Co-reporter:Minghan Cai, Dongdong Zhang, Tianyu Huang, Xiaozeng Song, and Lian Duan
ACS Applied Materials & Interfaces May 24, 2017 Volume 9(Issue 20) pp:17279-17279
Publication Date(Web):May 5, 2017
DOI:10.1021/acsami.7b04253
Organic light-emitting diodes (OLEDs) with simple structures are attracting a lot of attention nowadays, though their performances are always inferior to those of the more complicated structures as multifunctional materials are rare. Here, we have designed and synthesized multifunctional isomers by combining electron-donating carbazole (Cz) and triphenylamine (TPA) units with electron-accepting triazine (Trz), namely, 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-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-[1,1′-biphenyl]-4-amine (CzTPA-m-Trz). The use of multiple electron-donating groups gives them suitable highest occupied molecular orbitals 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 and hole transport layers show a higher hole current than the widely used 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile/4,4′-N,N′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl system. Interestingly, CzTPA-p-Trz is a fluorescent material with a high photoluminescence quantum yield (PLQY), while CzTPA-m-Trz shows weak thermally activated delayed fluorescence (TADF). As expected, a 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, among double-layer devices doped with an orange phosphorescent dopant, a 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, a high EQE of 21.8% was retained for CzTPA-m-Trz-based devices. These results are even comparable to those for the state-of-the-art phosphorescent devices based on the same dopant with more complicated structures. The above results indicate that well-designed multifunctional materials are promising for high-performance OLEDs with simple structures.Keywords: high power efficiency; low-efficiency roll-off; multifunctional material; organic light-emitting diode; simple structure; thermally activated delayed fluorescence;
Co-reporter:Dongdong Zhang, Pengcheng Wei, Deqiang Zhang, and Lian Duan
ACS Applied Materials & Interfaces June 7, 2017 Volume 9(Issue 22) pp:19040-19040
Publication Date(Web):May 15, 2017
DOI:10.1021/acsami.7b04391
A high triplet energy (T1) is usually taken as the prerequisite of the good exciton confinement of electron transporting materials (ETMs); however, there is usually a tradeoff with large mobility and stability. Here, we demonstrated that good exciton confinement can also be realized utilizing a low-T1 ETM with a sterically shielding low-T1 unit. Given the short-range interaction of the Dexter energy transfer, the large steric side groups of the low-T1 ETM can effectively hinder the T1 of the emitters from being quenched by increasing the intermolecular distance. Based on this concept, a maximum external quantum efficiency (EQEmax) as high as 21.3% was observed in the sky-blue thermally activated delayed fluorescence device using a low-T1 ETM, with the EQE remaining at 21.2% at 1000 cd/m2 and 17.8% at 5000 cd/m2. Further, an EQEmax as high as 25.5%, a low turn-on voltage of 2.3 V, as well as a long T90 of over 400 h at an initial luminance of 5000 cd/m2 were achieved for green phosphorescent devices. This work highlights a viable strategy for developing high-performance ETMs, paving their way toward practical applications.Keywords: anthracene derivative; electron transport; exciton confinement; phosphorescent organic light-emitting diodes; thermally activated delayed fluorescence;
Co-reporter:Dongxin Ma, Chen Zhang, Yong Qiu, Lian Duan
Organic Electronics 2017 Volume 42() pp:194-202
Publication Date(Web):March 2017
DOI:10.1016/j.orgel.2016.11.037
•Orange-red-emitting OLEDs are achieved by using cationic RuII complexes.•Counter-ion control improves the solubility and enable solution-processed OLEDs.•Counter-ion control improves the sublimability and enable vacuum-evaporated OLEDs.We designed and synthesized a novel series of red-emitting cationic ruthenium(II) complexes 1–6 with the same coordinated ruthenium(II) cation tris(2,2′-bipyridine)ruthenium(II) while different-sized negative counter-ions, namely chloride (1), tetrafluoroborate (2), hexafluorophosphate (3), perchlorate (4), tetrakis(pentafluorophenyl)borate (5) and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (6), respectively. Their physicochemical characteristics including solubility, photophysical properties and electrochemical behaviors were fully investigated. Experiments indicated that introduction of bulky anions could effectively lower the molecular polarity and lattice energy of these ionic materials, improve their solubility in organic solvents, thus enable constructing solution-processed organic light-emitting diodes (OLEDs) based on complexes 2–6. Furthermore, complex 6 was demonstrated evaporable, owing to the electrostatic interaction strongly reduced by its anion with the largest steric hindrance and well-dispersed charges. We succeeded in the preparation of orange-red-emitting OLEDs fabricated by vacuum evaporation deposition thereof, achieving a maximum brightness of 5345 cd m−2 with color coordinates of (0.51, 0.44). To our knowledge, this is the first report on sublimable cationic ruthenium(II) complexes, suggesting their great potential as a new material system for sustainable phosphorescence.
Co-reporter:Amjad Islam;Dongdong Zhang;Xinhua Ouyang;Rongjuan Yang;Tao Lei;Ling Hong;Ruixiang Peng;Ziyi Ge
Journal of Materials Chemistry C 2017 vol. 5(Issue 26) pp:6527-6536
Publication Date(Web):2017/07/06
DOI:10.1039/C7TC01597F
Highly efficient organic light-emitting diodes (OLEDs) with simplified device structures are widely desired for both scientific research and industrial applications. However, a very limited number of simplified OLEDs have been reported to date. In this work, two multifunctional blueish green emitters, BPTPETPAI and 2TPETPAI, are designed and synthesized. Owing to the presence of a tetraphenylethene (TPE) moiety, their aggregation induced emission (AIE) properties are also investigated. High photoluminescence efficiencies of the two compounds in non-doped films render them good emitters for non-doped devices. Multilayer non-doped devices based on these emitters achieve maximum external quantum efficiencies (EQEs) and current efficiencies (CEs) of 3.13% and 6.14 cd A−1 as well as 3.25% and 6.70 cd A−1 for BPTPETPAI and 2TPETPAI, respectively. Given their shallow highest occupied molecular orbital (HOMO) energy levels, both emitters can also be used as hole injection and hole transporting materials. Based on this, single layer devices show even higher efficiencies with extremely low efficiency roll-off, achieving maximum CEs as high as 7.12 cd A−1 and 7.80 cd A−1 using BPTPETPAI and 2TPETPAI, respectively. These results demonstrate a bright prospect for the development of highly desired multifunctional emitters as well as simplified OLEDs with significant reduction in the fabrication cost of the device.
Co-reporter:Zhengyang Bin;Jiangwei Li;Liduo Wang
Energy & Environmental Science (2008-Present) 2017 vol. 10(Issue 11) pp:2480-2480
Publication Date(Web):2017/11/08
DOI:10.1039/C7EE90060K
Correction for ‘Efficient n-type dopants with extremely low doping ratios for high performance inverted perovskite solar cells’ by Zhengyang Bin et al., Energy Environ. Sci., 2016, 9, 3424–3428.
Co-reporter:Zhengyang Bin;Jiangwei Li;Liduo Wang
Energy & Environmental Science (2008-Present) 2017 vol. 10(Issue 11) pp:2480-2480
Publication Date(Web):2017/11/08
DOI:10.1039/C7EE90060K
Correction for ‘Efficient n-type dopants with extremely low doping ratios for high performance inverted perovskite solar cells’ by Zhengyang Bin et al., Energy Environ. Sci., 2016, 9, 3424–3428.
Co-reporter:Minghan Cai;Xiaozeng Song;Dongdong Zhang;Juan Qiao
Journal of Materials Chemistry C 2017 vol. 5(Issue 13) pp:3372-3381
Publication Date(Web):2017/03/30
DOI:10.1039/C7TC00733G
A series of D–π–A type bipolar hosts based on triphenylene/carbazole were designed and synthesized. π–π stacking of the triphenylene units between two adjacent molecules renders these hosts high electron mobilities above 1 × 10−4 cm2 V−1 s−1, and their electron and hole mobilities can be regulated through varying the connection position and π moieties. Due to more balanced charge mobilities, a D1-based green thermally activated delayed fluorescence (TADF) device achieved the highest external quantum efficiency (EQE) of 16.3%, and its EQE could still maintain 15.3% and 13.8% at the high luminance of 5000 cd m−2 and 10 000 cd m−2, respectively. D1-based green phosphorescent devices also exhibited the highest EQE of 18.6% with a reduced roll-off to 17.6% at 5000 cd m−2 and 16.2% at 10 000 cd m−2.
Co-reporter:Pengcheng Wei, Dongdong Zhang, Minghan Cai, Xiaozeng Song, Zheyao Wang, Lian Duan
Organic Electronics 2017 Volume 49(Volume 49) pp:
Publication Date(Web):1 October 2017
DOI:10.1016/j.orgel.2017.05.013
•Achieving a TADF-based SEL-hybrid-WOLED with a high-efficiency, low roll-off, high CRI and superior CIE coordinates.•Demonstration zone of charge recombination is on the host of the white device.•Demonstrating energy transfer pathway between red and green dopants in the white device.Single-emitting-layer hybrid white organic light-emitting diodes (SEL-hybrid-WOLEDs) are promising candidates for large-area lightings, however, ideal hybrid WOLEDs with a simple structure and high-efficiency, low roll-off, high color rendering index (CRI) and superior CIE coordinates have been rarely reported. In this paper, high-performance SEL-hybrid-WOLEDs are demonstrated by utilizing a thermally activated delayed fluorescence (TADF) host emitter combined with green and red phosphors. The optimized WOLED exhibits an external quantum efficiency (EQE) of 20.2%, CIE coordinates of (0.360, 0.390) and a CRI of 85. Remarkably, an extremely low efficiency roll-off is also realized, with an EQE of 19.4% remained even at the practical luminance of 1000 cd/m2, resulting from the wide recombination zone as well as the well-tuned energy transfer in the emitting layer. Moreover, benefited from the stable recombination zone, superior color stability was also achieved. The intriguing results, we believe, greatly manifest the great potential of such a strategy and may pave the way towards real applications.Download high-res image (271KB)Download full-size image
Co-reporter:Dongdong Zhang, Chongguang Zhao, Yunge Zhang, Xiaozeng Song, Pengcheng Wei, Minghan Cai, and Lian Duan
ACS Applied Materials & Interfaces 2017 Volume 9(Issue 5) pp:
Publication Date(Web):January 17, 2017
DOI:10.1021/acsami.6b15272
Numerous efforts have been devoted to boost the efficiency of thermally activated delayed fluorescence (TADF) devices; however, strategies to suppress the device efficiency roll-off are still in urgent need. Here, a general and effective approach to suppress the efficiency roll-off of TADF devices is proposed, that is, utilizing TADF materials as the hosts for TADF emitters. Bearing small singlet–triplet splitting (ΔEST) with donor and acceptor units, TADF materials as the hosts possess the potential to achieve matched frontier energy levels with the adjacent transporting layers, facilitating balanced charge injection as well as bipolar charge transport mobilities beneficial to the balanced charges transportation. Furthermore, an enhanced Förster energy transfer from the host to the dopant can be anticipated, helpful to reduce the exciton concentration. Based on the principles, a new TADF material based on indeno[2,1-b]carbazole/1,3,5-triazin derivation is synthesized and used as the universal host for the full-color TADF devices. Remarkable low efficiency roll-off was achieved with above 90% of the maximum external quantum efficiencies (EQEmax’s) maintained even at a brightness of 2000 cd/m2, along with EQEmax’s of 23.2, 21.0, and 19.2% for orange, green, and sky-blue TADF devices, respectively. Through computational simulation, we identified the suppressed exciton annihilation rates compared with devices adopting conventional hosts. The state-of-the-art low efficiency roll-off of those TADF devices manifests the great potential of such host design strategy, paving an efficient strategy toward their practical application.Keywords: exciton annihilation; high efficiency; low efficiency roll-off; organic light-emitting diodes; thermally activated delayed fluorescence;
Co-reporter:Dongxin Ma;Yong Qiu
Advanced Functional Materials 2016 Volume 26( Issue 20) pp:3438-3445
Publication Date(Web):
DOI:10.1002/adfm.201505493
Recent development in the field of small molecular materials has led to great advances in the performance of vacuum-evaporated organic light-emitting diodes. However, as a significant class of phosphorescent emitters, ionic transition metal complexes are seldom sublimable due to the inherent ionic nature and low vapor pressure, restricting their applications in state-of-the-art devices fabricated by vacuum evaporation deposition. Here a facile, feasible and versatile strategy is shown to tune the volatility of ionic transition metal complexes through counter-ion control. By introducing counter-ions with large steric hindrance and well-dispersed charges, a series of evaporable ionic iridium complexes are developed, and efficient vapor-processed devices with a high brightness, small efficiency roll-off, and polychromic emission ranging from deep-blue to red-orange are achieved. Our findings unlock the utilization of ionic functional materials in vacuum-evaporated devices, and may open new doors for modern electronic materials technology.
Co-reporter:Jie Xue, Chen Li, Lijun Xin, Lian Duan and Juan Qiao
Chemical Science 2016 vol. 7(Issue 4) pp:2888-2895
Publication Date(Web):19 Jan 2016
DOI:10.1039/C5SC04685H
Though urgently needed, high-performance near-infrared organic light-emitting diodes (NIR-OLEDs) are still rare. NIR-OLEDs based on conventional NIR fluorescent materials usually suffer from low external quantum efficiencies (EQEs) because of the intrinsic obstacles according to the spin-statistics limit and energy-gap law. Herein, we realized high-efficiency and low efficiency roll-off fluorescent NIR-OLEDs through efficient triplet fusion of a bipolar host doped with a special naphthoselenadiazole emitter (4,9-bis(4-(2,2-diphenylvinyl)phenyl)-naphtho[2,3-c][1,2,5]selenadiazole, NSeD). Unlike typical NIR organic donor–acceptor (D–A) chromophores, NSeD features a non-D–A structure and a very large HOMO/LUMO overlap and displays a strong deep-red to NIR fluorescence and unique ambipolar character. The corresponding photoluminescence quantum efficiency of NSeD reaches 52% in solution and retains 17% in the blend film. The optimized NIR-OLEDs demonstrated a strong emission at 700 nm, a high maximum EQE of 2.1% (vs. the predicted theoretical maximum efficiency of 1.3%) and the EQE remained at around 2% over a wide range of current densities from 18 to 200 mA cm−2, which is amongst the highest performance for NIR-OLEDs based on organic fluorescent materials.
Co-reporter:Dongdong Zhang, Minghan Cai, Zhengyang Bin, Yunge Zhang, Deqiang Zhang and Lian Duan
Chemical Science 2016 vol. 7(Issue 5) pp:3355-3363
Publication Date(Web):
DOI:10.1039/C5SC04755B
Co-reporter:Dongdong Zhang, Minghan Cai, Yunge Zhang, Deqiang Zhang and Lian Duan
Materials Horizons 2016 vol. 3(Issue 2) pp:145-151
Publication Date(Web):04 Jan 2016
DOI:10.1039/C5MH00258C
Blue emitters with thermally activated delayed fluorescence (TADF) have the potential to achieve 100% internal quantum efficiency, though the strategy to design stable blue TADF emitters remains unexplored. Here, high efficiency and improved stability are achieved simultaneously for blue or sky blue TADF emitters by wise designing of molecular structures using tert-butyl units. The tert-butyl substituents act like a shield surrounding the luminance core, not only promoting the photoluminescence efficiency, but also improving the stability of the compounds. Consequently, the device with a sterically shielded emitter achieves a maximum external quantum efficiency as high as 21.2% and a record long T50 (time to 50% of initial luminance) of 770 h at an initial luminance of 500 cd m−2, corresponding to a half lifetime of 12873 h at 100 cd m−2.
Co-reporter:Dongxin Ma, Lian Duan and Yong Qiu
Journal of Materials Chemistry A 2016 vol. 4(Issue 22) pp:5051-5058
Publication Date(Web):05 May 2016
DOI:10.1039/C6TC00738D
Two novel red-emitting cationic iridium complexes, [Ir(ppy)2(pop)][B(5fph)4] (1) and [Ir(ppy)2(pop)][BArF24] (2), have been developed, where ppy is 2-phenylpyridine, pop is 2-(5-phenyl-1-1,3,4-oxadiazol-2-yl)pyridine, [B(5fph)4]− is tetrakis(pentafluorophenyl)borate and [BArF24]− is tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, respectively. Photophysical properties of 1 and 2 in both solution and neat film were fully investigated, along with their photochemical, thermal and electrochemical stability. Interestingly, by introducing bulky tetraphenylborate derivatives as negative counter-ions, the volatility of 1 and 2 has been extremely improved, enabling fabrication of organic light-emitting diodes (OLEDs) by vacuum evaporation deposition. By doping these two sublimable cationic emitters into a DIC-TRZ (2,4-diphenyl-6-bis(12-phenylindolo[2,3-a]carbazole-11-yl)-1,3,5-triazine) host, we succeeded in the preparation of orange-red-emitting devices with a peak wavelength of 596 nm. 1-Based OLEDs showed a current efficiency of 4.5 cd A−1 and maximum brightness of 19.4 × 103 cd m−2, whereas 2-based OLEDs furnished a higher efficiency of 5.1 cd A−1. Then, we attained a white emission by doping 1 or 2 into a TCTA (4,4′,4′′-tris(carbazol-9-yl)triphenylamine) host at low concentrations. The 1-based white device featured a high colour rendering index (CRI) of 86 and good Commission International de L'EClairage (CIE) coordinates of (0.33, 0.34), quite close to the equal-energy-white-point (i.e., CIEx,y = 0.33, 0.33), and the 2-based white device showed a rather higher CRI of 89. To the best of our knowledge, this is the first report of white OLEDs fabricated by vacuum evaporation deposition of sublimable cationic iridium complexes, indicating their great potential for use in full-colour flat-panel display and lighting applications.
Co-reporter:Dongxin Ma, Chen Zhang, Yong Qiu and Lian Duan
Journal of Materials Chemistry A 2016 vol. 4(Issue 24) pp:5731-5738
Publication Date(Web):27 May 2016
DOI:10.1039/C6TC01302C
We designed and synthesized a series of cationic iridium(III) complexes with the same coordinated iridium(III) cation but different-sized counter-ions, investigated their photophysical properties, electrochemical behaviours and thermal stability, then fabricated single-layer solution-processed organic light-emitting diodes (OLEDs) thereof, and demonstrated anionic migration in devices. By doping these cationic iridium(III) complexes at low concentrations (2 or 3 wt%) thus avoiding their anionic migration, we succeeded in the preparation of efficient blue-green OLEDs, achieving the highest current efficiency of 17.1 cd A−1, an external quantum efficiency of 6.8%, a maximum luminance of 14.2 × 103 cd m−2 and colour coordinates of (0.21, 0.48). To our knowledge, these values are among the best reported OLEDs based on ionic transition metal complexes as phosphorescent emitters in the blue-green region. The impact of different-sized counter-ions on carrier transport characteristics was also examined by single carrier devices. Notably, by doping these cationic iridium(III) complexes at high concentrations (20 wt%) and thus wisely employing their anionic migration, we developed simple-constructed OLEDs with a temporary p–i–n junction instead of active n-type dopants in the cathode and obtained a high current efficiency of 8.3 cd A−1 and a maximum luminance of 12.8 × 103 cd m−2, rather comparable to the corresponding OLEDs with the conventional Cs2CO3 active cathode.
Co-reporter:Dongdong Zhang, Deqiang Zhang, and Lian Duan
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 35) pp:23197
Publication Date(Web):August 19, 2016
DOI:10.1021/acsami.6b07107
Despite that the majority of practical organic light-emitting diodes (OLEDs) still rely on blue fluorophors with low triplet (T1) for creating blue light, hybrid white OLEDs based on low T1 blue fluorophors are still much lagged behind in power efficiency. Here, “ideal” hybrid WOLEDs with recorded efficiency as well as low roll-off, good color-stability and long lifetime were realized by utilizing the bipolar mixed materials as the host of green phosphor as well as the spacer to reduce T1 trap, while blue fluorophors with p-type delayed fluorescence to recycle the trapped T1. An electron transport material with both high electron mobility and good exciton confinement ability was used to boost the TTA efficiency. Hybrid WOLEDs with maximum current efficiency, external quantum efficiency and power efficiency of 49.6 cd/A, 19.1%, and 49.3 lm/W, respectively, together with a high color rendering index of 80 and a half lifetime of over 7000 h at an initial luminescence of 1000 cd/m2 were realized, manifesting the high potential of the strategy.Keywords: good color-stability; high efficiency; hybrid white organic light-emitting diodes; low efficiency roll-off; p-type delayed fluorescence
Co-reporter:Dongdong Zhang, Minghan Cai, Yunge Zhang, Zhengyang Bin, Deqiang Zhang, and Lian Duan
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 6) pp:3825
Publication Date(Web):January 22, 2016
DOI:10.1021/acsami.5b10561
Exciplex forming cohosts have been widely adopted in phosphorescent organic light-emitting diodes (PHOLEDs), achieving high efficiency with low roll-off and low driving voltage. However, the influence of the exciplex-forming hosts on the lifetimes of the devices, which is one of the essential characteristics, remains unclear. Here, we compare the influence of the bulk exciplex and interface exciplex on the performances of the devices, demonstrating highly efficient orange PHOLEDs with long lifetime at low dopant concentration by efficient Förster energy transfer from the interface exciplex. A bipolar host, (3′-(4,6-diphenyl-1,3,5-triazin-2-yl)-(1,1′-biphenyl)-3-yl)-9-carbazole (CzTrz), was adopted to combine with a donor molecule, tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA), to form exciplex. Devices with energy transfer from the interface exciplex achieve lifetime almost 2 orders of magnitude higher than the ones based on bulk exciplex as the host by avoiding the formation of the donor excited states. Moreover, a highest EQE of 27% was obtained at the dopant concentration as low as 3 wt % for a device with interface exciplex, which is favorable for reducing the cost of fabrication. We believe that our work may shed light on future development of ideal OLEDs with high efficiency, long-lifetime, low roll-off and low cost simultaneously.Keywords: Förster energy transfer; high efficiency; interface exciplex; long operational lifetime; phosphorescent organic light emitting diodes
Co-reporter:Dongxin Ma, Lian Duan and Yong Qiu
Dalton Transactions 2016 vol. 45(Issue 14) pp:6118-6123
Publication Date(Web):09 Nov 2015
DOI:10.1039/C5DT03776J
Spin-cast from various solvents, emissive layers show different film morphologies and performances in solution-processed organic light-emitting diodes (OLEDs). Here we fabricated and demonstrated highly efficient blue OLEDs based on bis[3,5-difluoro-2-(2-pyridyl)phenyl]-(2-carboxypyridy)iridium(III) by choosing several kinds of solvents for spin-coating. Experiments indicate that the single-layer device with an emissive film cast from chlorobenzene shows its best performance with a highest current efficiency of 18.99 cd A−1, a maximum luminance of 20.5 × 103 cd m−2 and an emission band centered at 474 nm. The efficiency achieved is the highest reported for solution-processed simple-manufactured OLEDs doped with transition metal phosphors emitting in the blue region.
Co-reporter:Lei He, Zhen Wang, Chunpeng Yang, Lian Duan, Ruiren Tang, Xiangzhi Song, Chunyue Pan
Dyes and Pigments 2016 Volume 131() pp:76-83
Publication Date(Web):August 2016
DOI:10.1016/j.dyepig.2016.04.006
•Cationic iridium complexes with phenyl-imidazole type CΛN ligands were developed.•The CΛN ligands destabilize both the HOMO and LUMO levels of the complexes.•Orange-red LEC using the complex gives a maximum current efficiency of 14.3 cd A−1.Phosphorescent cationic iridium complexes with phenyl-imidazole type cyclometalating ligands have been synthesized for the first time and their photophysical, electrochemical properties have been comprehensively investigated. By changing the ancillary ligands, the complexes give orange-red or green-blue light. Compared to 2-phenylpyridine (ppy), the phenyl-imidazole ligands destabilize simultaneously the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals of the complexes. Their emitting triplet states show dominant charge-transfer (iridium/cyclometalating ligands→ancillary ligands) character. The complexes have been used to fabricate solid-state light-emitting electrochemical cells (LECs). The orange-red LEC gives a high peak current efficiency of 14.3 cd A−1, which is among the highest reported for orange-red LECs; the green-blue LEC gives a peak current efficiency of 6.3 cd A−1. It is shown that the phenyl-imidazole cyclometalating ligands hold promise for the invention of iridium-based cationic phosphorescent dyes with tunable energy levels and emission properties.
Co-reporter:Haoyuan Li, Yong Qiu, Lian Duan
Organic Electronics 2016 Volume 33() pp:164-171
Publication Date(Web):June 2016
DOI:10.1016/j.orgel.2016.03.016
•A method is proposed to calculate the electric properties of organic-based devices from the molecular structure.•Snapshots from the dynamic trajectory are used to model the organic film in the device.•Calculated current densities of a hole-only device are reasonable comparing with experimental results.•This method can be used to aid the design of molecules and guide the optimization of devices.A method is proposed to calculate the electric properties of organic-based devices from the molecular structure. The charge transfer rate is obtained using non-adiabatic molecular dynamics. The organic film in the device is modeled using the snapshots from the dynamic trajectory of the simulated molecular system. Kinetic Monte Carlo simulations are carried out to calculate the current characteristics. A widely used hole-transporting material, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) is studied as an application of this method, and the properties of its hole-only device are investigated. The calculated current densities and dependence on the applied voltage without an injection barrier are close to those obtained by the Mott-Gurney equation. The results with injection barriers are also in good agreement with experiment. This method can be used to aid the design of molecules and guide the optimization of devices.
Co-reporter:Dongxin Ma, Chen Zhang, Yong Qiu, Lian Duan
Organic Electronics 2016 Volume 39() pp:16-24
Publication Date(Web):December 2016
DOI:10.1016/j.orgel.2016.09.012
•Deep-blue OLEDs are achieved by wisely choosing anions of cationic IrIII complexes.•Bulky anions avoid concentration quenching and improve photoluminescence efficiency.•Bulky anions reduce molecular aggregation thus blue-shift the emission spectra.A series of blue-emitting cationic iridium(III) complexes featuring the same coordinated iridium(III) cation while different-sized negative counter-ions are reported and investigated. Anionic influence on the physicochemical properties and device performance is studied and explained. Introduction of counter-ions with large steric hindrance, (i) effectively reduces the ionic interaction and molecular aggregation in solid states, improving the photoluminescence efficiency and avoiding the common bathochromic shift; (ii) also restricts anionic drift under the working bias in the optoelectronic devices, achieving solution-processed deep-blue-emitting diodes with a major peak at only 452 nm, among the shortest emission wavelengths of devices based on ionic transition metal complexes.
Co-reporter:Yilang Li;Dongdong Zhang;Yunge Zhang;Minghan Cai
Science China Chemistry 2016 Volume 59( Issue 6) pp:684-691
Publication Date(Web):2016 June
DOI:10.1007/s11426-015-0506-2
High cost of phosphors and significant efficiency roll-off at high brightness are the two main factors that limit the wide application of phosphorescent organic light-emitting diodes (PHOLEDs). Efforts have been paid to find ways to reduce the phosphors’ concentration and efficiency roll-off of PHOLEDs. In this work, we reported red emission PHOLEDs with low dopant concentration and low efficiency roll-off based on a novel host material 2,4-biscyanophenyl-6-(12-phenylindole[2,3-a]carbazole-ll-yl)-l,3,5-triazine (BCPICT), with thermally activated delayed fluorescent (TADF) properties. The device with 1.0% dopant concentration displayed a maximum external quantum efficiency of 10.7%. When the dopant concentration was increased to 2.0%, the device displayed a maximum external quantum efficiency of 10.5% and a low efficiency roll-off of 5.7% at 1000 cd/m2.
Co-reporter:YunGe Zhang;GenMao Huang;GuiFang Dong
Science China Technological Sciences 2016 Volume 59( Issue 9) pp:1407-1412
Publication Date(Web):2016 September
DOI:10.1007/s11431-016-6102-6
The full solution-processed oxide thin-film-transistors (TFTs) have the advantages of transparency, ease of large-area fabrication, and low cost, offering great potential applications in switching and driving fields, and attracting extensive research interest. However, the performance of the solution-processed TFTs is generally lower than that of the vacuum-deposited ones. In this article, the full-solution processed TFTs with zinc-tin-oxide (ZTO) semiconductor and aluminium (Al2O3) dielectrics were fabricated, and their mobilities in the saturation region are high. Besides, the effect of the Al2O3 dielectrics’ preparation technology on ZTO TFTs’ performance was studied. Comparing the ZTO TFTs using the spin-coated Al2O3 dielectrics of 1–4 layers, the ZTO TFT with 3-layer Al2O3 dielectrics achieved the optimal performance as its field-effect carrier mobility in the saturation region is 112 cm2/V s, its threshold voltage is 2.4 V, and its on-to-off current ratio is 2.8×105. This is also the highest reported carrier mobility of the solution-processed ZTO TFTs.
Co-reporter:Wei Jiang, Xinxin Ban, Muyang Ye, Yueming Sun, Lian Duan and Yong Qiu
Journal of Materials Chemistry A 2015 vol. 3(Issue 2) pp:243-246
Publication Date(Web):24 Nov 2014
DOI:10.1039/C4TC02485K
A novel cross-linkable hole-transporting material (HTM) has been synthesized and characterized. The HTM possesses high triplet energy, excellent film-forming and solvent-resistant abilities, suitable HOMO level and electrochemical stability. Solution processed multilayer blue electrophosphorescent devices using this cross-linked HTM show lower turn-on voltages and doubled efficiencies compared with the corresponding single-layer device.
Co-reporter:Xinxin Ban, Wei Jiang, Kaiyong Sun, Haiyong Yang, Yanan Miao, Fenghao Yang, Yueming Sun, Bin Huang and Lian Duan
Journal of Materials Chemistry A 2015 vol. 3(Issue 19) pp:5004-5016
Publication Date(Web):07 Apr 2015
DOI:10.1039/C5TC00691K
A series of bipolar hosts based on carbazole and phenyl benzimidazole (PBI) moieties, collectively named xCz–nPBI, were designed and synthesized. On the basis of different numbers, ratios and link-configurations of the functional groups, the influence of substitution on the chemical, photophysical and electrochemical properties of the host materials were investigated in detail. Both DFT calculations and single carrier devices demonstrate that the strategy of introducing more electron-withdrawing PBI groups in the molecules can effectively enhance the electron injection and transport ability of the bipolar host, while an increased number of carbazole units endows the hosts with a much smaller ΔEST for efficient hole injection at the cost of sacrificing their charge balance property. As a result, the solution-processed green-emitting PHOLEDs based on Cz–6PBI show an extremely low turn on voltage of 2.9 V and the highest current and power efficiency of 47.8 cd A−1 and 29.6 lm W−1, respectively. Even at luminance as high as 1000 cd m−2, their efficient roll-off was only 4.2%, which was far better than the 6Cz–PBI host device. Because the T1 energy levels and triplet state locations of these hosts are similar, their ΔEST and charge balance property should be the main factors that influence their EL performances. We conclude that it is not necessary to achieve a very small ΔEST by introducing more carbazole moieties at the cost of weakening of electron transporting ability. As for solution-processed devices, which suffer from solvent impurities and oxygen diffusion induced strong electron trapping effect, a systemic increase in the number of electron-withdrawing PBI groups in their host materials can significantly enhance the charge balance of their emission layers (EMLs) for highly power efficient solution-processed PHOLEDs.
Co-reporter:Dongdong Zhang, Minghan Cai, Yunge Zhang, Deqiang Zhang, and Lian Duan
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 51) pp:28693
Publication Date(Web):December 8, 2015
DOI:10.1021/acsami.5b10783
Single-emitting layer hybrid white organic light-emitting diodes (SEL-hybrid-WOLEDs) usually suffer from low efficiency, significant roll-off, and poor color stability, attributed to the incomplete energy transfer from the triplet states of the blue fluorophores to the phosphors. Here, we demonstrate highly efficient SEL-hybrid-WOLEDs with low roll-off and good color-stability utilizing blue thermally activated delayed fluorescence (TADF) materials as the host emitters. The triplet states of the blue TADF host emitter can be up-converted into its singlet states, and then the energy is transferred to the complementary phosphors through the long-range Förster energy transfer, enhancing the energy transfer from the host to the dopant. Simplified SEL-hybrid-WOLEDs achieve the highest forward-viewing external quantum efficiency (EQE) of 20.8% and power efficiency of 51.2 lm/W with CIE coordinates of (0.398, 0.456) at a luminance of 500 cd/m2. The device EQE only slightly drops to 19.6% at a practical luminance of 1000 cd/m2 with a power efficiency of 38.7 lm/W. Furthermore, the spectra of the device are rather stable with the raising voltage. The reason can be assigned to the enhanced Förster energy transfer, wide charge recombination zone, as well as the bipolar charge transporting ability of the host emitter. We believe that our work may shed light on the future development of highly efficient SEL-hybrid-WOLEDs with simultaneous low roll-off and good color stability.Keywords: blue thermally activated delayed fluorescence; good color-stability; high efficiency; hybrid white organic light-emitting diodes; low efficiency roll-off
Co-reporter:Chen Li, Lian Duan, Dongdong Zhang, and Yong Qiu
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 28) pp:15154
Publication Date(Web):July 9, 2015
DOI:10.1021/acsami.5b04090
Materials with thermally activated delayed fluorescence (TADF) realized 100% internal quantum efficiency (IQE) but suffered significant efficiency roll-off. Here, an exciton dynamics study reveals that materials with TADF may play opposite roles in affecting the efficiency roll-off: decreasing the triplet density due to the fast reverse intersystem crossing, on the one hand, and increasing the triplet density due to the weakened singlet radiation. We show theoretically and experimentally that TADF-sensitized phosphorescence can break this trade-off by exploiting the efficient Förster energy transfer and simultaneously achieve 100% IQE and low efficiency roll-off (with a critical current density of 460 mA cm–2).Keywords: efficiency roll-off; exciton annihilation; exciton dynamics; organic light-emitting diodes; quantum efficiency;
Co-reporter:Xinxin Ban, Wei Jiang, Kaiyong Sun, Xinyu Xie, Lang Peng, Hongshuang Dong, Yueming Sun, Bin Huang, Lian Duan, and Yong Qiu
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 13) pp:7303
Publication Date(Web):March 18, 2015
DOI:10.1021/acsami.5b00510
Two soluble bipolar host materials (mCP-BPBI and CP-QPBI), comprising different proportions of hole-transporting carbazole and electron-transporting benzimidazole, were synthesized. Their thermal, physical, and electrochemical properties were characterized. The designated bulky star-shaped structures efficiently suppress the direct intramolecular interaction between the donor and acceptor subunits to give high triplet energies. Through computational studies, varying the ratio of hole- and electron-transporting moieties could significantly change the carrier injection/transporting abilities and charge balance properties of the host materials. Indeed, CP-QPBI with more benzimidazole units shows extremely enhanced current density at the same voltage when compared to mCP-BPBI. The operating voltage of solution-processed phosphorescent light-emitting diodes with CP-QPBI as host were dramatically reduced by ∼3 V compared with the similar devices of mCP-BPBI. At the same time, the power efficiencies were improved for 2–2.5 times at the corresponding voltage. Importantly, both blue and green devices maintain their high efficiencies even at brightness up to 1000 cd m–2, which clearly demonstrates that the new strategy applied to improve electron-transporting ability and charge-balance property of the solution-processable host material by tuning the ratio of donor and acceptor unit is profitable.Keywords: bipolar host; high power efficiency; low operating voltage; OLEDs; reorganization energy; solution-process
Co-reporter:Zhengyang Bin, Lian Duan, and Yong Qiu
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 12) pp:6444
Publication Date(Web):March 13, 2015
DOI:10.1021/acsami.5b00839
Air-stable and low-temperature-evaporable n-type dopants are highly desired for efficient and stable organic light-emitting diodes (OLEDs). In this work, 2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-MeO-DMBI-I), a thermally decomposable precursor of organic radical o-MeO-DMBI, has been employed as a novel n-type dopant in OLEDs, because of its air stability, low decomposition temperature, and lack of atom diffusion. The n-type electrical doping is evidenced by the rapid increase in current density of electron-only devices and the large improvement in conductivity, originated from increased electron concentration in electron-transport layer (ETL) and reduced electron injection barrier. A highly efficient and stable OLED is created using o-MeO-DMBI as an n-type dopant in Bphen. Compared with the control device with its high-temperature-evaporable n-type dopant cesium carbonate (Cs2CO3), o-MeO-DMBI-doped device showed an incredible boom in current efficiency from 28.6 to 42.2 cd/A. Moreover, the lifetime (T70%) of o-MeO-DMBI-doped device is 45 h, more than 20 times longer than that of the Cs2CO3-doped device (2 h). The enhanced efficiency and stability are attributed to the improved balance of holes and electrons in the emissive layer, and the eliminated atom diffusion of cesium.Keywords: high efficiency; n-type dopant; organic light-emitting diodes; organic radical; organic salt
Co-reporter:Dongxin Ma, Lian Duan and Yong Qiu
Dalton Transactions 2015 vol. 44(Issue 18) pp:8521-8528
Publication Date(Web):04 Nov 2014
DOI:10.1039/C4DT02830A
Tetraimidazolylborate is first used as the counterion of two novel ionic iridium complexes, instead of tetraphenylborate. Both the electrochemical stabilities and electroluminescence are sharply improved. Organic light-emitting diodes have been successfully fabricated with a high current efficiency of 9.48 cd A−1 and a maximum brightness of 5163 cd m−2.
Co-reporter:Lian Duan;Taiju Tsuboi;Yong Qiu
Chinese Journal of Chemistry 2015 Volume 33( Issue 8) pp:859-864
Publication Date(Web):
DOI:10.1002/cjoc.201500266
Abstract
Electroluminescence (EL) characteristics have been studied for a hybrid tandem white organic light emitting diode (OLED) with a blue emitting fluorescent EL1 unit based on BCzVBi and a yellow emitting phosphorescent EL2 unit based on (fbi)2Ir(acac), where a MoO3 layer is inserted between EL1 and EL2 units as charge generation layer (CGL). Maximum current and power efficiencies of 68.1 cd/A and 29.2 lm/W were obtained, respectively, while the current and power efficiencies at luminance of 1000 cd/m2 were 68.0 cd/A and 24.6 lm/W. The yellow emission appears from about 4.5 V firstly, while the blue emission starts to appear from about 5.4 V. It was found that charge generation from CGL of MoO3/NPB bilayer occurred at high voltages of above 5.4 V but not at low voltages below 5.2 V.
Co-reporter:Dongdong Zhang, Lian Duan, Yunge Zhang, Minghan Cai, Deqiang Zhang and Yong Qiu
Light: Science & Applications 2015 4(1) pp:e232
Publication Date(Web):2015-01-01
DOI:10.1038/lsa.2015.5
To attain high efficiencies in hybrid white organic light-emitting diodes (WOLEDs), mutual quenching of the fluorophors and phosphors should be minimized. Efforts have been devoted to reducing the triplet quenching of phosphors; however, the quenching of fluorophors by the external heavy-atom effect (EHA) introduced by the phosphors is often ignored. Here, we observed that conventional fluorophors and fluorophors with thermally activated delayed fluorescence (TADF) behave differently in the presence of EHA perturbers. The efficiencies of the conventional fluorophors suffer greatly from the EHA, whereas the TADF fluorophors exhibit negligible changes, which makes TADF materials ideal fluorophors for hybrid devices. WOLEDs using a blue TADF fluorophor and an orange phosphor achieve a maximum forward viewing external quantum efficiency of 19.6% and a maximum forward viewing power efficiency of 50.2 lm W−1, among the best values for hybrid WOLEDs. This report is the first time that the EHA effect has been considered in hybrid WOLEDs and that a general strategy toward highly efficient hybrid WOLEDs with simple structures is proposed.
Co-reporter:Dongdong Zhang;Chen Li;Yilang Li;Haoyuan Li;Deqiang Zhang ;Yong Qiu
Advanced Materials 2014 Volume 26( Issue 29) pp:5050-5055
Publication Date(Web):
DOI:10.1002/adma.201401476
Co-reporter:Dongdong Zhang;Yilang Li;Haoyuan Li;Zhengyang Bin;Deqiang Zhang;Juan Qiao;Guifang Dong;Liduo Wang ;Yong Qiu
Advanced Functional Materials 2014 Volume 24( Issue 23) pp:3551-3561
Publication Date(Web):
DOI:10.1002/adfm.201303926
Orange-emitting phosphorescent organic light-emitting diodes (PHOLEDs) are drawing more and more attention; however, high-performance hosts designed for orange PHOLEDs are rare. Here, four indolocarbazole/1, 3, 5-triazine hybrids are synthesized to optimize the singlet and triplet energies, as well as transporting properties, for ideal orange PHOLEDs. By introducing moieties with different electronegativity, a graded reduction of the singlet and triplet energies is achieved, resulting in minimum injection barrier and minimum energy loss. Besides, the charge transporting abilities are also tuned to be balanced on the basis of the bipolar features of those materials. The optimized orange PHOLED shows a maximum external quantum efficiency (EQE) of 24.5% and a power efficiency of 64 lm W–1, both of which are among the best values for orange PHOLEDs. What is more, the efficiency roll-off is extremely small, with an EQE of 24.4% at 1000 cd m–2 and 23.8% at 10 000 cd m–2, respectively, which is the lowest efficiency roll-off for orange PHOLEDs to date, resulting in the highest EQE for orange PHOLEDs when the luminance is above 1000 cd m–2. Besides the balanced charges, the small roll-off is also attributed to the wide recombination zone resulting from the bipolar features of the hosts.
Co-reporter:Dongdong Zhang, Lian Duan, Yilang Li, Deqiang Zhang and Yong Qiu
Journal of Materials Chemistry A 2014 vol. 2(Issue 38) pp:8191-8197
Publication Date(Web):07 Aug 2014
DOI:10.1039/C4TC01289E
Hybrid white organic light-emitting diodes (WOLEDs) often undergo triplet energy loss through the triplet state of the blue fluorophors. Here, blue fluorophors with thermally activated delayed fluorescence (TADF) are introduced to solve this problem. The triplet excitons formed on blue TADF fluorophors can be harvested by either energy transfer to the low-lying triplet states of the phosphor or thermal upconversion to the emissive singlet states, eliminating the energy loss. Moreover, device structures are wisely designed to take full advantages of the charge trapping ability of the TADF dopant, 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene, achieving a color-stable warm white emission. Remarkably, a maximum forward viewing external quantum efficiency (EQE) of 22.5% and a maximum forward viewing power efficiency (PE) of 47.6 lm W−1 are achieved. These values are among the highest reported for hybrid WOLEDs and even comparable to full-phosphorescent ones, demonstrating that the strategy reported here is promising for OLED lighting.
Co-reporter:Dongdong Zhang, Lian Duan, Deqiang Zhang and Yong Qiu
Journal of Materials Chemistry A 2014 vol. 2(Issue 42) pp:8983-8989
Publication Date(Web):02 Sep 2014
DOI:10.1039/C4TC01757A
Phosphorescent organic-light emitting diodes (PHOLEDs) have achieved ultimate high efficiencies and long lifetimes. One of the remaining challenges in PHOLEDs is to reduce the cost, which can be accomplished by reducing the dopant concentration. Here, to promote energy transfer at low dopant concentrations, hosts with thermally activated delayed fluorescence (TADF) are utilized. The triplet excitons of the host with TADF can be thermally up-converted to their singlet states and then transferred to the guest through the long-range Förster energy transfer rather than the short-range Dexter one. Devices using 2-phenyl-4,6-bis(12-phenylindole[2,3-a]carbazole-11-yl)-1,3,5-triazine (PBICT) as the host for tris(2-phenylpyridine)iridium (Ir(ppy)3) achieve a maximum external quantum efficiency of 23.9% and a power efficiency of 77.0 lm W−1 at a low dopant concentration of 3 wt%. Moreover, unlike the traditional hosts, the lifetimes of devices using hosts with TADF are less sensitive to dopant concentration with the longest lifetime obtained at 3 wt% Ir(ppy)3. The findings may provide a novel strategy to simultaneously achieve high efficiency, low driving voltage and long lifetimes in PHOLEDs at a low phosphor concentration of ≤3 wt%.
Co-reporter:Wei Jiang, Jinan Tang, Xinxin Ban, Yueming Sun, Lian Duan, and Yong Qiu
Organic Letters 2014 Volume 16(Issue 20) pp:5346-5349
Publication Date(Web):September 26, 2014
DOI:10.1021/ol502531y
An ideal host material with high triplet energy, suitable HOMO energy level, excellent thermal and electrochemical stability, and bipolar charge carrier transport ability was synthesized. A high external quantum efficiency of 13.7% and a luminance efficiency of 48.2 cd A–1 with low efficiency roll-off were achieved in solution-processed green electrophosphorescent devices.
Co-reporter:Wei Jiang, Huange Xu, Xinxin Ban, Guolong Yuan, Yueming Sun, Bin Huang, Lian Duan, and Yong Qiu
Organic Letters 2014 Volume 16(Issue 4) pp:1140-1143
Publication Date(Web):February 11, 2014
DOI:10.1021/ol4037727
A novel alcohol-soluble electron-transport material was designed and synthesized. This material not only possesses a high triplet energy and a low HOMO level but also exhibits excellent electron-transport properties and good film-forming ability. Efficient fully solution-processed multilayer white electrophosphorescent devices have been fabricated by using this alcohol-processable material as an orthogonal electron-transport layer.
Co-reporter:Yan Zhao, Lian Duan, Deqiang Zhang, Guifang Dong, Juan Qiao, Liduo Wang, and Yong Qiu
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 6) pp:4570
Publication Date(Web):March 5, 2014
DOI:10.1021/am500399e
Various works on modification of the indium–tin oxide (ITO) substrate have been carried out so as to enhance hole injection in organic light-emitting devices. Herein, a simple and efficient approach to tuning the work function of the ITO substrate is described by surface modification of ITO with an organosiloxane self-assembled monolayer. The influences of the electronegativity on modification of the ITO substrate are systematically investigated by attaching electron-withdrawing groups (Cl, Br, and I) and an electron-donating group (NH2) to the organosiloxane materials. The preparation and modification of the ITO substrate has been studied using primarily atomic force microscopy and X-ray photoelectron spectroscopy and vacuum–ultraviolet spectroscopy, and remarkable changes have been observed after modification. The device based on a 3Cl–Si–ITO-modified anode exhibits the best efficiency among the devices, better than the control devices based on bare ITO, UV-treated ITO, and even Cl–ITO.Keywords: hole injection; organic light-emitting diodes; self-assembled monolayer; surface treatment;
Co-reporter:Genmao Huang, Lian Duan, Guifang Dong, Deqiang Zhang, and Yong Qiu
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 23) pp:20786
Publication Date(Web):November 6, 2014
DOI:10.1021/am5050295
Solution-processed metal oxide thin-film transistors (TFTs) operating in enhancement mode are promising for the next-generation flat panel displays. In this work, we report high-mobility TFTs based on SnO2 active layer derived from a soluble tin(II) 2-ethylhexanoate precursor. Densely packed polycrystalline SnO2 thin films with moderate oxygen vacancies and only a few hydroxides are obtained via systemically optimizing precursor concentrations and processing conditions. The utilization of a solution-processed high-κ Al2O3 insulating layer could generate a coherent dielectric/semiconductor interface, hence further improving the device performance. TFT devices with an average field-effect mobility of 96.4 cm2 V–1 s–1, a current on/off ratio of 2.2 × 106, a threshold voltage of 1.72 V, and a subthreshold swing of 0.26 V dec–1 have been achieved, and the driving capability is demonstrated by implementing a single SnO2 TFT device to tune the brightness of an organic light-emitting diode. It is worth noting that these TFTs work in enhancement mode at low voltages less than 4 V, which sheds light on their potential application to the next-generation low-cost active matrix flat panel displays.Keywords: enhancement mode; oxygen defects; solution-processed; thin-film transistor; tin oxide
Co-reporter:Dongxin Ma, Lian Duan, Yongge Wei, Lei He, Liduo Wang and Yong Qiu
Chemical Communications 2014 vol. 50(Issue 5) pp:530-532
Publication Date(Web):05 Nov 2013
DOI:10.1039/C3CC47362G
Phosphorescent quantum yields have been increased by 12 times by choosing bulky boracic anions as counterions for blue-emitting cationic iridium(III) complexes.
Co-reporter:Fuli Zhang, Dongxin Ma, Lian Duan, Juan Qiao, Guifang Dong, Liduo Wang, and Yong Qiu
Inorganic Chemistry 2014 Volume 53(Issue 13) pp:6596-6606
Publication Date(Web):June 10, 2014
DOI:10.1021/ic5001733
The development of pure-blue-to-deep-blue-emitting ionic phosphors is an ultimate challenge for full-color displays and white-light sources. Herein we report two series of short-wavelength light-emitting cationic iridium(III) complexes with nonconjugated ancillary and cyclometalating ligands, respectively. In the first series, nonconjugated 1-[(diphenylphosphino)methyl]-3-methylimidazolin-2-ylidene-C,C2′ (dppmmi) is used as the ancillary ligand and 2-phenylpyridine (ppy), 2-(2,4-difluorophenyl)pyridine (dfppy), and 1-(2,4-difluorophenyl)-1H-pyrazole (dfppz) are used as cyclometalating ligands. In the second one, nonconjugated 2,4-difluorobenzyl-N-pyrazole (dfbpz) is used as the cyclometalating ligand and 3-methyl-1-(2-pyridyl)benzimidazolin-2-ylidene-C,C2′ (pymbi) as the ancillary ligand. The synthesis and photophysical and electrochemical properties, together with the X-ray crystal structures of these complexes, have been investigated. At room temperature, blue-emitting complexes [Ir(ppy)2(dppmmi)]PF6 (1) and [Ir(dfppy)2(dppmmi)]PF6 (2; PF6– is hexafluorophosphate) show much larger photoluminescence quantum yields of 24% and 46%, respectively. On the contrary, for complexes [Ir(dfppz)2(dppmmi)]PF6 (3) and [Ir(dfbpz)2(pymbi)]PF6 (4), deep-blue luminescence is only observed at low temperature (77 K). Density functional theory calculations are used to rationalize the differences in the photophysical behavior observed upon changes of the ligands. It is shown that the electronic transition dipoles of cationic iridium complexes 1 and 2 are mainly confined to cyclometalated ligands (3MLCT and LC 3π–π*) and those of complex 3 are confined to all of the ligands (3MLCT, LC 3π–π*, and 3LLCT) because of the high LUMO energy level of dfppz. The emission of 4 mainly originates from the central iridium(III) ion and cyclometalated ligand to ancillary ligand charge transfer (3MLCT and 3LLCT), in contrast to commonly designed cationic complexes using carbene-type ancillary ligands, where emission originates from the cyclometalated main ligands. Solution-processed organic light-emitting diodes based on complexes 1 and 2 gave blue-green (498 nm) and blue (478 nm) electroluminescence with maximum current efficiencies of 3.8 and 3.4 cd A–1, respectively. The results indicate that introducing nonconjugated ligands into cationic iridium complexes is an effective means of achieving short-wavelength light-emitting phosphors.
Co-reporter:Zhengyang Bin, Lian Duan, Chen Li, Deqiang Zhang, Guifang Dong, Liduo Wang, Yong Qiu
Organic Electronics 2014 Volume 15(Issue 10) pp:2439-2447
Publication Date(Web):October 2014
DOI:10.1016/j.orgel.2014.07.002
•An insulating dopant BiF3 was employed in the hole-transporting layer.•The evaporation temperature of BiF3 is only about 350 °C.•BiF3 is a good dopant to improve the balance of holes and electrons in the emissive layer.•Doping of BiF3 leads to improved efficiency, thermal stability and lifetime of OLEDs.Bismuth Trifluoride (BiF3), with a high thermal stability and a low deposition temperature, has been studied as a novel dopant for the conventional hole transporting material of N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB). The efficiency and lifetime of organic light-emitting diodes (OLEDs) have been remarkably improved by using BiF3 doped NPB. For an optimized green device, a current efficiency of 21.6 cd/A is reached, 40% higher than the control device without BiF3. And the lifetime is increased from 115 h to 222 h at room temperature. The enhanced efficiency and lifetime are attributed to the improved balance of holes and electrons in the emissive layer. Most importantly, the thermal stability at an elevated temperature of the OLEDs with BiF3 doped NPB is largely improved, showing an order of magnitude longer lifetime than the control device at 80 °C.Graphical abstract
Co-reporter:Haoyuan Li;Chen Li;Yong Qiu
Israel Journal of Chemistry 2014 Volume 54( Issue 7) pp:918-926
Publication Date(Web):
DOI:10.1002/ijch.201400057
Abstract
In real devices, organic semiconductors are largely amorphous. Because accurate molecular packing in them cannot be obtained, the relationship between the molecular structure and the material properties can be difficult to understand. Nevertheless, knowing the charge transport processes is essential to material and device engineering. In amorphous organic semiconductors, charge transport is often apprehended as a hopping process that can be described using the Marcus or MillerAbrahams equations. The intrinsic disorder and frequently present traps have a great influence on the charge mobility. Carrier density, which would affect the effective density of states and create spacecharge perturbations, is also one important factor in the charge transport process. Herein, recent advances in the charge transport mechanism in amorphous organic semiconductors are summarized. The influences of disorder, carrier density, traps, and scatters are discussed in detail.
Co-reporter:Haoyuan Li ; Lian Duan ; Deqiang Zhang ;Yong Qiu
The Journal of Physical Chemistry C 2014 Volume 118(Issue 19) pp:9990-9995
Publication Date(Web):April 17, 2014
DOI:10.1021/jp5035618
It is known that the electric field is nonuniform inside organic electronic devices. However, the physics a few nanometers near interfaces and factors that influence the electric field distribution are still not fully understood. Moreover, the mobility might be nonuniform inside the device, since it is electric-field dependent. However, this has been overlooked in the apprehension of the space-charge-limited (SCL) current in the commonly used Mott–Gurney equation. Here, we carry out 3D multiparticle Monte Carlo simulations to study the electric field and energy diagram in a hole-only device under bias. Coulomb potential is obtained from the solution of the 1D Poisson equation of our system. The influences of the injection barrier, the energetic disorder and the applied bias are studied in detail. The SCL current is compared with that from the Mott−Gurney equation. It is found that the apparent charge mobilities are close to those calculated using the transit time corresponding to the cross point of asymptotes to the plateau and trailing edge of the current in the double logarithmic plot in the time-of-flight (TOF) simulations, which means that the space-charge-limited current (SCLC) measurement can be reliable for organic semiconductors.
Co-reporter:Haoyuan Li ; Lian Duan ;Yong Qiu
The Journal of Physical Chemistry C 2014 Volume 118(Issue 51) pp:29636-29642
Publication Date(Web):November 29, 2014
DOI:10.1021/jp510575q
Transition-metal oxides (TMOs), such as WO3, MoO3, V2O5, and ReO3, have been widely used as p-type dopants for organic semiconductors to improve device performances. However, it still remains unclear how charges transport after doping TMOs into organic materials. Here, Monte Carlo simulations are used to study the mechanisms of charge transport in TMO-doped organic semiconductors in a hole-only device. It is found that the charge carriers and the electric field are redistributed after the doping of TMOs, and the density of states distribution broadens. Furthermore, it is shown that new charge transport pathways are formed at medium-to-high doping ratios, leading to more efficient charge transport. At low energetic disorders, the charge mobility drops with the doping of TMOs. However, when the energetic disorder is high, the charge mobility will be improved at high doping ratios of TMOs. The conclusions will also help the understanding of the charge-transport process in electrochemical doped systems.
Co-reporter:Chen Li ; Lian Duan ; Haoyuan Li ;Yong Qiu
The Journal of Physical Chemistry C 2014 Volume 118(Issue 20) pp:10651-10660
Publication Date(Web):April 28, 2014
DOI:10.1021/jp5022906
In order to unravel the effect of trap energy on carrier transport in disordered organic semiconductors, a comprehensive study was conducted on hole transport in a series of organic molecular hosts with explicit traps at different trap energies. The mobility measured by time-of-flight experiments was found to decrease significantly at shallow trapping, but hardly decreased at deep trapping, where a sharp decrease of carrier density was observed. By analyzing temperature dependence of the mobility, the decreased mobility at shallow trapping were found to originate from increased energetic disorder and activation energy, whereas both energetic disorder and activation energy are changeless at deep trapping. We find it reasonable to cover the different effects of deep trapping and shallow trapping in a universal mechanism based on the Miller–Abrahams hopping model, and carry out multiple-carrier Monte Carlo simulations to elaborate how, within a universal mechanism, that deep and shallow traps affect energetic disorder, transporting trajectories and carrier density in different ways. The results suggest transporting at shallow trapping always is involved in a multiple-trapping-release process owing to frequent thermal reactivation, thus leading to winding transporting trajectory, increased effective energetic disorder, and increased activation energy; while deep traps tend to immobilize the carriers and act as ionized scattering centers, thus mainly decreasing the carrier density and just elongating the trajectory slightly. Investigating the change of mobility with continuous trap energy suggests a transition region, rather than a strict borderline existing between deep traps and shallow traps, where carrier transport is controlled by both deep traps and thermal reactivation from shallow traps.
Co-reporter:Dongxin Ma;Dr. Lian Duan; Yongge Wei ; Yong Qiu
Chemistry - A European Journal 2014 Volume 20( Issue 48) pp:15903-15912
Publication Date(Web):
DOI:10.1002/chem.201403278
Abstract
Trifluoromethylation of tetraphenlyborate counterions was successfully used to improve the electrochemical stabilities and device performance of cationic iridium(III) complexes. Melioration of the thermal, photoluminescent, electrochemical, and electrophosphorescent characteristics was achieved. Interionic hydrogen bonds were first found between the aromatic hydrogen atoms in the ancillary ligands of cations and the fluorine atoms in the trifluoromethyl groups of the anions. The strong impact of the counterions on the charge transport in the devices was investigated. A compound with two trifluoromethyl groups in the tetraphenlyborate ion shows the highest photoluminescent efficiency, the best electrochemical stability, and the greatest performance in green-blue-emitting devices, with a high current efficiency of 12.4 cd A−1 and an emission peak at λ=480 nm. The efficiencies achieved are the highest reported for OLEDs with ionic complexes emitting in the blue-green region.
Co-reporter:Dongdong Zhang, Lian Duan, Deqiang Zhang, Juan Qiao, Guifang Dong, Liduo Wang, Yong Qiu
Organic Electronics 2013 Volume 14(Issue 1) pp:260-266
Publication Date(Web):January 2013
DOI:10.1016/j.orgel.2012.11.003
In order to achieve low driving voltage, electrophosphorescent green organic light-emitting diodes (OLEDs) based on a host material with small energy gap between the lowest excited singlet state and the lowest excited triplet state (ΔEST) have been fabricated. 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)- 1,3,5-triazine (PIC–TRZ) with ΔEST of only 0.11 eV has been found to be bipolar and used as the host for green OLEDs based on tris(2-phenylpyridinato) iridium(III) (Ir(ppy)3). A very low onset voltage of 2.19 V is achieved in devices without p- or n-doping. Maximum current and power efficiencies are 68 cd/A and 60 lm/W, respectively, and no significant roll-off of current efficiency (58 cd/A at 1000 cd/m2 and 62 cd/A at 10,000 cd/m2) have been observed. The small roll-off is due to the improved charge balance and the wide charge recombination zone in the emissive layer.Graphical abstractHighlights► Host material with small energy gap between the lowest excited singlet state and the lowest excited triplet state. ► A very low onset voltage of 2.91 V is achieved. ► High efficiency and small roll-off have been observed.
Co-reporter:Yan Zhao, Lian Duan, Deqiang Zhang, Juan Qiao, Liduo Wang, Yong Qiu
Organic Electronics 2013 Volume 14(Issue 3) pp:882-887
Publication Date(Web):March 2013
DOI:10.1016/j.orgel.2013.01.001
We demonstrate the enhanced performance in organic light-emitting diodes by chlorinated indium tin oxide (ITO) in the presence of hydrogen peroxide. We adopt the approach of UV light irradiation of the ITO with o-dichlorobenzene and hydrogen peroxide. Adding hydrogen peroxide in the system can accelerate the rate of UV photolysis of o-dichlorobenzene and reduce the UV illumination time, and the device shows the highest performance of current efficiency 4.15 cd/A. Under UV radiation, hydrogen peroxide would generate hydroxyl radical HO. to attack o-dichlorobenzene, then further reducing the UV illumination time.Graphical abstractHighlights► UV light irradiation of the ITO with o-dichlorobenzene and hydrogen peroxide would improve the work function of ITO. ► Adding hydrogen peroxide in the system can accelerate the rate of UV photolysis of o-dichlorobenzene. ► XPS data prove the mechanism that adding H2O2 into the system can significantly reduce the UV illumination time.
Co-reporter:Chen Li, Lian Duan, Haoyuan Li, Yong Qiu
Organic Electronics 2013 Volume 14(Issue 12) pp:3312-3317
Publication Date(Web):December 2013
DOI:10.1016/j.orgel.2013.09.039
•Time of flight mobilities of Alq3:CBP co-evaporated mixtures are investigated.•The dependence of mobility upon CBP fraction well follows the percolation theory.•We construct a percolating model to quantify the percolation threshold.•This work give a good description of carrier mobility in mixed organic materials.Understanding the charge transport in molecular semiconductor mixtures remains challenging, largely due to the lack of a universal dependence of carrier mobility upon doping concentration. Here we demonstrate that it is feasible to use the percolation theory to explain the change of charge mobility in a model system of 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) and tris-(8-hydroxyquinoline) aluminum (Alq3) with various doping concentrations. As the fraction of CBP within the mixtures increases, the charge mobility firstly shows a reduction at low CBP fraction due to the scattering effect, and then increases well following a percolation model. Electron microscopy and atomic force microscopy analysis suggest that CBP and Alq3 are homogeneously mixed in their co-evaporated amorphous films, which meets the precondition for using percolation theory. We describe the possible microcosmic percolating mechanism with a model combining bond percolation with charge transfer integral calculation. Based on this model, the percolation threshold in molecular semiconductor mixtures can be predicated. For the hole and electron transport in our system, the predicated percolation thresholds are very close to the experimental values.Graphical abstract
Co-reporter:Yan Zhao, Lian Duan, Xiao Zhang, Deqiang Zhang, Juan Qiao, Guifang Dong, Liduo Wang and Yong Qiu
RSC Advances 2013 vol. 3(Issue 44) pp:21453-21460
Publication Date(Web):11 Sep 2013
DOI:10.1039/C3RA43017K
A white organic light emitting diode (OLED) with a simple structure using exciplex emission is fabricated. The white emission of exciplex is formed at the interface between new electron transport material anthracene-9,10-diylbis(diphenylphosphine oxide) (DPPA) and hole transporting layer N,N′-bis (naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB). Pure white emission with Commission International de L'Eclariage (CIE) coordinates (0.33, 0.33) at the equal-energy white point is obtained. The phosphine oxide type material DPPA based OLEDs work well when Al is directly deposited on them, without an electron injection layer. In order to investigate the mechanism of electron injection from Al to DPPA, X-ray photoelectron spectroscopy (XPS) measurements have been carried out and revealed that the improved electron injection is due to the existence of interaction between the phosphine oxide group of DPPA and Al at the DPPA/Al interface.
Co-reporter:Tao Hu, Lian Duan, Juan Qiao, Lei He, Deqiang Zhang, Liduo Wang, Yong Qiu
Synthetic Metals 2013 Volume 163() pp:33-37
Publication Date(Web):1 January 2013
DOI:10.1016/j.synthmet.2012.12.017
We demonstrate an efficient red emitting host-guest LEC based on cationic iridium complexes. The host-guest LEC has brightness of 8.4 cd m−2, external quantum efficiency (EQE) of up to 3.5% and peak current efficiency of 3.4 cd A−1, the highest efficiencies reported to date for red-emitting host-guest LECs based on cationic iridium complexes. It is confirmed that dispersing the guest molecules into the host matrix would greatly inhibit the self-quenching of the guest molecules in the emissive layer and thus improving the device efficiency, and offering an effective approach to improve device efficiencies of LECs.Graphical abstractHighlights► An efficient host-guest red emitting LEC is realized with cationic iridium complexes. ► High external quantum efficiency (EQE) of up to 3.5% and current efficiency of 3.4 cd A−1 are achieved. ► It is confirmed that doping could depress self-quenching in the emissive layer and improve device efficiencies.
Co-reporter:Lei He, Lian Duan, Juan Qiao, Deqiang Zhang, Guifang Dong, Liduo Wang, Yong Qiu
Synthetic Metals 2013 Volume 166() pp:52-56
Publication Date(Web):15 February 2013
DOI:10.1016/j.synthmet.2013.01.019
Light-emitting electrochemical cells (LECs) and organic light-emitting diodes (OLEDs) have been investigated based on three blue-green-emitting cationic iridium complexes (1–3). From complexes 1 to 2 and 3, the pendant phenyl ring on the ancillary ligand is gradually fluorinated. It is found that fluorination of the pendant phenyl ring reduces the electrochemical stability of the complexes, which in turn decreases the efficiency and stability of LECs based on the complexes, despite the fact that fluorination reinforces the intramolecular π–π stacking interactions. Fluorination of the pendant phenyl ring also makes the electroluminescent (EL) spectra of LECs red-shifted due to enhanced intermolecular interactions in films. When the complexes are used as dopants in OLEDs, fluorination of the pendant phenyl ring largely enhances the electron-trapping ability of the complexes in the light-emitting layer. Blue-green OLEDs based on complexes 1, 2 and 3 showed promising performances, with peak current efficiencies of 18.3, 9.0 and 14.7 cd A−1, respectively.Graphical abstractHighlights► LECs and OLEDs based on three cationic iridium complexes were characterized. ► Effect of fluorinating the pendant phenyl ring on device performance was studied. ► Fluorination reduces LEC stability despite enhanced intramolecular π–π interaction. ► Fluorination makes the electroluminescence of LEC red-shifted. ► Fluorination largely enhances electron-trapping ability of the complex in OLED.
Co-reporter:Yunlong Zhao, Lian Duan, Guifang Dong, Deqiang Zhang, Juan Qiao, Liduo Wang, and Yong Qiu
Langmuir 2013 Volume 29(Issue 1) pp:151-157
Publication Date(Web):December 5, 2012
DOI:10.1021/la304581c
Films of zinc tin oxide (ZTO), grown from solutions with zinc acetate dehydrate and tin(II) 2-ethylhexanoate dissolved in 2-methoxyethanol, have been used to fabricate thin-film transistors in combination with solution-processed aluminum oxide as the gate insulator. And the nonhomogeneity of the single-layer ZTO films, caused by both ZTO film–substrate interaction and surface crystallization, has been studied, which is essential to achieve high performance transistors. In the bottom-contact thin-film transistor based on a Sn-rich layer of ZTO, a high mobility of 78.9 cm2 V–1 s–1 in the saturation region has been obtained, with an on-to-off current ratio of 105 and a threshold gate voltage of 1.6 V.
Co-reporter:Yifu Jia ; Lian Duan ; Deqiang Zhang ; Juan Qiao ; Guifang Dong ; Liduo Wang ;Yong Qiu
The Journal of Physical Chemistry C 2013 Volume 117(Issue 27) pp:13763-13769
Publication Date(Web):June 13, 2013
DOI:10.1021/jp400003m
Transition-metal oxides (TMOs) are one of the most promising kinds of p-doping materials for organic semiconductors. However, to be compatible with organic materials, low-temperature evaporable TMOs are highly desirable. Rhenium(VII) oxide with a very low melting temperature of only 225 °C, which is the lowest among all TMO dopants, is first investigated as a p-dopant in N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4-diamine (NPB). Systematic studies are performed compared with ReO3, a different valence state oxide of rhenium. Hole mobility improvement from 5.38 × 10–4 to 5.88 × 10–3 cm2/(V s) at an electric field of 3 × 105 V/cm is achieved by doping Re2O7 into NPB. Lower valence states of Re species in Re2O7-doped NPB than ReO3 are observed by XPS study, indicating stronger charge transfer between Re2O7 and NPB. Temperature-dependent I–V study reveals lower hole injection barrier of Re2O7 than ReO3 in hole-only devices. Crystallinity of NPB films is found to be the same before and after doping by XRD study. Absorption spectrum study reveals higher stability of Re2O7-doped NPB than ReO3 in air. Hole current is enhanced by three orders of magnitude at 2 V when utilizing both rhenium-oxide-doped NPBs in hole-only devices. OLED devices with both rhenium-oxide-doped NPBs as hole injection layer (HIL) show a similar efficiency of 3.3 cd/A at 300 mA/cm2. Also, driving voltage is reduced from 2.6 V for pure NPB to 2.5 and 2.4 V for Re2O7 and ReO3 doped NPB, respectively.
Co-reporter:Haoyuan Li, Lian Duan, Yongduo Sun, Deqiang Zhang, Liduo Wang, and Yong Qiu
The Journal of Physical Chemistry C 2013 Volume 117(Issue 32) pp:16336-16342
Publication Date(Web):July 15, 2013
DOI:10.1021/jp4050868
An accurate description of charge transport in amorphous organic semiconductors is challenging. Many previously reported methods largely involve empirical parameters, which may hinder the understanding of the charge transport process in a specific material. In this paper, Born–Oppenheimer molecular dynamics (BOMD) is used to simulate the amorphous structure of a widely used small molecule 9,10-di-(2′-naphthyl)anthracene (ADN). Its hole and electron mobilities are calculated using an ab initio method. It is found that the inaccuracy in the calculation of the nonadiabatic couplings caused by the periodicity of the cell in the BOMD simulation can be greatly reduced by taking into account the mirror states in the surrounding cells. The calculated hole and electron mobilities both have the same order of magnitude with their corresponding experimental results, demonstrating the possibility to obtain reasonable charge transport mobilities for amorphous small-molecule semiconductors via the first-principles approach. Our work may shed light on the understanding of the charge transport process in amorphous organic semiconductors and the design of new charge transport materials.
Co-reporter:Tao Hu, Lei He, Lian Duan and Yong Qiu
Journal of Materials Chemistry A 2012 vol. 22(Issue 10) pp:4206-4215
Publication Date(Web):20 Jan 2012
DOI:10.1039/C2JM16185K
Solid-state light-emitting electrochemical cells (LECs) have aroused great interest in recent years as novel organic light-emitting devices and a promising lighting solution. LECs have many advantages over multilayered organic lighting-emitting diodes (OLEDs), such as single layer, solution process and air-stable cathodes. In recent years, ionic iridium complexes are widely used in LECs for their high efficiencies and tunable emission colours. In this feature article, we will review the proposed operation mechanisms of LECs, the development of ionic iridium complexes and the progress in improving colour, efficiency, stability and response time of the LECs. Finally, the prospects and remaining problems will be discussed.
Co-reporter:Lei He ; Dongxin Ma ; Lian Duan ; Yongge Wei ; Juan Qiao ; Deqiang Zhang ; Guifang Dong ; Liduo Wang ;Yong Qiu
Inorganic Chemistry 2012 Volume 51(Issue 8) pp:4502-4510
Publication Date(Web):March 30, 2012
DOI:10.1021/ic2021325
Intramolecular π–π stacking interaction in one kind of phosphorescent cationic iridium complexes has been controlled through fluorination of the pendant phenyl rings on the ancillary ligands. Two blue-green-emitting cationic iridium complexes, [Ir(ppy)2(F2phpzpy)]PF6 (2) and [Ir(ppy)2(F5phpzpy)]PF6 (3), with the pendant phenyl rings on the ancillary ligands substituted with two and five fluorine atoms, respectively, have been synthesized and compared to the parent complex, [Ir(ppy)2(phpzpy)]PF6 (1). Here Hppy is 2-phenylpyridine, F2phpzpy is 2-(1-(3,5-difluorophenyl)-1H-pyrazol-3-yl)pyridine, F5phpzpy is 2-(1-pentafluorophenyl-1H-pyrazol-3-yl)-pyridine, and phpzpy is 2-(1-phenyl-1H-pyrazol-3-yl)pyridine. Single crystal structures reveal that the pendant phenyl rings on the ancillary ligands stack to the phenyl rings of the ppy ligands, with dihedral angles of 21°, 18°, and 5.0° between least-squares planes for complexes 1, 2, and 3, respectively, and centroid-centroid distances of 3.75, 3.65, and 3.52 Å for complexes 1, 2, and 3, respectively, indicating progressively reinforced intramolecular π–π stacking interactions from complexes 1 to 2 and 3. Compared to complex 1, complex 3 with a significantly reinforced intramolecular face-to-face π–π stacking interaction exhibits a significantly enhanced (by 1 order of magnitude) photoluminescent efficiency in solution. Theoretical calculations reveal that in complex 3 it is unfavorable in energy for the pentafluorophenyl ring to swing by a large degree and the intramolecular π–π stacking interaction remains on the lowest triplet state.
Co-reporter:Fuli Zhang, Lian Duan, Juan Qiao, Guifang Dong, Liduo Wang, Yong Qiu
Organic Electronics 2012 Volume 13(Issue 7) pp:1277-1288
Publication Date(Web):July 2012
DOI:10.1016/j.orgel.2012.03.017
Two new blue emitting cationic iridium complexes with N-heterocyclic carbene–pyridine as the ancillary ligand, namely, [Ir(ppy)2(pymi)]PF6 and [Ir(dfppy)2(pymi)]PF6 (pymi is 1-pyridyl-3-methylimidazolin-2-ylidene-C,C2′, ppy is 2-phenylpyridine, dfppy is 2-(2,4-difluorophenyl)pyridine and PF6- is hexafluorophosphate), have been prepared, and the photophysical and electrochemical properties together with X-ray crystal structures have been investigated. In CH3CN solutions, [Ir(ppy)2(pymi)]PF6 and [Ir(dfppy)2(pymi)]PF6 exhibit blue light emission with the peaks at 472 and 451 nm, respectively. Both photophysical properties and quantum chemical calculations indicate that photoluminescences of these complexes are mainly from ppy- or dfppy-based 3π → π∗ states. Solution-processed organic light-emitting diodes (OLEDs) based on [Ir(ppy)2(pymi)]PF6 and [Ir(dfppy)2(pymi)]PF6 give blue–green electroluminescence (506 and 482 nm, respectively). At a doping concentration of 5 wt.%, the device based on [Ir(ppy)2(pymi)]PF6 reaches a maximum efficiency of 5.2 cd A−1, which indicates that this complex is a promising phosphor for achieving efficient electrophosphorescence in the blue–green region.Graphical abstractHighlights► Cationic iridium complexes with N-heterocyclic carbene–pyridine as the ancillary ligand. ► Blue phosphorescence emission of cationic iridium complexes. ► Solution-processed blue–green organic light-emitting diodes.
Co-reporter:Yunlong Zhao, Guifang Dong, Lian Duan, Juan Qiao, Deqiang Zhang, Liduo Wang and Yong Qiu
RSC Advances 2012 vol. 2(Issue 12) pp:5307-5313
Publication Date(Web):09 May 2012
DOI:10.1039/C2RA00764A
In order to study the impacts of precursors on solution-processed metal oxide films and the performance of their field-effect transistors (FETs), zinc acetate dehydrate and four different Sn precursors – tin(II) 2-ethylhexanoate, tin(IV) acetate, tin(II) chloride and tin(IV) isopropoxide – were utilized to prepare zinc–tin oxide (ZTO) thin films by metal–organic decomposition (MOD) and sol–gel processes. Through systematic analysis of these films and devices, it is demonstrated that Sn precursors, with different molecular geometrical configurations and organic ligands, greatly affect the thickness, density, morphology and composition of the ZTO thin films and, hence, the performance of their FETs. It is worth noting that although all of the ZTO thin films are amorphous, the morphologies of the ZTO thin films yielded from Sn(II) precursors, with RMS values below 0.5 nm, are much better than those yielded from Sn(IV) precursors. The ZTO-FET prepared from Zn(CH3COO)2·2H2O and SnCl2 shows a typical field-effect charge carrier mobility of 1.8 cm2 V−1 s−1, with an on/off current ratio of 6 × 105. Our research also indicates that tin(II) 2-ethylhexanoate and tin(IV) isopropoxide are promising Sn precursors for the fabrication of transistors.
Co-reporter:Tao Hu, Lian Duan, Juan Qiao, Lei He, Deqiang Zhang, Ruji Wang, Liduo Wang, Yong Qiu
Organic Electronics 2012 Volume 13(Issue 10) pp:1948-1955
Publication Date(Web):October 2012
DOI:10.1016/j.orgel.2012.06.005
Co-reporter:Fuli Zhang, Lian Duan, Juan Qiao, Guifang Dong, Liduo Wang, Yong Qiu
Organic Electronics 2012 Volume 13(Issue 11) pp:2442-2449
Publication Date(Web):November 2012
DOI:10.1016/j.orgel.2012.06.050
Recently, the stabilities of light-emitting electrochemical cells (LECs) based on cationic iridium(III) complexes with controlled intramolecular π–π stacking interactions by adopting pendant phenyl rings have been dramatically enhanced compared with those of the complexes without π–π stacking interactions within the molecule. Herein, a novel cationic iridium complex [Ir(ppy)2(pyphmi)]PF6[ppy = 2-phenylpyridine, pyphmi = 1-pyridyl-3-phenylimidazolin-2-ylidene-C,C2′] which exhibits intramolecular π-stacking interaction has been prepared and its X-ray crystal structure has been investigated. Unexpectedly, however, the corresponding LECs based on [Ir(ppy)2(pyphmi)]PF6 do not show significantly enhanced stabilities compared to the LECs based on [Ir(ppy)2(pymi)]PF6 [pymi = 1-pyridyl-3-methylimidazolin-2-ylidene-C,C2′] without pendant phenyl rings within the molecule. This phenomenon is attributed to the very long centroid–centroid distance between the π-stacked phenyl rings, which results from the larger tension of substituted five-membered ring moiety (imidazolin-2-ylidene). In addition, irreversible oxidation and reduction processes would also decrease the electrochemical stability of [Ir(ppy)2(pyphmi)]PF6. Thus, intramolecular π–π stacking interaction using pendant phenyl rings is not always effective to improve the stability of LECs.Graphical abstractHighlights► Cationic iridium(III) complexes with controlled intramolecular π–π stacking interactions. ► Light-emitting electrochemical cells. ► Not always effective to improve the stability.
Co-reporter:Chen Li, Lian Duan, Yongduo Sun, Haoyuan Li, and Yong Qiu
The Journal of Physical Chemistry C 2012 Volume 116(Issue 37) pp:19748-19754
Publication Date(Web):August 27, 2012
DOI:10.1021/jp307951h
The effects of trapping and scattering on the transporting properties of organic disorder semiconductors have been studied by time-of-flight (TOF) method. Tris-(8-hydroxyquinoline)-aluminum (Alq3), 2,2′,2″-(1,3,5-benzenetriyl)-tris-(1-phenyl-1H-benzimid-azole) (TPBi), and N,N-diphenyl-N,N-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′diamine (NPB) are doped into 4,4′-N,N′-dicarbazolebiphenyl (CBP) to form traps and scatters with various energy level differences. It is found that the low scatters significantly reduce the mobility and make the TOF transients, while the deep traps and high scatters would not significantly reduce the mobility and change the nondispersive behavior of the TOF transients. The main difference between deep traps and high scatters is that the deep traps induce a great reduction of the photocurrent, while the high scatters do not obviously decrease the photocurrent. The experimental results are well explained by the Miller–Abrahams hopping model and the effective energetic disorder. Furthermore, a theoretical method is established to determine the demarcation between the shallow trap (low scatter) and the deep trap (high scatter) in terms of energy level differences. These results may shed light on the understanding of charge transport in mixed organic semiconductors.
Co-reporter:Lei He, Lian Duan, Juan Qiao, Deqiang Zhang, Liduo Wang and Yong Qiu
Chemical Communications 2011 vol. 47(Issue 22) pp:6467-6469
Publication Date(Web):09 May 2011
DOI:10.1039/C1CC11263E
A new cationic iridium complex has been developed with 2-(1-phenyl-1H-pyrazol-3-yl)pyridine as the ancillary ligand, which bears a pendant protective phenyl ring within the molecule; blue-green light-emitting electrochemical cells (LECs) based on the complex show dramatically enhanced stability compared to the LEC based on a similar complex without pendant phenyl rings.
Co-reporter:Lei He, Lian Duan, Juan Qiao, Guifang Dong, Liduo Wang and Yong Qiu
Chemistry of Materials 2010 Volume 22(Issue 11) pp:3535
Publication Date(Web):May 10, 2010
DOI:10.1021/cm100993j
Blue-green-emitting cationic iridium complex with high luminescent efficiencies in both solutions and solid-states are essential for high-performance white light-emitting electrochemical cells (LECs). We report here an efficient blue-green-emitting cationic iridium complex [Ir(dfppz)2(tp-pyim)]PF6, using 1-(2,4-difluorophenyl)-1H-pyrazole (dfppz) as the cyclometalated ligand and 2-(1-(4-tritylphenyl)-1H-imidazol-2-yl)pyridine (tp-pyim) as the ancillary ligand. [Ir(dfppz)2(tp-pyim)]PF6 emits efficient blue-green light with a luminescent quantum yield of 0.54 in CH3CN solution. Because of the sterically bulky group 4-tritylphenyl that is attached to the ancillary ligand, the intermolecular interaction and excited-state self-quenching of [Ir(dfppz)2(tp-pyim)]PF6 in solid states is significantly suppressed. Theoretical calculations reveal that the emission from [Ir(dfppz)2(tp-pyim)]PF6 has both metal-to-ligand charge-transfer and ligand-centered 3π−π* character. LECs based on [Ir(dfppz)2(tp-pyim)]PF6 show highly efficient blue-green electroluminescence with peak current efficiency, external quantum efficiency, and power efficiency of 18.3 cd A−1, 7.6%, and 18.0 lm W−1, respectively. White LECs based on [Ir(dfppz)2(tp-pyim)]PF6 give warm-white light, with Commission Internationale de L’Eclairage coordinates of (0.37, 0.41), a color-rendering index up to 80, and a peak power efficiency of 11.2 lm W−1.
Co-reporter:Lian Duan, Byung Doo Chin, Nam Choul Yang, Mu-Hyun Kim, Hye Dong Kim, Seong Taek Lee, Ho Kyoon Chung
Synthetic Metals 2007 Volume 157(8–9) pp:343-346
Publication Date(Web):May 2007
DOI:10.1016/j.synthmet.2007.03.011
Employment of multilayer heterostructures is a common approach to achieve efficiency and stable organic light emitting diodes (OLEDs). In this work, we report multilayer blue polymer light-emitting devices (PLEDs) by using spin-coated fluorene-triarylamine copolymers as interlayers between the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT) and the emitting layer. A blue PLED with stepped hole injection profile yields an external quantum efficiency of 6.0% at a luminance of 9500 cd/m2 at 5.5 V and an extrapolated lifetime of more than 18,000 h from 100 cd/m2.
Co-reporter:Amjad Islam, Dongdong Zhang, Xinhua Ouyang, Rongjuan Yang, Tao Lei, Ling Hong, Ruixiang Peng, Lian Duan and Ziyi Ge
Journal of Materials Chemistry A 2017 - vol. 5(Issue 26) pp:NaN6536-6536
Publication Date(Web):2017/05/31
DOI:10.1039/C7TC01597F
Highly efficient organic light-emitting diodes (OLEDs) with simplified device structures are widely desired for both scientific research and industrial applications. However, a very limited number of simplified OLEDs have been reported to date. In this work, two multifunctional blueish green emitters, BPTPETPAI and 2TPETPAI, are designed and synthesized. Owing to the presence of a tetraphenylethene (TPE) moiety, their aggregation induced emission (AIE) properties are also investigated. High photoluminescence efficiencies of the two compounds in non-doped films render them good emitters for non-doped devices. Multilayer non-doped devices based on these emitters achieve maximum external quantum efficiencies (EQEs) and current efficiencies (CEs) of 3.13% and 6.14 cd A−1 as well as 3.25% and 6.70 cd A−1 for BPTPETPAI and 2TPETPAI, respectively. Given their shallow highest occupied molecular orbital (HOMO) energy levels, both emitters can also be used as hole injection and hole transporting materials. Based on this, single layer devices show even higher efficiencies with extremely low efficiency roll-off, achieving maximum CEs as high as 7.12 cd A−1 and 7.80 cd A−1 using BPTPETPAI and 2TPETPAI, respectively. These results demonstrate a bright prospect for the development of highly desired multifunctional emitters as well as simplified OLEDs with significant reduction in the fabrication cost of the device.
Co-reporter:Dongxin Ma, Chen Zhang, Yong Qiu and Lian Duan
Journal of Materials Chemistry A 2016 - vol. 4(Issue 24) pp:NaN5738-5738
Publication Date(Web):2016/05/27
DOI:10.1039/C6TC01302C
We designed and synthesized a series of cationic iridium(III) complexes with the same coordinated iridium(III) cation but different-sized counter-ions, investigated their photophysical properties, electrochemical behaviours and thermal stability, then fabricated single-layer solution-processed organic light-emitting diodes (OLEDs) thereof, and demonstrated anionic migration in devices. By doping these cationic iridium(III) complexes at low concentrations (2 or 3 wt%) thus avoiding their anionic migration, we succeeded in the preparation of efficient blue-green OLEDs, achieving the highest current efficiency of 17.1 cd A−1, an external quantum efficiency of 6.8%, a maximum luminance of 14.2 × 103 cd m−2 and colour coordinates of (0.21, 0.48). To our knowledge, these values are among the best reported OLEDs based on ionic transition metal complexes as phosphorescent emitters in the blue-green region. The impact of different-sized counter-ions on carrier transport characteristics was also examined by single carrier devices. Notably, by doping these cationic iridium(III) complexes at high concentrations (20 wt%) and thus wisely employing their anionic migration, we developed simple-constructed OLEDs with a temporary p–i–n junction instead of active n-type dopants in the cathode and obtained a high current efficiency of 8.3 cd A−1 and a maximum luminance of 12.8 × 103 cd m−2, rather comparable to the corresponding OLEDs with the conventional Cs2CO3 active cathode.
Co-reporter:Dongxin Ma, Lian Duan and Yong Qiu
Dalton Transactions 2015 - vol. 44(Issue 18) pp:NaN8528-8528
Publication Date(Web):2014/11/04
DOI:10.1039/C4DT02830A
Tetraimidazolylborate is first used as the counterion of two novel ionic iridium complexes, instead of tetraphenylborate. Both the electrochemical stabilities and electroluminescence are sharply improved. Organic light-emitting diodes have been successfully fabricated with a high current efficiency of 9.48 cd A−1 and a maximum brightness of 5163 cd m−2.
Co-reporter:Haoyuan Li ; Lian Duan ; Deqiang Zhang ;Yong Qiu
The Journal of Physical Chemistry C () pp:
Publication Date(Web):June 18, 2014
DOI:10.1021/jp504979x
The reorganization energy is one important parameter regarding the charge transfer in organic semiconductors. In the past, the influence of the molecular packing on the intramolecular reorganization energy was commonly neglected in theoretical estimations. Here, the quantum mechanic/molecular mechanic (qm/mm) method is used to study this influence for four small molecules: N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD), N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB), 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), and 4,7-diphenyl-1,10-phenanthroline (Bphen). An approximate linear relationship between the intramolecular reorganization energy and the change of the torsion angle during the molecular relaxation which causes the most steric hindrance is found. Furthermore, reorganization energies from qm/mm calculations are smaller depending on the degree of reduced conformational change.
Co-reporter:Dongxin Ma, Lian Duan and Yong Qiu
Dalton Transactions 2016 - vol. 45(Issue 14) pp:NaN6123-6123
Publication Date(Web):2015/11/09
DOI:10.1039/C5DT03776J
Spin-cast from various solvents, emissive layers show different film morphologies and performances in solution-processed organic light-emitting diodes (OLEDs). Here we fabricated and demonstrated highly efficient blue OLEDs based on bis[3,5-difluoro-2-(2-pyridyl)phenyl]-(2-carboxypyridy)iridium(III) by choosing several kinds of solvents for spin-coating. Experiments indicate that the single-layer device with an emissive film cast from chlorobenzene shows its best performance with a highest current efficiency of 18.99 cd A−1, a maximum luminance of 20.5 × 103 cd m−2 and an emission band centered at 474 nm. The efficiency achieved is the highest reported for solution-processed simple-manufactured OLEDs doped with transition metal phosphors emitting in the blue region.
Co-reporter:Dongxin Ma, Lian Duan and Yong Qiu
Journal of Materials Chemistry A 2016 - vol. 4(Issue 22) pp:NaN5058-5058
Publication Date(Web):2016/05/05
DOI:10.1039/C6TC00738D
Two novel red-emitting cationic iridium complexes, [Ir(ppy)2(pop)][B(5fph)4] (1) and [Ir(ppy)2(pop)][BArF24] (2), have been developed, where ppy is 2-phenylpyridine, pop is 2-(5-phenyl-1-1,3,4-oxadiazol-2-yl)pyridine, [B(5fph)4]− is tetrakis(pentafluorophenyl)borate and [BArF24]− is tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, respectively. Photophysical properties of 1 and 2 in both solution and neat film were fully investigated, along with their photochemical, thermal and electrochemical stability. Interestingly, by introducing bulky tetraphenylborate derivatives as negative counter-ions, the volatility of 1 and 2 has been extremely improved, enabling fabrication of organic light-emitting diodes (OLEDs) by vacuum evaporation deposition. By doping these two sublimable cationic emitters into a DIC-TRZ (2,4-diphenyl-6-bis(12-phenylindolo[2,3-a]carbazole-11-yl)-1,3,5-triazine) host, we succeeded in the preparation of orange-red-emitting devices with a peak wavelength of 596 nm. 1-Based OLEDs showed a current efficiency of 4.5 cd A−1 and maximum brightness of 19.4 × 103 cd m−2, whereas 2-based OLEDs furnished a higher efficiency of 5.1 cd A−1. Then, we attained a white emission by doping 1 or 2 into a TCTA (4,4′,4′′-tris(carbazol-9-yl)triphenylamine) host at low concentrations. The 1-based white device featured a high colour rendering index (CRI) of 86 and good Commission International de L'EClairage (CIE) coordinates of (0.33, 0.34), quite close to the equal-energy-white-point (i.e., CIEx,y = 0.33, 0.33), and the 2-based white device showed a rather higher CRI of 89. To the best of our knowledge, this is the first report of white OLEDs fabricated by vacuum evaporation deposition of sublimable cationic iridium complexes, indicating their great potential for use in full-colour flat-panel display and lighting applications.
Co-reporter:Xinxin Ban, Wei Jiang, Kaiyong Sun, Haiyong Yang, Yanan Miao, Fenghao Yang, Yueming Sun, Bin Huang and Lian Duan
Journal of Materials Chemistry A 2015 - vol. 3(Issue 19) pp:NaN5016-5016
Publication Date(Web):2015/04/07
DOI:10.1039/C5TC00691K
A series of bipolar hosts based on carbazole and phenyl benzimidazole (PBI) moieties, collectively named xCz–nPBI, were designed and synthesized. On the basis of different numbers, ratios and link-configurations of the functional groups, the influence of substitution on the chemical, photophysical and electrochemical properties of the host materials were investigated in detail. Both DFT calculations and single carrier devices demonstrate that the strategy of introducing more electron-withdrawing PBI groups in the molecules can effectively enhance the electron injection and transport ability of the bipolar host, while an increased number of carbazole units endows the hosts with a much smaller ΔEST for efficient hole injection at the cost of sacrificing their charge balance property. As a result, the solution-processed green-emitting PHOLEDs based on Cz–6PBI show an extremely low turn on voltage of 2.9 V and the highest current and power efficiency of 47.8 cd A−1 and 29.6 lm W−1, respectively. Even at luminance as high as 1000 cd m−2, their efficient roll-off was only 4.2%, which was far better than the 6Cz–PBI host device. Because the T1 energy levels and triplet state locations of these hosts are similar, their ΔEST and charge balance property should be the main factors that influence their EL performances. We conclude that it is not necessary to achieve a very small ΔEST by introducing more carbazole moieties at the cost of weakening of electron transporting ability. As for solution-processed devices, which suffer from solvent impurities and oxygen diffusion induced strong electron trapping effect, a systemic increase in the number of electron-withdrawing PBI groups in their host materials can significantly enhance the charge balance of their emission layers (EMLs) for highly power efficient solution-processed PHOLEDs.
Co-reporter:Tao Hu, Lei He, Lian Duan and Yong Qiu
Journal of Materials Chemistry A 2012 - vol. 22(Issue 10) pp:NaN4215-4215
Publication Date(Web):2012/01/20
DOI:10.1039/C2JM16185K
Solid-state light-emitting electrochemical cells (LECs) have aroused great interest in recent years as novel organic light-emitting devices and a promising lighting solution. LECs have many advantages over multilayered organic lighting-emitting diodes (OLEDs), such as single layer, solution process and air-stable cathodes. In recent years, ionic iridium complexes are widely used in LECs for their high efficiencies and tunable emission colours. In this feature article, we will review the proposed operation mechanisms of LECs, the development of ionic iridium complexes and the progress in improving colour, efficiency, stability and response time of the LECs. Finally, the prospects and remaining problems will be discussed.
Co-reporter:Dongdong Zhang, Lian Duan, Deqiang Zhang and Yong Qiu
Journal of Materials Chemistry A 2014 - vol. 2(Issue 42) pp:NaN8989-8989
Publication Date(Web):2014/09/02
DOI:10.1039/C4TC01757A
Phosphorescent organic-light emitting diodes (PHOLEDs) have achieved ultimate high efficiencies and long lifetimes. One of the remaining challenges in PHOLEDs is to reduce the cost, which can be accomplished by reducing the dopant concentration. Here, to promote energy transfer at low dopant concentrations, hosts with thermally activated delayed fluorescence (TADF) are utilized. The triplet excitons of the host with TADF can be thermally up-converted to their singlet states and then transferred to the guest through the long-range Förster energy transfer rather than the short-range Dexter one. Devices using 2-phenyl-4,6-bis(12-phenylindole[2,3-a]carbazole-11-yl)-1,3,5-triazine (PBICT) as the host for tris(2-phenylpyridine)iridium (Ir(ppy)3) achieve a maximum external quantum efficiency of 23.9% and a power efficiency of 77.0 lm W−1 at a low dopant concentration of 3 wt%. Moreover, unlike the traditional hosts, the lifetimes of devices using hosts with TADF are less sensitive to dopant concentration with the longest lifetime obtained at 3 wt% Ir(ppy)3. The findings may provide a novel strategy to simultaneously achieve high efficiency, low driving voltage and long lifetimes in PHOLEDs at a low phosphor concentration of ≤3 wt%.
Co-reporter:Dongdong Zhang;Minghan Cai;Zhengyang Bin;Yunge Zhang;Deqiang Zhang
Chemical Science (2010-Present) 2016 - vol. 7(Issue 5) pp:NaN3363-3363
Publication Date(Web):2016/04/26
DOI:10.1039/C5SC04755B
The high driving voltage of blue organic light-emitting diodes (OLEDs) based on emitters with thermally activated delayed fluorescence (TADF) remains a constraint for their portable application. A major reason for this is that the high triplet (T1) of the host required to match the blue TADF emitters would always lead to inferiority in terms of carrier injection. Therefore, a suitable host should possess not only a high T1 but also a relatively low singlet (S1) for improved carrier injection, indicating that small singlet–triplet splittings (ΔESTs) are highly desired. Here, four carbazolyl benzonitrile derivatives are facilely prepared in a one-step approach with restrained conjugate lengths to maintain high triplet energies while their highly twisted structures spatially separate the frontier orbital distribution to achieve relatively low ΔESTs. Meanwhile, the charge transporting mobilities of these hosts are effectively tuned by the different linker types of the host moieties. Consequently, high-triplet-energy hosts with favorable carrier injection/transporting abilities are realized, endowing blue TADF devices with a maximum external quantum efficiency of 21.5%, a maximum power efficiency of 42.0 lm W−1 and an ultra-low onset voltage of 2.8 V. It is noteworthy that a driving voltage of 4.9 V is achieved at a practical luminance of 1000 cd m−2, which is the lowest among the doped blue TADF OLEDs reported until now. This work suggests that manipulation of the molecular topologies not only leads to the flexible and feasible design of novel bipolar host materials, but also affords a promising method for fine-tuning physical properties and thus obtaining state-of-the-art device performances.
Co-reporter:Wei Jiang, Xinxin Ban, Muyang Ye, Yueming Sun, Lian Duan and Yong Qiu
Journal of Materials Chemistry A 2015 - vol. 3(Issue 2) pp:NaN246-246
Publication Date(Web):2014/11/24
DOI:10.1039/C4TC02485K
A novel cross-linkable hole-transporting material (HTM) has been synthesized and characterized. The HTM possesses high triplet energy, excellent film-forming and solvent-resistant abilities, suitable HOMO level and electrochemical stability. Solution processed multilayer blue electrophosphorescent devices using this cross-linked HTM show lower turn-on voltages and doubled efficiencies compared with the corresponding single-layer device.
Co-reporter:Dongxin Ma, Lian Duan, Yongge Wei, Lei He, Liduo Wang and Yong Qiu
Chemical Communications 2014 - vol. 50(Issue 5) pp:NaN532-532
Publication Date(Web):2013/11/05
DOI:10.1039/C3CC47362G
Phosphorescent quantum yields have been increased by 12 times by choosing bulky boracic anions as counterions for blue-emitting cationic iridium(III) complexes.
Co-reporter:Jie Xue, Chen Li, Lijun Xin, Lian Duan and Juan Qiao
Chemical Science (2010-Present) 2016 - vol. 7(Issue 4) pp:NaN2895-2895
Publication Date(Web):2016/01/19
DOI:10.1039/C5SC04685H
Though urgently needed, high-performance near-infrared organic light-emitting diodes (NIR-OLEDs) are still rare. NIR-OLEDs based on conventional NIR fluorescent materials usually suffer from low external quantum efficiencies (EQEs) because of the intrinsic obstacles according to the spin-statistics limit and energy-gap law. Herein, we realized high-efficiency and low efficiency roll-off fluorescent NIR-OLEDs through efficient triplet fusion of a bipolar host doped with a special naphthoselenadiazole emitter (4,9-bis(4-(2,2-diphenylvinyl)phenyl)-naphtho[2,3-c][1,2,5]selenadiazole, NSeD). Unlike typical NIR organic donor–acceptor (D–A) chromophores, NSeD features a non-D–A structure and a very large HOMO/LUMO overlap and displays a strong deep-red to NIR fluorescence and unique ambipolar character. The corresponding photoluminescence quantum efficiency of NSeD reaches 52% in solution and retains 17% in the blend film. The optimized NIR-OLEDs demonstrated a strong emission at 700 nm, a high maximum EQE of 2.1% (vs. the predicted theoretical maximum efficiency of 1.3%) and the EQE remained at around 2% over a wide range of current densities from 18 to 200 mA cm−2, which is amongst the highest performance for NIR-OLEDs based on organic fluorescent materials.
Co-reporter:Lei He, Lian Duan, Juan Qiao, Deqiang Zhang, Liduo Wang and Yong Qiu
Chemical Communications 2011 - vol. 47(Issue 22) pp:NaN6469-6469
Publication Date(Web):2011/05/09
DOI:10.1039/C1CC11263E
A new cationic iridium complex has been developed with 2-(1-phenyl-1H-pyrazol-3-yl)pyridine as the ancillary ligand, which bears a pendant protective phenyl ring within the molecule; blue-green light-emitting electrochemical cells (LECs) based on the complex show dramatically enhanced stability compared to the LEC based on a similar complex without pendant phenyl rings.
Co-reporter:Dongdong Zhang, Lian Duan, Yilang Li, Deqiang Zhang and Yong Qiu
Journal of Materials Chemistry A 2014 - vol. 2(Issue 38) pp:NaN8197-8197
Publication Date(Web):2014/08/07
DOI:10.1039/C4TC01289E
Hybrid white organic light-emitting diodes (WOLEDs) often undergo triplet energy loss through the triplet state of the blue fluorophors. Here, blue fluorophors with thermally activated delayed fluorescence (TADF) are introduced to solve this problem. The triplet excitons formed on blue TADF fluorophors can be harvested by either energy transfer to the low-lying triplet states of the phosphor or thermal upconversion to the emissive singlet states, eliminating the energy loss. Moreover, device structures are wisely designed to take full advantages of the charge trapping ability of the TADF dopant, 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene, achieving a color-stable warm white emission. Remarkably, a maximum forward viewing external quantum efficiency (EQE) of 22.5% and a maximum forward viewing power efficiency (PE) of 47.6 lm W−1 are achieved. These values are among the highest reported for hybrid WOLEDs and even comparable to full-phosphorescent ones, demonstrating that the strategy reported here is promising for OLED lighting.
Co-reporter:Minghan Cai, Xiaozeng Song, Dongdong Zhang, Juan Qiao and Lian Duan
Journal of Materials Chemistry A 2017 - vol. 5(Issue 13) pp:NaN3381-3381
Publication Date(Web):2017/03/07
DOI:10.1039/C7TC00733G
A series of D–π–A type bipolar hosts based on triphenylene/carbazole were designed and synthesized. π–π stacking of the triphenylene units between two adjacent molecules renders these hosts high electron mobilities above 1 × 10−4 cm2 V−1 s−1, and their electron and hole mobilities can be regulated through varying the connection position and π moieties. Due to more balanced charge mobilities, a D1-based green thermally activated delayed fluorescence (TADF) device achieved the highest external quantum efficiency (EQE) of 16.3%, and its EQE could still maintain 15.3% and 13.8% at the high luminance of 5000 cd m−2 and 10000 cd m−2, respectively. D1-based green phosphorescent devices also exhibited the highest EQE of 18.6% with a reduced roll-off to 17.6% at 5000 cd m−2 and 16.2% at 10000 cd m−2.
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rene-2,7-diyl)-1,4-phenylene]](/data/chemimg/3109700/220797-16-0.png)
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rene-2,7-diyl)-1,4-phenylene]](/data/chemimg/3109700/220797-16-0_b.png)
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