High-efficiency Fluorescent Organic Light-emitting Diodes Research Articles

Fine Modulation of the Higher-Order Excitonic States toward More Efficient Conversion from Upper-Level Triplet to Singlet.

Hot exciton luminogens capable of harvesting nonemissive triplet excitons via reverse intersystem crossing from high-order triplet (hRISC) to singlet have great potential in high-efficiency fluorescent organic light-emitting diodes (OLEDs). Although spin-orbit coupling (SOC) is regarded as a key factor affecting the RISC process, its effects on hot exciton materials are poorly understood. Herein, we design and synthesize two blue-emitting hot exciton luminogens, PABP and PAIDO, to study this issue by modulating the excited-state properties. Theoretical and experimental research contributions demonstrate that a stronger SOC between energetically close S1 (π-π*) and Tn (T3, n-π*) of PAIDO gives rise to faster and more efficient hRISC in comparison to that of PABP, leading to a higher external quantum efficiency and a higher exciton utilization efficiency. Crucially, the experimentally measured hRISC rate (khRISC) of hot exciton materials is on the order of 107 s-1, which is much faster than that of the thermally activated delayed fluorescence materials.

Realization of high-efficiency fluorescent organic light-emitting diodes with low driving voltage

It is commonly accepted that a full bandgap voltage is required to achieving efficient electroluminescence (EL) in organic light-emitting diodes. In this work, we demonstrated organic molecules with a large singlet-triplet splitting can achieve efficient EL at voltages below the bandgap voltage. The EL originates from delayed fluorescence due to triplet fusion. Finally, in spite of a lower quantum efficiency, a blue fluorescent organic light-emitting diode having a power efficiency higher than some of the best thermally activated delayed fluorescent and phosphorescent blue organic light-emitting diodes is demonstrated. The current findings suggest that leveraging triplet fusion from purely organic molecules in organic light-emitting diode materials offers an alternative route to achieve stable and high efficiency blue organic light-emitting diodes.

Highly efficient orange–red electroluminescence enabled by fluorenone-based thermally activated delayed fluorescent emitter

A butterfly shaped orange–red thermally activated delayed fluorescent (TADF) emitter, named ACFO, is developed by integrating electron-donating (D) 9,9-dimethyl-9,10-dihydroacridine units into γ-positions of an electron-accepting (A) fluorenone core to form a D-A-D configuration. ACFO emits intense orange–red light peaking at 600 nm in toluene solution, which is significantly red-shifted compared with its benzophenone-based counterpart. Imparted with a small singlet–triplet splitting energy of 0.13 eV, ACFO exhibits distinct TADF feature with a short delayed fluorescent lifetime of 3.8 μs and displays a moderate photoluminescence quantum yield of 31%. Consequently, employing ACFO as an emitter results in orange–red organic light-emitting diodes that exhibit a maximum external quantum efficiency of nearly 11%, corresponding to a nearly 100% exciton utilization efficiency in electroluminescence process. Our finding demonstrates for the first time that fluorenone derivatives can be employed as TADF emitters for high-efficiency fluorescent organic light-emitting diodes.

Highly Efficient Soluble Blue Delayed Fluorescent and Hyperfluorescent Organic Light-Emitting Diodes by Host Engineering.

Solution-processed high-efficiency fluorescent organic light-emitting diodes with an external quantum efficiency over 18% were developed by engineering a host material and device structure designed for solution process. A high triplet energy host material designed for the solution process, (oxybis(3-(tert-butyl)-6,1-phenylene))bis(diphenylphosphine oxide) (DPOBBPE), worked efficiently as the host of blue fluorescent devices because of good solubility, high photoluminescence quantum yield, and good film properties. The DPOBBPE host enabled a high external quantum efficiency of 18.8% in the fluorescent organic light-emitting diodes by the solution process. Moreover, 25.8% external quantum efficiency in the soluble blue thermally activated delayed fluorescent devices was also realized. The 25.8% external quantum efficiency of the DPOBBPE delayed fluorescent device and 18.8% external quantum efficiency of the fluorescent device are the highest efficiency values achieved in the solution-processed blue fluorescent organic light-emitting diodes. Moreover, the solution-processed fluorescent device showed an improved blue color coordinate of (0.14, 0.20) compared to (0.17, 0.31) of the delayed fluorescent device.

High-efficiency fluorescent organic light-emitting diodes with MoO3 and PEDOT : PSS composition film as a hole injection layer

The high efficiency and low efficiency roll-off can be achieved in C545T-based green OLEDs with MoO3/PEDOT : PSS as the interfacial modification layer on ITO.

Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion

High-efficiency fluorescent organic light-emitting diodes have been realized by employing custom-designed molecules that make it possible to convert non-radiative triplet states into radiative singlet states.

Dual efficiency enhancement by delayed fluorescence and dipole orientation in high-efficiency fluorescent organic light-emitting diodes

To explain the origin of extremely high efficiencies of deep-blue fluorescent organic light-emitting diodes (OLEDs) with anisotropic-shaped anthracene derivatives, the enhancements of singlet-exciton generation efficiency and outcoupling efficiency were investigated by transient electroluminescence measurement and variable angle spectroscopic ellipsometry, respectively. Both the delayed fluorescence from singlet excitons generated via triplet-triplet annihilation and the outcoupling enhancement by dipole orientation of emitters were found to contribute to the high external quantum efficiencies of the devices. This dual efficiency enhancement is important for understanding and further improving high-performance fluorescent OLEDs.