High-Performance, Solution-Processed, Embedded Multiscale Metallic Transparent Conductors

Simple Isopropanol-Assisted Direct Soft Imprint Lithography for Residue-Free Microstructure Patterning

A direct soft imprint lithography was proposed to realize the direct fabrication of residue-free, well-shaped functional patterns through a single step. This imprint method requires only a simply prepared isopropanol-treated polydimethylsiloxane (PDMS) stamp without any additional resists. Residue-free Ag patterns were successfully fabricated on different substrates by directly imprinting the Ag ink with the isopropanol-treated PDMS stamp. Furthermore, the coffee-ring effect of the imprinting Ag patterns can be eliminated by optimizing the imprinting time, isopropanol-treating time, and imprinting temperatures. Studies show that the residual Ag ink in the contact region can be absorbed by the isopropanol-treated PDMS stamp due to the "like dissolves like" principle. Finally, this method was employed to fabricate the Ag electrodes for the thin-film transistors, attaining a mobility of ∼8 cm2 V-1 s-1, which is comparable to those with vacuum-processed electrodes. This process provides a simple, low-cost, residue-free, coffee-ring-free, and fast patterning method in the field of microelectronics.

Effective film surface treatment for improving external quantum efficiency of photomultiplication type organic photodetector

A simple yet effective method based on methanol treatment is proposed to enhance the external quantum efficiency (EQE) of the photomultiplication type organic photodetector with a structure of Glass/ITO/PEDOT:PSS/P3H:PC71BM (100:1, wt./wt.)/Al. By modifying the PEDOT:PSS film surface with methanol, the EQE of photodetector significantly improved within a broad wavelength range of 300–700 nm. The maximum EQE of 25300% occurs at the wavelength of 350 nm in the methanol-treated device under −9 V bias, which more than doubles that (11500%) of the device without treatment. In addition, as a result of the methanol treatment, the detectivity of the device improved from 3.72 × 1012 to 7.24 × 1012 Jones at −9 V under 350 nm light illumination. The large improvement is attributed to the fact that the methanol treatment can improve the electrical performance of the PEDOT:PSS by removing the insulator PSS within the film and also result in PC71BM aggregations in the active layer. The latter can enhance the tunneling hole injection by the intensified energy-level bending, which is induced by both the trapped electrons in these aggregations and accumulated ones near Al electrode. As a result, the modification of both the PEDOT:PSS layer and the active layer increases the response current, resulting in the EQE improvement.

Selective multi-nanosoldering for fabrication of advanced solution-processed micro/nanoscale metal grid structures

Solution-processed metal grid transparent conductors with low sheet resistance, high optical transmittance and good mechanical flexibility have great potential for use in flexible optoelectronic devices. However, there are still remaining challenges to improve optoelectrical properties and electromechanical stability of the metallic structures due to random loose packings of nanoparticles and the existence of many pores. Here we introduce a selective multi-nanosoldering method to generate robust metallic layers on the thin metal grid structures (< a thickness of 200 nm), which are generated via self-pining assisted direct inking of silver ions. The selective multi-nanosoldering leads to lowering the sheet resistance of the metal grid transparent conductors, while keeping the optical transmittance constant. Also, it reinforces the electromechanical stability of flexible metal grid transparent conductors against a small bending radius or a repeated loading. Finally, organic light-emitting diodes based on the flexible metal grid transparent conductors are demonstrated. Our approach can open a new route to enhance the functionality of metallic structures fabricated using a variety of solution-processed metal patterning methods for next-generation optoelectronic and micro/nanoelectronic applications.

High Brightness Organic Light-Emitting Diodes with Capillary-Welded Hybrid Diameter Silver Nanowire/Graphene Layers as Electrodes

The development of silver nanowire electrodes is always limited due to some disadvantages, such as roughness, oxidative properties, and other disadvantages. In this research, a capillary-welded silver nanowire/graphene composite film was used as an electrode for organic light-emitting diode (OLED) devices. As an encapsulation layer, graphene reduced the surface roughness and the oxidation probability of silver nanowires. The composite electrode showed an excellent transmittance of 91.5% with low sheet resistant of 26.4 ohm/sq. The devices with the silver nanowire/graphene composite electrode emitted green electroluminescence at 516 nm, and the turn-on voltage was about 3.8 V. The maximum brightness was 50810 cd/cm², which is higher than the indium tin oxide-based (ITO-based) devices with the same configuration. Finally, it was proved that the silver nanowire/graphene composite electrodes possessed better heat dissipation than the ITO-based ones under energization. In summary, it means that this novel silver nanowires/graphene electrode has great potential in OLED device applications.

Controllable assembly of a hierarchical multiscale architecture based on silver nanoparticle grids/nanowires for flexible organic solar cells

In this work, an effective and facile strategy was developed to assemble a flexible hierarchical multiscale architecture by incorporating microscale silver nanoparticles (AgNPs) grids into random silver nanowires (AgNWs) networks combined with a room-temperature chemical sintering mechanism. The microscale AgNPs grids was fabricated by assemble AgNPs into a series of twin lines directly on a hydrophilic PET substrate based on coffee-ring effect with ink-jet printing technique. By regulating the assembly architecture, a flexible hierarchical multiscale conductor based on AgNPs grids/AgNWs was successfully fabricated and demonstrated a high transmittance of 87.5%, low sheet resistance of 16.5 Ω/sq and excellent mechanical flexibility. The hierarchical multiscale architecture was fairly favorable to efficiently collect free charges among the gaps in the AgNWs network, as well as to enhance the stability of conductivity by creating continuous conduction pathways. As an anode electrode in a flexible organic solar cell, the hierarchical multiscale AgNPs grids/AgNWs conductor demonstrated a more power photoelectric conversion efficiency, which was even superior to the corresponding properties of the ITO network at a similar transmittance. This simple, low-cost and nonlithographic solution-based approach would further enhance current fabrication approaches to create patterned microstructures, and have great potential to fabricate multifarious functional patterns in flexible electronic devices.

Emerging Novel Metal Electrodes for Photovoltaic Applications

Emerging novel metal electrodes not only serve as the collector of free charge carriers, but also function as light trapping designs in photovoltaics. As a potential alternative to commercial indium tin oxide, transparent electrodes composed of metal nanowire, metal mesh, and ultrathin metal film are intensively investigated and developed for achieving high optical transmittance and electrical conductivity. Moreover, light trapping designs via patterning of the back thick metal electrode into different nanostructures, which can deliver a considerable efficiency improvement of photovoltaic devices, contribute by the plasmon-enhanced light-mattering interactions. Therefore, here the recent works of metal-based transparent electrodes and patterned back electrodes in photovoltaics are reviewed, which may push the future development of this exciting field.

Temperature-Controlled Direct Imprinting of Ag Ionic Ink: Flexible Metal Grid Transparent Conductors with Enhanced Electromechanical Durability

Next-generation transparent conductors (TCs) require excellent electromechanical durability under mechanical deformations as well as high electrical conductivity and transparency. Here we introduce a method for the fabrication of highly conductive, low-porosity, flexible metal grid TCs via temperature-controlled direct imprinting (TCDI) of Ag ionic ink. The TCDI technique based on two-step heating is capable of not only stably capturing the Ag ionic ink, but also reducing the porosity of thermally decomposed Ag nanoparticle structures by eliminating large amounts of organic complexes. The porosity reduction of metal grid TCs on a glass substrate leads to a significant decrease of the sheet resistance from 21.5 to 5.5 Ω sq⁻¹ with an optical transmittance of 91% at λ = 550 nm. The low-porosity metal grid TCs are effectively embedded to uniform, thin and transparent polymer films with negligible resistance changes from the glass substrate having strong interfacial fracture energy (~8.2 J m⁻²). Finally, as the porosity decreases, the flexible metal grid TCs show a significantly enhanced electromechanical durability under bending stresses. Organic light‐emitting diodes based on the flexible metal grid TCs as anode electrodes are demonstrated.

Silver nanowire-graphene hybrid transparent conductive electrodes for highly efficient inverted organic solar cells

Silver nanowires (AgNWs) and graphene are both promising candidates as a transparent conductive electrode (TCE) to replace expensive and fragile indium tin oxide (ITO) TCE. A synergistically optimized performance is expected when the advantages of AgNWs and graphene are combined. In this paper, the AgNW-graphene hybrid electrode is constructed by depositing a graphene layer on top of the network of AgNWs. Compared with the pristine AgNWs electrode, the AgNW-graphene TCE exhibits reduced sheet resistance, lower surface roughness, excellent long-term stability, and corrosion resistance in corrosive liquids. The graphene layer covering the AgNWs provides additional conduction pathways for electron transport and collection by the electrode. Benefiting from these advantages of the hybrid electrodes, we achieve a power conversion efficiency of 8.12% of inverted organic solar cells using PTB7:PC₇₁BM as the active layer, which is compared to that of the solar cells based on standard ITO TCE but about 10% higher than that based on AgNWs TCE.

Flexible and Highly Photosensitive Electrolyte-Gated Organic Transistors with Ionogel/Silver Nanowire Membranes

Flexible and low-voltage photosensors with high near-infrared (NIR) sensitivity are critical for realization of interacting humans with robots and environments by thermal imaging or night vision techniques. In this work, we for the first time develop an easy and cost-effective process to fabricate flexible and ultrathin electrolyte-gated organic phototransistors (EGOPTs) with high transparent nanocomposite membranes of high-conductivity silver nanowire (AgNW) networks and large-capacitance iontronic films. A high responsivity of 1.5 × 10³ A·W^1-, high sensitivity of 7.5 × 10⁵, and 3 dB bandwidth of ∼100 Hz can be achieved at very low operational voltages. Experimental studies in temporal photoresponse characteristics reveal the device has a shorter photoresponse time at lower light intensity since strong interactions between photoexcited hole carriers and anions induce extra long-lived trap states. The devices, benefiting from fast and air-stable operations, provide the possibility of the organic photosensors for constructing cost-effective and smart optoelectronic systems in the future.