Influence of the average molar mass of poly(N-vinylpyrrolidone) on the dimensions and conductivity of silver nanowires

Improving the performance of Ag nanowire electrodes by adjusting the reaction conditions and the molar mass of PVP.

Nanopatterned graphene on a polymer substrate by a direct peel-off technique

A graphene (Gr) on a polyimide (PI) polymer film (Gr-PI film), obtained by a direct peel-off technique, is proposed and investigated. Thanks to its high transparency, electrical conductivity, mechanical strength, and chemical durability, the Gr-PI film is an ideal substrate for flexible electronic and optoelectronic devices, including transistors, light-emitting diodes, and plasmonic antennas. It is obtained using a straightforward method. After spin coating and curing a PI film on Gr previously grown on Cu, one can separate the Gr-PI film from the Cu foil thanks to the difference in the adhesive energy between the Gr-Cu and Gr-PI interfaces. The resulting Gr-PI film shows an average electrical sheet resistance ranging from 520 to 860 Ω/sq and a very high optical transmission (>90%), which have allowed the demonstration of a transparent heater. The surface morphology of the Gr-PI film follows that of the Cu foil, with the latter maintaining its surface properties and allowing in this way its reuse in subsequent chemical vapor deposition growth. The method can also be applied to patterned Gr, as is demonstrated for nanosize ribbons with a width of a few tens of nanometers.

An indium tin oxide-free polymer solar cell on flexible glass

Future optoelectronic devices and their low-cost roll-to-roll production require mechanically flexible transparent electrodes (TEs) and substrate materials. Indium tin oxide (ITO) is the most widely used TE because of its high optical transmission and low electrical sheet resistance. However, ITO, besides being expensive, has very poor performance under mechanical stress because of its fragile oxide nature. Alternative TE materials have thus been sought. Here we report the development of a multilayer TiO2/Ag/Al-doped ZnO TE structure and an ITO-free polymer solar cell (PSC) incorporating it. Electro-optical performances close to those of ITO can be achieved for the proposed TE and corresponding PSC with an additional advantage in their mechanical flexibility, as demonstrated by the fact that the cell efficiency maintains 94% of its initial value (6.6%) after 400 cycles of bending, with 6 and 3 cm maximum and minimum radii, respectively. Instead of common plastic materials, our work uses a very thin (0.14 mm) flexible glass substrate with several benefits, such as the possibility of high-temperature processes, superior antipermeation properties against oxygen and moisture, and improved film adhesion.

Ultrastable and atomically smooth ultrathin silver films grown on a copper seed layer

An effective method to deposit atomically smooth ultrathin silver (Ag) films by employing a 1 nm copper (Cu) seed layer is reported. The inclusion of the Cu seed layer leads to the deposition of films with extremely low surface roughness (<0.5 nm), while it also reduces the minimum thickness required to obtain a continuous Ag film (percolation thickness) to 3 nm compared to 6 nm without the seed layer. Moreover, the Cu seed layer alters the growth mechanism of the Ag film by providing energetically favorable nucleation sites for the incoming Ag atoms leading to an improved surface morphology and concomitant lower electrical sheet resistance. Optical measurements together with X-ray diffraction and electrical resistivity measurements confirmed that the Ag film undergoes a layer-by-layer growth mode resulting in a smaller grain size. The Cu seeded Ag growth method provides a feasible way to deposit ultrathin Ag films for nanoscale electronic, plasmonic and photonic applications. In addition, as a result of the improved uniformity, the oxidation of the Ag layer is strongly reduced to negligible values.

Graphene as an anti-permeation and protective layer for indium-free transparent electrodes

We show that graphene can be used as a protective layer for transparent electrodes made of materials which would otherwise deteriorate when exposed to the environment. In particular, we investigate aluminum-doped zinc oxides and ultrathin copper films capped with a one-atom graphene layer in damp heat (95% relative humidity and 95 °C) and high temperature (up to 180 °C) conditions. The results clearly indicate that a graphene layer can strongly reduce degradation of the electrodes' electrical, optical properties and surface morphology, thus preserving the functionality of the transparent electrodes. The proposed technique is particularly suitable for flexible optoelectronic devices thanks to the mechanical strength of graphene when subjected to bending.

Nickel as an alternative semitransparent anode to indium tin oxide for polymer LED applications

We report on the possibility of using a thin Ni layer, instead of ITO, as a semitransparent hole-injecting electrode for bottom polymer LEDs. Thin metal layers of Ni were deposited by a sputtering technique and their electrical and optical properties with different deposition times have been investigated. Both square resistance and transmittance were seen to decrease with deposition time (thickness). The films showed a transmittance of around 30-40%, which is quite low compared to the 86% of ITO, while their square resistance was higher than that of ITO. Nevertheless, diodes based on a blue emitting polymer, polyfluorene (PFO), showed the same efficiency for either ITO or thin Ni electrodes, although the Ni transmittance is around 2.5 times lower than the ITO transmittance. Such preliminary results definitively suggest that indium-free organic devices can be achieved.

Widely transparent electrodes based on ultrathin metals

Transparent electrodes made of single-component ultrathin (<10 nm) metal films (UTMFs) are obtained by sputtering deposition. We show that the optical transparency of the deposited films (chromium and nickel) is comparable to that of indium tin oxide (ITO) in the visible and near-infrared range (0.4-2.5 microm), while it can be significantly higher in the ultraviolet (175-400 nm) and mid-infrared (2.5-25 microm) regions. Despite their very small thickness, the deposited UTMFs are also uniform and continuous over the 10 cm substrate, as it is confirmed by the measured low electrical resistivity. The excellent optical and electrical properties, stability, compatibility with active materials, process simplicity, and potential low cost make UTMFs high-quality transparent electrodes for the optoelectronics industry, seriously competing with widely used transparent conductive oxides, such as ITO.