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Multifunctional materials for high performance double layer OLEDs: comparison of isomers with and without thermally activated delayed fluorescence Minghan Cai, Dongdong Zhang, Tianyu Huang, Xiaozeng Song, and Lian Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 7, 2017

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ACS Applied Materials & Interfaces

Multifunctional materials for high performance double layer OLEDs: comparison of isomers with and without thermally activated delayed fluorescence Minghan Cai, Dongdong Zhang, Tianyu Huang, Xiaozeng Song and Lian Duan*

Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China.

KEYWORDS: organic light emitting diode, thermally activated delayed fluorescence, simple structure, multifunctional material, high power efficiency, low efficiency roll-off

ABSTRACT: Organic light-emitting diodes (OLEDs) with simple structures are attracting much attention nowadays, though their performances are always inferior to the more complicated ones as multifunctional materials are rare. Here, we’ve designed and synthesized multifunctional isomers by combining electron-donating carbazole (Cz) and triphenylamine (TPA) units with

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electron-accepting triazine (Trz), namely, N-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-N-[4-(9phenyl-9H-carbazol-3-yl)phenyl]-[1,1'-Biphenyl]-4-amine

(CzTPA-p-Trz)

and

N-[3-(4,6-

diphenyl-1,3,5-triazin-2-yl)phenyl]-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-[1,1'-Biphenyl]-4amine (CzTPA-m-Trz). The use of multiple electron-donating groups renders them suitable highest occupied molecular orbitals (HOMOs) for hole injection and high mobilities for hole transport. Hole only devices with CzTPA-m-Trz or CzTPA-p-Trz as the hole injection layers (HILs) and hole transport layers (HTLs) show higher hole current than the widely used 1,4,5,8,9,11-hexaazatriphenylene

hexacarbonitrile

(HATCN)/4,4′-N,N′-bis[N-(1-naphthyl)-N-

phenylamino]biphenyl (NPB) system. Interestingly, CzTPA-p-Trz is a fluorescent material with high photoluminescence quantum yield (PLQY) while CzTPA-m-Trz shows weak thermally activated delayed fluorescence (TADF). As expected, CzTPA-p-Trz based undoped double layer green device achieved a higher external quantum efficiency (EQE) of 4.4% and a higher power efficiency (PE) of 11.8 lm/W. On the other hand, in double layer devices doped with an orange phosphorescent dopant, device based on TADF material CzTPA-m-Trz achieved higher peak EQE (23.5%) and PE (68.3 lm/W) than those of CzTPA-p-Trz (20.8% and 60.2 lm/W). Even at a high luminance of 5000 cd m-2, high EQE of 21.8% was retained for CzTPA-m-Trz based devices. These results are even comparable to the state-of-the-art phosphorescent devices based on the same dopant with more complicated structures. Above results indicate that well-designed multifunctional materials are promising for high performance OLEDs with simple structures.

1. INTRODUCTION


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In 1987, Tang fabricated first sandwiched structure electroluminescent device.1 Since then, organic light emitting diodes (OLEDs) started to become one of the most promising technologies in display area. Recently, new synthesized phosphorescent and thermally activated delayed fluorescence (TADF) materials have significantly promoted the external quantum efficiencies (EQEs) of OLEDs to above 30% because that they can harvest both singlet and triplet excitons.27

However, both phosphors and TADF materials should be dispersed in suitable hosts to avoid

triplet-triplet annihilation (TTA) and triplet-polaron annihilation caused by their long-lived triplets and aggregation induced quenching.8-9 Besides, to improve hole injection and to acquire balanced electrons and holes. OLEDs should also contain hole injection layers (HILs), hole transport layers (HTLs) and electron transport layers (ETLs). However, adopting multiple organic layers in an OLED will not only complicate the fabrication process but also increase the cost.10-11 Besides, interfacial problem should also be considered for guarantying ohmic contact and reducing energy gaps which will otherwise increase driving voltages and reduce power efficiencies.12-13 To solve these problems, simplified devices based on multifunctional bipolar materials have attracted much attention. In these devices, synthesized materials will not only be used for emission but also function as hosts, HTLs or ETLs. Among all simple structure OLEDs, green ones have been mostly investigated.14-22 For undoped devices, our group synthesized bipolar transporting benzanthracene derivative 7,12-bis-[4-(2,2diphenylvinyl)-phenyl] benzanthracene (BDPBan). Undoped single-layer green OLED based on BDPBan achieved a peak EQE of 3.2% and undoped double-layer green OLED achieved a slightly higher EQE (~3.5%).14 Huang compounded four dibenzothiophene S,S-dioxide derivatives and the maximum EQEs of single and double layer undoped green OLEDs using them were 3.1% and 4.9%, respectively.15 As for doped devices, Lu reported carbazole-based


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hosts for simplified single-layer green phosphorescent OLEDs (PHOLEDs). The maximum EQE could achieve 26.8%.11 Recently, Xue reported aromatic hydrocarbon macrocycles for highly efficient green PHOLEDs and the maximum efficiency was 22.8%.16 Further modification of these materials improved the efficiency to 24.8%.17 In addition to the green devices, orange devices also play a dominant role especially in fabricating white OLEDs23. However, as for simplified orange devices, seldom researches reported on them and their EQEs were usually lower than 10% which are far below requirement.24, 25 Besides, for most reported single and double layer OLEDs, they suffered from serious efficiency roll-off due to poor injection, imbalanced electron-hole transport and quenching effect. In 2013 our group firstly reported green PHOLEDs based on TADF hosts.26 Since then, TADF hosted PHOLEDs have been attracting much attention and orange devices based on them have achieved high efficiencies.26-34 Our group fabricated PHOLEDs using TADF material 2-phenyl4,6-bis(12-phenylindolo[2,3-a]carbazole-11-yl)-1,3,5-triazine (PBICT) as the host for orange phosphor iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2′)acetylacetonate (PO-01) and achieved the highest EQE of 24.5% with power efficiency (PE) of 64.5 lm/W.27 Kim’s group used horizontally oriented phosphorescent dyes in an exciplex-forming co-host and fabricated orange PHOLED.28 It achieved highest EQE of 32% and a PE of 68 lm/W. Usually, TADF materials are consisted by donor and acceptor parts and they render TADF materials bipolar properties. Which means, TADF materials may also be used for simplified device structures. However, as far as we know, few reports have studied on TADF material based simple structure OLEDs.29 In this work, we proposed that multifunctional materials toward simple structure devices can be designed by combining fragments with different functions. Triphenylamine (TPA) fragment was


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selected for hole injection. Due to shallow highest occupied molecular orbitals (HOMOs), materials based on TPA have been proven to show good hole injection properties, such as 1,1bis[(di-4-tolylamino)phenyl]cyclohexane

(TAPC)

and

4,4’,4’’-tris((3-methylphenyl)phenyl

amino)triphenylamine (m-MTDATA).35-36 Besides, previously synthesized multifunctional materials using TPA as building blocks have been widely used as HILs in fabricating simple structure OLEDs. In addition to that, carbazole (Cz) and triazine (Trz) fragments were selected as hole and electron transport fragments, respectively. They are classical p- and n-type units. Both fluorescent and TADF materials containing these two fragments have shown great potential as emitters and hosts for highly efficient OLEDs.37-40 By modulating the positions of three functional parts, we designed and synthesized N-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-N[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-[1,1'-Biphenyl]-4-amine (CzTPA-p-Trz) and N-[3-(4,6diphenyl-1,3,5-triazin-2-yl)phenyl]-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-[1,1'-Biphenyl]-4amine (CzTPA-m-Trz). The former one shows high photoluminescence quantum yield (PLQY) and the later one shows TADF property. Besides, they both exhibit extraordinary hole injection/hole transport and high hole/electron mobilities. Based on above characters, we fabricated undoped double layer green OLEDs. Due to high PLQY and absence of exciton quenching, CzTPA-p-Trz based undoped double layer device with a structure of indium tin oxide (ITO)/CzTPA-p-Trz/4,7-diphenyl-1,10-phenanthroline (BPhen)/LiF/Al achieved a high EQE of 4.4% and a high PE of 11.8 lm/W. This is among the highest PEs of undoped simple structure green fluorescent OLEDs. Furthermore, we fabricated orange devices using CzTPA-p-Trz and CzTPA-m-Trz as HILs, HTLs and hosts simultaneously and their structures were ITO/hosts/hosts:PO-01/BPhen/LiF/Al. TADF material CzTPA-m-Trz hosted orange device achieved higher maximum EQE of 23.5% and PE of 68.3 lm/W than these of CzTPA-m-Trz


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(20.8% and 60.2 lm/W). Even at a high luminance of 5000 cd m-2, its EQE maintained 21.8% and its PE maintained 49.3 lm/W. These performances are better than most recent reported PO01 doped PHOLEDs. The results manifest our design strategy and multifunctional fluorescent materials are more capable for undoped simple structure OLEDs and multifunctional TADF materials are more capable for doped simple structure OLEDs. 2. RESULTS AND DISCUSSION 2.1 Synthesis and Characterization Scheme 1 depicts the synthetic routes of CzTPA-p-Trz and CzTPA-m-Trz. All reagents were commercially available. The target compounds CzTPA-p-Trz and CzTPA-m-Trz were synthesized by palladium-catalyzed amination reaction between donor N-[4-(9-Phenyl-9Hcarbazol-3-yl)phenyl]-[1,1'-biphenyl]-4-amine with acceptor 2-(4-bromophenyl)-4,6-diphenyl1,3,5-triazine and 2-(3-bromophenyl)-4,6-diphenyl-1,3,5-triazine and their yields were above 90%.41 They were purified by column chromatography and sequentially by vacuum sublimation before characterization and device fabrication. The chemical structure of CzTPA-p-Trz and CzTPA-m-Trz were confirmed by 1H and

13

C NMR spectroscopy, mass spectrometry and

elemental analysis. The detailed synthetic methods and analytical data can be found in EXPERIMENTAL SECTION and Supporting Information.

2.2 Theoretical Calculations


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To fully understand the structural properties of CzTPA-p-Trz and CzTPA-m-Trz, the geometrical and electronic properties of them were simulated using density functional theory (DFT) at B3LYP/6-31G(d) level by Gaussian 09. As can be seen from Figure 1, for both hosts, their HOMOs mainly localize on their Cz and TPA fragments with lowest unoccupied molecular orbitals (LUMOs) mainly localize on their triazine fragments. As the meta-linkage has weaker conjugation than para-linkage, we can deduce that the donor/acceptor interaction in CzTPA-m-Trz is also weaker than CzTPA-p-Trz. This means that the donor and acceptor moieties of CzTPA-m-Trz can retain their own features (electron donating and withdrawing) with little influence from each other. Consequently, compared with CzTPA-p-Trz, CzTPA-m-Trz has higher HOMO and lower LUMO, which coordinately benefit to a smaller singlet energy. As a result, calculated HOMOs were -4.92 and -4.82 eV while calculated LUMOs were -1.67 and -1.77 eV for CzTPA-p-Trz and CzTPA-m-Trz, respectively (Table 1). Besides, according to time dependent density functional theory (TD-DFT) calculations, the singlet energies are 2.88 and 2.61 eV for CzTPA-p-Trz and CzTPA-m-Trz, respectively. Albeit different singlet energies, CzTPA-p-Trz and CzTPA-m-Trz exhibit similar triplet energies of 2.40 and 2.37 eV. This is because triplets of CzTPA-p-Trz and CzTPA-m-Trz both mainly localize on the same moieties (Figure S1). As a result, singlet−triplet splitting energies (∆Ests) are 0.48 eV and 0.24 eV for CzTPA-p-Trz and CzTPA-m-Trz, respectively.42

2.3 Thermal Properties Both CzTPA-p-Trz and CzTPA-m-Trz exhibit excellent thermal stability with sufficiently high decomposition temperatures (Td, corresponding to 5% weight loss in thermogravimetric analysis)


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of 437 °C for CzTPA-p-Trz and 457 °C for CzTPA-m-Trz, respectively. In addition, CzTPA-pTrz and CzTPA-m-Trz also show high glass transition temperatures (Tg) of 149 °C and 135 °C, respectively (Figure S2). Such high Td and Tg of CzTPA-p-Trz and CzTPA-m-Trz will be benefit for thermal evaporation and forming homogeneous amorphous films.43

2.4 Electrochemical Properties Cyclic voltammetry (CV) spectra of CzTPA-p-Trz and CzTPA-m-Trz were performed to evaluate their HOMO energy levels. As displayed in Figure S3, they both show quasi-reversible oxidation/reduction peaks. As the HOMOs of CzTPA-p-Trz and CzTPA-m-Trz both mainly localize on their Cz and TPA fragments, the difference between them is small. We can observe two oxidation peaks for both of them. As voltage increases, the first peak corresponds to TPA moiety and the second peak corresponds to Cz moiety. From the onsets of the oxidation diagrams, the HOMO energy levels of CzTPA-p-Trz and CzTPA-m-Trz were determined to be 5.25 and –5.21 eV, while their LUMO energy levels were calculated to be -2.43 and -2.51 eV from the onset of their absorption spectra, respectively. As expected, the experimental variation trend of HOMO and LUMO levels of CzTPA-p-Trz and CzTPA-m-Trz are in accord with the trend of the calculated values (Table 1).

2.5 Photophysical Properties The ultraviolet-visible (UV-vis) absorption, photoluminescence (PL) and phosphorescence spectra (measured at 77 K after a delay of 3 ms) of CzTPA-p-Trz and CzTPA-m-Trz in toluene


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(10-5 mol L-1) are shown in Figure 2 and summarized in Table 1. For UV-vis spectra, both CzTPA-p-Trz and CzTPA-m-Trz exhibit strong absorption peaking at 341 nm and 333 nm, respectively, which is in accordance with previous reported N-phenylcarbazole (NPC) derivatives.44 For CzTPA-p-Trz, a strong absorption peaking at 403 nm can also be observed. Whereas CzTPA-m-Trz only exhibits weak absorption at about 410 nm. These peaks can be attributed to the intramolecular charge-transfer (ICT) transition from the TPA and Cz fragments to TAZ fragment. As shown in Figure 1, owning to meta-linkage, the HOMO and LUMO overlap of CzTPA-m-Trz is much lower than that of CzTPA-p-Trz and the ICT of CzTPA-m-Trz is significantly suppressed. As a result, CzTPA-m-Trz exhibit much weaker ICT absorption intensity than CzTPA-p-Trz.45 Band gaps estimated from the absorption edge of the spectra are 2.82 eV for CzTPA-p-Trz and 2.70 eV for CzTPA-m-Trz. As for PL spectra, compared with CzTPA-p-Trz, the PL peak of CzTPA-m-Trz red shifts from 481 nm to 508 nm. Accordingly, singlet energy calculated from the onset of PL spectra is 2.82 eV for CzTPA-p-Trz and 2.70 eV for CzTPA-m-Trz, respectively. The red-shift of CzTPA-m-Trz is in accordance with the TDDFT results. We also record the UV-vis and PL spectra of CzTPA-p-Trz and CzTPA-m-Trz films (Figure S4). Compared with CzTPA-m-Trz, the much higher redshift of UV-vis and PL spectra for CzTPA-p-Trz film can be attributed to the partially formation of excimer (See also Figure S5). As the concentration increases, the spectra of CzTPA-p-Trz in toluene show slightly bathochromic shift but CzTPA-m-Trz does not. According to theoretical calculation, the dihedral angles between the phenyl rings connecting to Cz and the phenyl rings connecting to Trz in TPA increase from 65.3° for CzTPA-p-Trz to 70.1° for CzTPA-m-Trz. Thus, the ignorable emission bathochromic shift of CzTPA-m-Trz can be ascribed to more suppressed intermolecular aggregation and π–π interaction in solid state caused by relatively larger steric hindrance than


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CzTPA-p-Trz46-47. The triplet energies of CzTPA-p-Trz and CzTPA-m-Trz estimated by their first peaks of low temperature phosphorescence are 2.47 eV and 2.46 eV, respectively. They are similar to theoretical values. Accordingly, ∆Est of CzTPA-p-Trz calculated from the gap of singlet and triplet energy (0.35 eV) is much higher than that of CzTPA-m-Trz (0.24 eV). Thus, the two compounds were further subjected to transient PL spectroscopy to reveal the nature of their emission characteristics and their corresponding PLQYs are also marked (Figure 3 and Table 1). The transient PL decay curve of CzTPA-p-Trz in toluene merely consists a prompt component with a lifetime of 3.6 ns and its PLQY is 74.9%. After the air was bubbled out by nitrogen, the PLQY only slightly increased to 85.7%. In film its PLQY is 77.5%. The result show that CzTPA-p-Trz is a fluorescent material whose emission is less likely affected by oxygen. The large ∆Est of CzTPA-p-Trz can also confirm this. As for CzTPA-m-Trz, the transient PL decay consists of a prompt component with the lifetime of 17.0 ns and a distinct slow component with the lifetime of 0.74 µs and the PLQY is 14.5%. After removal of oxygen, its delayed lifetime increases to 1.51 µs and the PLQY significantly increases to 33.7% which is more than twice the value of original solution. For a neat CzTPA-m-Trz film, the PLQY can still maintain 32.6% with the prompt lifetime of 38.8 ns and the delayed lifetime of 1.36 µs. This confirms that CzTPA-m-Trz involves some degree of TADF characteristic. Calculated and experimental ∆Ests of CzTPA-m-Trz also manifest this. Besides, such short delayed lifetime of CzTPA-m-Trz will improve reverse internal system crossing (RISC) and then transfer the energy to guest in host-guest system. Thus, CzTPA-p-Trz with high PLQY and CzTPA-m-Trz with TADF provided us a good chance to compare non-TADF and TADF based undoped and doped simple structure devices.


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2.6 Electroluminescent (EL) Properties To confirm the hole injection and hole transport properties of CzTPA-p-Trz and CzTPA-m-Trz towards simple structure devices, we fabricated hole only devices. A widely used hole injection and hole transport system 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN)/ 4,4′N,N′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) was included for comparison, too. We used ITO as anode, HATCN, CzTPA-p-Trz and CzTPA-m-Trz as HILs and NPB, CzTPA-p-Trz and CzTPA-m-Trz as HTLs. Detailed structures and results are presented in Figure 4a. HATCN was proven to show 100% hole injection efficiency.53 When using HATCN as HILs, the current density of HATCN/NPB system based device is much higher than that of devices using CzTPAp-Trz and CzTPA-m-Trz as HTLs. This is due to relatively larger energy gaps of the LUMO level of HATCN (5.5 eV) and the HOMO levels of CzTPA-p-Trz (5.25 eV) and CzTPA-m-Trz (5.21 eV) than that of NPB (HOMO = 5.4 eV).54 Whereas, the hole only devices using CzTPAp-Trz and CzTPA-m-Trz as HILs and HTLs simultaneously exhibited even much higher current density than HATCN/NPB based device. This is because multifunctional CzTPA-p-Trz and CzTPA-m-Trz can eliminate the energy gaps of HILs and HTLs and interfacial problems. Thus, both CzTPA-p-Trz and CzTPA-m-Trz can be simultaneously used as good HILs and HTLs in devices, We further investigated the carrier mobilities of CzTPA-p-Trz and CzTPA-m-Trz by time-offlight (TOF) transient-photocurrent technique and the configurations are ITO/CzTPA-p-Trz or CzTPA-m-Trz (2000 nm)/Ag (150 nm). As shown in Figure 4b, both hole and electron mobilities of CzTPA-p-Trz and CzTPA-m-Trz can be well fitted with Poole-Frenkel function of = exp (E / / ). Recent reports showed that electron-transport dominant hosts are better than hole-transport hosts and can significantly improve the efficiencies and lifetime of devices.55-


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Both CzTPA-p-Trz and CzTPA-m-Trz exhibit slightly higher electron mobilities than their

corresponding hole mobilities. For CzTPA-p-Trz, the hole and electron mobilities are 2.14 × 10−4 cm2 V−1 s−1 and 1.18 × 10−4 cm2 V−1 s−1 at an electric field of 7 × 107 V m−1, respectively. Whereas CzTPA-m-Trz exhibits more balanced hole and electron mobilities of 1.84 × 10−4 cm2 V−1 s−1 and 1.48 × 10−4 cm2 V−1 s−1 (at 7 × 107 V m−1), respectively. As a result, due to distinguished hole injection/hole transport and equally high hole and electron mobilities, OLEDs using CzTPA-p-Trz and CzTPA-m-Trz as hosts, HILs and HTLs may realize lower driving voltages, higher efficiencies and more suppressed efficiency roll-off than multilayer OLEDs. Thus, we firstly fabricated undoped single organic layer OLEDs (SODs) and double organic layer OLEDs (DODs) based on CzTPA-p-Trz (SOD1 and DOD1) and CzTPA-m-Trz (SOD2 and DOD2). Their device structures and characteristics are presented in Figure S6 and Figure 5 and summarized in Table 2. As shown in Figure S6, EL spectra of SOD1 and SOD2 peaked at 508 nm and 518 nm, respectively. They were identical with PL spectra of CzTPA-p-Trz and CzTPAm-Trz films. Due to simple structures, the current densities of SOD1 and SOD2 were extremely high. However, the maximum EQEs of SOD1 and SOD2 were merely 0.012% and 0.014% with the maximum PEs of 0.018 and 0.021 lm/W for SOD1 and SOD2, respectively. This is probably due to poor electron injection caused by the lack of electron injection fragments for CzTPA-pTrz and CzTPA-m-Trz. For better electron injection and confinement of holes, we fabricated DOD1 and DOD2 (Figure 5). BPhen was selected as ETL and hole blocking layer (HBL). The electron mobility of BPhen is high and its deep HOMO level (≈ 6.0 eV) can effectively block the holes.48-50 Besides, the T1 energy level of BPhen (2.5 eV) is higher than these of CzTPA-p-Trz (2.47 eV) and CzTPA-m-Trz (2.46 eV), which will confine excitons in emitting layers. In some cases, exciplex formation will lower the efficiencies. To confirm the absence of exciplex


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formation between CzTPA-p-Trz with BPhen, we fabricated interface and mixed binary CzTPAp-Trz:BPhen (1:1, molar ratio) films. Comparison of them with CzTPA-p-Trz film and BPhen film is shown in Figure S7. We observed no new peaks in either interface or mixed binary films. UV-vis spectra and the PL spectra of two binary films are identical with the spectrum of CzTPAp-Trz film, too. The result confirms that CzTPA-p-Trz will not form exciplex with BPhen. As a result, the performance of DOD1 was superior to that of SOD1. The turn on voltage of DOD1 reduced to 3.10 V and it achieved a maximum EQE of 4.4%. Usually, under electrical excitation, the theoretical EQE of OLEDs can be estimated as follows: EQE = γηrηPLηout where γ is the recombination ratio of hole and electron; ηr is the excitation-production singlet-totriplet ratio; ηPL is PLQY and ηout is the outcoupling constant. Assuming that γ = 1, ηr = 25%, ηout =20%~30% and measured ηP of CzTPA-p-Trz film is 77.5%, calculated theoretical EQE is 3.9%~5.8%, which is in accordance with the experimental value. In addition to that, the double layer OLED had a very small EQE roll-off to 4.0% at 5000 cd m-2 and 3.6% at 10000 cd m-2. Besides, due to simple structure and low driving voltage the maximum PE of DOD1 could achieve 11.8 lm/W. As far as we know, this PE is the highest values of undoped simple structure green devices (Table S1). Similar to CzTPA-p-Trz, CzTPA-m-Trz will also not form exciplex with BPhen (Figure S8). However, DOD2 exhibited low maximum EQE and PE of 0.014% and 0.021 lm/W, respectively. This can be explained as follows. According to Figure 4, the current density of CzTPA-m-Trz based hole only device was much higher that of CzTPA-p-Trz based hole only device. Thus, holes will accumulated in DOD2. Besides, low PLQY and absence of dopant would accumulate CT excitons, too. As a result, serious triplet-polaron annihilation and TTA eventually leaded to extremely low efficiency of DOD2.


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Furthermore, we investigated the possibility of using these two compounds as hosts for simple structure OLEDs. A commonly used orange phosphor PO-01 was selected as the dopant due to its low triplet energy of 2.2 eV. At the very beginning, to confirm efficient energy transfer from CzTPA-p-Trz and CzTPA-m-Trz to PO-01, we fabricated doped films at concentration of 10 wt%. We observed no emission either from CzTPA-p-Trz or CzTPA-m-Trz, manifesting efficient host-dopant energy transfer. The absolute PLQYs of the doped films are as high as 80.2 and 82.4% for CzTPA-p-Trz and CzTPA-m-Trz (Figure S9a), accompanied by delayed lifetimes of 0.94 and 0.73 µs, respectively, which indicates the well-confined excitons on the dopants (Figure S9b). Device D1-D3 with different architectures were further designed to get fully understanding of the charge recombination area and exciton diffusion in devices. The detail structures and the corresponding characteristics of D1-D3 are presented in Figure 6 and summarized in Table 2. Previous studies showed that two different recombination and emission paths exist in host-guest system.51, 52 The first one is Langevin recombination. Holes and electrons will be injected to and recombine in hosts and produce excitons. Energy of host excitons will then be transferred to the dopants via Förster and Dexter mechanisms and emit light. The second one is trap-assisted recombination: Holes and electrons will be injected to the dopant and form excitons directly on the dopants. Given that the turn on voltages of D1-D3 were 2.7 eV and they all show similar current densities, Langevin recombination played a dominant role and trap-assisted recombination was negligible. Otherwise, the turn on voltage of D3 with extra 5 nm CzTPA-pTrz would be higher and its current density would be different from that of D1 and D3, whose emitting layers (EMLs) were directly adjacent to ETL. However, the maximum EQEs of D1 and D3 were 10.1% and 8.9% and the maximum EQE of D2 was 16.5%, which was much higher


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than those of D1 and D3. Similar to EQEs, the maximum PEs of D1 and D3 were 27.5 lm/W and 23.8 lm/W, respectively. Whereas, the maximum PE of D2 was as high as 46.7 lm/W. The remarkable performance of D2 can be explained as follows: Due to the energy gap between the LUMOs of CzTPA-p-Trz and BPhen, Langevin recombination mainly localized near the interface of EMLs and ETLs for D1-D3. In device D1, the energy of CzTPA-p-Trz would firstly be effectively transferred to PO-01 by both Förster and Dexter energy transfer. However, the excitons of PO-01 would then be transfered through EML by triplet diffusion and the triplets of PO-01 would eventually be quenched by ITO. This resulted in the poor performance of D1. As for D3, due to the inserted extra 5 nm CzTPA-p-Trz between EML and ETL, emission of CzTPA-p-Trz and PO-01 can simultaneously be observed in its EL spectrum. This confirms that Förster energy transfer from the singlets of CzTPA-p-Trz to PO-01 is ineffective. Besides, triplet of hosts had to diffuse through the extra CzTPA-p-Trz layer to EML by Dexter energy transfer. This would cause extra triplet loss. Simultaneously ineffective Förster and Dexter energy transfer eventually leaded to the low efficiency of D3. As a result, we employed the structure of D2 for further fabricating high performance double layer orange PHOLEDs. We optimized the thickness of different layers and the dopant concentrations. Structure of ITO/hosts (20 nm)/hosts:PO-01 (25nm, 10 wt%)/BPhen (40 nm)/LiF (1 nm)/Al (150 nm) were applied for orange PHOLEDs O1 and O2. The characters of all devices are presented in Figure 7 and summarized in Table 2. EL spectra of device O1 and device O2 showed typical emission of the phosphor PO-01 with single peaks at 565 nm, suggesting full energy transfer from CzTPA-p-Trz and CzTPA-m-Trz to PO-01. O2 using CzTPA-m-Trz as the host achieved a maximum EQE as high as 23.5%, higher than those found in device O1 hosted by CzTPA-p-Trz (maximum EQE of 20.8%). The better


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performance of O2 could be ascribe to the TADF property of CzTPA-m-Trz. The ∆Est of CzTPA-p-Trz is 0.35 eV and the RISC of CzTPA-p-Trz is not observed (Figure 3). Thus, triplet excitons of CzTPA-p-Trz could only transfer energy to PO-01 by Dexter energy transfer mechanism. Whereas, due to small ∆Est of 0.24 eV and short delayed lifetime of 1.36 µs, triplet excitons of CzTPA-m-Trz could be effectively converted to singlet excitons by RISC process and transfered the energy to PO-01 further. This provided another effective route to transfer the energy of hosts to guests. In addition to high efficiencies, both O1 and O2 exhibited low efficiency roll-off. The EQE of O1 maintained 20.5% at 5000 cd m-2 and 19.8% at 10000 cd m-2. Even when the brightness reached 25000 cd m-2, the EQE could still stay 18.0%. As for O2, its EQE reduced to 21.8% at 5000 cd m-2, 20.1% at 10000 cd m-2 and 16.7% at 25000 cd m-2. Besides, due to simple structures, good hole injection and balanced electron/hole transport, O1 and O2 also exhibited low turn on voltages of 2.67 V and 2.63 V with extremely low driving voltages of 3.63 V and 4.06 V at 5000 cd m-2 and 3.99 V and 4.52 V at 10000 cd m-2, respectively. Simultaneously high EQEs and low driving voltages resulted in high PEs of 60.2 lm/W and 68.3 lm/W for O1 and O2, respectively. Even at high brightness of 5000 cd m-2 they could still stay 51.5 lm/W and 49.3 lm/W. These values are better than most state-of-the-art PO01 based orange PHOLEDs (Table S2). 3. CONCLUSION In summary, we designed and synthesized multifunctional fluorescent material CzTPA-p-Trz and TADF material CzTPA-m-Trz by combining fragments with different functions. CzTPA-pTrz film exhibits high PLQY up to 77.5%. Albeit the low PLQY, CzTPA-m-Trz film exhibits TADF property with the short delayed lifetime of 1.36 µs. Both CzTPA-p-Trz and CzTPA-m-Trz can act as HILs, HTLs and hosts for PHOLEDs simultaneously. Fabricated undoped double layer


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green OLED based on CzTPA-p-Trz achieved high EQE up to 4.4% and PE of 11.8 lm/W. Whereas, for doped devices, CzTPA-m-Trz based double layer orange PHOLED achieved high EQE of 23.5% and PE of 68.3 lm/W. These excellent performances confirm that our design strategy for multifunctional materials is valid. Besides, multifunctional fluorescent materials are more capable for undoped simple structure OLEDs and multifunctional TADF materials are more capable for doped simple structure OLEDs. This sheds light on further development of multifunctional materials towards high performance simple structure OLEDs and will significantly broaden the application of simple structure OLEDs.

4 EXPERIMENTAL SECTION 4.1 General information The 1H NMR and

13

C NMR spectra were measured by a JEOLAL-600 MHz spectrometer at

ambient temperature with tetramethylsilane as the internal standard. The mass spectra were recorded on MALDI-TOF MS, Performance (Shimadzu, Japan). The elemental analyses were performed on a flash EA 1112 spectrometer. The thermogravimetric analysis (TGA) was performed on a STA 409PC thermogravimeter at a heating rate of 10 oC min-1 from ambient temperature to 600 oC under nitrogen atmosphere. The differential scanning calorimetry (DSC) measurements were performed on a DSC 2910 modulated calorimeter at a heating rate of 10 oC min-1 from ambient temperature to the temperature before decomposition under nitrogen atmosphere. The electrochemical measurements were performed with a Potentiostat/ Galvanostat Model 283 (Princeton Applied Research) electrochemical workstation by using Pt as working electrode, platinum wire as auxiliary electrode, and a Ag wire as reference electrode standardized


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against

ferrocene/ferrocenium.

The

oxidation

potentials

were

Page 18 of 37

measured

in

ultradry

dichloromethane solution containing 0.1 M n-Bu4NPF6 as supporting electrolyte at a scan rate of 100 mV s-1. The UV/Vis absorption spectra were recorded by an Agilent 8453 spectrophotometer. PLQYs were measured by PL quantum yield spectrometer C9920 (Hamamatsu). The quantum chemical calculations were performed by using the Gaussian 09 program package. The geometries of materials in the ground stat were optimized via density functional theory (DFT) calculations at the B3LYP/6-31G(d) level. Triplet states and singlet states of these materials were calculated by using time-dependent density TD-DFT calculations at B3LYP/6-31G(d) level. The HOMO and LUMO orbitals were visualized by using Gaussview.

4.2 Materials N-[4-(9-Phenyl-9H-carbazol-3-yl)phenyl]-[1,1'-biphenyl]-4-amine,

2-(4-bromophenyl)-4,6-

diphenyl-1,3,5-triazine and 2-(3-bromophenyl)-4,6-diphenyl-1,3,5-triazine were purchased from the

Suzhou

ge’ao

New

Material

bis(dibenzylideneacetone)palladium(0)

Co.

Ltd.

[Pd(dba)2]

and

Sodium

tert-butoxide

(NaOtBu),

2-(Dicyclohexylphosphino)-2',4',6'-

triisopropylbiphenyl (XPhos) were purchased from J&K Scientific Ltd. Besides the synthesized materials, all other reagents used in fabricating devices were purchased from Jilin Optical and Electronic Materials Co. Ltd. and Xi’an Polymer Light Technology Co. Ltd.

4.3 Synthesis of CzTPA-p-Trz


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2-(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine (1.17 g, 3 mmol), N-[4-(9-Phenyl-9H-carbazol-3yl)phenyl]-[1,1'-biphenyl]-4-amine (1.76 g, 3.6 mmol), NaOtBu (0.4 g, 4.2 mmol), Pd(dba)2 (0.035 g, 0.06 mmol) and XPhos (0.03 g, 0.06 mmol) were sequentially added to 150 ml round bottom flask. The mixture was heated to 100 oC and refluxed under nitrogen atmosphere for 20 hours. After the reaction completed, the mixture was extracted with 500 ml dichloromethane for two times. The organic layer was stored and evaporated under reduced pressure. Then, the crude product was isolated by silica gel column chromatography and further purified by vacuum sublimation to give pure product as white solid (2.18 g, yield 92%). 1H NMR (600 MHz, CDCl3, δ): 8.77 (d, J = 7.8Hz, 4H), 8.68 (d, J = 8.4 Hz, 2H), 8.38 (s, 1H), 8.21 (d, J = 7.8Hz, 1H), 7.71 (d, J = 8.4Hz, 2H), 7.68 (d, J = 8.4Hz, 1H), 7.66–7.54 (m, 14H), 7.52–7.42 (m, 6H), 7.38–7.29 (m, 8H);

13

C NMR (150 MHz, CDCl3, δ): 171.41, 171.15, 151.77, 146.23, 145.45, 141.44,

140.57, 140.40, 138.08, 137.74, 136.87, 136.53, 132.81, 132.47, 130.44, 130.05, 129.08, 129.03, 128.96, 128.71, 128.42, 128.23, 127.64, 127.24, 127.17, 126.94, 126.30, 126.16, 125.68, 125.33, 124.04, 123.56, 121.48, 120.50, 120.23, 118.63, 110.21, 110.07; MS (MALDI-TOF) m/z: [M]+ calcd for C57H39N5, 793.3205; found, 793.3506; Elemental analysis: calcd for C57H39N5: C, 86.23; H, 4.95; N, 8.82; found: C, 85.83; H, 5.05; N, 9.12.

4.4 Synthesis of CzTPA-m-Trz 2-(3-bromophenyl)-4,6-diphenyl-1,3,5-triazine (1.17 g, 3mmol), N-[4-(9-Phenyl-9H-carbazol-3yl)phenyl]-[1,1'-biphenyl]-4-amine (1.76 g, 3.6 mmol), NaOtBu (0.4 g, 4.2 mmol), Pd(dba)2 (0.035 g, 0.06 mmol) and XPhos (0.03 g, 0.06 mmol) were sequentially added to 150 ml round bottom flask. The mixture was heated to 100 oC and refluxed under nitrogen atmosphere for 20


19


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hours. After the reaction completed, the mixture was extracted with 500 ml dichloromethane for two times. The organic layer was stored and evaporated under reduced pressure. Then, the crude product was isolated by silica gel column chromatography and further purified by vacuum sublimation to give pure product as white solid (2.24 g, yield 94%). 1H NMR (600 MHz, CDCl3, δ): 8.72 (d, J = 7.5Hz, 4H), 8.61 (s, 1H), 8.45 (d, J = 7.5Hz, 1H), 8.40 (s, 1H), 8.20 (d, J = 7.5 Hz, 1H), 7.72 (d, J = 8.4Hz, 2H), 7.70 (d, J = 8.4Hz, 1H), 7.67–7.55 (m, 10H), 7.54–7.42 (m, 12H), 7.41–7.30 (m, 6H); 13C NMR (150 MHz, CDCl3, δ): 171.71, 171.64, 148.31, 147.00, 146.19, 141.44, 140.79, 140.32, 137.78, 137.14, 136.22, 135.80, 133.08, 132.66, 130.05, 129.79, 129.07, 128.93, 128.76, 128.33, 128.11, 127.94, 127.62, 127.18, 127.04, 126.87, 126.25, 125.33, 125.14, 124.47, 124.26, 124.03, 123.59, 123.38, 120.51, 120.19, 118.58, 110.18, 110.05; MS (MALDI-TOF) m/z: [M]+ calcd for C57H39N5, 793.3205; found, 793.2952; Elemental analysis: calcd for C57H39N5: C, 86.23; H, 4.95; N, 8.82; found: C, 86.07; H, 4.91; N, 9.02.

4.5 Device fabrication and measurement The organic layers were deposited consecutively on the precleaned ITO-coated glass substrates in a vacuum chamber with a pressure of 2×10-4 Torr. Next, the cathode was fabricated with thermal evaporation of a 1 nm LiF layer followed by a 150 nm aluminum layer. The deposition rates of all organic materials and aluminum were 1-2 Å s-1, while that of the LiF layer was 0.1 Å s-1. The electrical characteristics of the devices were measured with Keithley 2400 source meter. The EL spectra and luminance of the devices were obtained on a PR650 spectrometer. Characteristics of all the OLED devices were carried out in ambient laboratory conditions at room temperature.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Thermal properties and cyclic voltammetry spectra and ultraviolet-visible absorption and photoluminescence spectra and phosphorescence spectra of films and solutions (with different concentrations) and spin density calculations and electroluminescent properties of single organic layer devices and absence of exciplex formation proof and photophysical properties of doped films and 1H NMR spectra and

13

C NMR spectra and MALDI-TOF MS of CzTPA-p-Trz and

CzTPA-m-Trz are supplied as Supporting Information. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax: +86-10-62795137; Tel: +86-10-62782197 ORCID Minghan Cai: 0000-0003-0419-0799 Dongdong Zhang: 0000-0001-7095-2902 Lian Duan: 0000-0003-2750-0972 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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We would like to thank the National Key R&D Program of China (Grant Nos. 2016YFB0401003 and 2016YFB0400702), the National Key R&D Program of China (Grant No. 2015CB655002) and the CAS "Interdisciplinary Innovation Team" for ﬁnancial support. REFERENCES (1) Tang, C. W.; VanSlyke, S. A., Organic electroluminescent diodes. Appl Phys Lett 1987, 51 (12), 913-915. (2) Ma, Y.; Zhang, H.; Shen, J.; Che, C., Electroluminescence from triplet metal—ligand chargetransfer excited state of transition metal complexes. Synthetic Met 1998, 94 (3), 245-248. (3) Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R., Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 1998, 395 (6698), 151-154. (4) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C., Highly efficient organic lightemitting diodes from delayed fluorescence. Nature 2012, 492 (7428), 234-238. (5) Tao, Y.; Yuan, K.; Chen, T.; Xu, P.; Li, H.; Chen, R.; Zheng, C.; Zhang, L.; Huang, W., Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics. Advanced Materials 2014, 26 (47), 7931-7958. (6) Yang, Z.; Mao, Z.; Xie, Z.; Zhang, Y.; Liu, S.; Zhao, J.; Xu, J.; Chi, Z.; Aldred, M. P., Recent advances in organic thermally activated delayed fluorescence materials. Chem Soc Rev 2017, 46 (3), 915-1016. (7) Wong, M. Y.; Zysman-Colman, E., Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Advanced Materials 2017, DOI: 10.1002/adma.201605444


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(48) Small, C. E.; Tsang, S.-W.; Kido, J.; So, S. K.; So, F., Origin of Enhanced Hole Injection in Inverted Organic Devices with Electron Accepting Interlayer. Adv Funct Mater 2012, 22 (15), 3261-3266. (49) Shin, H.; Jung, H.; Kim, B.; Lee, J.; Moon, J.; Kim, J.; Park, J., Highly efficient emitters of ultra-deep-blue light made from chrysene chromophores. Journal of Materials Chemistry C 2016, 4 (17), 3833-3842. (50) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E., High-efficiency organic electrophosphorescent

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(56) Lee, J.-H.; Lee, S.; Yoo, S.-J.; Kim, K.-H.; Kim, J.-J., Langevin and Trap-Assisted Recombination in Phosphorescent Organic Light Emitting Diodes. Adv Funct Mater 2014, 24 (29), 4681-4688. (57) Higuchi, T.; Nakanotani, H.; Adachi, C., High-Efficiency White Organic Light-Emitting Diodes Based on a Blue Thermally Activated Delayed Fluorescent Emitter Combined with Green and Red Fluorescent Emitters. Advanced Materials 2015, 27 (12), 2019-2023.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Synthetic routes of CzTPA-p-Trz and CzTPA-m-Trz.

Figure 1. Calculated spatial distributions of HOMOs and LUMOs on optimized molecular geometries for CzTPA-p-Trz and CzTPA-m-Trz.


31


Figure 2. UV-vis absorption and PL spectra and phosphorescence spectra (measured at 77 K after a delay of 3 ms) of (a) CzTPA-p-Trz and (b) CzTPA-m-Trz in toluene (10-5 mol L-1).

(a)

(b) UV-Vis PL Phos

1.0

0.8 Intensity (a.u.)

Intensity (a.u.)

UV-Vis PL Phos

1.0

0.8 0.6 0.4 0.2

0.6 0.4 0.2

0.0

0.0 300

400

500

600

700

300

400

Wavelength (nm)

500

600

700

Wavelength (nm)

Figure 3. Transient PL decay curves of (a) CzTPA-p-Trz and (b) CzTPA-m-Trz in toluene (black), oxygen-free toluene (red) solutions and films (blue) at room temperature.

(a) 10

(b) 10

5

5

Sol (Air) Sol (N2) Film

4

10

PLQY=0.749 PLQY=0.857 PLQY=0.775

Sol (Air) Sol (N2) Film

4

10

3

10

Intensity

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

2

10

1

PLQY=0.145 PLQY=0.337 PLQY=0.326

3

10

2

10

1

10

10

0

0

10

10 0

100

200

300

400

500

0

Time (ns)

2000

4000

6000

8000

10000

Time (ns)


32

Page 33 of 37

Figure 4. (a) Current density versus voltage characteristics of hole only devices. (b) Charge mobilities of CzTPA-p-Trz and CzTPA-m-Trz under different electric fields.

(b) 1E-3

10000 X=HAT Y=CzTPA-p-Trz X=HAT Y=CzTPA-m-Trz X=HAT Y=NPB X=Y=CzTPA-p-Trz X=Y=CzTPA-m-Trz

8000

Mobility (cm2 V-1 S-1)

-2

Current Density (mA cm )

(a)

6000 Al 10 nm

HAT

100 nm

HTL(Y)

10 nm

HIL(X)

4000

2000

ITO

0 0

2

4

6

Hole mobility of CzTPA-p-Trz Electron mobility of CzTPA-p-Trz Hole mobility of CzTPA-m-Trz Electron mobility of CzTPA-m-Trz

1E-4

1E-5

8

7

7

6x10

Voltage (V)

7

7x10

8x10

Field intensity (V/m)

Figure 5. (a) EL spectrum (at 6V) (inset: device structure), (b) EQE and PE, (c) Current density (J)–voltage (V)–luminance (L) character of CzTPA-p-Trz based double organic layer OLED and (d) EL spectrum (at 6V) (inset: device structure), (e) EQE and PE, (f) J-V-L character of CzTPAm-Trz based double organic layer OLED.

(c)

5

10

0.4

ITO

3 8 2

6 4

0.2

1

0.0

0

2 400

500

600

700

0

2000

4000

6000

8000

3

10

2000 1500

2

10

1000 1

10

500 0

0 10000

0

10

0

1

2

2

Wavelength (nm)

(d)

3

4

5

6

7

2

Luminance (cd/m )

(e)

Luminance (cd/m )

(f)

0.04 1.0

-2

CzTPA-p-Trz

2500 Luminance (cd/m )

40 nm

12 10

EQE (%)

0.6

Bphen

Power Efficiency (lm/W)

4 40 nm

0.04 10000

CzTPA-m-Trz

0.4

0.03

0.02

0.02

ITO

0.01

0.01

0.2 0.0

0.00 400

500

600

Wavelength (nm)

700

0

100

200

300

0.00 400

2

Luminance (cd/m )

-2

10

8000

2

40 nm

0.03

2

Luminance (cd/m )

Bphen

Power efficiency (lm/W)

0.6

40 nm

EQE (%)

LiF/Al

0.8

-2

LiF/Al

0.8 Intensity (a.u.)

3000 4

14


(b) 1.0


(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6000 1

10

4000 2000

0

0

10

0

1

2

3

4

5

6

7

Voltage (V)


33


Figure 6. (a) Device structures, (b) EL spectra (at 6V), (c) J-V-L characters, (d) EQEs and (e) PEs of D1-D3.

Bphen

30 nm

CzTPA-p-Trz:PO-01

40 nm

Bphen

30 nm

CzTPA-p-Trz:PO-01

5 nm

CzTPA-p-Trz

ITO

Bphen

5 nm

CzTPA-p-Trz

30 nm

CzTPA-p-Trz:PO-01 ITO

ITO

(b)

(c)

10000 4

D1 D2 D3

1.0

D1 D2 D3

10

2

Luminance (cd/m )

0.8 0.6 0.4 0.2

8000

3

10

6000 2

10

4000 1

10

2000

0

0.0

0

10 400

500

600

700

0

1

Wavelength (nm)

2

3

4

5

6

7

Voltage (V)

(e)

20 D1 D2 D3

15

D1 D2 D3

50 Power Efficiency (lm/W)

(d)

40 nm

-2

40 nm

Intensity (a.u.)

LiF/Al

LiF/Al

LiF/Al


(a)

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

10

5

0

40 30 20 10 0

0

2000

4000

6000

8000 2

Luminance (cd/m )

10000

0

2000

4000

6000

8000

10000

2

Luminance (cd/m )


34

Page 35 of 37

Figure 7. (a) EQEs and PEs (inset: EL spectra at 6V) and (b) J-V-L characters of optimized orange PHOLEDs.

(b)

100

20 60 Intensity (a.u.)

15

5

40

20 400

500

600

0

1000

-2

5000

3

10

4000 3000

2

10

2000 1

10

1000

700

Wavelength (nm)

0

2

80

10

O1 O2

10

Luminance (cd/m )

25

6000 4

O1 O2

2000

3000

4000

0 5000


30


(a)

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

0

10

0

1

2

2

3

4

5

6

7

Voltage (V)

Luminance (cd/m )


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 37

Table 1. The summary of physical properties of CzTPA-p-Trz and CzTPA-m-Trz. Compound

Td/Tg [oC]

HOMO [eV]

LUMO [eV]

λAbs [nm]

λEm [nm]

S1 [eV]

T1 [eV]

∆EST [eV]

ηPL [%]

τp [ns]

τd [µs]

CzTPA-pTrz

437/149

-5.25a -4.92b

-2.43c -1.67b

341, 403d 350, 411e

481d 508e

2.82f 2.88h

2.47g 2.40h

0.35i 0.48h

74.9d, 85.7j 77.5e)

3.6d, 4.1j 4.1e

-

-5.21a -4.82b

-2.51c -1.77b

333d 339e

508d 518e

2.70f 2.61h

2.46g 2.37h

0.24i 0.24h

14.5d, 33.7j 32.6e

17.0d, 40.0j 38.8e

0.74d, 1.51j 1.36e

CzTPA-mTrz

457/135

a

Calculated according to the equation HOMO = 4.8 + onset voltage. b DFT calculation results. c Estimated according to the absorption bandgap and the HOMO energy level. d Measured in toluene solution (10-5 mol L-1). e Measured in evaporated films. f Estimated according to the onset of the PL spectra in toluene solution. g Estimated according to the onset of the phosphorescent spectra in toluene solution. h TD-DFT calculation results. i Calculated from measured S1 and T1. j Measured in toluene solution bubbled with N2 (10-5 mol L-1).

Table 2. The performances of all devices. Device

Structure

Voltagea

EQEb

PEb

λELc

[V]

[%]

[lm/W]

[nm]

CIEc)

SOD1

ITO/CzTPA-p-Trz/LiF/Al

3.78/-/-

0.012/-/-

0.018/-/-

508

SOD2

ITO/CzTPA-m-Trz/LiF/Al

3.71/-/-

0.014/-/-

0.021/-/-

518

(0.25,0.55) (0.30,0.56)

DOD1

ITO/CzTPA-p-Trz/BPhen/LiF/Al

3.10/5.49/6.32

4.4/4.0/3.6

11.8/7.5/6.1

514

(0.26,0.56)

DOD2

ITO/CzTPA-m-Trz/BPhen/LiF/Al

4.30/-/-

0.014/-/-

0.022/-/-

518

(0.29,0.56)

D1

ITO/CzTPA-p-Trz:PO-01/BPhen/LiF/Al

2.73/3.74/4.10

10.1/10.1/9.9

27.5/25.6/23.0

562

(0.49,0.50)

D2

ITO/CzTPA-p-Trz/CzTPA-p-Trz:PO-01/BPhen/LiF/Al

2.66/3.46/3.76

16.5/16.5/16.2

46.7/45.3/41.0

565

(0.50,0.50)

D3

ITO/CzTPA-p-Trz:PO-01/CzTPA-p-Trz/BPhen/LiF/Al

2.77/3.90/4.29

8.9/8.9/8.6

23.8/21.8/19.1

565

(0.49,0.50)

O1

ITO/CzTPA-p-Trz/CzTPA-p-Trz:PO-01/BPhen/LiF/Al

2.67/3.63/3.99

20.8/20.5/19.8

60.2/51.5/45.2

565

(0.51,0.49)

O2

ITO/CzTPA-m-Trz/CzTPA-m-Trz:PO-01/BPhen/LiF/Al

2.63/4.06/4.52

23.5/21.8/20.1

68.3/49.3/40.7

565

(0.51,0.49)

Abbreviations: λEL: peak wavelength of the EL spectrum. a Voltage measured in the order of turn on, at 5000 cd m-2 and 10000 cd m-2. b EQE and PE measured in the order of maximum, at 5000 cd m-2 and 10000 cd m-2. c Measured at 5V.


36

Page 37 of 37

Table of Contents

CzTPA-p-Trz Fluorescence High PLQY

Bphen

10 10


LiF/Al

100

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CzTPA-m-Trz TADF Low PLQY LiF/Al Bphen

CzTPA-m-Trz:PO-01 NPB

CzTPA-p-Trz 1

ITO

0

1000

2000

3000

4000

1 5000

CzTPA-m-Trz ITO

2

Luminance (cd/m )


37

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