Solution-processed optoelectronic and electronic devices are attractive owing to the potential for low-cost fabrication of large-area devices and the compatibility with lightweight, flexible plastic substrates. Solution-processed light-emitting diodes (LEDs) using conjugated polymers or quantum dots as emitters have attracted great interest over the past two decades. However, the overall performance of solution-processed LEDs--including their efficiency, efficiency roll-off at high current densities, turn-on voltage and lifetime under operational conditions-remains inferior to that of the best vacuum-deposited organic LEDs. Here we report a solution-processed, multilayer quantum-dot-based LED with excellent performance and reproducibility. It exhibits colour-saturated deep-red emission, sub-bandgap turn-on at 1.7 volts, high external quantum efficiencies of up to 20.5 per cent, low efficiency roll-off (up to 15.1 per cent of the external quantum efficiency at 100 mA cm(-2)), and a long operational lifetime of more than 100,000 hours at 100 cd m(-2), making this device the best-performing solution-processed red LED so far, comparable to state-of-the-art vacuum-deposited organic LEDs. This optoelectronic performance is achieved by inserting an insulating layer between the quantum dot layer and the oxide electron-transport layer to optimize charge balance in the device and preserve the superior emissive properties of the quantum dots. We anticipate that our results will be a starting point for further research, leading to high-performance, all-solution-processed quantum-dot-based LEDs ideal for next-generation display and solid-state lighting technologies.

Solution-processed, high-performance light-emitting diodes based on quantum dots

Blue organic light-emitting diodes that harness thermally activated delayed fluorescence are realized with an external quantum efficiency of 19.5% and reduced roll-off at high luminance.

Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence

Organic materials that exhibit thermally activated delayed fluorescence (TADF) are an attractive class of functional materials that have witnessed a booming development in recent years. Since Adachi et al. reported high-performance TADF-OLED devices in 2012, there have been many reports regarding the design and synthesis of new TADF luminogens, which have various molecular structures and are used for different applications. In this review, we summarize and discuss the latest progress concerning this rapidly developing research field, in which the majority of the reported TADF systems are discussed, along with their derived structure–property relationships, TADF mechanisms and applications. We hope that such a review provides a clear outlook of these novel functional materials for a broad range of scientists within different disciplinary areas and attracts more researchers to devote themselves to this interesting research field.

Recent advances in organic thermally activated delayed fluorescence materials.

The design and characterization of thermally activated delayed fluorescence (TADF) materials for optoelectronic applications represents an active area of recent research in organoelectronics. Noble metal-free TADF molecules offer unique optical and electronic properties arising from the efficient transition and interconversion between the lowest singlet (S1) and triplet (T1) excited states. Their ability to harvest triplet excitons for fluorescence through facilitated reverse intersystem crossing (T1→S1) could directly impact their properties and performances, which is attractive for a wide variety of low-cost optoelectronic devices. TADF-based organic light-emitting diodes, oxygen, and temperature sensors show significantly upgraded device performances that are comparable to the ones of traditional rare-metal complexes. Here we present an overview of the quick development in TADF mechanisms, materials, and applications. Fundamental principles on design strategies of TADF materials and the common relationship between the molecular structures and optoelectronic properties for diverse research topics and a survey of recent progress in the development of TADF materials, with a particular emphasis on their different types of metal-organic complexes, D-A molecules, and fullerenes, are highlighted. The success in the breakthrough of the theoretical and technical challenges that arise in developing high-performance TADF materials may pave the way to shape the future of organoelectronics.

Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics

We report a method to convert discrete representations of molecules to and from a multidimensional continuous representation. This model allows us to generate new molecules for efficient exploration and optimization through open-ended spaces of chemical compounds. A deep neural network was trained on hundreds of thousands of existing chemical structures to construct three coupled functions: an encoder, a decoder and a predictor. The encoder converts the discrete representation of a molecule into a real-valued continuous vector, and the decoder converts these continuous vectors back to discrete molecular representations. The predictor estimates chemical properties from the latent continuous vector representation of the molecule. Continuous representations allow us to automatically generate novel chemical structures by performing simple operations in the latent space, such as decoding random vectors, perturbing known chemical structures, or interpolating between molecules. Continuous representations also allow the use of powerful gradient-based optimization to efficiently guide the search for optimized functional compounds. We demonstrate our method in the domain of drug-like molecules and also in the set of molecules with fewer that nine heavy atoms.

Automatic chemical design using a data-driven continuous representation of molecules

The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10(6) decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.

/pdf/highly-efficient-organic-light-emitting-diodes-from-delayed-31otvjgv66.pdf

Highly efficient organic light-emitting diodes from delayed fluorescence

An organic light-emitting device having a light-emitting layer containing a compound represented by the general formula below has a high light emission efficiency. In the general formula, at least one of R1 to R5 represents a cyano group, at least one of R1 to R5 represents a 9-carbazolyl group, a 1,2,3,4-tetrahydro-9-carbazolyl group, a 1-indolyl group or a diarylamino group, and the balance of R1 to R3 represents a hydrogen atom or a substituent.

Organic light emitting element, and light emitting material and compound used in same

An organic light-emitting device having a light-emitting layer containing a compound represented by the general formula below has a high light emission efficiency. In the general formula, at least one of R 1 to R 5 represents a cyano group, at least one of R 1 to R 5 represents a 9-carbazolyl group, a 1,2,3,4-tetrahydro-9-carbazolyl group, a 1-indolyl group or a diarylamino group, and the balance of R 1 to R 5 represents a hydrogen atom or a substituent.

Organic light emitting device, and light emitting material and compound used therefor

This organic light emitting element, which has a compound represented by general formula in a light emitting layer, has high luminous efficiency. In general formula, at least one of R1-R5 represents a cyano group; at least one of R1-R5 represents a 9-carbazolyl group, a 1,2,3,4-tetrahydro-9-carbazolyl group, a 1-indolyl group or a diarylamino group; and the rest of R1-R5 represent a hydrogen atom or a substituent.

Organic electroluminescence device, and luminescence material and compound used therein

Two blueish materials were obtained in the acid-catalyzed condensation of 5-pentafluorophenyldipyrromethane-1,9-bis{(petafluorophenyl)-methanol} with 4,8-dihydro-4,8-ethano-pyrrol[3,4-f]isoindole followed by oxidation, although the yields were low. Their structures were unambiguously determined by the X-ray analysis. One was ethylene-connected bisdipyrromethene and another was bicyclo[2.2.2]octene-crosslinked dihydrohexaphyrin, which was gradually transformed to benzene-crosslinked hexaphyrin under air in solution via bicyclo[2.2.2]octadiene-crosslinked hexaphyrin. The benzene-crosslinked hexaphyrin was directly obtained in the similar acid-catalyzed condensation procedure of the dipyrromethane with 4,8-dihydropyrrol[3,4-f]isoindole, although the yield was also low. 

Hiroki Uoyama

Papers

Highly efficient organic light-emitting diodes from delayed fluorescence

Organic light emitting element, and light emitting material and compound used in same

Organic light emitting device, and light emitting material and compound used therefor

Organic electroluminescence device, and luminescence material and compound used therein

Benzene-crosslinked hexaphyrin: Molecules of benzene in hexaphyrin