New blue (DBA‐SAB) and deep‐blue (TDBA‐SAF) thermally activated delayed fluorescence (TADF) emitters are synthesized for blue‐emitting organic‐light emitting diodes (OLEDs) by incorporating spiro‐biacridine and spiro‐acridine fluorene donor units with an oxygen‐bridged boron acceptor unit, respectively. The molecules show blue and deep‐blue emission because of the deep highest occupied molecular energy levels of the donor units. Besides, both emitters exhibit narrow emission spectra with the full‐width at half maximum (FWHM) of less than 65 nm due to the rigid donor and acceptor units. In addition, the long molecular structure along the transition dipole moment direction results in a high horizontal emitting dipole ratio over 80%. By combining the effects, the OLED utilizing DBA‐SAB as the emitter exhibits a maximum external quantum efficiency (EQE) of 25.7% and 1931 Commission Internationale de l'éclairage (CIE) coordinates of (0.144, 0.212). Even a higher efficiency deep blue TADF OLED with a maximum EQE of 28.2% and CIE coordinates of (0.142, 0.090) is realized using TDBA‐SAF as the emitter.

Triplet harvesting is important for the realization of high‐efficiency fluorescent organic light‐emitting diodes (OLEDs). Triplet–triplet annihilation (TTA) is one triplet‐harvesting strategy. However, for blue‐emitting anthracene derivatives, the theoretical maximum radiative singlet‐exciton ratio generated from the TTA process is known to be 15% in addition to the initially generated singlets of 25%, which is insufficient for high‐efficiency fluorescent devices. In this study, nearly 25% of the radiative singlet‐exciton ratio is realized by TTA using an anthracene derivative, breaking the theoretical limit. As a result, efficient deep‐blue TTA fluorescent devices are developed, exhibiting external quantum efficiencies of 10.2% and 8.6% with Commission Internationale de l'Eclairage color coordinates of (0.134, 0.131) and (0.137, 0.076), respectively. The theoretical model provided herein explains the experimental results considering both the TTA and reverse intersystem crossing to a singlet state from higher triplet states formed by the TTA, clearly demonstrating that the radiative singlet ratio generated from TTA can reach 37.5% (total radiative singlet‐exciton ratio: 62.5%), well above 15% (total 40%), despite the molecule having S1, T2 < 2T1 < Q1 energy levels, which will lead to the development of high‐efficiency fluorescent OLEDs with external quantum efficiencies exceeding 28% if the outcoupling efficiency is 45%.

The use of exciplex hosts is attractive for high-performance phosphorescent organic light-emitting diodes (PhOLEDs) and thermally activated delayed fluorescence OLEDs, which have high external quantum efficiency, low driving voltage, and low efficiency roll-off. However, exciplex hosts for deep-blue OLEDs have not yet been reported because of the difficulties in identifying suitable molecules. Here, we report a deep-blue-emitting exciplex system with an exciplex energy of 3.0 eV. It is composed of a carbazole-based hole-transporting material (mCP) and a phosphine-oxide-based electron-transporting material (BM-A10). The blue PhOLEDs exhibited maximum external quantum efficiency of 24% with CIE coordinates of (0.15, 0.21) and longer lifetime than the single host devices.

Three new anthracene-cored molecules, 3,3′-(5-(10-(naphthalen-1-yl)­anthracen-9-yl)-1,3-phenylene)­dipyridine (AP3Py-Na), 3,3′-(5-(10-(naphthalen-2-yl)­anthracen-9-yl)-1,3-phenylene)­dipyridine (AP3Py-2Na), and 9,10-bis­(3,5-di­(pyridin-3-yl)­phenyl)­anthracene (ADP3Py), were synthesized to be used as an efficiency-enhancement layer (EEL) in blue fluorescent organic light-emitting diodes. Insertion of a very thin EEL (3 nm) between the deep blue emitting layer (EML) and the electron transport layer enhanced the external quantum efficiency (EQE) of the blue device by 44% compared to the device without the EEL, resulting in an EQE of 7.9% and a current efficiency of 9.0 cd A–1 at 1000 cd m–2; the CIE coordinates of the emitting color were (0.13, 0.14). The transient electroluminescence showed that the efficiency enhancement originates from the triplet–triplet annihilation (TTA) process in the EEL, followed by energy transfer to the emitting dye in the EML.

defined as the ratio of the number of photons escaping from the OLED stack to the number of photons generated in the emitting layer (EML), must be increased.The light extraction structure applied in WOLEDs should have a highly enhanced light extraction efficiency, minimal spectral shift (or high color quality after applying light extraction), and a simple manufacturing process. Because these restrictions are extremely difficult to achieve, many processes that have been reported for light extraction methods cannot be applied in indoor lightings. For example, the microlens array (MLA) has a simple manufacturing process and no spectral change, but it is difficult to achieve high efficiency because the MLA extracts the substrate mode only. [17][18][19] The application of a corrugated substrate between a transparent electrode and a glass substrate, whose internal light extraction method relies on extracting the waveguide and surface plasmon polariton (SPP) modes, is limited by the concentrated electric field around the electrodes, which results in electrical degradation of the device due to curvature of the Al electrode. [20][21][22][23][24][25] A vacuum nanohole array greatly increases the light extraction efficiency but has the disadvantage of a complicated manufacturing process, which includes dissolution of the silicon substrate, though it does maintain electrical stability by employing a flat surface. [26][27][28] To overcome these drawbacks, we used a direct printing technique, known as nanoimprint lithography (NIL), to replicate a randomly arrayed nanopillar structure. This NIL process has many advantages, including high throughput and resolution, applicability to large-area substrates, and a low fabrication cost due to its simple manufacturing process. [29][30][31] Via this NIL process, we could easily fabricate TiO 2 hollow-structured substrate without a complex process or high cost.In this report, a light extraction structure consisting of planarized high refractive index material (TiO 2 , n = 2.0) and air holes (n = 1.0) was easily fabricated using a simple process and then applied as a light extraction structure in a tandem WOLED. A patterned poly (benzyl methacrylate) (PBMA) layer was fabricated on the glass substrate, which was then spin-coated with TiO 2 nanoparticle dispersed resin and TiO 2 sol. The PBMA nanopattern was removed by annealing the substrate at 500 °C. The air-hole array filled the spaces where the PBMA escaped from, resulting in a structure with a mixture of high and low A simple fabrication method for a light extraction layer is required. In this report, an internal light extraction layer composed of a high refractive index material and an air void is fabricated in five steps. A direct printing process followed by annealing of the randomly arrayed poly(benzyl methacrylate) pillars after a planarization process using TiO 2 -nanoparticle dispersed resist and sol is used. These substrates are used for light extraction in white tandem organic light emitting diodes (WOLEDs). By combining the...