Deepak Kumar Dubey

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.

Nano-Structures Enabling Sunlight and Candlelight-Style OLEDs

Nano structures enable organic light-emitting diode (OLED) devices to be fabricated with relatively high efficiency and brightness, opening up a new era for high quality displays and lighting, wherein devising a pseudo-natural light is always a must. The uses of incandescent bulbs are the most friendly, electricity-driven lighting sources, lighting measure from the perspectives of human eye protection, melatonin generation, artifacts, ecosystems, the environment, and the night skies due to their intrinsically low blue emission. However, they are phasing out because of the energy wasting. To overcome these difficulties, researchers are focusing on developing a new light with high efficiency, whose emission spectra would also match with those of the natural lights. In 2009, Jou’s group invented the world’s first electrically powered sunlight-style OLED that yielded a sunlight-style illumination with various daylight chromaticities, whose color temperature ranges between 2,300 and 8,200 K, fully covering those of the entire daylight at different times and regions, and contributed a noteworthy incentive to OLED technology in general lighting. By putting more efforts on this technology, a blue hazard free, low color temperature candlelight-style OLED was developed by employing candlelight complementary emitters, namely orange-red, yellow, green, and sky-blue. The resultant candlelight OLED that exhibits a 1,900 K color temperature is exactly like candles or oil lamps, which is friendly to human eyes, physiologies, ecosystems, artifacts, and night-skies. Specifically, it is at least 10 times safer from the retina protection perspective or 5 times better for melatonin to naturally occur after dusk, as compared with the blue light-enriched white OLED, LED and CFL counterparts. In this article, we discuss the device structure, physics, and engineering behind the serendipity of the pseudo-natural light-style OLEDs.

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).

An Approach for Measuring the Dielectric Strength of OLED Materials

Surface roughness of electrodes plays a key role in the dielectric breakdown of thin-film organic devices. The rate of breakdown will increase when there are stochastic sharp spikes on the surface of electrodes. Additionally, surface having spiking morphology makes the determination of dielectric strength very challenging, specifically when the layer is relatively thin. We demonstrate here a new approach to investigate the dielectric strength of organic thin films for organic light-emitting diodes (OLEDs). The thin films were deposited on a substrate using physical vapor deposition (PVD) under high vacuum. The device architectures used were glass substrate/indium tin oxide (ITO)/organic material/aluminum (Al) and glass substrate/Al/organic material/Al. The dielectric strength of the OLED materials was evaluated from the measured breakdown voltage and layer thickness.

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.

High efficiency wet-processed green phosphorescent organic light-emitting diodes

Wet-processed organic light-emitting diodes (OLEDs) have drawn much attention because of large area-size and cost-effective fabrication feasibility via roll-to-roll manufacturing. However, fabrication of multilayered, wet-processed OLED using small molecules is still a challenge. In this work, we have fabricated wet-processed green OLEDs and enhanced the device performance by incorporating 4,4′,4′-tris(N-carbazolyl)-triphenylamine (TCTA) or N,N′-dicarbazolyl-3,5-benzene (mCP) as a hole-transport as well as electron-confining layer via spin coating. Accordingly, the resultant green OLED showed a maximum current efficiency of 65 cdA−1, a power efficiency of 56 lmW−1, and an external quantum efficiency of 18% at 100 cd/m−2. High efficiency may be attributed to balanced carrier-injection and excitons confinement in the emissive layer. The low roll-off in efficiency can be attributed to the bipolar nature of the host material 4, 4′-N, N′-dicarbazole-biphenyl, CBP.