a) V-TPAVTPD (insert: the second scan), (b) V-TPAVCBP (insert: the second scan); TGA thermograms of (c) V-TPAVTPD, (d) V-TPAVCBP. 3.4. Cross-linking temperature of HTLs characterized by solvent resistance 12
It is worth noting that both the cross-linking temperature of V-TPAVTPD and V-TPAVCBP are over 150 °C, which is beyond the tolerance of the plastic substrates used for flexible OLEDs. Consequently, pursuit for much lower cross-linking temperature is pressingly demanded. Inspired by the fact that vinyl groups can polymerize through photo-initiation, photoinitiator can be introduced to reduce the cross-linking temperature via a chemical photo cross-linking process. Here, cationic OPPI was selected as photoinitiator by virtue of its low toxicity and good solubility in low polarity solvents[38]. More importantly, OPPI can produce a super acid HSbF6 during photoinitiation, which promotes photo cross-linking process and then lowers the cross-linking temperature[39]. The molecular structure of OPPI and chemically photoinitiated polymerization mechanism were demonstrated in Fig. S2[40]. Cross-linking process is always proceeded with the aim of rendering the films with excellent solvent resistance (~100%), which is employed to characterize the cross-linking temperature. Herein, solvent resistance of undoped and OPPI-doped cross-linked films was characterized by UV-vis absorption before and after soaked in solvent (solvent: toluene). For undoped films, the cross-linking reaction was respectively carried out at 180 and 190 °C for 30 min. The films, doped with 3, 5, and 10 wt% OPPI, respectively, were irradiated by 365 nm UV for 5 min and heated at 120 °C for 30 min. Here, heating process ensured sufficient rotational diffusion of cross-linking groups, thereby increasing the degree of cross-linking[41]. Afterwards, cross-linked films treated with and without OPPI were immersed in toluene for 1 min, and their UV-vis absorption spectra before and after immersion were respectively exhibited in Fig. 4 and 5. 13
Fig. 4. UV-vis absorption spectra of cross-linked undoped V-TPAVTPD films heated at (a) 180 °C, (b) 190 °C and cross-linked undoped V-TPAVCBP films heated at (c) 180 °C, (d) 190 °C before and after toluene rinsing.
14
Fig. 5. UV-vis absorption spectra of cross-linked (a) V-TPAVTPD: 3 wt% OPPI film, (b) V-TPAVTPD: 5 wt% OPPI film, (c) V-TPAVTPD: 10 wt% OPPI film, (d) V-TPAVCBP: 3 wt% OPPI film, (e) V-TPAVCBP: 5 wt% OPPI film and (f) V-TPAVCBP: 10 wt% OPPI film heated at 120 °C before and after toluene rinsing. As revealed in Fig. 4, solvent resistance of undoped cross-linked films was 84%, 99% for V-TPAVTPD and 79%, 98% for V-TPAVCBP after heating at 180 and 190 °C, respectively. For OPPI doped films (Fig. 5), when heated at 120 °C, solvent resistance was achieved to 65%, 74%, 95% for V-TPAVTPD and 70%, 79%, 98% for V-TPAVCBP, respectively, corresponding to OPPI content at 3, 5, and 10 wt%. In addition, solvent resistance of 10 wt% OPPI doped films heated at 110 °C was also measured. As revealed in Fig. S3a and 3b, films could not resist the solvent dissolution effectively. 15
These results indicated that the cross-linking temperature could be vastly lowered from 190 to 120 °C as the films were doped with 10 wt% OPPI, where cross-linked films showed nearly 100% solvent resistance. Additionally, the HTL films also demonstrated perfect solvent resistance in chlorobenzene and o-dichlorobenzene, as revealed by Fig. S3c-f. Thus, 10 wt% OPPI and 120 °C were chosen for the stepwise research. To verify the occurrence of the cross-linking, FT-IR spectra, taking the obtained V-TPAVTPD as example, were measured before and after cross-linking procedure(Fig. S4). After thermal and photo cross-linking at 190 and 120 °C respectively, peaks at 990 and 905 cm-1 assigning to C–H out-of-plane bending vibration of terminal vinyl groups disappeared, which implies the complete cross-linking of films. 3.5. Hole transporting properties of the cross-linked HTLs The surface morphology of undoped and doped films on the ITO substrates was determined by AFM. As shown in Fig. S5, all the cross-linked films presented superior flatness with small root mean square (RMS). The RMS of the undoped films showed tiny differences before and after cross-linking, implying that the cross-linking groups were too small to affect the morphology of the films during the cross-linking process. In order to investigate the hole transporting properties of V-TPAVTPD and V-TPAVCBP, the hole mobility of undoped and doped cross-linked films was measured by space-charge-limited current method (SCLC). The HODs were prepared as follows[42-45]: 16
A: ITO/PEDOT: PSS (30 nm)/ V-TPAVTPD (300 nm)/MoO3 (10 nm)/Al (100 nm) B: ITO/PEDOT: PSS (30 nm)/ V-TPAVCBP (300 nm)/MoO3 (10 nm)/Al (100 nm) C: ITO/PEDOT: PSS (30 nm)/ V-TPAVTPD: OPPI (10 wt%, 300 nm)/MoO3 (10 nm)/Al (100 nm) D: ITO/PEDOT: PSS (30 nm)/ V-TPAVCBP: OPPI (10 wt%, 300 nm)/MoO3 (10 nm)/Al (100 nm) Fig. 6a was the J–V characteristics of fabricated HODs. The carrier mobility (μ) was calculated by the following equation (1) (2)[46]: 2
J
E 9 = μ0ε0εr exp(γ E) L 8
(1) μ(E)
= μ0exp(γ E)
(2)
Where μ0 is the zero-field mobility, 𝜀0 and 𝜀r are the vacuum dielectric constant and relative dielectric constant, respectively, L is the thickness of the organic layer, E is the electric field and 𝛾 is the Poole-Frenkel factor.
Fig. 6. (a) J-V characteristics, (b) fitting curves and (c) mobility curves for HODs. J-V characteristics of HODs, fitting curves and hole mobility were shown in Fig. 6. The zero-field hole mobility of undoped V-TPAVTPD and V-TPAVCBP were 1.1×10-4 and 5.5×10-5 cm2 V-1 s-1, 17
respectively, which were comparable to the small molecular HTMs used in the vacuum deposition process. This is because HTMs designed in this study are endowed with large π-conjugation area and little hole-blocking cross-linking groups. Furthermore, V-TPAVTPD exhibited higher hole mobility than that of V-TPAVCBP, corresponding to its better planarity and more uniform HOMO distribution. For 10% doped V-TPAVTPD and V-TPAVCBP, the hole mobilities were improved by two orders of magnitude and dramatically enhanced to 1.2×10-2 and 4.9×10-3 cm2 V-1 s-1, respectively. Other HODs, in which the organic layers are respectively doped with 3% and 5% OPPI, were also fabricated to evaluate their hole mobilities. As revealed in Fig. S6, hole mobility was in positive correlation with OPPI doping content. That is to say, the hole mobility of HTLs could be controlled by adjusting OPPI content. The super acid HSbF6 derived from OPPI after photoinitiation affords for the increase of film hole mobility. HSbF6, remained in the hole transporting layers after photo cross-linking processing, may promote triphenylamine forming triphenylamine cation radical through plundering an electron from N atom for its strong oxidizing property[47, 48]. Triphenylamine cation radicals are the fundamental unit for hole transporting, and thus the hole mobility of HTLs is enhanced. To verify the hypothesis, EPR spectrum of V-TPAVTPD film doped with 30 wt% OPPI was performed. As shown in Fig. 7a, the sample film had a paramagnetic signal with g factor of 2.0034, which was indicative of the existence of free radicals. UV-vis absorption spectra of V-TPAVTPD and V-TPAVCBP doped with 0 wt%, 10 wt%, 20 wt%, and 30 wt% OPPI were also carried out. As depicted in Fig. 7b and 7c, after doped with 18
OPPI, cross-linked V-TPAVTPD and V-TPAVCBP films showed new shoulder peaks around 650 and 610 nm, respectively, and the absorption peak intensity strengthened with the increase of OPPI, which might be assigned to oxidized V-TPAVTPD and V-TPAVCBP[49, 50]. The shorter wavelength obtained by V-TPAVCBP might be the consequence of poor planarity and small conjugation. Although an absorption band in long-wavelength area arose, it could hardly reduce the light transmission of HTLs and impact the device performance, which was ascribed to that its intensity was extremely low with 10% OPPI.
Fig. 7. (a) EPR spectrum of V-TPAVTPD: 30 wt% OPPI. UV-vis absorption spectra of (b) V-TPAVTPD: x wt% OPPI, (c) V-TPAVCBP: x wt% OPPI. (x=0, 10, 20, 30) 3.6. Green and red PHOLEDs characteristics using cross-linkable HTLs To our knowledge, photoinitiator usually leads to fluorescence quenching[28], but in this work, it can be firmly confined in the cross-linked HTLs which perform outstanding solvent resistance. Thus, the initiator will not enter the ELs and ruin the device performance. To confirm this point, samples with structures of quartz/ with or without HIL (PEDOT:PSS)/ with or without thin HTLs/ EL (TCTA: Ir(mppy)3) were prepared and the PL spectra of these samples were tested. As revealed in Fig. S7, 19
sample without HTLs showed the lowest emission peak intensity, indicating excitons quench at the interface of PEDOT:PSS/EL. On the contrary, samples with HIL and various HTLs demonstrated higher PL intensity, indicating that HTLs could reduce exciton quenching and the photoinitiator led to no fluorescence quenching. Green phosphorescent OLEDs were fabricated to evaluate the performance of the cross-linked HTLs. HTLs and ELs were prepared by solution processing. The electron transporting layers (ETLs) and the cathode were prepared by vacuum evaporation process. The chemical structures of PEDOT: PSS, TCTA, Ir(mppy)3 and TPBi were shown in Fig. S8. Devices were fabricated as follows: Device A: ITO/PEDOT: PSS (30 nm) /TCTA: Ir(mppy)3 (10 wt%, 25 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device B: ITO/PEDOT: PSS (30 nm) / V-TPAVTPD (25 nm)/TCTA: Ir(mppy)3 (10 wt%, 25 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device C: ITO/PEDOT: PSS (30 nm) / V-TPAVCBP (25 nm)/TCTA: Ir(mppy)3 (10 wt%, 25 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device D: ITO/PEDOT: PSS (30 nm) / V-TPAVTPD: OPPI (10 wt%, 25 nm)/TCTA: Ir(mppy)3 (10 wt%, 25 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device E: ITO/PEDOT: PSS (30 nm) / V-TPAVCBP: OPPI (10 wt%, 25 nm)/TCTA: Ir(mppy)3 (10 wt%, 25 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm)
20
Among them, Device A without HTLs was served for comparison. Device B and C were fabricated with thermally cross-linked HTLs only, while Device D and E were prepared using photo cross-linking HTLs. The energy level diagrams of these materials were drafted in Fig. 8a. All the device data were summarized in Table 2.
Fig. 8. (a) Energy level diagram of devices. (b) Current density (J)−voltage (V)−luminance (L) curves, (c) EQE−L curves (insert: the photo of Device B applied at 10 V) and (d) current efficiency (CE)−L−power efficiency (PE) curves of device characteristics. Fig. 8b displayed the J–V–L characteristics of fabricated green PHOLEDs. As revealed, compared with device A, devices with HTLs were endowed with higher current density and lower turn-on voltage 21
(Von). This manifested that the existence of HTLs contributed to the reduction of hole injection barrier and improved the hole injection efficiency. In addition, the current density and luminance of these devices ordered as D > E > B > C at operating voltages higher than 6 V, in accordance with the hole mobility sequence. Notably, as the operating voltage was lower than 6 V, the current density of device B and C were comparable although V-TPAVTPD possessed higher hole mobility. This mainly arose from lower hole injection barrier between V-TPAVCBP and TCTA. EQE-L, CE-L and PE-L curves of the devices below 10000 cd m-2 were depicted in Fig. 8c and 8d. The maximum CE were all above 50 cd A-1 for V-TPAVCBP based device C and E. Device E showed the highest EQE, CE and PE of 15.5%, 54.0 cd A−1 and 48 lm W−1, which attributed to the more suitable HOMO level and superior injection ability of V-TPAVCBP: OPPI. Due to the larger hole injection barrier, V-TPAVTPD based device B and D showed lower CE and PE. Device D exhibited the lowest CE among the devices with HTLs, only a little better than device A, which might attribute to the excessive injection of holes caused by the strictly high hole mobility of V-TPAVTPD: OPPI. Significantly, efficiency-off of device E, with the value of 4.4 % at 1000 cd m-2, was lower than any other devices containing HTLs. This can be ascribed to the carrier balance in EL profiting from the desirable hole mobility and HOMO level of OPPI doped V-TPAVCBP. J–V–L and EQE-L curves including the maximum brightness were demonstrated in Fig. S9a and 9b. To evaluate the device stability, operational lifetime of the devices was measured under a constant driving current with an initial luminance of 500 cd m−2. As revealed in Fig. S10, the operation lifetimes 22
(T50) for device A-E were respectively 1.0, 1.4, 3.3, 1.7 and 3.4 h. Clearly, all new HTLs based devices were endowed with preferable lifetime compared with that of the reference device. This result testified that thermally stable cross-linkable V-TPAVTPD and V-TPAVCBP could improve the stability of devices. Device C and E based on V-TPAVCBP demonstrated longer lifetime, which could be attributed to the smaller current at 500 cd m−2. Less heat was generated when the devices worked, which was responsible for the enhanced stability. Table 2 Device performances of the solution processed green PHOLEDs.
Device
Vona (V)
Maximum Luminance (cd m-2)
DeviceA: No HTL
3.9
51738
43.6/25.2/12.5
43.0/20.2/12.3
(0.34, 0.60)
Device B: V-TPAVTPD
3.2
77247
49.6/39.0/14.2
46.4/24.0/13.3
(0.34, 0.60)
Device C: V-TPAVCBP
3.0
80349
50.8/38.6/14.6
46.1/23.3/13.2
(0.34, 0.60)
2.6
59478
44.0/40.3/12.6
41.1/26.7/11.8
(0.34, 0.60)
2.9
72868
54.0/48.0/15.5
51.6/30.9/14.8
(0.34, 0.60)
Device D: V-TPAVTPD: OPPI Device E: V-TPAVCBP: OPPI aV : on
CE / PE /EQE (cd A−1 /lm W−1 /%) maximum
At 1000 cd
m-2
CIEb (x, y)
measured at 1 cd m−2; b CIE: measured at 10 V.
Since LUMO of V-TPAVTPD and V-TPAVCBP is lower than that of the emitter Ir(mppy)3, electron-blocking properties of the target HTLs are poor, which may lead to low EQEs of PhOLEDs. 23
To further evaluate the performance of the cross-linked HTLs, solution processed red PHOLEDs based on Ir(piq)2(acac) (LUMO = -2.70 eV) were fabricated. The chemical structures of mCP and Ir(piq)2(acac) were offered in Fig. S8. Devices were fabricated as follows: Device F: ITO/PEDOT: PSS (30 nm) / V-P-TPD (10 nm)/mCP: Ir(piq)2(acac) (7 wt%, 45 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device G: ITO/PEDOT: PSS (30 nm) / V-TPAVTPD (25 nm)/ V-P-TPD (10 nm)/mCP: Ir(piq)2(acac) (7 wt%, 45 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device H: ITO/PEDOT: PSS (30 nm) / V-TPAVCBP (25 nm)/ V-P-TPD (10 nm)/mCP: Ir(piq)2(acac) (7 wt%, 45 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device I: ITO/PEDOT: PSS (30 nm) / V-TPAVTPD: OPPI (10 wt%, 25 nm)/ V-P-TPD (10 nm)/mCP: Ir(piq)2(acac) (7 wt%, 45 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Device J: ITO/PEDOT: PSS (30 nm) / V-TPAVCBP: OPPI (10 wt%, 25 nm)/ V-P-TPD (10 nm)/mCP: Ir(piq)2(acac) (7 wt%, 45 nm)/TPBi (55 nm)/CsF (1 nm)/Al (100 nm) Among them, Device F was selected for comparison. Device (G, H) and (I, J) were respectively prepared using thermal and photo cross-linking HTLs. The energy level diagrams and devices’ performance were drafted in Fig. 9. Here, since the HOMO levels difference between HTLs and mCP was very large (0.65 and 0.53 eV), thin cross-linkable V-p-TPD films were added to reduce the hole injection barrier. The chemical structure, UPS and UV-Vis spectra of V-P-TPD were supplied in Fig. S11, All the device data were summarized in Table 3. 24
Fig. 9. (a) Energy level diagram of devices. (b) J−V−L curves, (c) EQE−L curves (insert: the photo of Device B applied at 10 V) and (d) CE−L−PE curves of device characteristics. As revealed in Fig. 9, because of the large hole injection barrier and thicker EL, the obtained red devices exhibited higher Von than that of green PHOLEDs. Luminance, efficiency and efficiency-off of devices G-J were superior than device F. Particularly, device J was endowed with the highest EQE (15.0%), CE (9.8 cd A−1) and PE (5.0 lm W−1), which attributed to the matched HOMO level and balanced carrier injection. In general, the narrow band gap of red light-emitting molecules can greatly enhance the non-radiative transition rate and results in low efficiency [51]. In this study, the highest EQE of 15.0% achieved by red PHOLEDs was even close to that of green PHOLEDs (15.5%), 25
indicating that V-TPAVTPD and V-TPAVCBP were more suitable for red devices. J–V–L and EQE-L curves including the maximum brightness were demonstrated in Fig. S9c and 9d. Table 3 Device performances of the solution processed red PHOLEDs. Device
Vona (V)
Maximum Luminance (cd m-2)
Device F: No HTL
5.7
9325
8.2/4.1/12.6
5.9/1.3/9.1
(0.68, 0.31)
Device G: V-TPAVTPD
4.6
15134
9.3/5.9/14.3
7.4/2.0/11.4
(0.68, 0.31)
Device H: V-TPAVCBP
5.2
12764
9.2/4.9/14.1
7.2/1.7/11.1
(0.68, 0.31)
5.1
11667
8.8/4.6/13.5
6.8/1.8/10.4
(0.68, 0.31)
5.2
13956
9.8/5.0/15.0
7.6/1.9/11.7
(0.68, 0.31)
Device I: V-TPAVTPD: OPPI Device J: V-TPAVCBP: OPPI aV : on
CE / PE /EQE (cd A−1 /lm W−1 /%) maximum
At 1000 cd
m-2
CIEb (x, y)
measured at 1 cd m−2; b CIE: measured at 10 V.
4. Conclusions In this study, two novel vinyl-based cross-linkable hole transporting materials, V-TPAVTPD and V-TPAVCBP, were designed and synthesized. In cross-linking stage, profiting from the introduction of cationic photoinitiator OPPI, the cross-linking temperature drastically dropped to 120 °C and the hole mobility was dramatically enhanced by two orders to 1.2×10-2 and 4.9×10-3 cm2 V-1 s-1 respectively for V-TPAVTPD and V-TPAVCBP. When served as HTLs in green and red phosphorescent OLEDs, the devices demonstrated excellent performance with higher current density and lower Von in contract to 26
device without HTLs. Especially, OLEDs based on V-TPAVCBP: OPPI exhibited maximum CE of 54.0 cd A−1 (green), 9.8 cd A−1 (red) and EQE of 15.5% (green), 15.0% (red). This work provided a simple strategy to develop hole transporting layers with low cross-linking temperature and high mobility, which is promising for solution-processing OLEDs. Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgements This work was supported by the National Key Research and Development Program of China (2016YFB0401303), the National Science Foundation for Young Scientists of China (No. 61804106) and the National Natural Science Foundation of China (No. 21676188). The calculation in this work was supported by high performance computing center of Tianjin University, China. Appendix A. Supplementary data Synthesis of V-TPAVTPD and V-TPAVCBP, Structure of OPPI and mechanisms of photoinitiation, AFM images, PL and Phos. spectra, FT-IR spectra, structures of materials used in OLEDs, NMR and Mass spectra are supplied. References
27
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· Two vinyl-based cross-linkable hole transporting materials are synthesized.
· Cross-linking temperature is markedly lowered by doping photoinitiator.
· Hole mobility is also enhanced by doping photoinitiator.
· The highest external quantum efficiencies are 15.5% (green) and 15.0% (red).
36
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
37