Solution-processed Nondoped Organic Light-emitting Diodes Research Articles

The role of a small molecular dipole moment for efficient non-doped deep blue thermally activated delayed fluorescence emitters

Non-doped and solution-processable deep blue emitters can greatly reduce the manufacturing cost of organic light-emitting diodes (OLEDs). However, influences of parameters concerning intermolecular interactions in solid state have not yet been explored. Herein, we proposed molecular dipole moment as a pre-experimental parameter to evaluate an emitter’s performance in the solid state. By extracting efficiency data of OLEDs based on blue-emitting molecules from literature and calculating the dipole moments of these molecules, we found a clear inverse relationship. Three deep blue TADF emitters with similar molecular emitting properties were designed and synthesized. The PL efficiency of their solid films and device performance was found to decrease as the molecular dipole increased. Specifically, TDBA-SPQ with the smallest dipole moments of 1.86 D delivered much better performance in both vacuum-processed and solution-processed non-doped OLEDs with record-high EQEmax of 22.7% and 15.2% with CIE-y coordinates of 0.136 and 0.123, respectively.

Propeller-shape isomers with turn-on through-space charge transfer for solution-processed non-doped organic light-emitting diodes

Purely organic thermally activated delayed fluorescence (TADF) emitters have developed rapidly in the last decade. Recently, the TADF materials with through-space charge transfer (TSCT) attract great attention due to the ease of tuning the optoelectronic properties for organic light-emitting diodes (OLEDs). Herein, we report the design and synthesis of three propeller-shape emitters via isomer engineering, which are named as TRZ-o-SDMAC, TRZ-m-SDMAC and TRZ-p-SDMAC respectively. Surprisingly, TRZ-o-SDMAC and TRZ-m-SDMAC simultaneously exhibit TADF and aggregation-induced emission. In contrast, the para-linked emitter TRZ-p-SDMAC exhibited no delayed fluorescence with only the nanosecond-scale (12 ns) transient decay. The solution-processed non-doped TADF-OLEDs based on the meta-linked compound TRZ-m-SDMAC render a maximum external quantum efficiency of 20.3 %, which is substantially high than those of the ortho- and para-linked analogues. This work manifests the unique feature of turn-on TSCT and thus TADF via isomeric strategy.

New electron-donating segment to develop thermally activated delayed fluorescence emitters for efficient solution-processed non-doped organic light-emitting diodes

Thermally activated delayed fluorescent (TADF) materials capable of efficient solution-processed non-doped organic light-emitting diodes (OLEDs) are of important and practical significance for further development of OLEDs. In this work, a new electron-donating segment, 2,7-di(9H-carbazol-9-yl)-9,9-dimethyl-9,10-dihydroacridine (2Cz-DMAC), was designed to develop solution-processable non-doped TADF emitters. 2Cz-DMAC can not only simultaneously increase the solubility of compounds and suppress harmful aggregation-caused quenching, but also efficiently broaden the delocalization of the highest occupied molecular orbital and promote the reverse intersystem crossing process. Three new TADF emitters, 2-(2,7-di(9H-carbazol-9-yl)-9,9-dimethylacridin-10(9H)-yl)dibenzo[b,d]thiophene 5,5-dioxide (2Cz-DMAC-BTB), 2-(2,7-di(9H-carbazol-9-yl)-9,9-dimethylacridin-10(9H)-yl)-9H-thioxanthen-9-one (2Cz-DMAC-TXO), 2-(2,7-di(9H-carbazol-9-yl)-9,9-dimethylacridin-10(9H)-yl)thianthrene 5,5,10,10-tetraoxide (2Cz-DMAC-TTR), were developed by using 2Cz-DMAC segment as the electron-donor. As anticipated, the solution-processed non-doped OLEDs employing 2Cz-DMAC-BTB, 2Cz-DMAC-TXO and 2Cz-DMAC-TTR as the emitters respectively exhibited green, orange and red emissions with maximum external quantum efficiencies of 14.0%, 6.6% and 2.9%. These results successfully demonstrate the feasibility and convenience of developing efficient solution-processable non-doped TADF emitters based on 2Cz-DMAC segment.

Novel molecular triad exhibiting aggregation-induced emission and thermally activated fluorescence for efficient non-doped organic light-emitting diodes.

A light-emitting molecular triad (BPCP-2CPC) with dual functionality was successfully synthesized and applied to solution-processed non-doped organic light-emitting diodes. The BPCP-2CPC triad contains 9-phenyl-9H-carbazole units as a host moiety tethered to a green-emitting core (BPCP) through a cyclohexane unit and exhibits thermally activated delayed fluorescence (TADF) and aggregation-induced emission (AIE) characteristics simultaneously. The BPCP-2CPC-based non-doped TADF-OLED devices showed a high external quantum efficiency (EQE) of 13.4%.

Constructing a Novel Dendron for a Self-Host Blue Emitter with Thermally Activated Delayed Fluorescence: Solution-Processed Nondoped Organic Light-Emitting Diodes with Bipolar Charge Transfer and Stable Color Purity.

Self-host thermally activated delayed fluorescence (TADF) materials have recently been identified as effective emitters for solution-processed nondoped organic light-emitting diodes (OLEDs). However, except for the carbazole unit, few novel dendrons have been developed to build self-host TADF emitters. This study reports two self-host blue materials, tbCz-SO and poCz-SO, with the same TADF emissive core and different dendrons. The influence of the peripheral dendrons on the photophysical properties and electroluminescent performances of the self-host materials were systematically investigated. The transient fluorescence and electroluminescence spectra indicated that the diphenylphosphoryl carbazole units could effectively encapsulate the emissive core to reduce the concentration quenching effect and to enhance reverse intersystem crossing. By using tbCz-SO and poCz-SO as host-free blue emitters, the performance of the solution-processed nondoped OLED device demonstrated that a more balanced charge transfer from the bipolar dendrons would offer a better current efficiency of 10.5 cd A-1 and stable color purity with Commission Internationale de L'Eclairage units of (0.18, 0.27).

Nondoped Deep‐Blue Organic Light‐Emitting Diodes with Color Stability and Very Low Efficiency Roll‐Off: Solution‐Processable Small‐Molecule Fluorophores by Phosphine Oxide Linkage

A series of solution-processable small molecules PO1-PO4 were designed and synthesized by linking N-phenylnaphthalen-1-amine groups to a phenyl phosphine oxide core through a π-conjugated bridge, and their thermal, photophysical, and electrochemical properties were investigated. The phosphine oxide linkage can disrupt the conjugation and allows the molecular system to be extended to enable solution processability and high glass transition temperatures (159-181 °C) while preserving the deep-blue emission. The noncoplanar molecular structures resulting from the trigonal-pyramidal configuration of the phosphine oxide can suppress intermolecular interactions, and thus these compounds exhibit strong deep-blue emission both in solution and the solid state with high photoluminescent quantum yield (PLQY) of 0.88-0.99 in dilute toluene solution. Solution-processed nondoped organic light-emitting diodes featuring PO4 as emitter achieve a maximum current efficiency of 2.36 cd A(-1) with CIE coordinates of (0.15, 0.11) that are very close to the NTSC blue standard. Noticeably, all devices based on these small-molecular fluorescent emitters show striking deep-blue electroluminescent color stability and extremely low efficiency roll-off.