Exciplex Organic Light-Emitting Diodes with Nearly 20% External Quantum Efficiency: Effect of Intermolecular Steric Hindrance between the Donor and Acceptor Pair

Superior Hole Injection Material PEGDT/TPF/PVDF with p-Doping Capability for Highly Efficient Solution-Processed Organic Light-Emitting Diode

The ability to charge injection is a key factor in determining the performance of the organic light-emitting diode (OLED) devices. Improving the work function of the anode surface via interface modification, thus lowering the hole injection barrier, stands as a crucial strategy for enhancing the performance of the OLED device. Herein, we propose an innovative p-doping hole injection material, namely, PEGDT/TPF/PVDF that exhibits excellent performance in OLED devices with the value of maximum current efficiency at 56.4 Cd A-1, maximum luminescence at 25,564 Cd m-2, and a high EQE of 19.8%. The results for PEGDT/TPF/PVDF showed good conductivity, excellent film-forming property, and high transmittance over 98% in the spectrum range of 500-700 nm. Changes in the hole-injection energy barriers observed from the surface of the anode suggest a modified anode with PEGDT/TPF/PVDF deepened the work function at a value of 0.2 eV, which dramatically improves the hole-injection properties. This work not only provides novel structural materials with exceptional hole-injection properties but also proposes a promising alternative to PEDOT/PSS.

Exciplex, Not Heavy-Atom Effect, Controls the Triplet Dynamics of a Series of Sulfur-Containing Thermally Activated Delayed Fluorescence Molecules

The efficiency of thermally activated delayed fluorescence (TADF) in organic materials relies on rapid intersystem crossing rates and fast conversion of triplet (T) excitons into a singlet (S) state. Heavy atoms such as sulfur or selenium are now frequently incorporated into TADF molecular structures to enhance these properties by increased spin-orbit coupling [spin orbit coupling (SOC)] between the T and S states. Here a series of donor-acceptor (D-A) molecules based on 12H-benzo[4,5]thieno[2,3-a]carbazole and dicyanopyridine is compared with their nonsulfur control molecules designed to probe such SOC effects. We reveal that unexpected intermolecular interactions of the D-A molecules with carbazole-containing host materials instead serve as the dominant pathway for triplet decay kinetics in these materials. In-depth photophysical and computational studies combined with organic light emitting diode measurements demonstrate that the anticipated heavy-atom effect from sulfur is overshadowed by exciplex formation. Indeed, even the unsubstituted acceptor fragments exhibit pronounced TADF exciplex emission in appropriate carbazole hosts. The intermolecular charge transfer and TADF in these systems are further confirmed by detailed time-dependent density functional theory studies. This work demonstrates that anticipated heavy-atom effects in TADF emitters do not always control or even impact the photophysical and electroluminescence properties.

Unraveling non-radiative decay channels of exciplexes to construct efficient red emitters for organic light-emitting diodes

Exciplex emitters naturally have thermally activated delayed fluorescence characteristics due to their spatially separated molecular orbitals. However, the intermolecular charge transfer potentially induces diverse non-radiative decay channels, severely hindering the construction of efficient red exciplexes. Thus, a thorough comprehension of this energy loss is of paramount importance. Herein, different factors, including molecular rigidity, donor-acceptor interactions and donor-donor/acceptor-acceptor interactions, that impact the non-radiative decay were systematically investigated using contrasting exciplex emitters. The exciplex with rigid components and intermolecular hydrogen bonds showed a photoluminescence quantum yield of 84.1% and a singlet non-radiative decay rate of 1.98 × 106 s-1 at an optimized mixing ratio, respectively, achieving a 3.3-fold increase and a 70% decrease compared to the comparison group. In the electroluminescent device, a maximum external quantum efficiency of 23.8% was achieved with an emission peak of 608 nm, which represents the state-of-the-art organic light-emitting diodes using exciplex emitters. Accordingly, a new strategy is finally proposed, exploiting system rigidification to construct efficient red exciplex emitters that suppress non-radiative decay.

Modulation of luminescence properties of circularly polarized thermally activated delayed fluorescence molecules with axial chirality by donor engineering

Multifunctional thermally activated delayed fluorescence (TADF) materials are currently a trending research subject for luminescence layer materials of organic light-emitting diodes (OLEDs). Among these, circularly polarized thermally activated delayed fluorescence (CP-TADF) materials have the advantage of being able to directly achieve highly efficient circularly polarized luminescence (CPL). The simultaneous integration of outstanding luminescence efficiency and excellent luminescence asymmetry factor (glum) is a major constraint for the development of CP-TADF materials. Therefore, on the basis of first-principles calculations in conjunction with the thermal vibration correlation function (TVCF) method, we study CP-TADF molecules with different donors to explore the feasibility of using the donor substitution strategy for optimizing the CPL and TADF properties. The results indicate that molecules with the phenothiazine (PTZ) unit as the donor possess small energy difference, a great spin-orbit coupling constant and a rapid reverse intersystem crossing rate, which endow them with remarkable TADF features. Meanwhile, compared with the reported molecules, the three designed molecules exhibit better CPL properties with higher glum values. Effective molecular design strategies by donor engineering to modulate the CPL and TADF properties are theoretically proposed. Our findings reveal the relationship between molecular structures and luminescence properties of CP-TADF molecules and further provide theoretical design strategies for optimizing the CPL and TADF properties.

Theoretical design and performance prediction of deep red/near-infrared thermally activated delayed fluorescence molecules with through space charge transfer

Thermally activated delayed fluorescence (TADF) molecules with through-space charge transfer (TSCT) have attracted much attention in recent years because of their ability to simultaneously reduce the energy difference (ΔEST) and enlarge the spin-orbit coupling (SOC). In this paper, 40 molecules are theoretically designed by changing the different substitution positions of the donors and acceptors, and systematically investigated based on the first-principles calculations and excited-state dynamics study. It is found that the emission wavelengths of v-shaped molecules with intramolecular TSCT are larger than those of the molecules without TSCT. Therefore, the intramolecular TSCT can induce the red-shift of the emission and realize the deep-red/near-infrared emission. Besides intramolecular TSCT can simultaneously increase the SOC as well as the oscillator strength and reduce the ΔEST. In addition, PXZ or PTZ can also favor the realization of smaller ΔEST and red-shift emission. Our calculations suggest that intramolecular TSCT and suitable donors (-PXZ or -PTZ) are an effective strategy for the design of efficient deep red/near-infrared TADF emitters.

Enhanced Efficiency of Exciplex Emission from a 9-Phenylfluorene Derivative

The exciplex-thermally activated delayed fluorescence (exciplex-TADF) system is an excellent candidate for the fabrication of high-efficiency organic light-emitting diodes (OLEDs) because of its more easily achieved small singlet-triplet energy splitting (ΔE_ST) and doping control. However, exciplex-TADF is still faced with the problems of low external quantum efficiency (η_ext) and unclear effect of structure modification in electron acceptors. Herein, we provide a steric hindrance increase strategy to obtain high-efficiency exciplex emissions. Through introducing a 9-phenylfluorene group into N-ethylcarbazole of the dicyano-substituted 9-phenylfluorene, an electron acceptor material with increased steric hindrance is obtained, which helps the exciplex harvest a larger driving force and higher emission efficiencies. Encouragingly, the obtained OLED displays a maximum η_ext of 25.8%, which is one of the best efficiency values among reported exciplex-OLEDs, simultaneously possessing excellent current efficiency of 83.6 cd A^-1 and power efficiency of 93.7 lm W^-1. It is expected that this work will offer a new avenue for designing electron acceptors for highly efficient exciplex emissions.

Exciplex-Forming Cohost Systems with 2,3-Dicyanopyrazinophenanthrene-based Acceptors to Achieve Efficient Near Infrared OLEDs

Two new 2,3-dicyanopyrazinophenanthrene-based acceptors (A) p-QCN and m-QCN were synthesized to blend with a donor (D) CPTBF for the exciplex formation. The energy levels of p-QCN and m-QCN are modulated by the peripheral substituents 4- and 3-benzonitrile, respectively. Exciplex-forming blends were identified by the observation of the red-shifted emissions from various D : A blends with higher ratios of donor for suppressing the aggregation of acceptor. The two-component relaxation processes observed by time-resolved photoluminescence support the thermally activated delayed fluorescence (TADF) character of the exciplex-forming blends. The device employing CPTBF : p-QCN and (2 : 1) and CPTBF : m-QCN (2 : 1) blend as the emitting layer (EML) gave EQE_max of 1.76 % and 5.12 %, and electroluminescence (EL) λ_max of 629 nm and 618 nm, respectively. The device efficiency can be further improved to 4.32 % and 5.57 % with CPTBF : p-QCN and (4 : 1) and CPTBF : m-QCN (4 : 1) as the EML, which is consistent with their improved photoluminescence quantum yields (PLQYs). A new fluorescent emitter BPBBT with photoluminescence (PL) λ_max of 726 nm and a high PLQY of 67 % was synthesized and utilized as the dopant of CPTBF : m-QCN (4 : 1) cohost system. The device employing CPTBF : m-QCN (4 : 1): 5 wt.% BPBBT as the EML gave an EQE_max of 5.02 % and EL λ_max centered at 735 nm, however, the weak residual exciplex emission remains. By reducing the donor ratio, the exciplex emission can be completely transferred to BPBBT and the corresponding device with CPTBF : m-QCN (2 : 1): 5 wt.% BPBBT as the EML can achieve EL λ_max of 743 nm and EQE_max of 4.79 %. This work manifests the high efficiency near infrared (NIR) OLED can be realized by triplet excitons harvesting of exciplex-forming cohost system, followed by the effective energy transfer to an NIR fluorescent dopant.

Achieving Balanced Electrical Performance of Host Material through Dual N-P═O Resonance Linkage for Efficient Electroluminescence

Developing high-performance host materials is one of the biggest challenges for blue and white thermally activated delayed-fluorescence (TADF) organic light-emitting diode (OLED) technology due to the rigorous requirements of both efficient carrier flux ability and high triplet energy (E_T) levels in static donor-acceptor molecules. Here, with the aid of a dual-resonance strategy, a host molecule showing dynamic adaption features in the acceptor-resonance-donor-resonance-acceptor (A-r-D-r-A) molecular configuration has been successfully developed through the implantation of two acceptors of diphenylphosphine oxide into electron-donating 5,10-dihydrophenazine with N-P═O resonance linkages. Owing to the dual enantiotropic N⁺═P-O^- resonances, the designed A-r-D-r-A molecule exhibits an extraordinarily balanced charge flux transportation attribute at high E_T (2.96 eV). Excitingly, blue and warm-white TADF OLEDs hosted by the A-r-D-r-A molecule exhibit outstanding external quantum efficiencies of 14.7 and 20.3%, respectively. Our studies not only broaden the scope of resonance molecules but also indicate that a resonance structure is an effective linkage to develop optoelectronic materials with dynamically adaptive properties.

Efficient Exciplex-Based Deep-Blue Organic Light-Emitting Diodes Employing a Bis(4-fluorophenyl)amine-Substituted Heptazine Acceptor

The realization of a deep-blue-emitting exciplex system is a herculean task in the field of organic light-emitting diodes (OLEDs) on account of a large red-shifted and broadened exciplex emission spectrum in comparison to those of the corresponding single compounds. Herein, 2,5,8-tris(di(4-fluorophenyl)amine)-1,3,4,6,7,9,9b-heptaazaphenalene (HAP-3FDPA) was designed as an electron acceptor by integrating three bis(4-fluorophenyl)amine groups into a heptazine core, while 1,3-di(9H-carbazol-9-yl)benzene (mCP) possessing two electron-donating carbazole moieties was chosen as the electron donor. Excitingly, the exciplex system of 8 wt% HAP-3FDPA:mCP exhibited deep-blue emission and a high photoluminescence quantum yield of 53.2%. More importantly, an OLED containing this exciplex system as an emitting layer showed deep-blue emission with Commission Internationale de l'Eclairage coordinates of (0.16, 0.12), a peak luminance of 15,148 cd m-2, and a rather high maximum external quantum efficiency of 10.2% along with a low roll-off. This study not only reports an efficient exciplex-based deep-blue emitter but also presents a feasible pathway to construct highly efficient deep-blue OLEDs based on exciplex systems.

Heptazine-Based π-Conjugated Materials for Light-Emitting

On the basis of planar and relatively rigid nitrogen-rich heterocyclic system of the heptazine core, heptazine-based π-conjugated materials have aroused widespread attention over the past decade by virtue of the fascinating electronic, optical, thermal, and mechanical properties in the fields of light-emitting, photocatalysis, sensors, environmental remediation, and so forth. However, there are still several obstacles to be solved before practical applications, such as low photoluminescence quantum efficiencies for light-emitting and weak visible absorption for photocatalysis. To further enhance various properties of heptazine-based π-conjugated materials, a series of strategies have been developed, including ingenious molecular design and modification, novel synthetic, and preparation methods. In this review, the significant progress of monomeric and polymeric heptazine-based π-conjugated materials and their applications typically in light-emitting are reviewed, which is beneficial for the acceleration of practical applications of heptazine-based materials and devices.

Enhanced Electroluminescence Based on a π-Conjugated Heptazine Derivative by Exploiting Thermally Activated Delayed Fluorescence

Heptazine derivatives have attracted much attention over the past decade by virtue of intriguing optical, photocatalytic as well as electronic properties in the fields of hydrogen evolution, organic optoelectronic technologies and so forth. Here, we report a simple π-conjugated heptazine derivative (HAP-3DF) possessing an n→π* transition character which exhibits enhanced electroluminescence by exploiting thermally activated delayed fluorescence (TADF). Green-emitting HAP-3DF shows relatively low photoluminescence quantum efficiencies (Φ_p) of 0.08 in toluene and 0.16 in doped film with bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO) as the matrix. Interestingly, the organic light-emitting diode (OLED) incorporating 8 wt% HAP-3DF:DPEPO as an emitting layer achieved a high external quantum efficiency (EQE) of 3.0% in view of the fairly low Φ_p of 0.16, indicating the presence of TADF stemming from n→π* transitions. As the matrix changing from DPEPO to 1,3-di (9H-carbazol-9-yl)benzene (mCP), a much higher Φ_p of 0.56 was found in doped film accompanying yellow emission. More importantly, enhanced electroluminescence was observed from the OLED containing 8 wt% HAP-3DF:mCP as an emitting layer, and a rather high EQE of 10.8% along with a low roll-off was realized, which should be ascribed to the TADF process deriving from exciplex formation.

Thermally activated delayed fluorescence exciplex emitters for high-performance organic light-emitting diodes

Owing to their natural thermally activated delayed fluorescence (TADF) characteristics, the development of exciplex emitters for organic light-emitting diodes (OLEDs) has witnessed booming progress in recent years. Formed between electron-donating and electron-accepting molecules, exciplexes with intermolecular charge transfer processes have unique advantages compared with unimolecular TADF materials, offering a new way to develop high-performance TADF emitters. In this review, a comprehensive overview of TADF exciplex emitters is presented with a focus on the relationship between the constituents of exciplexes and their electroluminescence performance. We summarize and discuss the latest and most significant developments of TADF exciplex emitters. Notably, the design principles of efficient TADF exciplex emitters are systematically categorized into three systems within this review. These progressive achievements of TADF exciplex emitters point out future challenges to trigger more research endeavors in this growing field.

Exciplex energy transfer through spacer: White electroluminescence with enhanced stability based on cyan intermolecular and orange intramolecular thermally activated delayed fluorescence

Capability of exciplex energy transfer through a spacer was investigated using three exciplex-forming solid mixtures which contained the well-known electron accepting 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine and appropriately designed bipolar cyanocarbazolyl-based derivatives functionalized by attachment of carbazolyl, acridanyl or phenyl units. These novel cyanocarbazolyl-based derivatives were used as both the spacer and exciplex-forming donor. Efficient organic light-emitting diodes with electroluminescence in cyan-yellow region and maximum external quantum efficiency of up to 7.7% were fabricated owing to efficient thermally activated fluorescence (TADF) of the newly discovered exciplexes. An approach of exciton separation by the spacer between the studied exciplexes and selected orange TADF emitter was proposed for the fabrication of white electroluminescent devices with prolonged lifetime comparing to that of single-color exciplex-based devices. Exciplex-forming systems were tested for exciton separation between inter- and intramolecular TADF. Exciplex energy transfer through a spacer was observed on relatively long distance for one system due to the energy resonance between triplet levels of the exciplex and spacer. First time observed here exciplex energy transfer through a spacer can be useful for both improvement of device stability and obtaining of white electroluminescence.