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We report fluorescent deep-blue organic light emitting diodes (OLEDs) with significantly high efficiency by enhancing singlet exciton generation via triplet-triplet annihilation (TTA). Here, electron transport layers (ETL) and exciton blocking layers (EBL) with different characteristics (especially mobility) are applied to increase singlet harvesting (using TTA process) through appropriate charge carrier injection in the emissive layer. OLED with high electron mobility ETL, 1,4-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (p-bPPhenB) and high hole mobility EBL, 9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-dihydroacridine (PCzAc) exhibit relatively high external quantum efficiency of 9.18% with deep-blue color coordinates of (0.135, 0.115) at the luminance of 1000 cd/m2 and low driving voltage (4.5 V). The transient electroluminescence measurements are conducted to investigate the relation between prompt fluorescence from the initial singlet excitons and delayed fluorescence from the TTA process. This analysis shows significant enhancement in the singlet exciton generation by TTA process and charge recombination rate.

In this paper, we report a full-color reflective display device by combining reflective electrochromic device (ECD) and different color production units (conventional color filter (CF) and transmittance controllable electrochromic color filter (TCECF)). A full-color reflective device with TCECF showed an excellent diffuse reflectance of 47.2% in the white state owing to the high transmittance of TCECF in the bleached state than that of CF (19.5%). This device structure can easily provide various colors with high brightness and saturation with a broad grayscale. Particularly, in the colored state, TCECF and CF-based full-color reflective device displayed color coordinates of (0.59, 0.34), (0.31, 0.66), (0.24, 0.30) and (0.65, 0.33), (0.27, 0.61), (0.15, 0.06), for red, green and blue, respectively, and alongside also exhibited color gamut of 36.8% (for TCECF) and 73.1% (for CF) for full color reflective devices.

Highly efficient single-stack hybrid cool white organic light-emitting diodes (OLEDs) having blue-yellow-blue multiple emitting layers (EMLs) are designed and constructed by utilizing blue thermally activated delayed fluorescent (TADF) and yellow phosphorescent emitters. The out-coupling efficiencies of yellow and blue emissions are maximized by tuning the ITO and total device thickness that satisfies both of antinode positions for yellow and blue emissions in a limited multiple EML thickness. To obtain a cool white emission, the exciton formation ratio in the blue-yellow-blue multiple EML system is controlled by manipulating the recombination zone through charge conductivity variation of host medium in the blue TADF EML. The resulting device exhibits cool white emission with very high maximum external quantum efficiency of 23.1% and CIE color coordinates of (0.324, 0.337). We anticipate that the studied approach will raise the viability of single-stack hybrid cool white OLEDs for high performance display applications.

In this work, a series of hole transporting materials with carbazole and triphenylamine cores have been synthesized and characterized. In the carbazole's 3rd and 6th positions, two site tryphenylamine para positions are end capped with the same types of branching derivatives to compare the overall performances of constructed devices. All of our hole transporting materials showed good thermal stabilities without any crystallized features which expressed in higher decomposition temperature (Over 500 °C at 5% weight reduction). All synthesized materials revealed HOMO energy levels between −5.62 and −5.48 eV, which values are lying between HOMO energy values of anode and emission layer; as a result, it made an effective path for hole transportation. Higher lying LUMO values between −2.51 and −2.31 can block the electrons from adjacent layer to ensure the perfect recombination in the middle layer. Triphenylamine based HTMs indicated better performances than carbazole based HTMs. Further comparisons were done by using NPB as hole transporting material with the same red phosphorescent based OLED device. HTM2A based device IV was exhibited higher maximum current efficiency of 30.6 cd/A and higher maximum external quantum efficiency of 26.7% than reference NPB based device. Measured Hole mobility value of HTM2A with hole dominant device was 5.3 × 10 −4 cm 2 V −1 s −1 , which was better than NPB. Synthesized HTM2A would be a promising hole transporting material for various phosphorescent based OLEDs.

In this paper, we report two new phenanthroline-based compounds, 1,4-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (p-bPPhenB) and 1,3-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (m-bPPhenB), for the charge generation unit of tandem organic light-emitting diodes (OLEDs). These two compounds exhibited high electron mobility of (5.8–4.4) × 10–3 cm2/(V s), a very small injection barrier at the p–n junction interface, a high glass transition temperature of 123.9–182.1 °C, and exceptionally good operational stability. Because of such excellent characteristics, a single-stack red phosphorescent OLED (PhOLED) with p-bPPhenB showed a low driving voltage (2.7 V) and significantly improved maximum power efficiency (56.8 lm/W), external quantum efficiency (30.8%), and device lifetime (LT95, 130 h) compared to those of the control device using bathophenanthroline (Bphen) (3.7 V, 39 lm/W, 27.1%, and 13 h). Furthermore, a two-stack (tandem) red PhOLED using p-bPPhenB in the charge generation unit exhibited superior cha...