Elastic-Plastic Fully π-Conjugated Polymer with Excellent Energy Dissipation Capacity for Ultra-Deep-Blue Flexible Polymer Light-Emitting Diodes with CIEy = 0.04.

Emerging intrinsically flexible fully π-conjugated polymers (FπCPs) are a promising functional material for flexible optoelectronics, attributed to their potential interchain interpenetration and entanglement. However, the challenge remains in obtaining elastic-plastic FπCPs with intrinsic robust optoelectronic property and excellent long-term and cycling deformation stability simultaneously for applications in deep-blue flexible polymer light-emitting diodes (PLEDs). This study, demonstrates a series of elastic-plastic FπCPs (P1-P4) with an excellent energy dissipation capacity via side-chain internal plasticization for the ultra-deep-blue flexible PLEDs. First, the freestanding P1 film exhibited a maximum fracture strain of 34.6%. More interestingly, the elastic behavior is observed with a low strain (≤10%), and the stretched film with a high deformation (>10%) attributed to plastic processing revealed the robust capacity to realize energy absorption and release. The elastic-plastic P1 film exhibits outstanding ultra-deep-blue emission, with an efficiency of 56.38%. Subsequently, efficient PLEDs are fabricated with an ultra-deep-blue emission of CIE (0.16, 0.04) and a maximum external quantum efficiency of 1.73%. Finally, stable and efficient ultra-deep-blue electroluminescence are obtained from PLEDs based on stretchable films with different strains and cycling deformations, suggesting excellent elastic-plastic behavior and deformation stability for flexible electronics.

Triplet Homoleptic Iridium(III) Complex as a Potential Donor Material for Organic Solar Cells.

Triplet photovoltaic materials have been rarely investigated in organic solar cells (OSCs) because the role and mechanism of triplet excitons are still unclear. Cyclometalated heavy metal complexes with triplet features are expected to increase exciton diffusion lengths and improve exciton dissociation in OSCs, while the power conversion efficiencies (PCEs) of their bulk-heterojunction (BHJ) OSCs are still limited to <4%. We herein report an octahedral homoleptic tris-Ir(III) complex TBz3Ir as a donor material for BHJ OSCs with a PCE of over 11%. In comparison with the planar organic TBz ligand and heteroleptic TBzIr, TBz3Ir demonstrates the highest PCE and best device stability in both fullerene- and non-fullerene-based devices, owing to the long triplet lifetime, enhanced optical absorption, increased charge transport, and improved film morphology. From transient absorption, triplet excitons were deduced to participate in the photoelectric conversion process. In particular, the more significant 3D structure of TBz3Ir induces an unusual film morphology in TBz3Ir:Y6 blends, showing obviously large domain sizes suitable for triplet excitons. Thus, a high PCE of 11.35% with a high circuit current density of 24.17 mA cm-2 and a fill factor of 0.63 is achieved for small-molecular Ir complex-based BHJ OSCs.

Microdroplet-Facilitated Assembly of Thermally Activated Delayed Fluorescence-Encoded Microparticles with Non-interfering Color Signals.

Encoded microparticles (EMPs) have shown demonstrative value for multiplexed high-throughput bioassays such as drug discovery and diagnostics. Herein, we propose for the first time the incorporation of thermally activated delayed fluorescence (TADF) dyes with low-cost, heavy metal-free, and long-lived luminescence properties into polymer matrices via a microfluidic droplet-facilitated assembly technique. Benefiting from the uniform droplet template sizes and polymer-encapsulated structures, the resulting composite EMPs are highly monodispersed, efficiently shield TADF dyes from singlet oxygen, well preserve TADF emission, and greatly increase the delayed fluorescence lifetime. Furthermore, by combining with phase separation of polymer blends in the drying droplets, TADF dyes with distinct luminescent colors can be spatially separated within each EMP. It eliminates optical signal interference and generates multiple fluorescence colors in a compact system. Additionally, in vitro studies reveal that the resulting EMPs show good biocompatibility and allow cells to adhere and grow on the surface, thereby making them promising optically EMPs for biolabeling.

BODIPY Cored A-D-A'-D-A Type Nonfused-Ring Electron Acceptor for Efficient Polymer Solar Cells.

In this work, boron dipyrromethene (BODIPY) is for the first time employed as electron-deficient core (A') to construct an A-D-A'-D-A type nonfused-ring electron acceptor (NFREA) for polymer solar cells (PSCs). Among, cyclopentadithiophene (CPDT) and fluorinated dicyanoindanone (DFIC) are involved as electron-donating (D) bridges and terminal A groups, respectively. Bearing with the steric BODIPY core, tMBCIC exhibits twisted configuration with dihedral angles >45°  between BODIPY and CPDT bridges. Thus, compared with the BODIPY-free planar A-D-D-A structured bCIC, reduced aggregation, weakened intramolecular D-A interactions with up-shifted lowest unoccupied molecular orbital by 0.4 eV as well as blueshifted absorption by up to 150 nm is observed in tMBCIC. Moreover, owing to the intrinsic large molar extinction coefficient from BODIPY, promoted light-harvest ability is achieved for tMBCIC, particularly in its blend films. Therefore, PSCs by using PBDB-T as donor, tMBCIC as NFREA afford superior power conversion efficiency (PCE) of 9.22% and higher open-circuit voltage (Voc ) of 0.954 V compared to 4.47% and 0.739 V from bCIC-devices. Moreover, compared to other BODIPY-flanked electron acceptors (<5%) reported so far, BODIPY-cored tMBCIC realizes a remarkable progress in PCE.

Cyclometalated Ir(III) complexes as potential electron acceptors for organic solar cells.

Cyclometalated iridium(iii) complexes have been investigated as promising electron donor (D) materials in organic solar cells (OSCs) due to their unique octahedral configuration for optimized morphology and their significantly long lifetimes potentially for enhanced exciton dissociation. However, the application as electron acceptor (A) materials has never been reported. In order to fill this blank, herein, two cyclometalated heteroleptic Ir complexes, TRIr and 2TRIr, based on electron donating-accepting type organic ligands with different π-conjugation lengths are reported as electron acceptor materials in comparison with their corresponding main organic ligands. The two Ir complexes exhibit suitable HOMO/LUMO energy levels of -5.55/-3.47 eV and -5.44/-3.48 eV, which are ∼0.1 eV higher in the HOMO and ∼0.15 eV deeper in the LUMO than the TR and 2TR ligands, respectively. 2TRIr with extended ligand π-conjugation displays a poor triplet feature, while TRIr demonstrates obvious metal-to-ligand charge transfer (MLCT) transition absorption, with a triplet component photoluminescence (PL) lifetime of 85 ns in neat films. When blended with PBDB-T in bulk heterojunction (BHJ) OSCs, the power conversion efficiencies (PCEs) are 2-3 times higher than their relevant ligands, with values of 1.20% and 1.62% for TRIr and 2TRIr, and 0.58% and 0.47% for the TR and 2TR ligand-based devices, respectively. TRIr and 2TRIr based active layer blends exhibit poorer hole and electron mobilities, whereas compared with their relatively linear planar ligands, both of the two octahedral Ir complexes exhibit an optimized surface morphology for less bimolecular recombination and more efficient exciton dissociation, thus contributing to improved photovoltaic performance.

Design, synthesis and application in biological imaging of a novel red fluorescent dye based on a rhodanine derivative.

A novel acceptor-donor-acceptor type molecule, namely 2-triphenylamine-1,3-dia[2-(3-ethyl-4-oxo-thiazolidin-2-ylidene)-malononitrile] (2RDNTPA), is designed and synthesized. 2RDNTPA exhibits a large Stokes shift of 244 nm and red fluorescence emission of 629 nm with a decent photoluminescence quantum yield of 13%. Furthermore, as a potential red fluorescent dye, 2RDNTPA can be applied in fluorescence imaging of living cancer cells (HepG2) with negligible cytotoxicity and a half maximal inhibitory concentration much more than 100 µM.

Enabling long-lived organic room temperature phosphorescence in polymers by subunit interlocking.

Long-lived room temperature phosphorescence (LRTP) is an attractive optical phenomenon in organic electronics and photonics. Despite the rapid advance, it is still a formidable challenge to explore a universal approach to obtain LRTP in amorphous polymers. Based on the traditional polyethylene derivatives, we herein present a facile and concise chemical strategy to achieve ultralong phosphorescence in polymers by ionic bonding cross-linking. Impressively, a record LRTP lifetime of up to 2.1 s in amorphous polymers under ambient conditions is set up. Moreover, multicolor long-lived phosphorescent emission can be procured by tuning the excitation wavelength in single-component polymer materials. These results outline a fundamental principle for the construction of polymer materials with LRTP, endowing traditional polymers with fresh features for potential applications.

Short-Axis Methyl Substitution Approach on Indacenodithiophene: A New Multi-Fused Ladder-Type Arene for Organic Solar Cells.

Indacenodithiophene (IDT) is a promising building block for designing organic semiconductors. In this work, a new pentacyclic ladder-type arene IDMe was designed and synthesized by introducing methyl substitution on the short-axis of IDT. Two non-fullerene electron acceptors (IDIC and ID-MeIC) without and with methyl substitution were designed and synthesized for further study. Compared with IDIC, ID-MeIC with methyl substitution on the short-axis of IDT shows smaller bandgap, stronger extinction coefficient, and better crystallinity. Besides, PBDB-T: ID-MeIC blend film shows more efficient exciton generation and dissociation and more balanced charge transport mobility. Therefore, polymer solar cells based on PBDB-T: ID-MeIC can achieve better photovoltaic performance with a PCE of 6.46% and substantial increase in J SC to 14.13 mA cm-2 compared to 4.94% and 9.10 mA cm-2 of PBDB-T: IDIC. These results suggest that short-axis substitution on multi-fused ladder-type arenes, such as IDT is an effective way to change the optical and electronic properties of the organic semiconductors for high-performance OPVs.

N-Benzoimidazole/Oxadiazole Hybrid Universal Electron Acceptors for Highly Efficient Exciplex-Type Thermally Activated Delayed Fluorescence OLEDs.

Recently, donor/acceptor type exciplex have attracted considerable interests due to the low driving voltages and small singlet-triplet bandgaps for efficient reverse intersystem crossing to achieve 100% excitons for high efficiency thermally activated delayed fluorescence (TADF) OLEDs. Herein, two N-linked benzoimidazole/oxadiazole hybrid electron acceptors were designed and synthesized through simple catalyst-free C-N coupling reaction. 24iPBIOXD and iTPBIOXD exhibited deep-blue emission with peak at 421 and 459 nm in solution, 397 and 419 nm at film state, respectively. The HOMO/LUMO energy levels were -6.14/-2.80 for 24iPBIOXD and -6.17/-2.95 eV for iTPBIOXD. Both compounds could form exciplex with conventional electron donors such as TAPC, TCTA, and mCP. It is found that the electroluminescent performance for exciplex-type OLEDs as well as the delayed lifetime was dependent with the driving force of both HOMO and LUMO energy offsets on exciplex formation. The delayed lifetime from 579 to 2,045 ns was achieved at driving forces close to or larger than 1 eV. Two TAPC based devices possessing large HOMO/LUMO offsets of 1.09-1.34 eV exhibited the best EL performance, with maximum external quantum efficiency (EQE) of 9.3% for 24iPBIOXD and 7.0% for iTPBIOXD acceptor. The TCTA containing exciplex demonstrated moderate energy offsets (0.88-1.03 eV) and EL efficiency (~4%), while mCP systems showed the poorest EL performance (EQE <1%) and shortest delayed lifetime of <100 ns due to inadequate driving force of 0.47-0.75 eV for efficient exciplex formation.

A cyclometalating organic ligand with an Iridium center toward dramatically improved photovoltaic performance in organic solar cells.

Organometallic compounds as photoactive materials are relatively new in organic solar cells. Upon cyclometalation, the octahedral heteroleptic Ir complex TBzIr exhibits significantly enhanced optical-absorption and improved film-morphology compared to the planar organic 2-(5''-hexyl-[2,2':5',2''-terthiophen]-5-yl)benzo[d]thiazole (TBz) ligand. Thus, a dramatically improved power conversion efficiency (PCE) from ∼0 to 3.81% is attained when combined with PC71BM.

Sulfenate anions as organocatalysts for benzylic chloromethyl coupling polymerization via C=C bond formation.

Organocatalytic polymerization reactions have a number of advantages over their metal-catalyzed counterparts, including environmental friendliness, ease of catalyst synthesis and storage, and alternative reaction pathways. Here we introduce an organocatalytic polymerization method called benzylic chloromethyl-coupling polymerization (BCCP). BCCP is catalyzed by organocatalysts not previously employed in polymerization processes (sulfenate anions), which are generated from bench-stable sulfoxide precatalysts. The sulfenate anion promotes an umpolung polycondensation via step-growth propagation cycles involving sulfoxide intermediates. BCCP represents an example of an organocatalyst that links monomers by C=C double bond formation and offers transition metal-free access to a wide variety of polymers that cannot be synthesized by traditional precursor routes.

Long Electron-Hole Diffusion Length in High-Quality Lead-Free Double Perovskite Films.

Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead-based perovskite solar cells. Here, the first double perovskite (Cs2 AgBiBr6 ) solar cells using the planar structure are demonstrated. The prepared Cs2 AgBiBr6 films are composed of high-crystal-quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high-quality double perovskite films show long electron-hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO2 exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.

Thermally Activated Delayed Fluorescence Organic Dots (TADF Odots) for Time-Resolved and Confocal Fluorescence Imaging in Living Cells and In Vivo.

The fluorophores with long-lived fluorescent emission are highly desirable for time-resolved fluorescence imaging (TRFI) in monitoring target fluorescence. By embedding the aggregates of a thermally activated delayed fluorescence (TADF) dye, 2,3,5,6-tetracarbazole-4-cyano-pyridine (CPy), in distearoyl-sn-glycero-3-phosphoethanolamine-poly(ethylene glycol) (DSPE-PEG2000) matrix, CPy-based organic dots (CPy-Odots) with a long fluorescence lifetime of 9.3 µs (in water at ambient condition) and high brightness (with an absolute fluorescence quantum efficiency of 38.3%) are fabricated. CPy-Odots are employed in time-resolved and confocal fluorescence imaging in living Hela cells and in vivo. The green emission from the CPy-Odots is readily differentiated from the cellular autofluorescence background because of their stronger emission intensities and longer lifetimes. Unlike other widely studied DSPE-PEG2000 encapsulated Odots which are always distributed in cytoplasm, CPy-Odots are located mainly in plasma membrane. In addition, the application of CPy-Odots as a bright microangiography agent for TRFI in zebrafish is also demonstrated. Much broader application of CPy-Odots is also prospected after further surface functionalization. Given its simplicity, high fluorescence intensity, and wide availability of TADF materials, the method can be extended to develop more excellent TADF Odots for accomplishing the challenges in future bioimaging applications.

A Significantly Twisted Spirocyclic Phosphine Oxide as a Universal Host for High-Efficiency Full-Color Thermally Activated Delayed Fluorescence Diodes.

A universal thermally activated delayed fluorescence (TADF) host, 4'-diphenylphosphinoylspiro[fluorene-9,9'-xanthene] (SFXSPO), is constructed with a highly distorted and asymmetric configuration and disordered molecular packing in its solid state. SFXSPO successfully endows its full-color TADF diodes with state-of-the-art performance, e.g., the record external quantum efficiency of 22.5% and 19.1% and internal quantum efficiency of ≈100% for its yellow TADF diodes and single-host full-TADF nearly-white-emitting devices, respectively.

The inductive-effect of electron withdrawing trifluoromethyl for thermally activated delayed fluorescence: tunable emission from tetra- to penta-carbazole in solution processed blue OLEDs.

The non-conjugated inductive effect of trifluoromethyl (CF3) was found to be a new acceptor instead of other conjugated moieties. Two blue emissive compounds, 4CzCF3Ph and 5CzCF3Ph, were synthesized using a simple catalyst-free C-N coupling reaction. Solution-processed TADF devices based on the 5CzCF3Ph dopant exhibit significantly higher efficiency (11.8 cd A(-1), 5.2%) than 4CzCF3Ph (1.03 cd A(-1), 0.67%) due to the smaller ΔE(ST) value for efficient RISC.

Dramatic Enhancement of Power Conversion Efficiency in Polymer Solar Cells by Conjugating Very Low Ratio of Triplet Iridium Complexes to PTB7.

Various low ratios of triplet iridium complexes (0, 0.5, 1, 1.5, 2.5, and 5 mol%) are conjugated to the backbone of the famous champion donor polymer PTB7. At the same conditions, the power conversion efficiency for polymer containing 1% of Ir increases by 45%, 39%, and 31% in three batches of devices compared with control Ir-free PTB7.

A versatile efficient one-step approach for carbazole-pyridine hybrid molecules: highly efficient host materials for blue phosphorescent OLEDs.

By using various fluoro-substituted pyridines and carbazole as starting materials, we have synthesized a series of carbazole-pyridine hybrid compounds through an efficient catalyst free C-N coupling reaction with high yields of 85-95%. The bi-, tri- and tetra-carbazole substituted pyridine derivatives exhibit increased thermal stabilities and comparatively high triplet energy levels of ∼3.0 eV. By using them as host materials for blue phosphorescent OLEDs, maximum current efficiencies are achieved ranging from 32-42 cd A(-1).

Organic host materials for phosphorescent organic light-emitting diodes.

Phosphorescent organic light-emitting diodes (PhOLEDs) unfurl a bright future for the next generation of flat-panel displays and lighting sources due to their merit of high quantum efficiency compared with fluorescent OLEDs. This critical review focuses on small-molecular organic host materials as triplet guest emitters in PhOLEDs. At first, some typical hole and electron transport materials used in OLEDs are briefly introduced. Then the hole transport-type, electron transport-type, bipolar transport host materials and the pure-hydrocarbon compounds are comprehensively presented. The molecular design concept, molecular structures and physical properties such as triplet energy, HOMO/LUMO energy levels, thermal and morphological stabilities, and the applications of host materials in PhOLEDs are reviewed (152 references).

Morphologically and electrochemically stable bipolar host for efficient green electrophosphorescence.

A new host tBu-o-CzOXD is facilely synthesized through a simple aromatic nucleophilic substitution reaction between 3,6-di-tert-butyl-9H-carbazole and 2,5-bis(2-fluorophenyl)-1,3,4-oxadiazole. Its thermal, electrochemical, electronic absorption and photoluminescent properties are fully investigated. A high glass transition temperature (T(g)) of 149 degrees C is observed for tBu-o-CzOXD due to the introduction of bulky tert-butyl moiety, significantly higher than 97 degrees C of o-CzOXD without tert-butyl substituent. Moreover, encapsulation of tert-butyl on the 3- and 6-positions of carbazole greatly enhances the electrochemical stability as compared to o-CzOXD. Green phosphorescent OLEDs hosted by tBu-o-CzOXD show a maximum luminance of 48293 cd m(-2) at 17.1 V, a maximum current efficiency of 38.4 cd A(-1) and a maximum power efficiency of 34.7 lm W(-1). Furthermore, the devices exhibit a slow current efficiency roll-off. The device merits, together with the excellent morphological and electrochemical stability, make the new compound an ideal host material for phosphorescent emitters.

Morphological stable spirobifluorene/oxadiazole hybrids as bipolar host materials for efficient green and red electrophosphorescence.

A series of 9,9'-spirobifluorene/oxadiazole hybrids with various linkages between two components, namely SBF-p-OXD (1), SBF-m-OXD (2), and SBF-o-OXD (3) are designed and synthesized through Suzuki cross-coupling reactions. The incorporation of a rigid and bulky spirobifluorene moiety greatly improves their thermal and morphological stability, with T(d) (decomposition temperature) and T(g) (glass transition temperature) in the ranges of 401-480 degrees C and 136-210 degrees C, respectively. 2 and 3 with meta- and ortho-linkage display higher triplet energy and blue-shifted absorption and emission than their para-linked analogue 1 owing to the decreasing pi-conjugation between the two components. Their HOMO and LUMO energy levels depend on the linkage modes within the range of 5.57-5.64 eV and 2.33-2.49 eV, respectively. Multilayer deep red electrophosphorescent devices with 1-3 as hosts were fabricated and their EL efficiencies follow the order of 3 (o)>2 (m)>1 (p), which correlates with their triplet energy and the separation of HOMO and LUMO distributions at molecular orbitals. The maximum external quantum efficiencies of 11.7% for green and 9.8% for deep red phosphorescent organic light-emitting diodes (OLEDs) are achieved by using 2 and 3 as host materials, respectively.