Thermally Induced Phase Separation of the PEDOT:PSS Layer for Highly Efficient Laminated Polymer Light-Emitting Diodes.

Among various conductive polymers, the poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) film has been studied as a promising material for use as a transparent electrode and a hole-injecting layer in organic optoelectronic devices. Due to the increasing demand for the low-cost fabrication of organic light-emitting diodes (OLEDs), PEDOT:PSS has been employed as the top electrode by using the coating or lamination method. Herein, a facile method is reported for the fabrication of highly efficient polymer light-emitting diodes (PLEDs) based on a laminated transparent electrode (LTE) consisting of successive PEDOT:PSS and silver-nanowire (AgNW) layers. In particular, thermally induced phase separation (TIPS) of the PEDOT:PSS film is found to depend on the annealing temperature (Tanneal) during preparation of the LTE. At Tanneal close to the glass transition temperature of the PSS chains, a PSS-rich phase with a large number of PSS- molecules enhances the work function of the PEDOT:PSS on the glass-side surface relative to the air side. By using the optimized LTEs, bidirectional laminated PLEDs are obtained with a total external quantum efficiency of 2.9% and a turn-on voltage of 2.6 V, giving a comparable performance to that of the reference bottom-emitting PLED based on a costly evaporated metal electrode. In addition, an analysis of the angular characteristics, including the variation in the electroluminescence spectra and the change in luminance according to the emission angle, indicates that the laminated PLED with the LTE provides a more uniform angular distribution regardless of the direction of emission. Detailed optical and electrical analyses are also performed to evaluate the suitability of LTEs for the low-cost fabrication of efficient PLEDs.

Controlled Growth of Perovskite Nanocrystals on Nanotubes via a Nanoseeding Intermediate Stage: Toward Novel Optoelectronic Applications.

CsPbBr3 perovskite nanocrystals (CNCs) were densely anchored on multiwalled carbon nanotubes (MWNTs) via a nanoseeding intermediate stage, in which lead-based nuclei are formed on the nanotube surface. After the formation of the intermediate, a cesium precursor was added to promote the growth of CNCs from the surface nuclei and to thereby obtain CNC-decorated MWNT nanohybrids (CMNHs). The morphology and properties of the CMNHs were determined by the reaction temperature employed during their synthesis. Importantly, the use of MWNTs promoted the formation of larger CNCs that emitted intense green light and modified the electronic structure and bandgap energy of the CNCs. Consequently, the CMNHs could function as optoelectronic transducers and exhibit a "turn-on" photocurrent response when exposed to UV light of narrow specific-range wavelengths. In a novel approach for preventing counterfeit products, the CMNHs were used as a light-emitting black ink to create quick-response codes with fake pixels.

Gradual Morphological Change in PEDOT:PSS Thin Films Immersed in an Aqueous Solution.

The poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) film is a promising material for electrodes, biomolecular sensor channels, and probes for physiological signals because the electrical conduction of PEDOT:PSS is tuned simply through the electrochemical reaction with the target analyte. However, forming a specific morphology or nanostructure on PEDOT:PSS thin films immersed in an aqueous solution is still a challenge. Herein, we report the mechanism for the stepwise morphological change in the highly conductive PEDOT:PSS layer that successfully explains the electrical and structural modulations that occur after a soaking test in various pH conditions. The change in PEDOT:PSS begins with the rapid swelling and dissolution of PSS-rich domains and the simultaneous structural rearrangement of the remaining PEDOT chains within 1 s of dipping. Analysis confirms that the pH conditions of an aqueous solution govern the oxidation state and the form of the PEDOT chains. After removing the water molecules, additional PEDOT-rich grains were generated and accumulated on the surface of the film, which exhibited hydrophobic barrier characteristics. With the help of this intrinsic barrier on the PEDOT:PSS surface, the sheet resistance slightly increased from 72 to 144 Ω/sq even after dipping in a water bath for 350 h. We also demonstrate the usability of the proposed approach on a sensor to detect vitamin C in an aqueous medium. Utilizing the electrochemical reaction of PEDOT:PSS films, the simple resistor sensor showed a response time of less than 150 s, which is 10 times faster than that observed in a previous report. The soaked samples also showed a more reliable linear correlation between the current change and the amount of ascorbic acid compared with pristine PEDOT:PSS. Both the proposed mechanism and the role of accumulated PEDOT-rich regions illustrate the versatile potential of highly conductive PEDOT:PSS films in the field of bioelectronic applications, owing to the increased design architecture.

Effect of Laser-Induced Direct Micropatterning on Polymer Optoelectronic Devices.

Solution-processed polymer devices have been studied as a low-cost alternative to the conventional vacuum-processed organic devices. However, forming a specific pattern on polymer semiconductor films without costly lithography is still challenging. Herein, we report a low-cost direct patterning method for polymer optoelectronic devices, which can successfully engrave designated patterns on the polymer semiconductor layer regardless of its size and even after device encapsulation. Irradiation of a 100 ns pulse laser forms high-resolution patterns on devices such as polymer light-emitting diodes and polymer memory devices. The biggest advantage of the proposed patterning method is that it does not produce any physical damage in the device, such as leakage current or degraded light-emitting efficiency. Analysis confirms that the laser-induced heat alters the solid or crystal structure of the polymer semiconducting layers so that the designated areas of the polymer devices can be selectively and deliberately deactivated. We demonstrate the usability of the developed laser-induced direct-patterning method on the polymer devices by engraving a digital image onto "ON-state" light-emitting devices and by generating multiple states onto a 4 × 4 matrix polymer nonvolatile memory.

Luminescent CeO2:Eu3+ nanocrystals for robust in situ H2O2 real-time detection in bacterial cell cultures.

Hydrogen peroxide (H2O2) quantification in biomedicine is valuable as inflammation biomarker but also in assays employing enzymes that generate or consume H2O2 linked to a specific biomarker. Optical H2O2 detection is typically performed through peroxidase-coupled reactions utilizing organic dyes that suffer, however, from poor stability/reproducibility and also cannot be employed in situ in dynamic complex cell cultures to monitor H2O2 levels in real-time. Here, we utilize enzyme-mimetic CeO2 nanocrystals that are sensitive to H2O2 and study the effect of H2O2 presence on their electronic and luminescent properties. We produce and dope with Eu3+ these particles in a single-step by flame synthesis and directly deposit them on Si and glass substrates to fabricate nanoparticle layers to monitor in real-time and in situ the H2O2 concentrations generated by Streptococcus pneumoniae clinical isolates. Furthermore, the small CeO2:Eu3+ nanocrystals are combined in a single-step with larger, non-responsive Y2O3:Tb3+ nanoparticles during their double-nozzle flame synthesis to engineer hybrid luminescent nanoaggregates as ratiometric robust biosensors. We demonstrate the functionality of these biosensors by monitoring their response in the presence of a broad range of H2O2 concentrations in vitro from S. pneumoniae, highlighting their potential for facile real-time H2O2 detection in vitro in cell cultures.

Simultaneously enhanced optical, electrical, and mechanical properties of highly stretchable transparent silver nanowire electrodes using organic surface modifier.

We report on a new surface modifier which simultaneously improves electrical, optical, and mechanical properties of silver nanowire-based stretchable transparent electrodes. The transparent electrodes treated with 11-aminoundecanoic acid achieve a low sheet resistance of 26.0 ohm/sq and a high transmittance of 90% with an excellent stretchability. These improvements are attributed to the effective formation of a strong chemical bond between silver nanowire networks and elastomeric substrates by 11-aminoundecanoic acid treatment. The resistance change of the optimized silver nanowire/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin-films is only about 10% when the film is stretched by 120%. In addition, the chemical stability of stretchable silver nanowire films is significantly improved by the introduction of conductive PEDOT:PSS overcoat film. The optimized electrodes are utilized as high-performance stretchable transparent heaters, successfully illustrating its feasibility for future wearable electronics.

Facile and Scalable Fabrication of Flexible Reattachable Ionomer Nanopatterns by Continuous Multidimensional Nanoinscribing and Low-temperature Roll Imprinting.

We develop a facile route to the scalable fabrication of flexible reattachable ionomer nanopatterns (RAINs) by continuous nanoinscribing and low-temperature roll imprinting, which are repeatedly attachable to and detachable from arbitrarily shaped surfaces. First, by sequentially performing continuous nanoinscribing over a polymer substrate along the multiple directions, we readily create the multidimensional nanopattern, which otherwise demands complex nanofabrication. After its transfer to an elastomer pad for use as a soft nanoimprinting stamp, we then conduct a low-temperature roll imprinting of the ionomer film to fabricate a flexible and highly transparent RAIN. Reversible loosening of ionic units in the ionomer material at the mild temperature as low as ∼60-70 °C enables the faithful nanopatterning over thermosensitive organic compounds and fragile materials under a slight pressure. The excellent adhesion purely emerging from ionic interactions uniquely realizes the conformal attachability and clean detachability of RAINs for universal targets in ambient conditions, particularly beneficial for individual wearable and mobile devices requiring the user-specific "on/off" of the nanopattern-driven functionalities. As one vivid example, we demonstrate that a single light-emitting device can be switched from the focused pointer to the widespread flashlight depending on the RAIN application upon user's purpose.

Silver Nanowire-Based Stretchable Transparent Electrode for Flexible Organic Light-Emitting Diode.

The new stretchable transparent electrode was proposed and fabricated based on a process which utilizes silver nanowires (AgNWs) on a Polyurethane (PU) substrate. In order to overcome the rough surface nature of the silver nanowire electrode, a titanium oxide (TiO2) buffer layer was over-coated and then followed by a heat treatment of organo-metalic sol-gel solution. The fabricated stretchable electrodes exhibit electrical sheet resistance of 24 Ω/□, transmittance of 78% at 550 nm wavelength and average surface roughness of less than 5 nm. In addition, without adding additional conductive polymer layers, the fabricated AgNW-based electrode can maintain its initial electrical resistance even if nearly 130% strain is applied. In this letter, the critical role of a TiO2 buffer layer in achieving the high performance of the AgNW stretchable electrode is discussed.

Highly Conductive PEDOT:PSS Films with 1,3-Dimethyl-2-Imidazolidinone as Transparent Electrodes for Organic Light-Emitting Diodes.

Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT: PSS) films as transparent electrodes for organic light-emitting diodes (OLEDs) are doped with a new solvent 1,3-dimethyl-2-imidazolidinone (DMI) and are optimized using solvent post-treatment. The DMI doped PEDOT: PSS films show significantly enhanced conductivities up to 812.1 S cm(-1) . The sheet resistance of the PEDOT: PSS films doped with DMI is further reduced by various solvent post-treatment. The effect of solvent post-treatment on DMI doped PEDOT: PSS films is investigated and is shown to reduce insulating PSS in the conductive films. The solvent posttreated PEDOT: PSS films are successfully employed as transparent electrodes in white OLEDs. It is shown that the efficiency of OLEDs with the optimized DMI doped PEDOT: PSS films is higher than that of reference OLEDs doped with a conventional solvent (ethylene glycol). The results present that the optimized PEDOT: PSS films with the new solvent of DMI can be a promising transparent electrode for low-cost, efficient ITO-free white OLEDs.

Improvement of phosphor modeling based on the absorption of Stokes shifted light by a phosphor.

We have found that the emission spectrum of phosphors measured in the powder state differs from that measured for a single phosphor. When the emission spectrum of the powder state is adopted in an optical simulation, the simulated optical properties e.g., the correlated color temperature, color rendering index, and chromaticity coordinates, show a remarkable discrepancy from those of the fabricated LED package. However, the discrepancy is significantly improved when the emission spectrum from a low concentration of phosphor in a silicone binder is employed. We suggest that the discrepancy originates from the absorption of Stokes shifted light by a phosphor.

Enhanced and balanced efficiency of white bi-directional organic light-emitting diodes.

We report on the characteristics of enhanced and balanced white-light emission from bi-directional organic light-emitting diodes (BiOLEDs) enabled by the introduction of micro-cavity effects. The insertion of an additional metal layer between the indium tin oxide anode and the hole transporting layer results in similar light output of our BiOLEDs in both top and bottom direction and in reduced distortion of the electroluminescence spectrum. Furthermore, we find that by utilizing MC effects, the overall current efficiency can be improved by 26.2% compared to that of a conventional device.

Digital-mode organic vapor-jet printing (D-OVJP): advanced jet-on-demand control of organic thin-film deposition.

Digital-mode organic vapor-jet printing (D-OVJP) is demonstrated by producing a series of organic vapor jets. D-OVJP not only inherits all the benefits of a conventional OVJP but also provides an advanced, straightforward control over organic deposition with a pixel-to-pixel precision. Digitally-controlled film thickness and high-performance thin-film transistors are demonstrated with D-OVJP, proving its potential applicability to organic electronics and related areas.

Multilayer transparent electrode for organic light-emitting diodes: tuning its optical characteristics.

The optical properties of dielectric-metal-dielectric (DMD) transparent electrodes are investigated from the perspectives of organic light-emitting diodes (OLEDs). A joint experimental and theoretical study showed that the optical characteristics of OLEDs based on DMD electrodes can be widely tuned to fulfill the requirements of a target application through careful control of the microcavity effect, transmittance of DMD electrodes, and a correlation of these two factors with the emission spectra of the emitted materials. Upon variation of the DMD structure, near-Lambertian emission and a 100% improvement in the luminous efficiency are demonstrated, respectively. Optimization strategies are also discussed that are relevant to forward luminous efficiency, total optical power, and angular/ spectral characteristics.