Flexible, transparent patterned electrodes based on graphene oxide/silver nanowire nanocomposites fabricated utilizing an accelerated ultraviolet/ozone process to control silver nanowire degradation.

We developed flexible, transparent patterned electrodes, which were fabricated utilizing accelerated ultraviolet/ozone (UV/O3)-treated graphene oxide (GO)/silver nanowire (Ag-NW) nanocomposites via a simple, low-cost pattern process to investigate the feasibility of promising applications in flexible/wearable electronic and optoelectronic devices. The UV/O3 process of the GO/Ag-NW electrode was accelerated by the pre-heat treatment, and the degradation interruption of Ag NWs was removed by the GO treatment. After the deposition of the GO-treated Ag NW electrodes, the sheet resistance of the thermally annealed GO-treated Ag-NW electrodes was significantly increased by using the UV/O3 treatment, resulting in a deterioration of the GO-treated Ag NWs in areas exposed to the UV/O3 treatment. The degradation of the Ag NWs caused by the UV/O3 treatment was confirmed by using the sheet resistances, scanning electron microscopy images, X-ray photoelectron microscopy spectra, and transmittance spectra. While the sheet resistance of the low-density Ag-NW electrode was considerably increased due to the pre-thermal treatment at 90 °C for 10 min, that of the high-density Ag-NW electrode did not vary significantly even after a UV/O3 treatment for a long time. The degradation interference phenomenon caused by the UV/O3 treatment in the high-density Ag NWs could be removed by using a GO treatment, which resulted in the formation of a Ag-NW electrode pattern suitable for promising applications in flexible organic light-emitting devices. The GO treatment decreased the sheet resistance of the Ag-NW electrode and enabled the pattern to be formed by using the UV/O3 treatment. The selective degradation of Ag NWs due to UV/O3 treatment decreased the transparency of the Ag-NW electrode by about 8% and significantly increased its sheet resistance more than 100 times.

Degradation mechanisms of silver nanowire electrodes under ultraviolet irradiation and heat treatment.

We report the degradation mechanisms of the silver nanowire (Ag NW) electrodes that play a significantly important role in the stability of wearable and flexible devices. The degradation mechanisms behind the increase in the sheet resistances of Ag NW electrodes were clarified by investigating the variations in the structure and the chemical composition of the Ag NW electrodes caused by ultraviolet irradiation and thermal treatment. While the shapes of the Ag NWs were affected by melting during the thermal degradation process, the chemical composition of the polyvinylpyrrolidone protective layer on the surfaces of the Ag NWs was not changed. Ultraviolet irradiation deformed the shapes of the Ag NWs because nitrogen or oxygen atoms were introduced to the silver atoms on the surfaces of the Ag NWs. A graphene-oxide flake was coated on the Ag NW electrodes by using a simple dipping method to prevent ultraviolet irradiation and ozone contact with the surfaces of the Ag NWs, and the increase in the sheet resistance in the graphene-oxide-treated Ag NWs was suppressed. These observations will be of assistance to researchers trying to find novel ways to improve the stability of the Ag NW electrodes in next-generation wearable devices.

Efficiency Enhancement in Polymer Light-Emitting Devices Fabricated Utilizing a MoO3 Hole Injection Layer Coated with Different Solvents.

Polymer light-emitting devices (PLEDs) with a MoO3 hole injection layer (HIL) were fabricated to enhance their luminance efficiency. Ultraviolet photoelectron spectroscopy spectra showed that the valence band maximum level of the MoO3 layer was located between the work function of the the indium-tin-oxide anode and the highest occupied molecular orbital level of the poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine] hole transport layer. The surface of the MoO3 layer formed by using an ethanol solvent was smoother than that of the MoO layer formed by using a deionized water solvent due to a decrease in the aggregation of the MoO3 resolved in ethanol. The MO3 HIL decreased the operating voltage of the PLED and increased the luminance and the luminance efficiency of the PLED due to a decrease in the hole injection barrier. The operating voltage, the luminance, and the luminance efficiency of the PLEDs with the MoO3 HIL formed by using an ethanol solvent were enhanced in comparison with those of the PLEDs with a MoO3 HIL formed by using a deionized water solvent due to a decrease in the surface roughness of the HIL.

Transparent Conducting Silver-Nanowire-Embedded Poly(methyl methacrylate) Nanocomposite Films Formed by Using a Transfer Method.

Poly(methyl methacrylate) (PMMA) substrates containing silver nanowires (Ag NWs) were fabricated by using a transfer method. Ag NWs with a length of 20 µm and a width of 80 nm were synthesized by using a modified polyol process. Ag NW electrodes with a high surface roughness value on glass substrates were significantly improved by using both a transfer method and a poly(3,4-ethylenedioxythiophene):polystyrene sulfonate layer coating. The peak-to-valley roughness of the Ag NWs decreased from 210 to 26 nm resulting from the filling of the PMMA into the vacancies among the Ag NWs, and the corresponding root-mean-square roughness decreased from 74 to 6 nm. Atomic force microscopy images showed a dramatic decrease in the surface roughness of the PMMA substrates containing Ag NWs. The optical transmittance and the sheet resistance of the optimized Ag NW-embedded PMMA substrates were 80% and 14 Ω/sq, respectively.

Flexible white organic light-emitting devices with a porous red polymer layer and a blue small molecular layer utilizing a phase separation of blended polymer.

Flexible white organic light-emitting devices (WOLEDs) with an emitting layer consisting of a porous red poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) polymer layer and a blue 4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi) small molecular layer were fabricated on polyethylene terephthalate substrates. The current density of the flexible WOLEDs fabricated with a blend layer formed with a higher spincoating speed was significantly higher than that of a device fabricated with a lower spincoating speed, due to the higher pore density. The ratio between the red and the blue color peak intensities of the electroluminescence spectra for the flexible WOLEDs with a porous red MEH-PPV polymer layer and a blue DPVBi small molecular layer was controlled by the spincoating speed of the blend layer.

Carrier transport mechanisms of phosphorescent organic light-emitting devices fabricated utilizing a N,N'-dicarbazolyl-3,5-benzene: 1,3,5-tri(phenyl-2-benzimidazole)-benzene mixed host emitting layer.

The electrical and the optical properties of phosphorescent organic light-emitting devices (PHOLEDs) fabricated utilizing a mixed host emitting layer (EML) consisting of N,N'-dicarbazolyl-3,5-benzene (mCP) and 1,3,5-tri(phenyl-2-benzimidazole)-benzene (TPBi) were investigated to clarify the carrier transport mechanisms of PHOLEDs. While the operating voltage of the PHOLEDs with a mixed host EML significantly decreased due to the insertion of TPBi with a high electron mobility, the quantum efficiency of the PHOLEDs decreased due to the hindrance of the exciton energy transfer by TPBi molecules. The electroluminescence spectra for the PHOLEDs with an tris(2-phenylpyridine)iridium-doped mixed host EML showed that the TPBi molecules in the mixed host EML increased the electron injection into the mixed host EML, resulting in a decrease of the shift length of the recombination zone in comparison with a single host EML.

White organic light-emitting devices utilizing a mixed color-conversion phosphor layer consisting of CaAl12O19:Mn and Zn2SiO4:Mn.

White organic light-emitting devices (WOLEDs) were fabricated utilizing a mixed color-conversion layer consisting of CaAl12O19:Mn and Zn2SiO4:Mn phosphors. The ratio between the CaAl12O19:Mn and the Zn2SiO4:Mn phosphor determined the rate of the red and the green lights. The color rendering index was improved by using a mixed color-conversion phosphor layer.

Luminance mechanisms in green organic light-emitting devices fabricated utilizing tris(8-hydroxyquinoline)aluminum/4,7-diphenyl-1, 10-phenanthroline multiple heterostructures acting as an electron transport layer.

The electrical and the optical properties in green organic light-emitting devices (OLEDs) fabricated utilizing tris(8-hydroxyquinoline)aluminum (Alq3)/4,7-diphenyl-1,10-phenanthroline (BPhen) multiple heterostructures acting as an electron transport layer (ETL) were investigated. The operating voltage of the OLEDs with a multiple heterostructure ETL increased with increasing the number of the Alq3/BPhen heterostructures because more electrons were accumulated at the Alq3/BPhen heterointerfaces. The number of the leakage holes existing in the multiple heterostructure ETL of the OLEDs at a low voltage range slightly increased due to an increase of the internal electric field generated from the accumulated electrons at the Alq3/BPhen heterointerface. The luminance efficiency of the OLEDs with a multiple heterostructure ETL at a high voltage range became stabilized because the increase of the number of the heterointerface decreased the quantity of electrons accumulated at each heterointerface.

Luminance efficiency enhancement in green organic light-emitting devices fabricated utilizing a cesium fluoride/fullerene heterostructure electron injection layer.

Enhancement mechanisms of the luminance efficiency in green organic light-emitting devices (OLEDs) fabricated utilizing a cesium fluoride (CsF)/fullerene (C60) heterostructure acting as an electron injection layer (EIL) were investigated. The luminance efficiencies as functions of the current density showed that the luminance efficiency in the green OLEDs fabricated utilizing a CsF/C60 heterostructure acting as an EIL was higher than that in the green OLEDs fabricated utilizing a CsF, a Liq, or a C60 single EIL. The interfacial dipole existing at the CsF layer decreased the electron injection barrier, and the stability of the OLEDs with a CsF EIL was enhanced due to the lower diffusion rate of Cs atoms in comparison with Li atoms. The enhancement of the luminance efficiency of the OLEDs with a heterostructure EIL was attributed to the increase in the electron injection. These results can help improve understanding of the enhancement mechanisms of the luminance efficiency in green OLEDs utilizing a CsF/C60 heterostructure acting as an EIL.

Color stabilization in white organic light emitting devices utilizing trapping layers inserted in both an electron transport layer and an emitting layer.

The electrical and the optical properties of white organic light emitting devices (OLEDs) utilizing trapping layers inserted into both an electron transport layer (ETL) and an emitting layer (EML) were investigated. The current density of OLEDs with an ETL containing a 5,6,11,12-tetraphenylnaphthacene (rubrene) layer was slightly smaller than those of other devices. The luminance-current density and luminance efficiency-current density of the OLEDs with rubrene layers embedded in only an ETL or an EML were similar to the blue reference device. While the electroluminescence (EL) spectrum for the OLEDs with a rubrene layer in the ETL in the low voltage range showed the white color, that with rubrene layers in both the EML and the ETL exhibited white color, regardless of the applied voltage. The Commission International de l'Eclairage coordinates of the white OLEDs became stabilized by inserting rubrene layers into both the EML and the ETL.