Nengquan Li

Bio: Nengquan Li is an academic researcher from Nanjing University. The author has contributed to research in topics: Triphenylamine & OLED. The author has an hindex of 5, co-authored 6 publications receiving 59 citations.

Tuning Charge Balance in Solution-Processable Bipolar Triphenylamine-diazafluorene Host Materials for Phosphorescent Devices.

Abstract: Three bipolar hosts, namely TPA-DAF, TPA-DAF2, and TPA-DAF3, comprising an electron-donating triphenylamine (TPA) group and electron-accepting 4,5-diazafluorene (DAF) units are investigated for phosphorescent organic light-emitting diodes (PhOLEDs). Given the nonplanar structure of the sp3-hybridized C9 atom in DAF unit, these molecules have a highly nonplanar configuration, good film-forming property, and high triplet energy (ET) of 2.88–2.89 eV. Among them, TPA-DAF shows more balanced carrier injecting/transporting ability, suitable highest occupied molecular orbital (MO) energy level and higher current density, and therefore TPA-DAF-based devices exhibit the best performances, having an extremely slight efficiency roll-off with current efficiency of 20.0 cd/A at 973 cd/m2, 19.5 cd/A at 5586 cd/m2, and 17.6 cd/A at 9310 cd/m2 for blue PhOLEDs; 23.5 cd/A at 1059 cd/m2 and 15.3 cd/A at 8850 cd/m2 for green PhOLEDs; and 12.2 cd/A at 1526 cd/m2, 10.5 cd/A at 5995 cd/m2, and 9.2 cd/A at 8882 cd/m2 for red Ph...

A solution-processable triphenylamine-fluorene host for exciplex based white phosphorescent organic light-emitting diodes

Abstract: A novel triphenylamine-fluorene oligomer with macro-spirocyclic structure was designed and prepared as a host for exciplex based white phosphorescent organic light-emitting diodes (white PhOLEDs), in which only iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic) was employed as the dopant. The device exhibited a comparatively high performance with a maximum luminance and current efficiency of 14 213 cd m−2 and 22.6 cd A−1, respectively.

A Macrospirocyclic Carbazole–Fluorene Oligomer as a Solution-Processable Matrix Host Material for Blue Phosphorescent Organic Light-Emitting Diodes with Low Turn-On Voltage and Efficiency Roll-Off

Abstract: The use of mixed hosts has been demonstrated to be an effective way to improve the efficiency of phosphorescent organic light-emitting diodes (PhOLEDs). In this study, a macrospirocyclic oligomer (Cz-F)4 based on carbazole and fluorene is synthesized as a solution-processable matrix host material for blue PhOLEDs. The oligomer exhibits a high triplet level, good film-forming ability, solution processability, and doping compatibility. As a result, a blue solution-processed PhOLED based on a (Cz-F)4/TAPC [1,1-bis((di-4-tolylamino) phenyl) cyclohexane] mixed host achieves a maximum current efficiency of 18.8 cd/A with a turn-on voltage of 3.3 V. When a ternary mixed-host system composed of (Cz-F)4, TAPC, and OXD-7 [1,3-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene] is employed, the device exhibits a low turn-on voltage of 2.8 V, and the driving voltages are only 3.8–4.6 V at the practical luminance of 100–1000 cd/m2. Even at 5000 and 10 000 cd/m2, the driving voltages are only 5.7 and 6.6 V, resp...

Pyrene‐functionalized oligofluorenes as non‐doped deep blue emitters for solution‐processed organic light‐emitting diodes

Abstract: Two pyrene-functionalized oligofluorenes (TPA-PyF3 and CBP-PyF3) are prepared using the condensation reaction by the Friedel–Crafts procedure. In the produced oligomers, the triphenylamine or N,N′-dicarbazolyl-4,4′-biphenyl core serves as a spacer bearing spiro-linked fluorene moieties to form a multi-H shaped structure. This specific structure efficiently retards the crystallization tendency of the pyrene groups, and gives the materials completely amorphous morphological structure and film forming ability. Solution-processed OLEDs with the structure of ITO/PEDOT:PSS (25 nm)/TPA-PyF3 or CBP-PyF3 (40 nm)/TPBI (35 nm)/Ca (10 nm)/Ag (100 nm) show low turn-on voltages of 3.6 V, and the maximum external quantum efficiencies reach 1.78% and 2.07% for TPA-PyF3 and CBP-PyF3, respectively. Moreover, both devices exhibit stable deep-blue light emission with Commission International de I'Eclairage (CIE) coordinates of around (0.16, 0.09) at the brightness of 100–1000 cd m−2. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015

Self-host thermally activated delayed fluorescent dendrimers with flexible chains: an effective strategy for non-doped electroluminescent devices based on solution processing

Abstract: Two self-host dendrimers TZ-Cz and TZ-3Cz with TADF nature were designed and synthesized for solution-processed nondoped fluorescent organic light-emitting diodes (OLEDs). The first and second generation carbazole dendrons were conveniently introduced into the TADF core through non-conjugated aliphatic chains, which favourably make the fluorescence of the core independent of the peripheral dendrons. Photophysical investigation showed that the prevalent concentration quenching effect of the TADF materials can be effectively restrained by the encapsulation of the emissive core. Moreover, the key role of the end-capping moieties on the molecular frontier orbital of the dendrimers was revealed by electrochemical testing and theoretical calculations. Solution-processed OLEDs featuring TZ-3Cz as the host-free emitter achieved a superior external quantum efficiency of 10.1%, which is almost ten times higher than that of the emissive core TZ (1.09%). Thus, a facile strategy to design a solution-process with highly emissive self-host TADF materials for nondoped OLEDs by flexible alkyl chain is demonstrated.

Multiple Stimuli-Responsive Fluorescence Behavior of Novel Polyamic Acid Bearing Oligoaniline, Triphenylamine, and Fluorene Groups

Abstract: Multiple stimuli-responsive fluorescent materials have gained increasing attention for their fundamental investigation and intelligent applications. In this work, we report design and synthesis of a novel polyamic acid bearing oligoaniline, triphenylamine, and fluorene groups, which served as sensitive units and fluorescence emission unit, respectively. The resulting polymer exhibits multiple stimuli-responsive fluorescence switching behavior triggered by redox species, pH, electrochemical, and pressure stimuli. Every fluorescence switching mechanism upon each stimulus was studied in detail. The interactions and energy transfer between sensitive units and emission unit are largely responsible for this fascinating fluorescent switching behavior. This work provides a deep understanding of the optical switching essence upon these stimuli, opening the way for the development of new fluorescent sensing applications.

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

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

White Polymer Light-Emitting Diodes Based on Exciplex Electroluminescence from Polymer Blends and a Single Polymer

Abstract: In this Article, we designed and synthesized a series of polyfluorene derivatives, which consist of the electron-rich 4,4′-(9-alkyl-carbazole-3,6-diyl)bis(N,N-diphenylaniline) (TPA-Cz) in the side chain and the electron-deficient dibenzothiophene-5,5-dioxide (SO) unit in the main chain. The resulting copolymer PF-T25 that did not comprise the SO unit exhibited blue light-emission with the Commission Internationale de L’Eclairage coordinates of (0.16, 0.10). However, by physically blending PF-T25 with a blue light-emitting SO-based oligomer, a novel low-energy emission correlated to exciplex emerged due to the appropriate energy level alignment of TPA-Cz and the SO-based oligomers, which showed extended exciton lifetime as confirmed by time-resolved photoluminescent spectroscopy. The low-energy emission was also identified in copolymers consisting of SO unit in the main chain, which can effectively compensate for the high-energy emission to produce binary white light-emission. Polymer light-emitting diodes...