Snehasis Sahoo

A wet- and dry-process feasible carbazole type host for highly efficient phosphorescent OLEDs

A wet- and dry-process feasible host material is crucial to realize, respectively, low cost roll-to-roll fabrication of large area and high performance organic light-emitting diodes (OLEDs) with precise deposition of organic layers. We demonstrate in this study high efficiency phosphorescent OLED devices by employing a newly synthesized carbazole based host material 1,6-bis[3-(2-methoxy-3-pyridinyl)carbazol-9-yl]hexane (compound 5). Moreover, two other carbazole hosts 1,6-bis[3-(6-methoxy-3-pyridinyl)carbazol-9-yl]hexane (compound 4) and 3,6-di(2-methoxy-3-pyridinyl)-9-ethylcarbazole (compound 6) are also synthesized for comparison. By doping a typical green emitter fac tris(2-phenylpyridine)iridium (Ir(ppy)3) in compound 5, for example, the resultant wet-processed device exhibits at 100 cd m−2 a current efficiency of 27 cd A−1 and a power efficiency of 16.1 lm W−1. The dry-processed device shows a current efficiency of 61 cd A−1 and a power efficiency of 62.8 lm W−1. The high efficiency may be attributed to the host possessing an effective host-to-guest energy transfer, effective carrier injection balance, and the device architecture enabling excitons to generate on both the host and the guest.

Multi-substituted deep-blue emitting carbazoles: a comparative study on photophysical and electroluminescence characteristics

A series of carbazole derivatives containing various degrees of 2,3,6,7-substitutions with triphenylamine or carbazole chromophores have been synthesized by Suzuki–Miyaura coupling reaction in good yields and characterized by optical, electrochemical, thermal, theoretical and electroluminescence studies. The physicochemical properties of the dyes are significantly influenced by the linking topology and chromophore density around the central carbazole unit. The triphenylamine substituted derivatives showed red-shifted absorption/emission in the series attributable to their extended π-conjugation. Multiple substitutions on the carbazole core resulted in twisting of chromophores from the central carbazole and disturbed the effective inter-chromophoric electronic interactions, which led to shorter wavelength absorptions for tri- and tetra-substituted star-bursts when compared to di-substituted analogues. However, the starburst derivatives exhibited relatively high molar extinction coefficients attributable to the increased chromophore density. In contrast, the emission profiles of the starbursts are red-shifted when compared to triads. Also, the triphenylamine containing dyes displayed positive solvatochromism in emission attributable to the structural reorganization induced electronic perturbations. The solvatochromic data are suggestive of the general solvent effect in the excited state with some charge migration. Carbazole featuring dyes showed superior thermal stability over the triphenylamine derivatives due to the rigidity of the former chromophore. Electrochemical studies revealed comparatively high oxidation propensity for the triphenylamine-containing dyes and exhibited variations attributable to chromophore enrichment. Solution processable multilayer OLEDs were fabricated by employing these materials as emitting dopants in CBP which exhibited deep-blue electroluminescence with CIE coordinates, 0.16 ≤ x ≤ 0.17, 0.05 ≤ y ≤ 0.08. Interestingly, the linear di-substituted derivatives exhibited superior device performance over the tri- and tetra-substituted star-bursts attributable to the efficient trapping of excitons generated in the host matrix.

Wet-process feasible candlelight OLED

Candlelight-based, blue-hazard free lighting sources are friendly to the human eye and physiology, museum artifacts, ecosystems, the environment, and the night sky. Wet processing enables organic devices to be fabricated cost-effectively with a large area-size via continuous roll-to-roll manufacturing. We demonstrate here a candlelight organic light-emitting diode (OLED) using a wet process to deposit at least the layers of emission, hole-injection, and hole-transportation. The resulting 1918 K candlelight OLED is about 50 and 15 times safer to the retina and in terms of melatonin generation protection, respectively, as compared with the 5000 K white LED, CFL and OLED. It can reach a maximum brightness of 38 000 cd m−2, equivalent to 42 000 candlelight in one square meter, and its efficacy is 300 times that of candle and 3 times that of an incandescent bulb. This study may serve as a starting point to begin a healthy-light based “Lighting Renaissance” with feasible commercialization.

Solution-processable naphthalene and phenyl substituted carbazole core based hole transporting materials for efficient organic light-emitting diodes

Solution-processable molecular hole transporting materials (HTMs) are extremely crucial in order to realize low cost, high throughput, and roll-to-roll fabrication of large area organic light emitting diodes for display and lighting applications. In this report, a series of naphthalene and phenyl substituted carbazole core based HTMs, 3-(1-naphthyl)-9-(2-phenylvinyl)carbazole (NPVCz), 3,6-di-(1-naphthyl)-9-phenylvinylcarbazole (DNPVCz), and 3,6-diphenyl-9-(2-phenylvinyl)carbazole (DPPVCz) are successfully synthesized and characterized. The synthesized HTMs possess excellent solubility in common organic solvents. By using a fluorescent tris(8-hydroxyquinolinato)aluminium emitter, we demonstrate an enhancement of 135%, from 1.7 to 4.5 cd A−1, in the current efficiency of an organic light emitting diode (OLED) by replacing the conventional HTM, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), with the NPVCz counterpart. Moreover, the current efficiency of a conventional tris[2-phenylpyridinato-C2,N]iridium(III) based phosphorescent green OLED device increases from 46.4 to 66.2 cd A−1 by substituting the NPB with NPVCz. These findings suggest that this type of solution-processable molecular HTM will be a promising contender for high efficiency OLED devices.

High light-quality OLEDs with a wet-processed single emissive layer

High light-quality and low color temperature are crucial to justify a comfortable healthy illumination. Wet-process enables electronic devices cost-effective fabrication feasibility. We present herein low color temperature, blue-emission hazards free organic light emitting diodes (OLEDs) with very-high light-quality indices, that with a single emissive layer spin-coated with multiple blackbody-radiation complementary dyes, namely deep-red, yellow, green and sky-blue. Specifically, an OLED with a 1,854 K color temperature showed a color rendering index (CRI) of 90 and a spectrum resemblance index (SRI) of 88, whose melatonin suppression sensitivity is only 3% relative to a reference blue light of 480 nm. Its maximum retina permissible exposure limit is 3,454 seconds at 100 lx, 11, 10 and 6 times longer and safer than the counterparts of compact fluorescent lamp (5,920 K), light emitting diode (5,500 K) and OLED (5,000 K). By incorporating a co-host, tris(4-carbazoyl-9-ylphenyl)amine (TCTA), the resulting OLED showed a current efficiency of 24.9 cd/A and an external quantum efficiency of 24.5% at 100 cd/m2. It exhibited ultra-high light quality with a CRI of 93 and an SRI of 92. These prove blue-hazard free, high quality and healthy OLED to be fabrication feasible via the easy-to-apply wet-processed single emissive layer with multiple emitters.

Tuning Photophysical and Electroluminescence Properties in Asymmetrically Tetrasubstituted Bipolar Carbazoles by Functional Group Disposition

Carbazoles decorated with both donor and acceptor fragments offer a classical way to optimize bipolar functional properties. In this work, a series of carbazoles featuring triphenylamine donors and cyano acceptors are synthesized and their structure-property relationship is studied. The effects of connectivity and the chromophore number density on photophysical and electroluminescence properties are investigated. The position of the triphenylamine donor on the 3,6-dicyanocarbazole nucleus significantly affected the photophysical and electroluminescence properties. The dye possessing triphenylamine on C2 and C7 displayed a red shift in absorption when compared with the structural analogue with triphenylamine tethered to C1 and C8. The emission wavelength of the dyes are tunable from blue to green, by altering the position of triphenylamine and cyano substituents. All of the dyes exhibited positive solvatochromism in emission, attributable to the photoinduced intramolecular charge transfer from the triphenylamine donor to the cyano acceptor. However, the extent of charge transfer and hybridization of local and charge-transfer-excited states is highly dependent on the position of triphenylamine and cyano groups on the carbazole nucleus. Dyes containing cyano substituents at C2 and C7 showed a prolonged excited state lifetime, broad emission, and large Stokes shifts, indicating the presence of a higher charge transfer component in the excited state. The dyes displayed exceptional thermal stability with the onset decomposition temperature (10% weight loss) > 350 °C. Electrochemical measurements revealed low oxidation potential for dyes containing triphenylamine at C3 and/or C6. Addition of a cyano acceptor on carbazole led to the stabilization of lowest unoccupied molecular orbital. Furthermore, the materials were tested as emitting dopants in solution-processable multilayer organic light emitting diodes and found to display deep-blue/sky-blue electroluminescence with external quantum efficiency as high as 6.5% for a deep-blue emitter (CIE y ∼ 0.06).

Molecule based monochromatic and polychromatic OLEDs with wet process feasibility

Wet-process enables organic light-emitting diodes (OLEDs) to be made cost-effectively via a continuous process, such as roll-to-roll manufacturing. Initially, wet-process based OLEDs used to be fabricated with polymer-based emitters and/or host materials. However, small molecules that can be processed using a wet-process are more promising for commercialization due to easier control over their molecular weight, purification, solution viscosity, layer thickness, and uniformity. We have hence reviewed herein (i) what constitutes the essential materials for wet-process feasible OLEDs, (ii) applicable wet-process technologies, and (iii) reported wet-processed mono- and polychromatic OLED devices with sound efficiency performance. And last, but not least, we have pointed out the most critical challenges that wet-processed electronics including OLED must face before gaining sufficient ground for competition and turning into disruptive technology. These include issues such as multiple-layer feasibility, film integrity, film forming capability, and device lifetime.