Iridium(III)-Complexed Polydendrimers for Inkjet-Printing OLEDs: The Influence of Solubilizing Steric Hindrance Groups

Suppression of the Coffee Ring Effect in a Single Solvent-Based Silicon Nanoparticle Ink

Silicon (Si) is made printable by dispersing Si nanoparticles in a single organic solvent. Viscoelastic properties of the prepared inks as well as the uniformity of inkjet-printed thin films are investigated in dependence on the Si volume fraction. It has been demonstrated that no ink additives are needed to completely suppress the occurrence of the coffee ring effect. This is obtained by increasing the ink's volume fraction to induce gelation in order to generate elasticity. The printability of our inks is investigated in terms of Weber, Reynolds, and Ohnesorge numbers and found to be maintained even at high particle loads due to shear-thinning viscosity behavior. When printed onto tungsten (W) substrates, Si inks with ϕ(Si) = 0.4% and ϕ(Si) = 2.1% leave a ring stain after drying, whereas coffee rings are absent for inks with ϕ(Si) = 3.0% and above. The reason for this is a significant ink elasticity achieved by the buildup of a gel network for higher particle loads, which leads to thixotropy-like properties. These are low viscosity for printability and elevated elasticity during ink drying, made possible by a breakup of the gel network during drop formation in conjunction with a rapid network reformation after deposition.

Versatile Electroluminescence Color-Tuning Strategy of an Efficient Light-Emitting Electrochemical Cell (LEC) by an Ionic Additive

The color-tuning strategies of solid-state light-emitting devices (ss-LEDs) are mainly focused on engineering molecular structures. In this paper, for the first time, we developed a facile strategy for tuning the electroluminescence (EL) color from orange to green through the addition of the ionic additive TBAP (tetrabutylammonium perchlorate). To achieve the active ionic emissive compound for use in a light-emitting electrochemical cell (LEC), the neutral biscyclometalated bromo tetrazole iridium(III) [Ir(ppy)₂(BrTz)] was exchanged to its cationic complex, [Ir(ppy)₂(BrTz-Me)]ClO₄ (ppy = 2-phenyl pyridine, BrTz = 4-bromo-2-pyridine tetrazole, BrTz-Me = 4-bromo-2-pyridine methyl tetrazole) with a new synthetic strategy. This method allows employing neutral Ir-cyclometalated complexes, which are ruled out for use in LECs because of their non-ionic behaviors. In the following, an LEC based on the new cationic [Ir(ppy)₂(BrTz-Me)]ClO₄ as the emissive layer was fabricated between the FTO (fluorine-doped tin oxide) anode and Ga:In alloy cathode without using any additive or polymers, which makes this configuration the simplest ss-LED so far. By adding the ionic additives, the electroluminescence characteristics of [Ir(ppy)₂(BrTz-Me)]ClO₄ were dramatically increased, including luminance (L) from 162.8 cd/m² for the device with an additive to 212.9 and 355.9 cd/m² for devices containing LiTFSI (bis(trifluoromethane)sulfonamide lithium salt) and TBAP, respectively. In particular, when TBAP was added to the [Ir(ppy)₂(BrTz-Me)]ClO₄ complex, the irradiance was significantly increased from 166.4 to 220.8 μW/cm² with an efficacy of 1.78 cd/A and external quantum efficiency (EQE) value of 2.14%. The obtained EL results clearly showed that adding TBAP and LiTFSI significantly improved the electroluminescence characteristics and tuned the electroluminescence color.

Tuning the Photophysical Properties of Homoleptic Tris-Cyclometalated Ir(III) Complexes by Facile Modification of the Imidazo-Phenanthridine and Their Application to Phosphorescent Organic Light-Emitting Diodes

To explore the excited-state electronic structure of the blue-emitting Ir(dmp)₃ dopant material (dmp = 3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridine), which is notable for durable blue phosphorescent organic light-emitting diode (PhOLED), a series of homoleptic dmp-based Ir(III) complexes (DMP-R, tris[3-(2,6-dimethylphenyl)-7-R-imidazo[1,2-f]phenanthridin-12-yl-κC ¹²,κN ¹]iridium, R = H, CH₃, F, and CF₃) were prepared by introducing an electron-donating group (EDG; -CH₃) or an electron-withdrawing group (EWG; -F and -CF₃) at the 7-position of the imidazo-phenanthridine ligand. The photophysical analysis demonstrated that the alteration from EDG to EWGs led to redshifted structureless emission profiles, which were correlated with variations in the ³MLCT/³ILCT ratio in the T₁ excited state. From electrochemical studies and density functional theory calculations, it turned out that the excited-state nature of the dmp-based Ir(III) complexes was significantly affected by the inductive effect of the 7-substituent of the cyclometalating dmp ligand. As a result of the lowest unoccupied molecular orbital energy stabilization by the EWGs that suppressed the non-radiative pathway from the emissive triplet excited state to the ³ d-d state, the F- and CF₃-modified Ir(dmp)₃ complexes (DMP-F and DMP-CF ₃ ) showed quantum yields of 27-30% in the solution state, which were at least 4- or 5-fold higher than those shown by DMP-H and DMP-CH ₃ . A PhOLED device based on DMP-CF ₃ [CIE chromaticity (0.17, 0.39)], which demonstrated a distinct ³MLCT characteristic, exhibited better electroluminescent efficiencies with an external quantum efficiency of 13.5% than that based on DMP-CH ₃ .

Novel carbazole- and dioxino[2,3-b]pyrazine-based bipolar hosts for red PhOLEDs with a high brightness

The target compounds offer new synthetic ideas for red bipolar host materials.

Flexible Polymer X-ray Detectors with Non-fullerene Acceptors for Enhanced Stability: Toward Printable Tissue Equivalent Devices for Medical Applications

There is growing interest in the development of novel materials and devices capable of ionizing radiation detection for medical applications. Organic semiconductors are promising candidates to meet the demands of modern detectors, such as low manufacturing costs, mechanical flexibility, and a response to radiation equivalent to human tissue. However, organic semiconductors have typically been employed in applications that convert low energy photons into high current densities, for example, solar cells and LEDs, and thus existing design rules must be re-explored for ionizing radiation detection where high energy photons are converted into typically much lower current densities. In this work, we report the optoelectronic and X-ray dosimetric response of a tissue equivalent organic photodetector fabricated with solution-based inks prepared from polymer donor poly(3-hexylthiophene) (P3HT) blended with either a non-fullerene acceptor (5Z,5'Z)-5,5'-((7,7'-(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (o-IDTBR) or a fullerene acceptor, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Indirect detection of X-rays was achieved via coupling of organic photodiodes with a plastic scintillator. Both detectors displayed an excellent response linearity with dose, with sensitivities to 6 MV photons of 263.4 ± 0.6 and 114.2 ± 0.7 pC/cGy recorded for P3HT:PCBM and P3HT:o-IDTBR detectors, respectively. Both detectors also exhibited a fast temporal response, able to resolve individual 3.6 μs pulses from the linear accelerator. Energy dependence measurements highlighted that the photodetectors were highly tissue equivalent, though an under-response in devices compared to water by up to a factor of 2.3 was found for photon energies of 30-200 keV due to the response of the plastic scintillator. The P3HT:o-IDTBR device exhibited a higher stability to radiation, showing just an 18.4% reduction in performance when exposed to radiation doses of up to 10 kGy. The reported devices provide a successful demonstration of stable, printable, flexible, and tissue-equivalent radiation detectors with energy dependence similar to other scintillator-based detectors used in radiotherapy.

Iridium(III) Complexes with [-2, -1, 0] Charged Ligand Realized Deep-Red/Near-Infrared Phosphorescent Emission

A series of [-2, -1, 0] charged-ligand based iridium(III) complexes of [Ir(bph)(bpy)(acac)] (1), [Ir(bph)(2MeO-bpy)(acac)] (2), [Ir(bph)(2CF₃ -bpy)(acac)] (3), [Ir(bph)(bpy)(2^t Bu-acac)] (4) and [Ir(bph)(bpy)(CF₃ -acac)] (5), which using biphenyl as dianionic ligand [-2], acetylacetone (or its derivatives) as monoanionic ligand [-1], and 2,2'-bipyridine (or its derivatives) as neutral ligand [0] were designed and synthesized. The chemical structures were well characterized. All of the ligands have simple chemical structures, thus further making the complexes have excellent thermal stability and are easy to sublimate and purify. Phosphorescent characteristics with short emission lifetime were demonstrated for these emitters. Notably, all of the complexes exhibit remarkable deep red/near infrared emission, which is quite different from the reported [-1, -1, -1] charged-ligand based iridium(III) complexes. The photophysical properties of these complexes are regularly improved by introducing electron-donating or -withdrawing groups into [-1] or [0] charged-ligand. The related organic light-emitting diodes exhibited deep red/near infrared emission with acceptable external quantum efficiency and low turn-on voltage (<2.6 V). This work provides a new idea for the construction of new type phosphorescent iridium(III) emitters with different valence states of [-2, -1, 0] charged ligands, thus offering new opportunities and challenges for their optoelectronic applications.

High Performance Near-Infrared Emitters with Methylated Triphenylamine and Thiadiazolo[3,4-g]quinoxaline-Based Fluorophores

Three near-infrared emitters (2TPA-QBT, 2MeTPA-BT and TPA-QBT-MeTPA) were rationally designed and synthesized. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations showed that the introduction of mono- or di-methyl groups between the donors and acceptor could result in the spatial configuration changing greatly for 2MeTPA-QBT and TPA-QBT-MeTPA compared to their parent compound 2TPA-QBT. The emission of TPA-QBT-MeTPA had a more obvious hybridized local and charge transfer feature (HLCT) based on the influence of the steric hindrance of the methyl substituent. Attributed to their different spatial configurations and luminescence mechanisms, different emission wavelengths with photoluminescent quantum yields of 26%, 38% and 34% in toluene, as well as 24%, 27% and 31% in 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) doped film, were observed for 2TPA-QBT, 2MeTPA-QBT and TPA-QBT-MeTPA, respectively. The constructed organic light-emitting devices (OLEDs) displayed electroluminescence with emission peaks at 728, 693 and 710 nm, with maximum external quantum efficiencies of 1.58%, 1.33% and 3.02% for the 2TPA-QBT, 2MeTPA-QBT and TPA-QBT-MeTPA-doped OLEDs, respectively. This work illustrated the effect of spatial configuration changes on the luminescence properties of donor-acceptor-type organic emitters.

Ink synthesis and inkjet printing of electrostatically stabilized multilayer graphene nanoshells

HYPOTHESIS

Most functional inkjet inks are sterically stabilized nanoparticle dispersions that require a post-printing-process to remove stabilizing materials and gain functionality. This post-process limits material selection and increases fabrication time and complexity for printed devices. By optimizing the electrostatic stability of a carbon nanomaterial dispersed in water or ethylene glycol via pH adjustment, a stable and printable ink should be attainable without a steric stabilizing material and hence the post-process may be avoided.

EXPERIMENTS

The electrostatic stability of multilayer graphene nanoshells (MGNS)-an inexpensive and net carbon-negative nanomaterial-dispersed in water and ethylene glycol was studied by measuring zeta potential as a function of pH and modeling energetic potentials between particles. Requirements for electrical percolation of printed MGNS were analyzed and corroborated with electrical measurements.

FINDINGS

Electrostatic stability improved with increased zeta potential caused by an increased pH. Ionic strength also increased with pH, causing strong destabilization. By increasing zeta potential while minimizing ionic strength, the maximum solid-loading of MGNS in DI water and ethylene glycol was increased up to 20%. For the MGNS solid-loading achieved here, electrical percolation occurs with 20-30 consecutively printed layers producing a resistivity of 30 Ω-cm. The inexpensive, environmentally-friendly MGNS are a promising material for printed, flexible electronics.

Realizing 22.3% EQE and 7-Fold Lifetime Enhancement in QLEDs via Blending Polymer TFB and Cross-Linkable Small Molecules for a Solvent-Resistant Hole Transport Layer

Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4'-(N-(4-butylphenyl)))] (TFB) has been widely used as a hole transport layer (HTL) material in cadmium-based quantum dot light-emitting diodes (QLEDs) because of its high hole mobility. However, as the highest occupied molecular orbital (HOMO) energy level of TFB is -5.4 eV, the hole injection from TFB to the quantum dot (QD) layer is higher than 1.5 eV. Such a high oxidation potential at the QD/HTL interface may seriously degrade the device lifetime. In addition, TFB is not resistant to most solvents, which limits its application in inkjet-printed QLED display. In this study, the blended HTL consisting of TFB and cross-linkable small molecular 4,4'-bis(3-vinyl-9H-carbazol-9-yl)1,1'-biphenyl (CBP-V) was introduced into red QLEDs because of the deep HOMO energy level of CBP-V (-6.2 eV). Compared with the TFB-only devices, the external quantum efficiency (EQE) of devices with the blended HTL improved from 15.9 to 22.3% without the increase of turn-on voltage for spin-coating-fabricated devices. Furthermore, the blended HTL prolonged the T90 and T70 lifetime from 5.4 and 31.1 to 39.4 and 148.9 h, respectively. These enhancements in lifetime are attributed to the low hole-injection barrier at the HTL/QD interface and high thermal stability of the blended HTL after cross-linking. Moreover, the cross-linked blended HTL showed excellent solvent resistance after cross-linking, and the EQE of the inkjet-printed red QLEDs reached 16.9%.

Efficient blue, green and red iridium(iii) complexes with noncovalently-linked pyrazole/pyrazolide rings for organic light-emitting diodes

Blue, green and red iridium(iii) complexes using a new pyrazole/pyrazolide ancillary ligand system, and their application in OLEDs with EQEs of up to 21.0% and low efficiency roll-off.