Silu Tao

Efficient blue and white organic light-emitting devices based on a single bipolar emitter

Excellent bipolar carrier transport properties of 2,7-dipyrenyl-9,9′-dimethyl-fluorene (DPF) have been elucidated by using different device structures. A nondoped device using DPF as host emitter showed highly-efficient blue emission with a maximum efficiency of 6.0cd∕A and CIE coordinates of x=0.15 and y=0.19. Another device based on rubrene-doped DPF as emission layer gave pure high-efficiency white emission with good color stability, a maximum efficiency of 10.5cd∕A, and CIE coordinates of x=0.28 and y=0.35. The excellent bipolar transport capability and high performance as both emitter and host suggest that DPF is an efficient and versatile material for various applications in organic light-emitting devices.

Long-lifetime thin-film encapsulated organic light-emitting diodes

Multiple fluorocarbon (CFx) and silicon nitride (Si3N4) bilayers were applied as encapsulation cap on glass-based organic light-emitting diodes (OLEDs). When CFx/Si3N4 bilayers were deposited onto the OLED structure, the devices showed performance worse than one without any encapsulation. The adverse effects were attributed to the damage caused by reaction species during the thin-film deposition processes. To solve this problem, a CuPc interlayer was found to provide effective protection to the OLED structure. With a structure of CuPc/(CFx/Si3N4)×5, the encapsulated device showed an operation lifetime over 8000 h (higher than 80% of that achieved with a conventional metal encapsulation).

A novel bipolar phenanthroimidazole derivative host material for highly efficient green and orange-red phosphorescent OLEDs with low efficiency roll-off at high brightness

A new bipolar fluorophore, N,N-diphenyl-4′-(9-(4′-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-[1,1′-biphenyl]-4-yl)-9H-fluoren-9-yl)-[1,1′-biphenyl]-4-amine (PPI-F-TPA), consisting of an electron-withdrawing phenanthro[9,10-d]imidazole (PI) chromophore and an electron-donating triphenylamine group, based on an indirect linkage, has been designed and synthesized. The sp3-hybridized C9 atom of the fluorene linkage efficiently interrupts molecular conjugation and inhibits π–π intermolecular interactions, resulting in efficient violet-blue emission, excellent thermal stability and high triplet energy. Equipped with balanced carrier mobility, PPI-F-TPA shows impressive performance as the emitting layer in non-doped OLEDs, which achieved an external quantum efficiency (EQE) of 3.11% with a CIE coordinate of (0.16, 0.05). Furthermore, the high triplet energy allows PPI-F-TPA to be used as a host for PhOLEDs. High performance green and orange-red PhOLEDs with the maximum EQEs, current efficiencies (CE) and power efficiencies (PE) of 15.6% and 12.5%, 57 cd A−1 and 27 cd A−1, 60 lm W−1 and 28.3 lm W−1, respectively, have been successfully obtained. More importantly, all the devices exhibit low efficiency roll-off; in particular, that of the orange-red PhOLEDs is extremely small. The orange-red PhOLED has a decay rate of EQE less than 1% at 1000 cd m−2, 13.6% at 10 000 cd m−2 and 29.5% even at 50 000 cd m−2, which is very rare among orange or orange-red PhOLEDs at such high brightness.

A triphenylamine derivative as a single-emitting component for highly-efficient white electroluminescent devices

A new triphenylamine-based compound has been designed and synthesized for application in white organic light-emitting devices (WOLEDs), and investigated in terms of photoluminescence (PL) and electroluminescence (EL) properties. The PL of the compound in thin film showed a white emission due to the combination of exciton and excimer emissions. Taking advantage of this property, a WOLED was fabricated by using the compound as the sole emitter. The WOLED exhibited highly-efficient white emission with a low turn-on voltage of 3 V and a maximum brightness of 12320 cd m−2 at 8 V, and a maximum luminous efficiency of 7.0 cd A−1 (7.1 lm W−1) with CIE coordinates of (x = 0.29, y = 0.34). The device performance characteristics are among the best ever achieved in single-emitter OLEDs.

Single zinc-doped indium oxide nanowire as driving transistor for organic light-emitting diode

Zn-doped In2O3 nanowires (NWs) were prepared by simple chemical vapor deposition and were systematically characterized. Field-effect transistors (FETs) constructed from the Zn-doped In2O3 nanowires exhibit excellent performance characteristics such as high mobility, “high-on-state” current of 105A and large on/off current ratio of 107. Single-NW-FETs can successfully drive an organic light-emitting diode, revealing the application potential of Zn-doped In2O3 NW-FETs in high-performance displays.

High-performance fluorescent/phosphorescent (F/P) hybrid white OLEDs consisting of a yellowish-green phosphorescent emitter

The color rendering index (CRI) of a white organic-light emitting device (WOLED) employing standard red + green + blue emitters is typically limited by the deep valley between the red and the green emission peaks. To address this issue without increasing device complexity, we synthesized a yellowish-green iridium emitter, iridium(III) bis(2-phenylpyridine)(2-(benzo[d]oxazol-2-yl)phenol) (Ir(ppy)2bop), for replacing the standard green emitter. By combining emissions from Ir(ppy)2bop with those from a blue fluorescent emitter and a red phosphor, a high performance fluorescent/phosphorescent (F/P) WOLED has been fabricated. The device gives white emission with a maximum efficiency of 55.2 cd A−1 (49.6 lm W−1) and an EQE value of 20% without any light extraction technologies. It is noteworthy that the color rendering index (CRI) of the white OLED reaches up to 89. Considering both the efficiency and the CRI, these results are among the best-reported white OLEDs.

Non-doped deep blue emitters based on twisted phenanthroimidazole derivatives for organic light-emitting devices (CIE y ≈ 0.04)

The π-conjugation length of donor–acceptor molecules is not conducive to blue emission and the color purity of devices. Hence, by using a twisted donor–acceptor molecular design, we developed three deep-blue emitters, mtp, Tmtp and Cmtp. Compared to the TPA-BPI we reported previously, the subtle molecular modification and optimization shows extremely good color purity without impairing the excellent photophysical and electrical properties. The nondoped mtp-based device emitted deep-blue emission at 436 nm with CIE of (0.15, 0.05) and a maximum EQE of 3.89%. The Cmtp-based device emitted blue light of at 445 nm with CIE of (0.15, 0.07). Especially, the Tmtp-based device showed a violet-blue CIE coordinate of (0.15, 0.04).

Mechanochromic asymmetric sulfone derivatives for use in efficient blue organic light-emitting diodes

Typical π–π stacking is suppressed by the asymmetric molecular design for high fluorescence quantum yield (Φf) blue light emission, overcoming the aggregation caused quenching (ACQ) limitation. In this research, two novel blue fluorescent materials with asymmetric structure: 2-(4′-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)-[1,1′-biphenyl]-4-yl)-1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole (PSC) and 2-(4′-((4′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-4-yl)sulfonyl)-[1,1′-biphenyl]-4-yl)-1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole (PSBC), consisting of a sulfone group as the electron acceptor and two different electron donors, carbazole and phenanthroimidazole, were designed and synthesized. The two compounds have high Φf (95.3% for PSC and 81.1% for PSBC) in film because of the restricting π–π stacking, and show apparent mechanochromic properties, i.e., an emission change from deep blue to blue-green resulting from external mechanical stimuli. The emissions display 50 nm/23 nm red shifts after grinding. Organic light emitting diodes (OLEDs) using the two compounds as emitters exhibited good efficiencies: the doped PSC-based device emitted blue light at 444 nm with CIE co-ordinates of (0.151, 0.072). The PSBC-based device also emitted blue light at 444 nm with CIE co-ordinates of (0.151, 0.068). A maximal external quantum efficiency (EQE) of 5.43% was also achieved.

High-performance organic red-light-emitting devices based on a greenish-yellow-light-emitting host and long-wavelength emitting dopant

We demonstrated an organic red-light-emitting device (ORLED) using a host, 5,6-bis-[4-(naphthalene-1-yl-phenyl-amino)-phenyl]-pyrazine-2,3-dicarbonitrile (BNPPDC), and a dopant, 2,3-bis[[[(2-hydroxy-4-diethylamino)phenyl] (methylene)] amino]-2-butanedinitrile (BDPMB). The device achieved a brightness of 9730cd∕m2 at a 11V, a power efficiency of 2.35lm∕W, a current efficiency of 3.36cd∕A at 4.5V, and a low turn-on voltage of 3.0V, with nearly saturated red emission. The device is superior or equal to the best fluorescent ORLEDs reported. BNPPDC generally induced a significant blueshift in dopant emission, thus it may serve as a host for dopants emitting at long wavelengths in ORLEDs with improved performance.

Ternary Organic Solar Cells with Coumarin7 as the Donor Exhibiting Greater Than 10% Power Conversion Efficiency and a High Fill Factor of 75%

Ternary bulk heterojunction (BHJ) is a brilliant photovoltaic technology for improving the performance of organic solar cells (OSCs), because the light absorption range can be significantly extended by using multiple donors or acceptor materials. In this paper, coumarin7 (C7), a small organic molecule typical led used in organic light-emitting diodes, was initially exploited as second electron-donor component in ternary bulk heterojunction OSCs along with conventional blend system spolythieno[3,4-b]-thiophene/benzodithiophene(PTB7) and [6,6]-phenyl-C71 -butyric acid methyl(PC71 BM). A champion PCE value of 10.28% was realized in the ternary OSCs when incorporated with 10 wt % C7 doping ratio in the donors, corresponding to about 35% enhancement compared with the PTB7:PC71BM-based OSCs, a high fill factor (FF) of 75.03%, a short-circuit currentdensity (Jsc) of 18.72 mA cm-2 and an open-circuit voltage (Voc) of 0.73 V. The enhanced performance of the ternary OSCs can be attributed to the simultaneous improvement of the FF and the Jsc. In addition to extended light absorption, a perfect nanofiber filament active layer morphology is obtained due to the good compatibility between C7 and PTB7, which facilitates the balance of charge transportation and the suppression of charge recombination. This investigation suggests that coumarin derivatives, which have completely different structure with polymer donors, can also be used to fabricate ternary solar cells and have the potential applications to obtain amazing performance after further device engineering and optimization.