How optimized emitter design leads to saturated color highperformance organic light-emitting diodes

In an article recently published in Sci.China Chem.,Liao,Jiang,and their co-workers[1]report two N/carbonyl-based multi-resonant thermally activated delayed fluorescence(MR-TADF)emitters,the organic light-emitting diode(OLED)that uses one of these,MTDMQAO,showed record maximum external quantum efficiencies(EQEmax)for devices using this class of compound.The reason for this impressive device performance stems from a rationale and judicious molecular design,building on a core QAO(aka DiKTa)[2]MR-TADF emitter structure that the authors helped to pioneer[3,4],and decorating a fused derivative of it with an electron-withdrawing triazine unit.

Thermally activated delayed fluorescence materials for nondoped organic light-emitting diodes with nearly 100%exciton harvest

High-performance nondoped organic light-emitting diodes(OLEDs)are promising technologies for future commercial applications.Herein,we synthesized two new thermally activated delayed fluorescence(TADF)emitters that enable us,for the first time,to combine three effective approaches for enhancing the efficiency of nondoped OLEDs.First,the two emitters are designed to have high steric hindrances such that their emitting cores will be suitably isolated from those of their neighbors to minimize concentration quenching.On the other hand,each of the two emitters has two stable conformations in solid films.In their neat films,molecules with the minority conformation behave effectively as dopants in the matrix composing of the majority conformation.One hundred percent exciton harvesting is thus theoretically feasible in this unique architecture of“self-doped”neat films.Furthermore,both emitters have relatively high aspect ratios in terms of their molecular shapes.This leads to films with preferred molecular orientations enabling high populations of horizontal dipoles beneficial for optical outcoupling.With these three factors,OLEDs with nondoped emitting layers of the respective emitters both achieve nearly 100%exciton utilization and deliver over 30%external quantum efficiencies and ultralow efficiency roll-off at high brightness,which have not been observed in reported nondoped OLEDs.

Rational molecular design of efficient yellow-red dendrimer TADF for solution-processed OLEDs: a combined effect of substitution position and strength of the donors

The development of high-performance solution-processed red organic light-emitting diodes(OLEDs) remains a challenge,particularly in terms of maintaining efficiency at high luminance. Here, we designed and synthesized four novel orange-red thermally activated delayed fluorescence(TADF) dendrimers that are solution-processable: 2GCz BP, 2DPACz BP, 2FBP2GCz and 2FBP2DPACz. We systematically investigated the effect of substitution position and strength of donors on the optoelectronic properties. The reverse intersystem crossing rate constant(kRISC) of the emitters having donors substituted at positions 11and 12 of the dibenzo[a,c]phenazine(BP) is more than 10-times faster than that of compounds substituted having donors substituted at positions 3 and 6. Compound 2DPACz BP, containing stronger donors than 2GCz BP, exhibits a red-shifted emission and smaller singlet-triplet energy splitting, ΔE_(ST), of 0.01 e V. The solution-processed OLED with 10 wt% 2DPACz BP doped in m CP emitted at 640 nm and showed a maximum external quantum efficiency(EQE_(max)) of 7.8%, which was effectively maintained out to a luminance of 1,000 cd m-2. Such a device's performance at relevant display luminance is among the highest for solution-processed red TADF OLEDs. The efficiency of the devices was improved significantly by using 4Cz IPN as an assistant dopant in a hyperfluorescence(HF) configuration, where the 2DPACz BP HF device shows an EQEmaxof 20.0% at λEL of 605 nm and remains high at 11.8% at a luminance of 1,000 cd m-2, which makes this device one of the highest efficiency orange-to-red HF SP-OLEDs to date.

Flexible organic light-emitting diodes for antimicrobial photodynamic therapy

Bacterial infection and the growth of antibiotic resistance is a serious problem that leads to patient suffering,death and increased costs of healthcare.To address this problem,we propose using flexible organic light-emitting diodes(OLEDs)as light sources for photodynamic therapy(PDT)to kill bacteria.PDT involves the use of light and a photosensitizer to generate reactive oxygen species that kill neighbouring cells.We have developed flexible top-emitting OLEDs with the ability to tune the emission peak from 669 to 737 nm to match the photosensitizer,together with high irradiance,low driving voltage,long operational lifetime and adequate shelf-life.These features enable OLEDs to be the ideal candidate for ambulatory PDT light sources.A detailed study of OLED–PDT for killing Staphylococcus aureus was performed.The results show that our OLEDs in combination with the photosensitizer methylene blue,can kill more than 99%of bacteria.This indicates a huge potential for using OLEDs to treat bacterial infections.

Organic photovoltaics for simultaneous energy harvesting and high-speed MIMO optical wireless communications

We show that organic photovoltaics(OPVs)are suitable for high-speed optical wireless data receivers that can also harvest power.In addition,these OPVs are of particular interest for indoor applications,as their bandgap is larger than that of silicon,leading to better matching to the spectrum of artificial light.By selecting a suitable combination of a narrow bandgap donor polymer and a nonfullerene acceptor,stable OPVs are fabricated with a power conversion efficiency of 8.8%under 1 Sun and 14%under indoor lighting conditions.In an optical wireless communication experiment,a data rate of 363 Mb/s and a simultaneous harvested power of 10.9 mW are achieved in a 4-by-4 multipleinput multiple-output(MIMO)setup that consists of four laser diodes,each transmitting 56 mW optical power and four OPV cells on a single panel as receivers at a distance of 40 cm.This result is the highest reported data rate using OPVs as data receivers and energy harvesters.This finding may be relevant to future mobile communication applications because it enables enhanced wireless data communication performance while prolonging the battery life in a mobile device.

A hybrid organic-inorganic polariton LED

Polaritons are quasi-particles composed of a superposition of excitons and photons that can be created within a strongly coupled optical microcavity.Here,we describe a structure in which a strongly coupled microcavity containing an organic semiconductor is coupled to a second microcavity containing a series of weakly coupled inorganic quantum wells.We show that optical hybridisation occurs between the optical modes of the two cavities,creating a delocalised polaritonic state.By electrically injecting electron–hole pairs into the inorganic quantum-well system,we are able to transfer energy between the cavities and populate organic-exciton polaritons.Our approach represents a new strategy to create highly efficient devices for emerging‘polaritonic’technologies.

Insights into enhanced electrochemiluminescence of a multiresonance thermally activated delayed fluorescence molecule

The electrochemiluminescence(ECL)behavior of a multiresonance thermally activated delayed fluorescence molecule has been investigated for the first time by means of ECL‐voltage curves,newly designed ECL‐time observatory,and ECL spectroscopy.The compound,Mes3DiKTa,shows complex ECL behavior,including a delayed onset time of 5 ms for ECL generation in both the annihilation pathway and the coreactant route,which we attribute to organic long‐persistent ECL(OLECL).Triplet‐triplet annihilation,thermally activated delayed fluorescence and uncompensated solution resistance cannot be ruled out as contributing mechanisms to the ECL.A very long ECL emission decay was attributed to OLECL as well.The absolute ECL efficiencies of Mes3DiKTa were enhanced and reached 0.0013%in annihilation route and 1.1%for the coreactant system,which are superior to those of most other organic ECL materials.It is plausible that ECL materials with comparable behavior as Mes3DiKTa are desirable in applications such as ECL sensing,imaging,and light‐emitting devices.

Triple-cation perovskite solar cells for visible light communications

Hybrid perovskite materials are widely researched due to their high absorptivity,inexpensive synthesis,and promise in photovoltaic devices.These materials are also of interest as highly sensitive photodetectors.In this study,their potential for use in visible light communication is explored in a configuration that allows for simultaneous energy and data harvesting.Using a triple-cation material and appropriate device design,a new record data rate for perovskite photodetectors of 56 Mbps and power conversion efficiencies above 20%under white LED illumination are achieved.With this device design,the−3 dB bandwidth is increased by minimizing the dominating time constant of the system.This correlation between the bandwidth and time constant is proved using measurements of time-resolved photoluminescence,transient photovoltage,and device resistance.