Highly efficient color-stable deep-blue multilayer PLEDs: preventing PEDOT:PSS-induced interface degradation

Mechanically robust stretchable semiconductor metallization for skin-inspired organic transistors

Despite recent remarkable advances in stretchable organic thin-film field-effect transistors (OTFTs), the development of stretchable metallization remains a challenge. Here, we report a highly stretchable and robust metallization on an elastomeric semiconductor film based on metal-elastic semiconductor intermixing. We found that vaporized silver (Ag) atom with higher diffusivity than other noble metals (Au and Cu) forms a continuous intermixing layer during thermal evaporation, enabling highly stretchable metallization. The Ag metallization maintains a high conductivity (>10⁴ S/cm) even under 100% strain and successfully preserves its conductivity without delamination even after 10,000 stretching cycles at 100% strain and several adhesive tape tests. Moreover, a native silver oxide layer formed on the intermixed Ag clusters facilitates efficient hole injection into the elastomeric semiconductor, which transcends previously reported stretchable source and drain electrodes for OTFTs.

Improving the Performance of Blue Polymer Light-Emitting Diodes Using a Hole Injection Layer with a High Work Function and Nanotexture

For light-emitting polymers with a deep highest occupied molecular orbital energy level used for polymer light-emitting diodes (PLEDs), the hole injection barrier and hole transport of the anode buffer layer are of vital importance for optimizing electroluminescent performance. In this study, high-work-function hole injection layers with nanotextures were achieved by modifying poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) with a perfluorinated ionomer (PFI) and n-butyl alcohol and were used to achieve a single-layer device without a hole transport layer. With such an interlayer, the PLEDs based on PPF-SO25 exhibit remarkable current efficiency over 13.0 cd A^-1, which significantly outperform the devices with regular PEDOT:PSS. To our knowledge, this performance is among the best reported for single-layer blue PLEDs. The bias-dependent capacitance curves of these PLEDs suggest a nonuniform surface distribution of PFI. Our findings show that the PFI-modified PEDOT:PSS not only operates as a high-work-function hole injection layer to facilitate hole injection but also as a potential inner scattering medium for light extraction.

Modulating the luminance of organic light-emitting diodes via optical stimulation of a photochromic molecular monolayer at transparent oxide electrode

Self-assembled monolayers (SAMs) deposited on bottom electrodes are commonly used to tune charge carrier injection or blocking in optoelectronic devices. Beside the enhancement of device performance, the fabrication of multifunctional devices in which the output can be modulated by multiple external stimuli remains a challenging target. In this work, we report the functionalization of an indium tin oxide (ITO) electrode with a SAM of a diarylethene derivative designed for optically control the electronic properties. Following the demonstration of dense SAM formation and its photochromic activity, as a proof-of-principle, an organic light-emitting diode (OLED) embedding the light-responsive SAM-covered electrode was fabricated and characterized. Optically addressing the two-terminal device by irradiation with ultraviolet light doubles the electroluminescence. The original value can be restored reversibly by irradiation with visible light. This expanded functionality is based on the photoinduced modulation of the electronic structure of the diarylethene isomers, which impact the charge carriers' confinement within the emissive layer. This approach could be successfully exploited in the field of opto-communication technology, for example to fabricate opto-electronic logic circuits.

Improving the Electroluminescent Performance of Blue Light-Emitting Polymers by Side-Chain Modification

Blue light-emitting polymers are in urgent demand for new-generation display and solid-state lighting devices fabricated through low-cost wet processing. However, their current performances are far from satisfactory. Here, we developed a series of poly(fluorene-co-dibenzothiophene-S,S-dioxides) (PFSOs) bearing different alkyl chains, alkoxyphenyl chains, or both alkylaryl and alkoxyphenyl side chains. The introduction of alkoxyphenyl groups moderately enhanced the electron-donating ability of the polymers, leading to more balanced charge carrier fluxes. Meanwhile, asymmetric bulky side chains enabled more pronounced variation of molecular conformation while restraining the intermolecular aggregation of polymers, resulting in a lower refractive index, thus facilitating light extraction compared with polymers based on the same two alkyl or alkoxyphenyl side chains. Polymer light-emitting devices based on PFSO-BMD with asymmetric side chains exhibited a maximum luminous efficiency of 8.58 cd A^-1, associated with pure blue Commission Internationale de l'Eclairage coordinates of (0.14, 0.14). These findings demonstrated that side-chain modification can be an effective strategy for developing efficient solution-processable blue light-emitting polymers.

Highly Efficient Solution-Processed Thermally Activated Delayed Fluorescence Bluish-Green and Hybrid White Organic Light-Emitting Diodes Using Novel Bipolar Host Materials

Two pyridine-containing bipolar host materials with high triplet energy, 9,10-dihydro-9,9-dimethyl-10-(3-(6-(3-(9,9-dimethylacridin-10(9H)-yl)phenyl)pyridin-2-yl)phenyl acridin (DDMACPy) and N-(3-(6-(3-(diphenyl amino)phenyl)pyridin-2-yl)phenyl)-N-phenylbenzenamine (DTPAPy), are synthesized from the modification of the commonly adapted host material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (DCzPPy). The highest occupied molecular orbital levels of DDMACPy (5.50 eV) and DTPAPy (5.60 eV) are found to be shallower than that of DCzPPy (5.90 eV) that leads to the improvement in hole injection from the hole transport layer PEDOT:PSS (WF = 5.10 eV). These host materials are used in the emitting layer of bluish-green organic light-emitting diode (OLED) with the thermally activated delayed fluorescence (TADF) emitter, 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine, as the guest. The DDMACPy-based device shows the highest performance among them, with the maximum external quantum efficiency (EQE_max), current efficiency (CE_max), and power efficiency (PE_max) of 21.0%, 53.1 cd A^-1, and 44.0 lm W^-1 at CIE (0.17, 0.42), respectively. By further doping with the red-emitting phosphor iridium(III) bis(2-phenylquinoline)(2,2,6,6-tetramethylheptane-3,5-ionate) [Ir(dpm)PQ₂] and yellow-emitting phosphor iridium(III) bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C20)acetylacetonate (PO-01-TB) emitters into the bluish-green emitting layer, a TADF-phosphor hybrid white OLED (T-P WOLED) is obtained with excellent EQE_max, CE_max, and PE_max of 17.4%, 48.7 cd A^-1, and 44.5 lm W^-1 at CIE (0.35, 0.44), respectively. Moreover, both the bluish-green and WOLED show a low efficiency roll-off with external quantum efficiencies at the brightness of 1000 cd m^-2 (EQE₁₀₀₀) of 18.7 and 16.2%, respectively, which are the highest performance records among the solution-processed TADF bluish-green and T-P WOLEDs.

Enhancing Phosphorescence through Rigidifying the Conformation to Achieve High-Efficiency OLEDs by Modified PEDOT

Bis(diphenylphosphinomethyl)phenylphosphine (dpmp)-supported Pt₂Au heterotrinuclear complexes [Pt₂Au(dpmp)₂(C≡CPh)₄](ClO₄) (1), [Pt₂Au(dpmp)₂(DEBf)(C≡CPh)₂](ClO₄) (2), and [Pt₂Au(dpmp)₂(DECz)(C≡CPh)₂](ClO₄) (3) were prepared and used in organic light-emitting diodes (OLEDs) as a new class of light emitters, where DEBf = dibenzofuran-4,6-diacetylide and DECz = 3,6-di-tert-butylcarbazole-1,8-diacetylide. Although the flexible structure of Pt₂Au complex 1 (λ_em = 503 nm, Φ_em < 0.1%) results in weak photoluminescence in fluid CH₂Cl₂, complexes 2 (λ_em = 585 nm, Φ_em = 4.9%) and 3 (λ_em = 589 nm, Φ_em = 3.2%) with a rigid conformation give a much stronger phosphorescence. The displacement of two σ-bonded phenylacetylide ligands with a diacetylide ligand such as DEBf and DECz to fasten Pt₂Au structures facilitates greatly luminescent emission so that the emissive quantum yield in doping film is as high as 89% for 2 and 93% for 3. As revealed by a theoretical study, the severe structural distortion of diacetylide-linked Pt₂Au complexes 2 (λ_em = 585 nm) and 3 (λ_em = 589 nm) in a triplet excited state gives rise to significant red shifts of phosphorescent emission spectra relative to that of complex 1 (λ_em = 503 nm). By means of Pt₂Au complexes as phosphorescent emitters, solution-processed OLEDs achieved a relatively low external quantum efficiency (EQE < 9.5%) when commercial poly(ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) was used as the hole-injection layer (HIL). In contrast, the peak EQE was increased to 18.3% with a dramatic increase of efficiency by the use of modified HILs composed of PEDOT:PSS and PSS-Na, which provide a higher work function and a better film morphology.

Towards Monodisperse Star-Shaped Ladder-Type Conjugated Systems: Design, Synthesis, Stabilized Blue Electroluminescence, and Amplified Spontaneous Emission

A novel series of monodisperse star-shaped ladder-type oligo(p-phenylene)s, named as TrL-n (n=1-3), have been explored. Their thermal and electrochemical properties, fluorescence transients, photoluminescence quantum yields, density functional theory calculations, electroluminescence (EL) and amplified spontaneous emission (ASE) properties have been systematically investigated to unravel the molecular design on optoelectronic properties. The resulting materials showed excellent structural perfection, free of chemical defects, and exhibited great thermal stability (T_d : 404-418 °C and T_g : 147-184 °C) and amorphous glassy morphologies. Compared with their corresponding linear counterparts FL-m (m=1-3), TrL-n showed only little bathochromic shifts (5-12 nm) for the absorption maxima λ_max in both solution and films. The star-shaped ladder-type compounds exhibited enhanced optical stability and suppressed low-energy emission. Their EL spectra exhibited excellent stability with increasing the driving voltage from 6 to 12 V. Moreover, superior low ASE thresholds were recorded for TrL-n compared with FL-m. Rather low ASE threshold (29 nJ per pulse or 1.60 μJ cm^-2 ) was recorded for TrL-3, demonstrating their promising potential as excellent gain media. This study provides a novel design concept to develop monodisperse star-shaped ladder-type materials with excellent structural perfection, which are vital for shedding light on exploring robust organic emitters for optoelectronic applications.

Synthesis and performance of cross-linked PEDOT:MOI-P(SS-HEA) transparent conductive films by UV irradiation

A novel multi-functional counteranion of methacryloyloxyethyl isocyanate grafting poly(styrene sulfonate-co-2-hydroxyethyl acrylate) [MOI-P(SS-HEA)] for PEDOT was designed to improve the humidity stability and water resistance of PEDOT:PSS-based conductive films.

Synthesis and application of photolithographically patternable deep blue emitting poly(3,6-dimethoxy-9,9-dialkylsilafluorene)s

Poly(silafluorene)s (PSFs) are promising light-emitting materials with brilliant solid-state blue luminescence, high quantum efficiency, excellent solubility, and improved thermal and chemical stability. PSFs are reported to have high electron affinity and conductivity originating from σ*-π* conjugation between the σ*-antibonding orbital of the exocyclic Si-C bond and the π* antibonding orbital of the butadiene fragment, a promising characteristic for improved charge carrier balance in OLEDs. In this paper, we present a protocol for photopatterning derivatives of poly(3,6-dimethoxy-9,9-dialkylsilafluorenes) with resolutions exceeding 10 μm. The procedure begins by converting polymers (Mn = 50-55 kg/mol, PDI = 1.8) with cyclohexenyl and norbornenyl containing side chains to their respective epoxides using the Prilezhaev reaction and m-chloroperoxybenzoic acid (m-CPBA). Using the I-line (365 nm) of a Karl Suss MA6 mask aligner, a 1 s UV light exposure of the photoacid generator (PAG) bis(4-tert-butylphenyl)iodonium hexafluoro-phosphate (DtBPI-PF6) generates sufficient protons to catalyze epoxide ring-opening and form a bridging network of covalent C-O bonds which renders the material insoluble in developing solvents such as toluene or THF. The resultant cross-linked material possess characteristic blue photoluminescence with solid state quantum yields >80%. Polymer films have excellent transparency (with a measured Eg ≈ 3.0 eV). Energy levels determined using cyclic voltammetry were -5.7 and -2.7 eV for HOMO and LUMO, respectively. Additionally, several device applications are demonstrated which incorporate cross-linked films. These include examples of solid state lasing in the region of 420-450 nm from cross-linked films on second order corrugated silica substrates (Λ = 200 nm). OLEDs were also prepared with a cross-linked emitting layer as part of a trilayer device which we report to have a maximum external quantum efficiency of 3.2% at 33 mA/cm(2) and a stable blue-violet emission with an electroluminescence maximum at 410 nm. Photopatternable PSF epoxides are also shown to be efficient hosts for Förster energy transfer and we provide examples of pattern layers incorporating small molecule emitters which emit in both the red and green while blue emission of the host is effectively suppressed.