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

This article summarises the industrial applications of poly-(3,4-ethylenedioxythiophene)
				(PEDT, PEDOT). The basic chemical and physical properties of PEDT are discussed to outline the fundamentals which lead to PEDT as a highly valuable electric and electronic material. PEDT applications are reviewed depending on the two different ways of preparation: in situ polymerisation of the monomer, usually carried out by the user, and applying the prefabricated polymer in the form of its water-based complex with poly(styrene sulfonic acid). Several applications like antistatic coatings, cathodes in capacitors, through-hole plating, OLED's, OFET's, photovoltaics, and electrochromic applications are discussed in detail.

Scientific importance, properties and growing applications of poly(3,4-ethylenedioxythiophene)

A number of materials have been explored for their use as artificial muscles Among these, dielectric elastomers (DEs) appear to provide the best combination of properties for true muscle-like actuation DEs behave as compliant capacitors, expanding in area and shrinking in thickness when a voltage is applied Materials combining very high energy densities, strains, and efficiencies have been known for some time To date, however, the widespread adoption of DEs has been hindered by premature breakdown and the requirement for high voltages and bulky support frames Recent advances seem poised to remove these restrictions and allow for the production of highly reliable, high-performance transducers for artificial muscle applications

Advances in dielectric elastomers for actuators and artificial muscles.

Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand-alone electrodes for organic solar cells have been optimized using a solvent post-treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm−1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO-free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C60 bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre-heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post-treatment can be a very promising electrode material for highly efficient flexible ITO-free organic solar cells.

Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post-Treatment for ITO-Free Organic Solar Cells

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

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

A detailed investigation of the processing parameters influencing the oxidative polymerization of 3,4-ethylenedioxythiophene (EDOT) and a methanol-substituted derivative (EDOT-CH2OH) was performed with the goal of maximizing the conductivity of the polymer. We show that the conductivity can be significantly enhanced by varying the monomer, oxidant (iron(III) p-toluenesulfonate (Fe(OTs)3)), weak base (imidazole (Im)), solvent (various alcohols), and solution concentrations. The effect of each variable on the final materials properties is investigated, and the parameters have been optimized to achieve conductivities as high as 900 S cm−1. Surface resistance below 150 Ω/□ for 80-90 nm thick films with visible-spectrum transparency exceeding 80 % is achieved. The combination of these properties makes the films highly suitable for numerous device applications.

Towards a Transparent, Highly Conductive Poly(3,4-ethylenedioxythiophene)

We report the synthesis and physical studies of a liquid crystalline elastomer fiber consisting of two side-chain liquid crystalline acrylates and a nonmesogennic comonomer side group that acts as a reactive site for cross-linking. The terpolymer was synthesized by radical polymerization, and the cross-linking of the network was achieved by using a diisocyanate unit. The fiber formed shows good liquid crystal alignment texture under a cross-polarizer microscope. Thermoelastic response shows strain changes through the nematic−isotropic phase transition of about 30−35%. A retractive force of nearly 300 kPa was measured in the isotropic phase. Static work loop studies show the viscoelastic losses in these materials to be very small. We also present preliminary studies on the effect of doping carbon nanotubes on the induced strain at the nematic−isotropic transition.

Nematic Elastomer Fiber Actuator

A photoemission study of the interface between spin-cast films of a conducting polymer blend consisting of poly(3,4-ethylenedioxythiophene) (PEDOT), poly(4-styrenesulfonate) (PSS) and glycerol as an additive, and vacuum-evaporated hole transport layers (HTL) of 4,4′-bis(carbazol-9-yl)biphenyl, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1-1′biphenyl-4,4′-diamine and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′- biphenyl-4,4′-diamine reveals a hole injection barrier between 0.5 and 0.9 eV at the glycerol-modified PEDOT-PSS/HTL interface. The measured energy barriers imply a reasonable charge injection, which is very encouraging for further development of the novel anode structures based on a conducting polymer/small molecule interface to be utilized in electro-optic applications such as organic light-emitting devices.

Nikolay Nikolov

Papers

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

Towards a Transparent, Highly Conductive Poly(3,4-ethylenedioxythiophene)

Nematic Elastomer Fiber Actuator

Hole injection barriers at polymer anode/small molecule interfaces

Hydroxylated secondary dopants for surface resistance enhancement in transparent poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) thin films