jing li

The twist angle and rotation freedom between the donor (D) and acceptor (A) in D–A materials plays an important role in their photophysical properties. Here, the authors select the asymmetric acceptor pyridal[2,1,3]thiadiazole (PT) to construct two D–A isomers (p-TPA-PT and d-TPA-PT) with different twist angle and rotation freedom due to the donor triphenylamine (TPA) proximal or distal to N-atom (pyridyl), as well as a symmetric bis-triphenylamine-substituted compound DTPA-PT for their investigation. On the basis of experimental and theoretical analysis, the authors have explained how the difference in twist-angle and freedom of rotation affects the photophysical properties of these materials. The p-TPA-PT has small twist angles with a relative large k_r, however, the more freedom of rotation of the D–A bond causes larger k_nr, which is comparable to the k_r, and thus a lower photoluminescence (PL) efficiency is obtained in the doped film. Although the d-TPA-PT has relative large twist angles with low k_r, the suppressed rotation of the DA bond significantly reduces k_nr, resulting in more competitive k_r when compared with the p-TPA-PT, consequently relatively higher PL efficiency is observed. The DTPA-PT, combination of p-TPA-PT and d-TPA-PT, shows highest k_r, much larger than its k_nr, therefore shows highest PL efficiency. As a result, the DTPA-PT based organic light emitting diode (OLED) shows beautiful deep-red emission, maximum external quantum efficiency of 3.87%, L_max = 12 000 cd m⁻², which is among the best deep-red OLED devices.

The twist angle and freedom of rotation differences on the photophysical properties of donor–acceptor materials are systematically investigated by elegant molecular design. From the experimental and theoretical analysis, how these differences affect their photophysical properties is explained. A highly efficient deep-red emission organic light-emitting diode (OLED) based on these materials is also obtained with a maximum external quantum efficiency of 3.87% and L_max = 12 000 cd m⁻².

A series of pyrimidine-conjugated thermally activated delayed fluorescent (TADF) emitters is developed. Using a highly emissive green pyrimidine conjugate emitter and sophisticated device engineering, the optimized device operates on 2.2 V at 1 cd m⁻² and exhibits a superior performance with an η_ext of over 25%, an η_p of over 110 lm W⁻¹, and exceptionally low efficiency roll-off.

The spatio-temporal dynamics of excitons in functional polymer light-emitting diodes reveal that singlet–triplet annihilation (STA) is significantly higher than the often invoked singlet–polaron annihilation (SPA). STA holds back the efficiencies of PLEDs and further realization of organic injection lasers. Additional singlets are found in F8BT that are related to thermally activated delayed fluorescence (TADF), as triplet–triplet annihilation (TTA) alone cannot account for the observed high external quantum efficiencies.

High-performance blue thermally activated delayed fluorescence (TADF) emitters containing a phenoxaphosphine oxide or phenoxathiin dioxide acceptor unit coupled with a dimethylacridan donor unit are developed by T. Yasuda and co-workers, as desribed on page 4626. These emitters can allow efficient up-conversion of triplet excitons into singlet excitons, leading to both photoluminescence and internal electroluminescence quantum efficiencies of up to nearly 100%.

J.-J. Kim and co-workers achieve highly efficient blue organic light-emitting diodes (OLEDs) using a low-refractive-index layer. As described on page 4920, an external quantum efficiency over 34% is achieved, owing to the low refractive index of the materials. A milepost and a shining entrance of the castle are the metaphor indicating the way to highly efficient blue OLEDs. On the way to the castle, the depicted chemical structures serve as the light-emitting layer.

Efficient sky-blue organic light-emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) display a three orders of magnitude increase in lifetime, which is superior to those of controlled phosphorescent OLEDs used in this study. The combination of electro-oxidation and photo-oxidation of the TADF emitters in their triplet excited-states is suppressed through molecule design and device engineering.

Donor–acceptor–acceptor′ small-molecule donors are synthesized to investigate regioisomeric effects on organic photovoltaic device performance. Cross-conjugation in 2-((7-(N-(2-ethylhexyl)-benzothieno[3,2-b]thieno[3,2-d]pyrrol-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene)malononitrile leads to an increased open-circuit voltage compared with its isomer 2-((7-(N-(2-ethylhexyl)-benzothieno[3,2-b]thieno[2,3-d]pyrrol-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene)malononitrile. A correlation is then established between molecular conjugation length and orbital energies, and hence open-circuit voltage.

Leveraging on plasmonic hot electrons is an emerging strategy for electron–hole separation towards efficient photovoltaics and photocatalysis. On page 960, X. Huang, S. J. Wang, S. J. Chua, and co-workers extend this concept to electron–hole recombination for illumination, which is rarely reported. By taking advantage of the good energy match between the dopant-correlated green luminescence in Cu-doped ZnO nanowires and the localized surface plasmon resonance of Au nanoparticles, an intense, ultrafast, and temperature-robust UV luminescence is achieved in the nanowires after coating with Au nanoparticles.

A rapid, scalable, and noninvasive lithography is reported as a pixel patterning method for large-area flexible organic light-emitting diodes (OLEDs). Effective pixel separation is achieved by highly aligned printed organic fibers with diameters from several hundreds of nanometers to approximately 1 μm. This method, developed by T.-W. Lee and co-workers on page 967, suggests a possible replacement of photolithography for the large-area patterning of passive matrix OLEDs, banks in active matrix OLEDs, and grid patterns in white OLEDs.

Highly efficient, solution-processed, deep-blue fluorescent emitters are urgently required to realize inexpensive organic light-emitting diodes (OLEDs) for full-color displays and lighting applications. Herein, two new bipolar fluorescent emitters: 2-(4-(7-(9,9-dimethylacridin-10(9H)-yl)-9,9-diethyl-9H-fluoren-2-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (AFpPPI) and 2-(3-(7-(9,9-dimethylacridin-10(9H)-yl)-9,9-diethyl-9H-fluoren-2-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (AFmPPI) are rationally designed and synthesized. These two are afforded from 9,9-dimethyl-9,10-dihydroacridine (DMACR) as an electron donor and phenylphenanthroimadazole (PPI) as an electron acceptor, using 9,9-diethylfluorene as a spacer with different substitution isomers (para or meta). The photophysical, electrochemical, thermal, and charge-transport properties, as well as the electronic distribution of AFpPPI and AFmPPI are investigated and the results are well supported by density functional theory (DFT) and semi-classical Marcus theory. Interestingly, AFpPPI and AFmPPI display deep-blue emission with high fluorescence quantum yields (Φ_F). Furthermore, solution-processed, non-doped OLEDs were fabricated with AFpPPI and AFmPPI as an emitter, in which AFpPPI delivered a maximum external quantum efficiency (EQE) of 5.76% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.10) and a maximum current efficiency (CE) of 5.39 cd A⁻¹, which is the best performance for deep-blue, solution-processed fluorescent OLEDs based on non-doped bipolar emitters.

Highly efficient deep-blue fluorescent emitters are rationally designed and synthesized for solution-processed non-doped organic light-emitting diodes. The photophysical/electrochemical properties are investigated and the results are supported by theoretical calculations. In particular, AFpPPI as an emitter exhibits a maximum external quantum efficiency of 5.76% with CIE coordinates of (0.15, 0.10) and a maximum current efficiency of 5.39 cd A⁻¹.

A high triplet energy host material for improved efficiency and elongated lifetime is developed by modifying carbazole linked terphenyl with a CN substituent. The CN modification strengthens electron injection and transport of the host material, which reduces the driving voltage, improved quantum efficiency, and power efficiency of blue phosphorescent organic light-emitting diodes. Furthermore, the CN modification also extends the lifetime of the blue device due to the chemical bond stabilizing effect of the CN moiety in radical, anion, and cation states. Therefore, the molecular design modifying the backbone structure with CN is an efficient approach for high efficiency and long lifetime.

The chemical bond of carbazole based host material is strengthened by CN modification of carbazole, which improves the lifetime of blue phosphorescent organic light-emitting diodes.