Triplet Annihilation Process Research Articles

Direct observation of room-temperature high-level triplet–triplet annihilation process in 1%rubrene-doped OLEDs with a CzDBA sensitizer

Using the magneto-electroluminescence as a sensitive and fingerprint probing tool, a “hot exciton” channel, named the high-level triplet–triplet annihilation (HL-TTA or T2T2A, T2 + T2 → Sn → S1 → S0 + hv) process from the high-lying triplet (T2) to the lowest singlet states, is observed in 1%rubrene-doped organic light-emitting diodes with a thermally activated delayed fluorescence sensitizer of 9,10-bis(4-(9H-carbazol-9-yl)-2,6-dimethylphenyl)-9,10-diboraanthracene (CzDBA) at room temperature. The addition of a sensitizer simultaneously promotes the Dexter energy transfer channels of host-sensitizer and sensitizer-guest triplets, thereby increasing the amounts of T2 states and inducing the occurrence of the HL-TTA process. Additionally, the HL-TTA enhances with the increase in the bias current and decreases with lowering the working temperature, which is consistent with the current dependence of the conventional low-level TTA (T1T1A, T1 + T1 → S1 + S0) process but contrary to its temperature dependence. More interestingly, the high concentration of CzDBA induces the H-type aggregation of rubrene molecules, promoting the singlet fission process but suppressing the HL-TTA process. These findings enrich the physical understanding of hot exciton channels and provide ideas for the preparation of high-performance devices.

9‐(Diphenylphosphoryl)‐10‐(phenylethynyl)anthracene Derivatives: Synthesis and Implications for the Substituent and Solvent Effects on the Light‐Emitting Properties

AbstractHerein, we report a series of 9‐(diphenylphosphoryl)‐10‐(phenylethynyl)anthracenes (DPPPEAs) as novel fluorescent 9,10‐disubstituted anthracene derivatives. The DPPPEAs were prepared by Sonogashira coupling of (10‐bromoanthracen‐9‐yl)diphenylphosphine oxide with terminal arylacetylenes, and their structures were fully characterized. UV/Vis fluorescence spectroscopy and theoretical calculations were used to evaluate substituent effects on fluorescence properties of DPPPEAs. The nature of emissive excited state of DPPPEAs was found to vary greatly depending on the substituent, solvent, and temperature. Unsubstituted DPPPEA emitted intense fluorescence from locally excited (LE) state, whereas donor‐acceptor (D‐A) type DPPPEAs substituted with diphenylamino groups showed strong solvatofluorochromism derived from the charge‐transfer (CT) state. Notably, carbazolyl derivatives exhibited fluorescence from LE hybridized with CT state. The disappearance of CT emission in rigid media suggests that solvent reorganization plays a crucial role in producing large Stokes shifts of D‐A type derivatives. The DPPPEAs have also been found to function as annihilators in porphyrin‐sensitized triplet–triplet annihilation processes.

Observation of Vibrational Phosphorescence Peaks at Room Temperature and Their Impacts on Triplet–Triplet Annihilation

AbstractTriplet–triplet annihilation (TTA) is a feasible approach for utilizing electrically generated triplet excitons in organic light‐emitting diodes. However, low TTA efficiency is often observed in solids. Owing to the optically inaccessible nature of triplets, the main loss routes of the TTA process remain unknown. In this work, three vibrational emission peaks of the first excited triplet state (T1) are identified in the sensitized room‐temperature phosphorescence of TTA molecules. These peaks provide strong evidence of the loss channels of TTA. It is found that the monomolecular recombination of T1 represents an energy‐loss channel that combines the transient photoluminescence of both upconversion fluorescence and phosphorescence. Moreover, the study demonstrates that low‐energy vibrational T1 acts as an energy‐loss channel with only slight contributions to TTA. This work presents constructive insights into the improvement of TTA efficiency in solid films.

In Situ Quantifying the Physical Parameters Determining the Efficiency of OLEDs Relying on Triplet–Triplet Annihilation Up‐Conversion

AbstractTriplet–triplet annihilation (TTA) up‐conversion is an effective way to utilize triplet excitons in organic light‐emitting diodes (OLEDs). However, the parameters characterizing the triplet excitons and relevant TTA process in OLEDs under working conditions have not been quantified. Here, an in situ method is established to map these parameters for further ascertaining their impact on device efficiency. The physical parameters, including triplet recombination rate, TTA rate, typical current JTTA, and saturated ratio, can be in situ quantified by transient electroluminescence technique. The expression of JTTA shows that minimizing the triplet quenching and maximizing the TTA rate are effective ways to lower JTTA. While highly efficient devices require a lower JTTA. Guided by these criteria, the device efficiency is promoted by weakening the triplet quenching via blending two materials. These investigations establish an in situ method to quantify the physical parameters that allow identifying the useful TTA materials and optimizing the design of device structures.

Suppressing singlet-triplet annihilation processes to achieve highly efficient deep-blue AIE-based OLEDs.

Aggregation-induced emission (AIE) materials are attractive for the fabrication of high efficiency organic light-emitting diodes (OLEDs) by harnessing "hot excitons" from the high-lying triplet exciton states (Tn, n ≥ 2) and high photoluminescence (PL) quantum efficiency in solid films. However, the electroluminescence (EL) efficiency of most AIE-based OLEDs does not meet our expectation due to some unrevealed exciton loss processes. Herein, we further enhance the efficiency of blue AIE-based OLEDs, and find experimentally and theoretically that the serious exciton loss is caused by the quenching of radiative singlet excitons and long-lived triplet excitons [singlet-triplet annihilation (STA)]. In order to suppress the STA process, 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP) with triplet-triplet annihilation up-conversion was doped in two AIE emitters to reduce the triplet excitons on the lowest triplet excited state (T1) of AIE molecules. It can be seen that the external quantum efficiency (EQE) of the resulting blue OLEDs was enhanced to 11.8% with CIE coordinates of (0.15, 0.07) and a negligible efficiency roll-off, realizing the efficiency breakthrough of deep-blue AIE-based OLEDs. This work establishes a physical insight in revealing the exciton loss processes and the fabrication of high-performance AIE-based OLEDs.

Dendritic Interfacial Exciplex Hosts for Solution‐Processed TADF‐OLEDs with Power Efficiency Approaching 100 lm W−1

AbstractTwo kinds of interfacial exciplex hosts formed between a dendritic oligocarbazole donor (H2) and two pyridine‐containing isomeric acceptors (B4PyMPM and B3PyMPM) are developed for solution‐processed thermally activated delayed fluorescent (TADF) organic light‐emitting diodes (OLEDs). The exciplex hosts exhibit small singlet−triplet energy splitting (90–110 meV) and distinct TADF effect which can convert triplet excitons into singlet ones and thus are beneficial for alleviating the triplet annihilation process. Notably, it is found that a small structural variation of acceptor from B4PyMPM (para‐linked pyridyl units) to B3PyMPM (meta‐linked pyridyl units) has a significant effect on enhancing carrier balance between donor and acceptor layers of interfacial exciplex, leading to greatly improved electroluminescent efficiency for the resultant devices. Solution‐processed TADF‐OLEDs based on H2/B3PyMPM interfacial exciplex host achieve the maximum power efficiency (PE) of 95.0 lm W−1 (85.5 cd A−1, 26.4 %), which is much better than that of H2/B4PyMPM devices (69.9 lm W−1) and represents the record PE for solution‐processed TADF‐OLEDs.

A Dimeric π‐Stacking of Anthracene Inducing Efficiency Enhancement in Solid‐State Fluorescence and Non‐Doped Deep‐Blue Triplet–Triplet Annihilation Organic Light‐Emitting Diodes

AbstractTriplet–triplet annihilation (TTA) mechanism utilizing the conversion of low triplet energy excitons to generate singlet excitons has been successfully employed in realizing highly efficient fluorescent organic light‐emitting diodes (OLEDs). Herein, new anthracene‐based TTA molecules (TPNACN and TPBACN) are developed as deep‐blue emitters for high‐efficiency non‐doped TTA‐OLEDs. Their structural, physical, and photophysical properties are experimentally and theoretically investigated. These compounds in solid‐state exhibit different photophysical properties due to a discrepancy in the molecular packing. Particularly, in the crystal of TPNACN, anthracene moieties are arranged with dimeric π–π stacking, and the material shows a strong excimer emission in the deep‐blue region with ΦPL close to the ideal theoretical value. The non‐doped TTA‐OLED based on TPNACN attains a high maximum external quantum efficiency of 7.89% (6.63 cd A−1) with a low turn‐on voltage of 2.6 V, and displays deep‐blue emission with CIE coordinates of (0.146, 0.101). These results prove that a separated dimeric π‐stacked molecular alignment of anthracene enhances not only the fluorescence efficiency in the solid state but also the ratio of singlet exciton harvested by the TTA process in the device, bringing about excellent device electroluminescent properties. This can be a new tactic to designing new emissive materials for efficient OLED devices.

Single‐Photon Near‐Infrared‐Responsiveness from the Molecular to the Supramolecular Level via Platination of Pentacenes

AbstractNear‐infrared (NIR) responsiveness is important for various applications. Currently, single‐photon NIR‐responsive systems are rare compared to systems that display two‐photon absorption and triplet–triplet annihilation processes. Supramolecular stacking of photo‐responsive chromophores results in decreased efficiency due to space‐confinement effects. Herein we show that σ‐platination of pentacenes is a feasible protocol to build single‐photon NIR‐responsive systems, with advantages including a low HOMO–LUMO energy gap, high photochemical efficiency, and pathway specificity. The pentacene‐to‐endoperoxidation transformation is accompanied by color and absorbance changes. The high photo‐oxygenation efficiency of σ‐platinated pentacenes facilitates NIR responsiveness in one‐dimensional supramolecular polymers, resulting in the disappearance of supramolecular chirality signals and disruption of self‐assembled nanofibers. Overall, the σ‐platination strategy opens up new avenues toward NIR photo‐responsive materials at the molecular and supramolecular levels.

Direct Transition from Triplet Excitons to Hybrid Light-Matter States via Triplet-Triplet Annihilation.

Strong light–matter coupling generates hybrid states that inherit properties of both light and matter, effectively allowing the modification of the molecular potential energy landscape. This phenomenon opens up a plethora of options for manipulating the properties of molecules, with a broad range of applications in photochemistry and photophysics. In this article, we use strong light–matter coupling to transform an endothermic triplet–triplet annihilation process into an exothermic one. The resulting gradual on–off photon upconversion experiment demonstrates a direct conversion between molecular states and hybrid light–matter states. Our study provides a direct evidence that energy can relax from nonresonant low energy molecular states directly into hybrid light–matter states and lays the groundwork for tunable photon upconversion systems that modify molecular properties in situ by optical cavities rather than with chemical modifications.

Role of nanosize and defect trapping upon singlet fission yield and singlet fission dynamics of 1,6-Diphenyl-1,3,5-hexatriene nanoaggregates

Nanoaggregate formation of 1,6-Diphenyl-1,3,5-hexatriene (DPHT) has been observed to introduce energy perturbation which results nearly isoergic singlet fission (SF), as compared to the endoergic SF in the crystalline forms of DPHT. It results prompt triplet pair production with a time constant ∼11.4 ps and subsequently it goes for production of separated triplet excitons with time constant of ∼152 ps. The separated triplet exciton yield is estimated to be 115 ± 10 %. The reverse process of singlet exciton production i.e. triplet fusion or triplet triplet annihilation process is also energetically feasible in this material due to the isoergic nature of SF process. It causes generation of back transferred singlet excitons which can again participate in SF process or it may decay through defect trapping. The defect trapping of photogenerated as well as re-generated singlet excitons plays detrimental role in the overall triplet yield as compared to that of the crystalline forms of same molecule.

Delayed photoacidity produced through the triplet-triplet annihilation of a neutral pyranine derivative.

A novel pyranine derivative, EtHPTA-OH, was synthesized via the substitution of the anionic sulfonate groups with neutral diethylsulfonamide groups. The photophysical and photochemical properties of EtHPTA-OH were studied using photoluminescence quenching and transient absorption spectroscopy. The singlet state of EtHPTA-OH was found to be highly photoacidic (pKa* = 8.74 in acetonitrile). A series of aniline and pyridine bases were used to investigate excited-state proton transfer (ESPT) from singlet EtHPTA-OH, and rate constants for singlet quenching via ESPT were determined (kq = 5.18 × 109 to 1.05 × 1010 M-1 s-1). EtHPTA-OH was also found to exhibit a long-lived triplet state which reacts through a triplet-triplet annihilation (TTA) process to reform singlet EtHPTA-OH on timescales of up to 80 μs. Detection of ESPT photoproducts on timescales comparable to that of TTA singlet regeneration provides strong evidence for photoacidic behavior stemming from the regenerated singlet EtHPTA-OH.

Electronic energy pooling in organic systems: a cross-disciplinary tutorial review

Electronic energy pooling via excited state (exciton) annihilation, primarily in organic systems, is reviewed in tutorial form. Cross-disciplinary terminologies and references are used and reference is made to the historical origins of the phenomena. Applications in organic photovoltaic and electroluminescent devices are addressed. Particular attention is paid to the kinetics of the processes involved; a standard format for all systems is developed. Within the organic materials framework, all triplet–triplet, triplet–singlet, and singlet–singlet annihilation processes are discussed. Examples from gas, liquid, and solid phase systems, including both homo- and hetero-species interactions, are employed. Particular attention is given to triplet–triplet annihilation processes in which product states other than the lowest excited singlet state are formed.

Investigation of β-cyclodextrin complex formation with 2,2′-dipyridine in ground and excited triplet states

It was found that 2,2′-dipyridine ( $$DP$$ ) forms inclusion complex with beta-cyclodextrin ( $$CD$$ ) in basic water solution by using NMR and spectrophotometry. Complex formation does not have an effect on the yield of triplet excited 2,2′-dipyridine. Triplet excited 2,2′-dipyridine $$({}^{3}DP)$$ also forms inclusion complex with beta-cyclodextrin and the value of complex formation constant is close to that for $$DP$$ in the ground state. The electron transfer rate constant between the inclusion complex of $${}^{3}DP$$ with $$CD$$ and N-acetyl-l-tyrosine ( $$TyrOH$$ ) is about ten times less than between $${}^{3}DP$$ in free (non complexed) state and $$TyrOH$$ in basic (pH 11) solution. Also it was found that inclusion complex formation of $${}^{3}DP$$ with $$CD$$ slightly decreases the rate constant of triplet–triplet annihilation process.

Theoretical Analyses of Triplet–Triplet Annihilation Process of 9,10-Diphenylanthracene in Solution

Theoretical schemes to evaluate electronic coupling elements for triplet–triplet annihilation (TTA) processes are proposed using fragment molecular orbital-linear combination of molecular orbital methods. Combining them with the Marcus formula, we estimate rate constants of TTA for 9,10-diphenylanthracene (DPA). We also performed molecular dynamics simulations of diffusion of DPAs in solution to estimate rate constants of molecular encounter in an assumed diffusion-controlled reaction. On the basis of the calculated rate constants, the mechanism of TTA process for DPAs in solution are discussed.

Superior upconversion fluorescence dopants for highly efficient deep-blue electroluminescent devices.

In this study, we revealed a new approach for the development of new triplet-triplet annihilation (TTA) materials with highly efficient deep-blue fluorescence via the incorporation of a styrylpyrene core and an electron-donating group. The resulting deep-blue emitters (PCzSP, DFASP, and DPASP) exhibit intramolecular charge transfer emissions with remarkably high emission quantum yields. The electroluminescent devices based on these three fluorophores as dopants using CBP as a host exhibit very high device efficiencies; in particular, the DPASP-doped device reveals an extremely high EQE of 12%, reaching the limit of a TTA-based device. The EL characteristics of DPASP-doped CBP-based devices at various doping concentrations (0-5%) suggest that the dopant DPASP is responsible for the TTA-type delayed fluorescence in the device; no delayed fluorescence was observed for the device using CBP as the host emitter. Moreover, when using DMPPP with ambipolar characteristics as the host, the deep-blue DPASP-doped device also gives outstanding performance with an EQE of nearly 11% with an extremely small efficiency roll-off, which was ascribed to the excellent charge balance in the emitting layer of the EL device. The TTA process of the SP-based dopants accounts significantly for the superior efficiencies of the EL devices.

Achieving above 30% external quantum efficiency for inverted phosphorescence organic light-emitting diodes based on ultrathin emitting layer

High efficiency inverted phosphorescence organic light-emitting diodes (PhOLEDs) based on ultrathin undoped and doped emitting layer (EML) have been developed. Compared to conventional device, the inverted PhOLED with 0.5nm undoped EML exhibits significantly larger external quantum efficiency (EQE), due to effective energy transfer from the excited host to the emitter. According to the atomic force microscopy image of EML, the 0.5nm emitter sandwiched by two hosts can be considered as the emitter doped in two hosts. The inverted device with intentionally doped ultrathin EML (1.5nm) exhibits the maximum EQE of 31.1%, which is attributed to optimized charge balance and preferred horizontal orientation of emitter. However, such inverted device has large efficiency roll-off at high brightness because of triplet–triplet annihilation process within the ultrathin EML. This can be improved by broadening the doped EML. The inverted device with 10.5nm doped EML has about EQE of 20 % at 10,000cd/m2. It is expected that our inverted PhOLED will promote development of high efficiency active-matrix organic light-emitting diodes based on the n-type Indium Gallium Zinc Oxide thin film transistor.

Plasmon-Enhanced Triplet–Triplet Annihilation Using Silver Nanoplates

Photon upconversion processes have attracted substantial interest as a means of circumventing the Shockley–Queisser limit for single-junction photovoltaic devices. Despite this promise, the quantum yield of most upconversion processes is very low at the light intensities typical of solar radiation (∼100 mW/cm2). Additionally, bimolecular upconversion processes that rely on molecular diffusion (e.g., triplet–triplet annihilation) typically see further reductions in quantum yield when the upconverting chromophores are confined to a solid state or thin film matrix. Here we report a plasmon-based enhancement of the triplet–triplet annihilation process when silver nanoplates are embedded in poly(methyl methacrylate) thin films containing the upconverting materials palladium(II) octaethylporphyrin and 9,10-diphenylanthracene. The silver nanoplates are synthesized with localized surface plasmon resonance bands tailored to overlap strongly with the Q-band of the porphyrin, leading to enhanced light absorption within the film and higher overall triplet concentrations. Optimization of the silver nanoplate loading leads to a nearly 10-fold increase in the upconverted light intensity compared with control samples containing no silver.

Magnetic field effects on electroluminescence emanated simultaneously from blue fluorescent and red phosphorescent emissive layers of an organic light-emitting diode

A simple method by employing white organic light-emitting diode (OLED) to simultaneously study the effects of magnetic fields (MFEs) on the fluorescent and phosphorescent emission was presented. The white OLED consisting of the blue fluorescent and red phosphorescent materials was fabricated. The low-pass filter chopped at 500nm and the high-pass filter chopped at 550nm were used to separate the MFEs on the blue fluorescence and red phosphorescence, separately. The magnetoelectroluminescences (MELs) on the fluorescence sharply increased at low magnetic field then tended to saturation at high magnetic field. While the MELs on the phosphorescence sharply increased at low magnetic field then slowly monotonically increased as the magnetic field further increased. Our results indicated that the magnetic-field modulated singlet-to-triplet electron–hole pair conversion combining with the magnetic-field modulated triplet–triplet annihilation process are the origins of the MFEs on the electroluminescence.

30.4: High‐Efficiency Fluorescent Red‐ and Yellow‐Emitting OLED Devices

AbstractWe report high‐efficiency fluorescent red OLEDs with 11.5% EQE (15 cd/A) at 3 V, and yellow OLEDs with 8.6% EQE (28 cd/A) at 2.9 V. This performance was obtained using a nonemitting assist layer adjacent to the emission layer, and low‐voltage electron‐transporting and electron‐injecting layers. Time‐resolved electroluminescence measurements indicate a significant efficiency contribution (up to 50%) from triplet—triplet annihilation processes.

Driving current and temperature dependent magnetic-field modulated electroluminescence in Alq 3-based organic light emitting diode

Driving current and temperature dependences of magnetic-field modulated electroluminescence (EL) in tris(8-hydroxyquinoline) aluminum (Alq 3)-based OLEDs have been thoroughly investigated. At low temperatures, the applied magnetic-field induces a sharp increase of the EL in low field regime ( B ⩽ 35 mT) and a slow but apparent decrease at high fields (35 ⩽ B ⩽ 500 mT). The low-field increase in EL (LFE) survives at all working temperatures while the high-field decrease (HFE) gradually disappears as temperature is increased. At a given temperature, the higher the current level, the smaller LFE and stronger HFE are observed. To explain the observed MFEs a composite model based on magnetic-field dependent singlet-to-triplet conversion of electron-hole pairs and magnetic-field mediated triplet–triplet annihilation process is proposed in this paper.