Screen‐Printed Poly(3,4‐Ethylenedioxythiophene):Poly(Styrenesulfonate) Grids as ITO‐Free Anodes for Flexible Organic Light‐Emitting Diodes

Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) grids have been successfully constructed by roll‐to‐roll compatible screen‐printing techniques and have been used as indium tin oxide (ITO)‐free anodes for flexible organic light‐emitting diodes (OLEDs). The grid‐type transparent conductive electrodes (TCEs) can adopt thicker PEDOT: PSS grid lines to ensure the conductivity, while the mesh‐like grid structure can play an important role to maintain high optical transparency. By adjusting grid periods, grid thickness and treatment of organic additives, PEDOT: PSS TCEs with high optical transparency, low sheet resistance, and excellent mechanical flexibility have been achieved. Using the screen‐printed PEDOT: PSS grids as the anodes, ITO‐free OLEDs achieved peak current efficiency of 3.40 cd A^?1 at the current density of 10 mA cm^?2, which are 1.56 times better than the devices with ITO glass as the anodes. The improved efficiency is attributed to the light extraction effect and improved transparency by the grid structure. The superior optoelectronic performances of OLEDs based on flexible screen‐printed PEDOT: PSS grid anodes suggest their great prospects as ITO‐free anodes for flexible and wearable electronic applications.     

Stable solution processed hole injection material for organic light-emitting diodes

A new aqueous based polymer, Plexcore® OC AQ1200 (AQ1200) was used as a hole injection layer (HIL) in organic light emitting diodes (OLEDs) and the device results are compared with polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS) in terms of injection efficiency and stability. Dark injection transient measurements show a higher injection efficiency in hole-only devices using AQ1200 HIL compared with PEDOT:PSS. Using AQ1200 as an HIL, high efficiency phosphorescent OLEDs are demonstrated to have a long lifetime, with an estimated operational half lifetime (LT 50) of more than 8000 h from an initial luminance of 1000 cd/m².     

Synthesis and optoelectronic properties of novel fluorene-bridged dinuclear cyclometalated iridium (III) complex with A–D–A framework in the single-emissive-layer WOLEDs

To explore the influence of push–pull chromophores on properties of emitter in organic light-emitting devices (OLEDs), an acceptor–donor–acceptor (A–D–A)-based dinuclear iridium (III) complex of (dfppy)₄Ir₂(dipic-FL) was synthesized via Suzuki coupling reaction, in which dfppy is 2-(2,4-difluorophenyl)pyridine and dipic-FL is 2,7-di(5-pyridyl-2-carboxyl)-9,9-dioctyl-9H-fluorene. An intense emission peak at about 480 nm resulting from the (dfppy)₂Ir(pic) chromophore and a weak long-wavelength emission band at 580–660 nm attributed to intramolecular charge transfer transition were exhibited for (dfppy)₄Ir₂(dipic-FL) in dichloromethane solution. But a remarkably hypsochromic photoluminescence profile with an intense characteristical emission peak at 422 nm was observed, which is attributed to the intraligand (IL) π–π^∗ excited states in its thin film. White emission with a maximum luminance of 1040 cd/m² and current efficiency of 1.2 cd/A was obtained in its single-emissive-layer (SEL) OLEDs with a configuration of ITO/PEDOT:PSS/(dfppy)₄Ir₂(dipic-FL) (10 wt%):TCTA/TPBi/LiF/Al. To our knowledge, this is one of the best examples in term of dinuclear iridium complex as single dopant in the high-performance white-emitting SEL-OLEDs.     

Comparison of dimethyl sulfoxide treated highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) electrodes for use in indium tin oxide-free organic electronic photovoltaic devices

Indium tin oxide (ITO)-free organic photovoltaic (OPV) devices were fabricated using highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent conductive electrode (TCE). The intrinsic conductivity of the PEDOT:PSS films was improved by two different dimethyl sulfoxide (DMSO) treatments – (i) DMSO was added directly to the PEDOT:PSS solution (PEDOT:PSS_ADD) and (ii) a pre-formed PEDOT:PSS film was immersed in DMSO (PEDOT:PSS_IMM). X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (CAFM) studies showed a large amount of PSS was removed from the PEDOT:PSS_IMM electrode surface. OPV devices based on a poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk hetrojunction showed that the PEDOT:PSS_IMM electrode out-performed the PEDOT:PSS_ADD electrode, primarily due to an increase in short circuit current density from 6.62 mA cm⁻² to 7.15 mA cm⁻². The results highlight the importance of optimising the treatment of PEDOT:PSS electrodes and demonstrate their potential as an alternative TCE for rapid processing and low-cost OPV and other organic electronic devices.     

Modification effect of hole injection layer on efficiency performance of wet-processed blue organic light emitting diodes

We examined the performance of solution-processed organic light emitting diodes (OLEDs) by modifying the hole injection layer (HIL), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS). Atomic force microscopy (AFM) showed morphological changes with surface roughness (R_RMS) of 1.47, 1.73, and 1.37 nm for pristine PEDOT: PSS, PEDOT: PSS modified with a 40 v% deionized water and with a 30 v% acetone, respectively. The surface hydrophobicity of the acetone modified PEDOT:PSS HIL layer was decreased by 34% as comparing with the water modified counterpart. Electrical conductivity was increased to two orders of magnitude for the water and acetone modified PEDOT:PSS as compared to pristine. We observed a low refractive index and high transmittance for the modified HILs. We fabricated and explored electroluminescent properties of bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) based sky blue device by utilizing HIL with and without modification. The changes in electrical conductivity, surface roughness, refractive index, and transmittance of the modified HILs strongly influenced the performance of devices. By utilizing a 30% acetone modified HIL, the power efficiency was increased from 14.2 to 24.2 lm/W, an increment of 70% and the EQE from 8.5 to 13.1% at 100 cd/m², an increment of 54%. The maximum luminance also increased from 11,780 to 18,190 cd/m². The findings revealed herein would be helpful in designing and fabricating high efficiency solution processed OLEDs.     

The application of single-layer graphene modified with solution-processed TiOx and PEDOT:PSS as a transparent conductive anode in organic light-emitting diodes

There are many challenges for a direct application of graphene as the electrodes in organic electronics due to its hydrophobic surfaces, low work function (WF) and poor conductance. The authors demonstrate a modified single-layer graphene (SLG) as the anode in organic light-emitting diodes (OLEDs). The SLG, doped with the solution-processed titanium suboxide (TiO_x) and poly(3,4-ethylenedio-xythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS), exhibits excellent optoelectronic characteristics with reduced sheet resistance (R_sq), increased work function, as well as over 92% transmittance in the visible region. It is notable that the R_sq of graphene decreased by ∼86% from 628 Ω/sq to 86 Ω/sq and the WF of graphene increased about 0.82 eV from 4.30 eV to 5.12 eV after a modification by using the TiO_x–PEDOT:PSS double interlayers. In addition, the existence of additional TiO_x and PEDOT:PSS layers offers a good coverage to the PMMA residuals on SLG, which are often introduced during graphene transfer processes. As a result, the electrical shorting due to the PMMA residues in the device can be effectively suppressed. By using the modified SLG as a bottom anode in OLEDs, the device exhibited comparable current efficiency and power efficiency to those of the ITO based reference OLEDs. The approach demonstrated in this work could potentially provide a viable way to fabricate highly efficient and flexible OLEDs based on graphene anode.     

Efficient nondoped organic light-emitting diodes with Cu complex emitter

In Cu^I complex based organic light emitting diodes (OLEDs) a host matrix is traditionally thought to be required to achieve high efficiency. Herein, it is found that the device ITO/MoO₃ (1 nm)/4,4′-N,N′-dicarbazole-biphenyl (CBP, 35 nm)/[Cu(μ-I)dppb]₂ (dppb = 1,2-bis[diphenylphosphino]benzene, 20 nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi, 65 nm)/LiF (1 nm)/Al (100 nm) with a vacuum thermal evaporated nondoped Cu^I complex emissive layer (EML) showed external quantum efficiency and current efficiency of 8.0% and 24.3 cd/A at a brightness of 100 cd/m², respectively, which are comparable to the maximum efficiencies reported in an optimized doped OLED with the same emitter, higher efficiency than the OLED with a [Cu(μ-I)dppb]₂:CBP EML, and much higher efficiencies than the nondoped OLED with a bis(2-phenylpyridine)(acetylacetonate)iridium [Ir(ppy)₂(acac)] EML. A series of reference films and single carrier devices were fabricated and studied to understand the difference between Cu^I and Ir^III complex based nondoped OLEDs.     

Modified physico–chemical properties and supercapacitive performance via DMSO inducement to PEDOT:PSS active layer

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was treated with dimethyl sulfoxide (DMSO) in order to modify it physico–chemical properties in an effort to improve its electrochemical performance. In this study, the conductivity of PEDOT:PSS was controlled through post-solvent treatment in DMSO for various dipping times. The maximum conductivity of the DMSO-treated PEDOT:PSS electrode was ∼1890 S cm⁻¹. The electrical conductivity, PEDOT to PSS ratio, chemical compositions, wettability, and surface roughness are correlated and suitably explained. The supercapacitive performance of the DMSO-treated PEDOT:PSS electrodes was studied in 1 M H₂SO₄ within an optimized potential window of 0 to −0.6 V versus Ag/AgCl. The maximum specific capacitance of the PEDOT:PSS electrode treated in DMSO for 60 min was found to be ∼153 F g⁻¹ at a 1 mA cm⁻² discharge current density (∼145 F g⁻¹ at a 10 mV s⁻¹ scan rate), along with an energy density of 93 W h kg⁻¹ and a power density of 4.6 kW kg⁻¹.     

Blade coating of Tris(8-hydroxyquinolinato)aluminum as the electron-transport layer for all-solution blue fluorescent organic light-emitting diodes

Organic light-emitting diodes (OLEDs) with a low driving voltage and efficient blue fluorescence were fabricated through blade coating. Tris(8-hydroxyquinolinato)aluminum (Alq₃) is a relatively stable electron-transporting material commonly used in evaporation. However, depositing Alq₃ through a solution process is difficult because of its extremely low solubility organic solvents, a result of its symmetrical molecular structure. In this study, Alq₃ was successfully deposited through blade coating at a very low concentration below 0.1wt%. The OLEDs contained co-dopants BUBD-1 and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph), and a high-band-gap host 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) as the emission layer with the following structure: ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:2% BUBD-1:1% DSA-Ph (50 nm)/TPBI (30 nm)/LiF (0.8 nm)/Al (100 nm)or ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:3% BUBD-1 (50 nm)tris(8-hydroxyquinolinato)aluminum (Alq₃; 10 nm)/LiF (0.8 nm)/Al (100 nm). 2,7-disubstituted fluorene-based triaryldiamine(VB-FNPD)is the cross-linking transporting material. The device exhibited a peak current efficiency of 5.67 cd/A for Alq₃ and 5.76 cd/A for TPBI. The device with Alq₃ has operated lifetime seven times higher than the device with TPBI.     

Highly efficient,solution-processed orange–red phosphorescent OLEDs by using new iridium phosphor with thieno[3,2-c]pyridine derivative as cyclometalating ligand

A new orange iridium phosphor of (EtPy)₂Ir(acac) with thieno[3,2-c]pyridine derivative as cyclometalating ligand was designed and synthesized. The combination of thieno[3,2-c]pyridine with rigid fluorene moiety enlarged the π conjugation of ligand, and consequently caused the peak emission of (EtPy)₂Ir(acac) red-shift to 588 nm. By using (EtPy)₂Ir(acac) as the orange phosphor, the fully solution-processed PhOLEDs were fabricated with the following device configuration: ITO/PEDOT:PSS/PVK: PBD: (EtPy)₂Ir(acac)/CsF/Al. With PEDOT:PSS 8000 as the hole-injecting material, the orange device achieved a maximum current efficiency of 13.4 cd A⁻¹, a maximum power efficiency of 5.9 lm W⁻¹ and a maximum external quantum efficiency (EQE) of 11.2% with a CIE coordinate of (0.62, 0.38) that falls into the orange–red region. Moreover, at high luminance of 1000 cd m⁻², the device still remained high current efficiency of 8.7 cd A⁻¹ and EQE of 7.3%. To the best of our knowledge, these efficiencies were among the highest ever reported for solution-processed orange–red PhOLEDs.