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Polarity Tunable Host Materials and Their Applications in Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes Gaozhan Xie, Dongjun Chen, Xianglong Li, Xinyi Cai, Yunchuan Li, Dongcheng Chen, Kunkun Liu, Qian Zhang, Yong Cao, and Shi-Jian Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08738 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016
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ACS Applied Materials & Interfaces
Polarity Tunable Host Materials and Their Applications in Thermally Activated Delayed Fluorescence Organic LightEmitting Diodes Gaozhan Xie, Dongjun Chen, Xianglong Li, Xinyi Cai, Yunchuan Li, Dongcheng Chen, Kunkun Liu, Qian Zhang, Yong Cao, Shi-Jian Su* State Key Laboratory of Luminescent Materials and Devices and Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, China ABSTRACT: A series of polarity tunable host materials were developed based on oligocarbazoles and diphenylphosphine oxide, and their polarities can be tuned through increasing distance of acceptor and donor units. Density functional theory calculations were employed and photoluminescence spectra in different polar solvents were measured to illustrate different polarities of these host materials. As CZPO has relatively stronger polarity, electroluminescence (EL) spectrum of solution-processed device based on 6 wt% PXZDSO2: CZPO is 7 nm red-shifted than other host materials based devices. Besides, a comparable impressive external quantum efficiency (EQE) value of 18.7% is achieved for evaporation-processed yellow device consisting of FCZBn, which is superior to that of the device based on CBP (4,4’-dicarbazolyl-1,1’-biphenyl) (17.0%), and its efficiency roll-off is also obviously reduced, giving an EQE value as high as 16.3% at the luminance of 1000 cd/m2. In addition, from CZPO to FCZBn, as the polarities of host materials decrease, EL spectra of solution-processed devices based on DMAC-DPS emitter blue-shift constantly from 496 nm to 470 nm. The current work gives a constructive approach to control EL spectra of organic light-emitting diodes with a fixed thermally activated delayed fluorescence emitter by tuning the polarities of host materials. KEYWORDS: oligocarbazoles, polarity tunable host materials, thermally activated delayed fluorescence, organic light emitting diodes, spectra shift INTRODUCTION Thermally activated delayed fluorescence (TADF) materials have recently drawn extensive attention in the field of organic light-emitting diodes (OLEDs) as they can
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realize 100% internal quantum efficiency through efficient reverse intersystem crossing (RISC) of triplet excitons without using any noble and nonrenewable metals.1-3 To date, plenty of TADF emitters have been developed and applied in evaporation- or solutionprocessed devices successfully, with maximum external quantum efficiencies (EQEs) comparable to phosphorescent OLEDs.4-10 For most TADF emitters, in principle, strong electron donor (D) and electron acceptor (A) units are introduced to spatially separate their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) to obtain tiny energy gap (ΔEST) between the lowest singlet excited state (S1) and triplet excited state (T1) since small enough ΔEST plays a determined role in RISC process.11 However, donor-acceptor (D-A) molecules with strong intramolecular charge transfer (ICT) are inevitably sensitive to neighboring environments and would exhibit disparate photophysical properties in different surroundings.12-14 For instance, highly efficient TADF emitter ACRDSO2 (2-[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]thianthrene-9,9,10,10- tetraoxide) in planar pn-type OLEDs exhibits green (a peak wavelength of 538 nm) and yellow emission (a peak wavelength of 552 nm) respectively when doped in a p-type material TXFCz (9-(spiro[fluorene-9,9'-thioxanthen]-2-yl)-9Hcarbazole) and a n-type material TmPyTZ (2,4,6-tris(3-(pyridin-3-yl)phenyl)-1,3,5triazine).15 On the other hand, delayed components of TADF emitters currently possess long decay lifetime in microsecond or millisecond range16-18 so that excited excitons can be quenched easily due to triplet-triplet annihilation (TTA), singlet-triplet annihilation (STA), and triplet-polaron annihilation (TPA). Similar to phosphorescence materials, most TADF emitters should also be doped in host materials to restrain exciton quenching, thus their devices would attain high efficiency and reduced efficiency roll-off. Undoubtedly, physico-chemical properties of host materials would directly influence luminescent characteristics of guest materials in the host-guest system, and then determine device performances to a large extent. Besides, TADF emitters tend to be captious and only a few host materials have been well used in evaporation-processed TADF OLEDs,19-20 let alone solution-processed TADF OLEDs, which require more for host materials, such as good solubility.8 Therefore, more excellent host materials, especially for solution-processed host materials, need to be exploited at present. More
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than that, suitable and elaborate design criteria for host materials should be developed to further excavate potential performance of TADF emitters. For superior host materials, their T1 energy should be high enough to prevent energy back transfer from guest to host.21 As a consequence, T1 excitons generated in the dispersed emitters can be effectively confined and harvested in luminescence process. In addition, appropriate HOMO and LUMO levels of host materials are significant to ensure efficient charge injection from the adjacent layers, in that way balanced charge transport could facilitate hole and electron recombination in the emitting layer.22 Nowadays, much attention has been paid to the above-mentioned requirements for TADF host materials, while their polarities are almost neglected in molecular design, though they have obvious effect on luminescent properties of TADF emitters, especially on luminous intensity and emitting color. What is more, it might be possible to control emitting colors of OLEDs through tunning polarities of host materials by utilization of the same emitters. Herein, a series of polarity tunable host materials based on diphenylphosphine oxide and oligocarbazoles were designed and synthesized to study their effects on TADF emitters. Meanwhile, diphenylphosphine oxide with excellent electron injection and transport ability was introduced as electron acceptor unit,23 while oligocarbazoles formed by linkages through the 3(9') position were explored as donor units. 3(9') linkages of oligocarbazoles were designed to maintain high triplet energy levels of target compounds since nitrogen heteroatoms and large dihedral angles between adjacent carbazoles could break their conjugation to some degree.24 Also, twisted three-dimensional molecular conformations of oligocarbazoles would increase material solubility a lot, making them applicable to solution process.25-26 More importantly, it has been reported that HOMO electron density of D-A molecules containing oligocarbazoles tends to distribute on peripheral carbazoles, and their intermolecular interactions would become weaker as generation of oligocarbazoles increases due to longer dispersion distance between D and A units.27 Therefore, target D-A molecules, xCZPO (x = the quantities of carbazoles, see Scheme 1), might be of tunable polarities according to the quantities of carbazoles. Besides, non D-A molecule, FCZBn, which should have extremely weak CT characteristic, was also synthesized for comparison in this work. Low temperature phosphorescence spectra illustrate all the developed compounds have almost the same T1
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energy levels of around 3.04 eV, which can be attributed to the contribution of their counterpart oligocarbazoles, and PL spectra in different polar solutions reveal these materials possess various polarities. In addition, TADF emitter PXZDSO2 (2-(4-phenoxazinephenyl)thianthrene9,9',10,10'-tetraoxide) was employed as guest material and its luminescent properties doped in the designed host materials were also investigated.4 Since CT effects of PXZDSO2 are far stronger in comparison with those host materials, the guest-guest interactions have dominant influence on luminescent colors in solution-processed doped films, whereas the influence of host-guest interactions can be almost negligible, resulting in their similar PL spectra. Furthermore, the prompt decay lifetimes (around 30 ns) and delayed decay lifetimes (around 2 s) in solution-processed doped films are quite the same as well. As CZPO has relatively stronger polarity, the electroluminescence (EL) spectrum of solution-processed device based on 6 wt% PXZDSO2: CZPO is 7 nm redshifted than other devices with the same structure but using different host materials. Besides, a comparable impressive EQE value of 18.7% is achieved for the evaporationprocessed yellow OLED based on 6 wt% PXZDSO2: FCZBn emitting layer, and even at the luminance of 1000 cd/m2, its EQE maintains as high as 16.3%. Moreover, the polarity effects of host materials on EL peak become obvious when adopting weaker CT state emitter DMAC-DPS, giving EL peaks blue-shifted from 496 nm for CZPO to 470 nm for FCZBn. EXPERIMENTAL SECTION General. 1H and 13C NMR spectra were tested on a Bruker NMR spectrometer operating at 500 and 125 MHz, respectively, using tetramethylsilane (TMS) as the internal standard. Mass spectra (MS) were measured using a JEOL JMS-K9 mass spectrometer. Matrixassisted laser-desorption/ionization time-of-flight (MALDI-TOF) mass spectra were obtained from a Bruker BIFLEXIII TOF mass spectrometer. Thermogravimetric analyses (TGA) were performed on a Netzsch TG 209 under a N2 flow at a heating rate of 10 °C min–1. Differential scanning calorimetry (DSC) measurements were operated on a Netzsch DSC 200 F3 under a N2 flow at a heating rate of 10 °C min–1 and a cooling rate of 20 °C min–1. UV-vis absorption spectra were measured on a HP 8453
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spectrophotometer while PL spectra were gained using a Jobin-Yvon spectrofluorometer. Cyclic voltammetry (CV) was measured on a CHI600D electrochemical work station with a platinum working electrode and a platinum wire counter electrode against a Ag/AgCl reference electrode with a nitrogen-saturated anhydrous acetonitrile and dichloromethane
solution
(ratio,
4:1)
of
0.1
mol
L–1 tetrabutylammonium
hexafluorophosphate. Photoluminescence quantum yields (PLQYs) were measured on a HAMAMATSU absolute PL quantum yield spectrometer C11347-11 in doping films. Transient PL spectra measurement was performed on Quantaurus-Tau fluorescence lifetime measurement system (C11367-03, Hamamatsu Photonics Co.). Device Fabrication and Characterization. Indium tin oxide (ITO) coated glass with a sheet resistance of 10 Ω per square was used as the substrate. The substrates were carefully cleaned by acetone, isopropyl alcohol, detergent, deionized water, and isopropyl alcohol under ultrasonic bath and treated with O2 plasma for 20 min. For solution process, a poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer was spin-coated onto a pre-cleaned ITO glass substrate, then dried at 150 °C for 10 min. A PVK (polyvinylcarbazole, 8 mg/mL, in chlorobenzene) layer was subsequently spincoated onto a dried PEDOT:PSS layer, and annealed at 120 °C for 20 min. And the emitting layers (8.6 mg/mL, in chlorobenzene) were spin-coated onto the PVK or PEDOT:PSS layer, followed by drying at 100 °C for 10 min. Finally, an electrontransport layer TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene), a LiF layer, and an Al layer were deposited consecutively onto the spin-coated film by thermal evaporation under high vacuum (
