Transparent electrodes are a necessary component in many modern devices such as touch screens, LCDs, OLEDs, and solar cells, all of which are growing in demand. Traditionally, this role has been well served by doped metal oxides, the most common of which is indium tin oxide, or ITO. Recently, advances in nano-materials research have opened the door for other transparent conductive materials, each with unique properties. These include CNTs, graphene, metal nanowires, and printable metal grids. This review will explore the materials properties of transparent conductors, covering traditional metal oxides and conductive polymers initially, but with a focus on current developments in nano-material coatings. Electronic, optical, and mechanical properties of each material will be discussed, as well as suitability for various applications.

Emerging Transparent Electrodes Based on Thin Films of Carbon Nanotubes, Graphene, and Metallic Nanostructures

On the mechanism of conductivity enhancement in poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) film through solvent treatment

The conductivity of PEDOT:PSS films was significantly enhanced from 0.3 S cm(-1) to 3065 S cm(-1) through a treatment with dilute sulfuric acids. PEDOT:PSS films with a sheet resistance of 39 Ω sq(-1) and transparency of around 80% at 550 nm are obtained. These PEDOT:PSS films with conductivity and transparency comparable to ITO can replace ITO as the transparent electrode of optoelectronic devices.

Solution‐Processed Metallic Conducting Polymer Films as Transparent Electrode of Optoelectronic Devices

Polymers for flexible displays: From material selection to device applications

The conductivity of a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film can be enhanced by more than two orders of magnitude by adding a compound with two or more polar groups, such as ethylene glycol, meso-erythritol (1,2,3,4-tetrahydroxybutane), or 2-nitroenthanol, to an aqueous solution of PEDOT:PSS. The mechanism for this conductivity enhancement is studied, and a new mechanism proposed. Raman spectroscopy indicates an effect of the liquid additive on the chemical structure of the PEDOT chains, which suggests a conformational change of PEDOT chains in the film. Both coil and linear conformations or an expanded-coil conformation of the PEDOT chains may be present in the untreated PEDOT:PSS film, and the linear or expanded-coil conformations may become dominant in the treated PEDOT:PSS film. This conformational change results in the enhancement of charge-carrier mobility in the film and leads to an enhanced conductivity. The high-conductivity PEDOT:PSS film is ideal as an electrode for polymer optoelectronic devices. Polymer light-emitting diodes and photovoltaic cells fabricated using such high-conductivity PEDOT:PSS films as the anode exhibit a high performance, close to that obtained using indium tin oxide as the anode.

High-Conductivity Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) Film and Its Application in Polymer Optoelectronic Devices†

Films fabricated from commercially available poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) aqueous dispersions have been widely used in many electronic and optoelectronic applications. Previous attempts to utilize them as anodes in organic light-emitting diodes (OLEDs) were not satisfactory due to their low conductivity. In this letter we report on the fabrication and characterization of an OLED device made using a highly conductive form of PEDOT:PSS as anode and demonstrate its superior performance relative to that of a similar device using the commercial conducting polymer as an anode. An external electroluminescence quantum efficiency of ∼0.73% was measured at 100 A/m2.

Molecular organic light-emitting diodes using highly conducting polymers as anodes

Doped ZnO thin films as anode materials for organic light-emitting diodes

Zr-doped ZnO (ZZO) thin films have been investigated as an anode material, a potential alternative to indium tin oxide (ITO), for organic light emitting diode (OLED) devices. ZZO films have been deposited on glass substrates by pulsed laser deposition. The electrical and optical properties of these films were studied as a function of substrate temperature and oxygen pressure during deposition. For a 200-nm-thick ZZO film grown at 250 °C in 1 mTorr of oxygen, a resistivity of 5.6×10−4 Ω cm was measured and an average optical transmittance of 84% was measured in the visible range (400–700 nm). The ZZO films, grown at different oxygen pressures, were used as an anode contact for OLED devices. External electroluminescence quantum efficiencies (0.8%–0.9%) comparable to those (0.9%–1.0%) measured for control devices fabricated on commercial ITO anodes were obtained at high current densities (1000 A/m2). These results demonstrate that ZZO is a good anode material. In addition, it is an attractive alternative to ...

Anode material based on Zr-doped ZnO thin films for organic light-emitting diodes

Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) aqueous dispersion (PEDOT:PSS) is a highly conductive polymer. Attempts to utilize them as an anode in an organic light emitting diode have been made but a satisfactory result was not obtained due to low device efficiency caused by the relatively high surface resistance and poor optical transparency. We recently reported that the conductivity of the PEDOT:PSS (Baytron P by Bayer) was dramatically increased without losing the optical transparency by addition of a small amount of a polyalcohol. Here, we present the improved I-V-L characteristics of the molecular organic light emitting diodes fabricated using the highly conductive and transparent PEDOT:PSS as an anode.

Molecular organic light-emitting diodes using highly conductive and transparent polymeric anodes

We report a photoemission study of the interfaces between spin-cast films of a new variation of a polymer blend consisting of poly(3,4-ethylenedioxy-thiophene) (PEDOT) and poly(4-styrenesulfonate) (PSS) and glycerol as an additive, and vacuum-evaporated hole transport layers (HTL) of 4,4'-bis(carbazol-9-yl)biphenyl (CBP),N,N'-diphenyl-N,N'-bis(1-naphthyl)-1-1'biphenyl-4,4'di amine (NPD) and N,N'-diphenyl-N,N'-bis(3methylphenyl)-1,1'-biphenyl-4,4'-dia mine (TPD). The hole injection barrier, as deduced from photoemission spectroscopy, is 0.5 - 0.9 eV at the PEDOT-PSS / HTL interface, which compares very well with the previously reported barrier heights for oxygen plasma -treated indium-tin oxide (ITO)/NPD and ITO/TPD interfaces, and which is, most notably, a factor of two smaller than barriers measured for a PEDOT-PSS/hole-transporting luminescent polymer, e.g. poly(bis-(2-dimethyloctylsilyl)-1,4-phenylvinylene, interface. The measured energy barriers imply a sufficiently efficient charge injection at the studied PEDOT-PSS/HTL interface, which is very encouraging for further development of anode structures based on similar conducting polymer blends and chemically modified structures to be utilized in molecular organic light-emitting device applications.

W. H. Kim

Papers

Molecular organic light-emitting diodes using highly conducting polymers as anodes

Doped ZnO thin films as anode materials for organic light-emitting diodes

Anode material based on Zr-doped ZnO thin films for organic light-emitting diodes

Molecular organic light-emitting diodes using highly conductive and transparent polymeric anodes

Hole injection energetics at highly conducting polymer anode: small molecule interfaces studied with photoemission spectroscopy