Transient Photoluminescence Decay Research Articles

Diindolocarbazole-Based Rigid Donor-Acceptor TADF Molecules for Energy and Electron Transfer Photocatalysis.

The design and synthesis of four twisted donor-acceptor (D-A) thermally activated delayed fluorescence (TADF) molecules CBZ-IQ, CBZ-2FIQ, DI-IQ and DI-2FIQ is reported in this work based on diindolocarbazole (DI) and phenyl carbazole as donor and indoloquinoxalines as acceptor. These compounds serve as photocatalysts for organic transformations. Theoretical calculations and experimental data showed reasonable singlet and triplet energy gaps of 0.17-0.26 eV for all compounds. All molecules showed increase in fluorescence quantum yields after degassing the solution and the transient photoluminescence decay showed two components: shorter prompt components (11.4 ns to 31 ns) and longer delayed components (36.4 ns to 1.5 μs) which further indicate the occurrence of TADF process. Cyclic voltammetry studies indicated well-suited excited state redox potentials of all compounds to catalyze organic transformations such as heteroarene arylation. Accordingly, photocatalytic C-H arylation of heteroarenes were performed using these compounds with excellent isolated yields of upto 80 %. Due to their suitable efficient triplet energy levels, all the emitters were also employed as energy transfer photocatalysts in E to Z isomerization of stilbene with the excellent conversion of ~90 %.

Trinuclear Re(I)-Coordinated Organic Cage as the Supramolecular Photocatalyst for Visible-Light-Driven CO2 Reduction.

Photocatalytic reduction of excess CO2 in the atmosphere to value-added chemicals by visible light can be an effective solution to fuel shortage and global warming. Considering these issues, we designed and successfully synthesized a trinuclear Re(I)-coordinated organic cage (Re-C4R) as the supramolecular photocatalyst. Photophysical, electrochemical properties, and photocatalytic performance comparison of Re-C4R and its mononuclear analogue Re-bpy are discussed in detail. Notably, the covalent linkage of three Re(I) subunits in Re-C4R leads to TONCO = 691 (per Re(I) site in 4 h) more than three times as much as TONCO = 208 of Re-bpy. Compared to Re-bpy, higher current enhancement in the control CV experiments under CO2 was observed for Re-C4R. CO2 adsorption process can be promoted because of the cryptand structure and multiple amine groups of Re-C4R. Moreover, decay lifetimes of Re-C4R are shorter than those of Re-bpy in the ultrafast transient absorption (TA) and photoluminescence (PL) decay spectra, indicating that the trinuclear cryptate structure of Re-C4R could facilitate electron transfer efficiency during CO2 reduction.

Red‐Emitting Fluorophores Featuring Combined Hybridized Local and Charge‐Transfer Excited State and Aggregation‐Induced Emission as Efficient Emitters for Electroluminescent Devices

AbstractHerein, donor (D)‐acceptor (A) type fluorophores (TTBz and TTNz) were designed and synthesized with different acceptor units. The two acceptor units benzo[c][1,2,5]thiadiazole (Bz) and naphtho[2,3‐c][1,2,5]thiadiazole (Nz) are asymmetrically functionalized with triphenylamine (TPA) as a strong donor and tetraphenylethylene (TPE) as an aggregation‐induced emission (AIE)‐luminogen to realize combined AIE and hybridized local and charge transfer (HCLT) excited state features as well as long‐wavelength emission. Their HCLT and AIE properties are verified by theoretical calculations, solvatochromic effects, and transient photoluminescence decay experiments. They exhibit strong orange‐red and deep‐red fluorescence emission with good thermal stability and hole mobility, leading to effective utilization as emitters in organic light‐emitting diodes (OLEDs). The resulting devices show decent electroluminescent (EL) performance (current efficiency of 12.04–12.58 cd A−1, maximum brightness of 11206–20160 cd m−2, and external quantum efficiency of 5.11–5.47 %). Particularly, the TTNz‐based OLED demonstrates steady deep‐red emission with a peak at 661 nm and CIE coordinates of (0.67, 0.33).

Highly Emissive Luminogens in Both Solution and Aggregate States

Highly Emissive Luminogens in Both Solution and Aggregate States

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Comprehensive understanding of multiple resonance thermally activated delayed fluorescence through quantum chemistry calculations

Molecules that exhibit multiple resonance (MR) type thermally activated delayed fluorescence (TADF) are highly efficient electroluminescent materials with narrow emission spectra. Despite their importance in various applications, the emission mechanism is still controversial. Here, a comprehensive understanding of the mechanism for a representative MR-TADF molecule (5,9-diphenyl-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracene, DABNA-1) is presented. Using the equation-of-motion coupled-cluster singles and doubles method and Fermi’s golden rule, we quantitatively reproduced all rate constants relevant to the emission mechanism; prompt and delayed fluorescence, internal conversion (IC), intersystem crossing, and reverse intersystem crossing (RISC). In addition, the photoluminescence quantum yield and its prompt and delayed contributions were quantified by calculating the population kinetics of excited states and the transient photoluminescence decay curve. The calculations also revealed that TADF occurred via a stepwise process of 1) thermally activated IC from the electronically excited lowest triplet state T1 to the second-lowest triplet state T2, 2) RISC from T2 to the lowest excited singlet state S1, and 3) fluorescence from S1.

Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes

Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes

Room-Temperature Persistent Luminescence in Metal Halide Perovskite Nanocrystals for Solar-Driven CO 2 Bioreduction

Room-Temperature Persistent Luminescence in Metal Halide Perovskite Nanocrystals for Solar-Driven CO ₂ Bioreduction

Facile synthesis of C–Ta4+ co-doped NaTaO3 and rGO nanocomposites with enhanced visible light photocatalytic performance

Nanocomposites of C–Ta4+ co-doped NaTaO3 and reduced graphene oxide (C-NTO/rGO) with highly efficient photocatalytic activity were fabricated by a one-step solvothermal reaction. The optimum composites C-NTO/3rGO (with 3 wt% rGO) exhibited superior photocatalytic activity for the degradation of RhB over a mono-component counterpart under visible light irradiation. The photocatalytic efficiency was found to be as high as 97% for 90 min. Both XRD and XPS characterizations indicated that the doping of carbon into the NaTaO3 lattice leads to the reduction of Ta5+ to Ta4+. Furthermore, the co-doping significantly narrows the band gap (about 2.8 eV). Photo-luminescence (PL), time resolved transient PL decay spectra and photo-current results confirmed that photo-induced hole-electron pairs can be effectively separated resulting from the introduction of rGO. The synergistic effect of a decreased band gap and highly effective separation of photo-generated electrons and holes may be responsible for the enhanced photocatalytic activity of the fabricated nanocomposites. Both h+ and •OH active radicals were found to be the key factors for the photo-decomposition process of RhB over the nanocomposites, while •O2− was also involved to a smaller extent. A possible photo-degradation mechanism for C-NTO/rGO was proposed on the basis of the experiments results.

Quantifying Exciton Annihilation Effects in Thermally Activated Delayed Fluorescence Materials

AbstractImportant parameters for the design and performance of thermally activated delayed fluorescence (TADF) emitters are the forward and reverse intersystem crossing rates between singlet and triplet states. The magnitude of these rates is determined from the prompt and delayed transient photoluminescence decay. It is demonstrated that this photoluminescence decay strongly depends on the initial photoexcited population density due to exciton–exciton annihilation processes. By kinetic modeling of the power‐dependent time‐resolved photoluminescence of the TADF emitter 9,10‐bis(4‐(9H‐carbazol‐9‐yl)‐2,6‐dimethylphenyl)‐9,10‐diboraanthracene (CzDBA), singlet–triplet annihilation and triplet–triplet annihilation are identified as the main loss processes with rate constants in the order of 10−17 m3 s−1. Neglecting these quenching processes leads to erroneous estimates of the (reverse) intersystem crossing rates.

Two-Channel Space Charge Transfer-Induced Thermally Activated Delayed Fluorescent Materials for Efficient OLEDs with Low Efficiency Roll-Off.

Enhancing the reverse intersystem crossing (RISC) process of thermally activated delayed fluorescent (TADF) emitters is an effective approach to realize efficient organic light-emitting diodes (OLEDs) with low efficiency roll-off. In this work, we designed two novel TADF emitters, SAT-DAC and SATX-DAC, via a spiro architecture. Efficient maximum external quantum efficiencies (EQEs) of 22.6 and 20.9% with reduced efficiency roll-off (EQEs of 17.9 and 17.0% at 1000 cd m-2) were achieved via a "two-RISC-channel" strategy. X-ray diffraction shows close donor (D)/acceptor (A) spacing and suitable D/A orientation in crystals of the two emitters favoring both intra- and intermolecular through-space charge transfer (TSCT) processes. Transient photoluminescence decay measurements show that both emitters have two RISC channels leading to kISCT exceeding 106 s-1. These results suggest that the "two-RISC-channel" design can be a novel approach for enhancing performance of TADF emitters, in particular at high excitation densities.

Coupled time resolved and high frequency modulated photoluminescence probing surface passivation of highly doped n-type InP samples

Electronic passivation of III–V surfaces is essential for applications in optoelectronic devices. A key aspect is the measurement of the surface recombination properties, which can be done by various techniques including transient photoluminescence (TRPL) or luminescence quantum yield. These measurement techniques are always indirect since they suppose postulating a mathematical model for the data interpretation. Most common models use the notion of surface recombination velocity to quantify the surface recombination. In this paper, we demonstrate on the InP substrate that this notion is not always sufficient to represent the injection dependence of the surface phenomena. The study of power dependence of TRPL decays coupled with modulated photoluminescence (MPL) spectra on four samples from the same wafer with different surface treatments (epi-ready, freshly cleaned, after air exposure, and with poly-phosphazen passivation) allows us to discriminate between bulk and surface properties. We introduce surface defect trapping as an alternative to explain TRPL decays and MPL phase excitation power dependences of the three non-passivated samples. Surface trap parameters such as capture cross section and defect density are extracted. The passivated sample exhibits an invariant response shape on eight orders of magnitude of illumination. The stability of the PL response at high flux is in agreement with the perfect stability of the passivation layer, which is able to protect the InP surface without chemical changes over more than one year. They are linked to the nature of the passivation layer/InP interface. Other surface treatments were found to have an injection dependent response at high flux corresponding to different surface defect distributions but also possibly to surface chemical changes for the freshly cleaned sample.

Pure-Blue Fluorescence Molecule for Nondoped Electroluminescence with External Quantum Efficiency Approaching 13%

A pure-blue light-emitting material is one of the key components in the preparation of organic light-emitting diode (OLED) displays. Although high-efficiency blue OLEDs have been realized in therma...

Synthesis and Delayed Fluorescent Properties of p‐Nido‐Carborane‐Triarylborane Conjugates with a Methyl‐Substituted Phenylene Linker

A series of p‐nido‐carborane‐triarylborane conjugates (nido‐1‐3) in which a methyl group is introduced at the ortho‐position to the carborane cage in the phenylene linker was prepared and characterized. All compounds exhibit broad low‐energy absorptions (λabs = ca. 350–400 nm) attributable to the intramolecular charge transfer transition from the nido‐carborane donor to the (MePh)BMes2 acceptor. Electrochemical studies confirm that oxidation occurs at the nido‐carborane while the boryl moieties are responsible for the reduction. All nido‐compounds show broad green emissions in tetrahydrofuran (THF) with good photoluminescence (PL) quantum yields (ΦPL = 24%–78%). In particular, different from the almost non‐thermally activated delayed fluorescence (TADF) properties of unsubstituted para‐conjugates, the transient PL decay curves of nido‐1‐3 show the existence of weak TADF (τd = 0.9–1.4 μs in THF). The TADF properties are further supported by the very small singlet‐triplet energy splitting below 0.15 eV and are also observed in poly(methyl methacrylate) (PMMA) films doped with nido‐1‐3.

Fully Vacuum‐Processed Perovskite Solar Cells on Pyramidal Microtextures

Solar cells based on metal halide perovskites have attracted tremendous attention due to the rapid increase in performance of single junctions and tandem solar cells. Recently, highest perovskite/silicon tandem efficiencies are realized with front‐side polished silicon wafers or adapted microstructure of textured silicon solar cells. One way to integrate perovskite top cells on typical micrometer‐sized pyramidal structures, is conformal vacuum‐based perovskite deposition. Herein, fully vacuum‐based perovskite solar cells are developed on top of random pyramidal microtextured glass substrates with a pyramid size up to 9 μm. This method allows improvement of the light management of the textured perovskite solar cell and resembles the typical pyramid topography of silicon solar cells as a step toward monolithic tandem integration. Moreover, to improve the quality of the perovskite on the textured substrates, three different methylammonium lead iodide (MAPbI3) films are tested by adjusting the rate ratio of the precursors. Optimized ratios for textured substrates with higher PbI2 rates enable a transient photoluminescence decay time above 0.75 μs approaching that of planar substrates at around 1.2 μs. Finally, a efficiency over 15% is achieved, which is, to the best of our knowledge, the first reported device on microscopically textured glass by co‐evaporated ion.

Visualization of Carrier Transport in Luminescent Polymer Thin Film by Using Transient Photoluminescence Decay Imaging

Transient photoluminescence (PL) decay imaging is conducted for visualizing carrier transport in luminescent polymer semiconductor thin films. The method is simple, but enables a real‐time observation of carrier motion on the scale of several millimeters in luminescent thin films. Decrease in the PL intensity of the channel region in organic field‐effect transistor (OFET) is observed in accordance with the device operation. Upon application of pulse voltages to the source electrode for carrier injection, the transient PL decay motion is visualized using a conventional charge‐coupled device (CCD) camera. By analyzing the PL decay motion, hole and electron mobilities of poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) (F8BT) thin film are evaluated as 1.3 × 10−4 cm2 (V s)−1 and 3.2 × 10−5 cm2 (V s)−1, respectively. Mobility anisotropy is also visualized by using round‐shaped electrode as the source electrode, and the difference of mobility anisotropy for hole and electron of F8BT is clearly identified.

Exciton-polaron interaction in blue fluorescent organic light-emitting diodes

OLEDs are popular as display technology nowadays, which have been widely used in commercial application. However, there are still some problems that blue light devices are not as efficient or stable as red and green light devices. Although the use of phosphorescent dyes can significantly improve the internal quantum efficiency, the high production cost and unstable performance limit the industrialization of phosphorescent OLEDs. In the development of OLEDs, the researchers found that OLEDs suffered from a decline in their efficiency at high brightness levels, a behavior known as “efficiency roll-off”. The efficiency roll-off is more pronounced in phosphorescent devices due to the longer lifetime of triplet exciton than singlet exciton, so that it has been widely investigated in recent years. Little is known, still, about fluorescent devices. Accordingly, unraveling the exciton loss mechanism in blue fluorescent OLEDs is particularly important, as it is a limiting factor for the improvement of efficiency. In this work, the efficiency roll-off in blue fluorescent OLEDs is investigated by observing the quenching of DPAVBI excitons. Firstly, the effects of electron current and hole current on photoluminescence(PL) behavior of unipolar devices are studied by steady-state and transient-state measurements, and we analyze PL spectrum and calculate the exciton quenching rate constant according to the transient PL decay curves to clarify the exciton quenching dynamics. The results show that the holes are much more efficient in quenching the excitons when the host is a hole transport material. This is different from the general understanding that exciton-polaron quenching effect with higher carrier mobility is weaker. Because the existence of bound charges produces additional charge density, and it is inferred that the exciton is mainly quenched by trapped charge rather than moving charge. We also exclude the effect of exciton–exciton annihilation and electric-field-induced dissociation on the efficiency degradation of the OLEDs. It is confirmed experimentally that exciton-polaron interaction is the dominant mechanism of the efficiency roll-off in fluorescent OLEDs. We then fabricate organic light-emitting diode devices with different doping concentrations to figure out the effect of doping concentration on exciton-polaron interaction, and obtain a blue fluorescence device with good comprehensive performance. We also summarize some feasible methods to optimize the efficiency of the OLEDs. In this paper, our findings about exciton-polaron interaction might provide a viable source for efficiency improvement by regulating charge trapping in light emitting layer.

Bright Deep Blue TADF OLEDs: The Role of Triphenylphosphine Oxide in NPB/TPBi:PPh3O Exciplex Emission

AbstractVery bright (≈14 000 cd m−2) deep blue exciplex organic light emitting diodes (OLEDs) peaking at ≈435 nm, where the photopic response is ≈0.033, and with CIE color coordinates (0.1525, 0.0820), are described. The OLED properties are interestingly linked to PPh3O (triphenylphosphine oxide) and attributes of the emitting layer (EML) comprising NPB interfacing host:guest TPBi:PPh3O 5:1 weight ratio. A neat PPh3O layer that is central for device performance follows the EML (NPB/TPBi:PPh3O 5:1/PPh3O). The bright electroluminescence originates from NPB/TPBi:PPh3O exciplexes involving triplets via thermally activated delayed fluorescence, as evident from the strong quenching of the photoluminescence (PL) by oxygen and interestingly, the monomolecular emission process. The transient PL decay times of a NPB/TPBi:PPh3O 5:1/PPh3O film are 43 ns in air versus 136, 610, and weak ≈2000 ns in N2. For comparison, the respective PL decay times of films of NPB:TPBi are 16 ns in air versus 131 and 600 ns in N2, and of NPB:PPh3O they are 29 ns in air versus 56, 483, and weak ≈2000 ns in N2. It is suspected that slow emitting states are associated with a PPh3O aggregate interacting with NPB.

Photocatalytic recovery of H2 from H2S containing wastewater: Surface and interface control of photo-excitons in Cu2S@TiO2 core-shell nanostructures

This study focuses on the synthesis of Cu2S particles as the core wrapped-up with thin layer of TiO2-shell for improved surface and interface control of photo-excitons for H2 production. Chemical states of metal Cu+, Ti4+ ions were confirmed by X-ray Photo-electron and Electron Paramagnetic Resonance (g-factor of 2.03) Spectroscopy. Transmission Electron Microscopic images revealed the hexagonal shape of Cu2S wrapped-up with TiO2 having the shell thickness varying from 12.0 to 16.7 nm. Time resolved transient photoluminescence decay spectra confirmed that three folds prolonged life-time of excitons generated on Cu2S@TiO2 core-shell than pristine Cu2S. The batch photo-ractor used to evaluate the performance of photocatalysts in an aqueous solution containing Na2S/Na2SO4 promoted oxidation reaction for H+ generation. The control over shell thickness directly influenced the optical and surface-interface properties, resulting in the enhanced H2 production of 41.6 mmol h−1 g−1 of catalyst for an optimized catalyst with UV–vis light to H2 production efficiency of 10.3%. The same catalyst also exhibited excellent stability for un-interrupted H2 production upto 30 h, along with good results on recyclability and reusable efficiency. Scale-up operation performed to recover H2 from sulphide wastewater in tank photo-reactor triggered H2 gas 16.1 mmol h−1 g−1 of catalyst.

Dendritic Carbene Metal Carbazole Complexes as Photoemitters for Fully Solution-Processed OLEDs

Light-emitting carbene-metal amide complexes bearing first- and second-generation carbazole dendron ligands are reported, (AdL)M(Gn), (M = Cu and Au; Gn carbazole dendrimer generation, where n = 1 and 2; AdL = adamantyl-substituted cyclic (alkyl)(amino)carbene). The thermal stability of the complexes increases with each dendrimer generation. Cyclic voltammetry indicates that the highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels are largely unaffected by the size of the dendron, while first reduction and oxidation processes show a quasi-reversible character. The gold complexes in toluene at room temperature show photoluminescent quantum yields of up to 51.5% for the first and 78% for the second generation. Varied temperature transient photoluminescence decay is consistent with a thermally activated process, indicating a delayed fluorescence-type emission mechanism. Neat films show excited state lifetimes composed of prompt and dominant sub-microsecond delayed components, ...