We present an indium-free transparent conducting composite electrode composed of silver nanowires (AgNWs) and ZnO bilayers. The AgNWs form a random percolating network embedded between the ZnO layers. The unique structural features of our ZnO/AgNW/ZnO multilayered composite allow for a novel transparent conducting electrode with unprecedented excellent thermal stability (∼375 °C), adhesiveness, and flexibility as well as high electrical conductivity (∼8.0 Ω/sq) and good optical transparency (>91% at 550 nm). Cu(In,Ga)(S,Se)₂ (CIGSSe) thin film solar cells incorporating this composite electrode exhibited a 20% increase of the power conversion efficiency compared to a conventional sputtered indium tin oxide-based CIGSSe solar cell. The ZnO/AgNW/ZnO composite structure enables effective light transmission and current collection as well as a reduced leakage current, all of which lead to better cell performance.

Fully solution‐processed Al‐doped ZnO/silver nanowire (AgNW)/Al‐doped ZnO/ZnO multi‐stacked composite electrodes are introduced as a transparent, conductive window layer for thin‐film solar cells. Unlike conventional sol–gel synthetic pathways, a newly developed combustion reaction‐based sol–gel chemical approach allows dense and uniform composite electrodes at temperatures as low as 200 °C. The resulting composite layer exhibits high transmittance (93.4% at 550 nm) and low sheet resistance (11.3 Ω sq‐1), which are far superior to those of other solution‐processed transparent electrodes and are comparable to their sputtered counterparts. Conductive atomic force microscopy reveals that the multi‐stacked metal‐oxide layers embedded with the AgNWs enhance the photocarrier collection efficiency by broadening the lateral conduction range. This as‐developed composite electrode is successfully applied in Cu(In1‐x,Gax)S2 (CIGS) thin‐film solar cells and exhibits a power conversion efficiency of 11.03%. The fully solution‐processed indium‐free composite films demonstrate not only good performance as transparent electrodes but also the potential for applications in various optoelectronic and photovoltaic devices as a cost‐effective and sustainable alternative electrode.

Copper nanowire (CuNW)-network film is a promising alternative to the conventional indium tin oxide (ITO) as a transparent conductor. However, thermal instability and the ease of oxidation hinder the practical applications of CuNW films. We present oxidation-resistive CuNW-based composite electrodes that are highly transparent, conductive and flexible. Lactic acid treatment effectively removes both the organic capping molecule and the surface oxide/hydroxide from the CuNWs, allowing direct contact between the nanowires. This chemical approach enables the fabrication of transparent electrodes with excellent properties (19.8 X sq À1 and 88.7% at 550 nm) at room temperature without any atmospheric control. Furthermore, the embedded structure of CuNWs with Al-doped ZnO (AZO) dramatically improves the thermal stability and oxidation resistance of CuNWs. These AZO/CuNW/AZO composite electrodes exhibit high transparency (83.9% at 550 nm) and low sheet resistance (35.9 X sq À1 ), maintaining these properties even with a bending number of 1280 under a bending radius of 2.5 mm. When implemented in a Cu(In 1 Àx ,Gax)(S,Se) 2 thin-film solar cell, this composite electrode demonstrated substantial potential as a low-cost (Ag-, In-free), high performance transparent electrode, comparable to a conventional sputtered ITO-based solar cell. NPG Asia Materials (2014) 6, e105; doi:10.1038/am.2014.36; published online 13 June 2014Keywords: chemical reduction treatment; copper nanowire; indium-free transparent electrodes; photovoltaic; thermal oxidation resistance INTRODUCTION Transparent conductive materials are a crucial, basic element in the realization of various optoelectronic devices, such as flat panel displays, touch screens, organic light emitting diodes and thin-film solar cells. 1-3 Indium tin oxide (ITO) has been the most widely explored material, because of its excellent transparency (B90% at 550 nm) and low sheet resistance (B20 O sq À1 ). However, with the rising demand for transparent electrodes, the development of cost-effective alternatives to ITO has been of great importance. The quest for alternative transparent conductors, including conducting polymers, carbon nanotubes, graphenes and nanostructured electrodes, has been a topic of active research for the last decade. 4 Among these materials, metal-nanowire network films are thought to be a substantial candidate. The metallic nanowire film is capable of providing high transparency and conductivity by virtue of its characteristic structure, which consists of a percolated random network of nanowires. [5][6][7][8][9] Unlike the brittle metal oxide skeletonbased ITO, metal-nanowire films are easily applicable to bendable, wearable active devices, because of the inherently flexible nature of metals. A high aspect ratio is a critical factor for high transparency and conductivity, but metal nanowires possessing this quality suffer

Recent advances in unconventional foldable and stretchable electronics have forged a new field in electronics. However, traditional conducting metal oxides and metal thin films are inappropriate as electrodes for stretchable devices because they are vulnerable to tensile strain as well as bending strain. In this study, we describe the fabrication of annealing-free, copper nanowire (CuNW)-based stretchable electrodes using an inexpensive metal source through a simple and scalable process at low temperature without a vacuum. We also introduce a reversible and extremely stretchable (up to 700% of strain) helical, CuNW-based conducting spring, which has not been previously used for stretchable electrodes. NPG Asia Materials (2014) 6, e132; doi:10.1038/am.2014.88; published online 26 September 2014 INTRODUCTION Recent advances in unconventional foldable and stretchable electronics show great potential for future wearable applications, including smart skins, electronic eye-type imagers, skin-like pressure sensors, electronic textiles and muscle-like soft actuators. 1-5 Most of these applications require sufficient elasticity for bending, stretching, twisting and deformation into complex, non-planar shapes while maintaining good electrical properties and reliability. Stretchability is the most crucial property for the development of next-generation wearable devices. To date, two primary strategies have been suggested for the fabrication of stretchable electronic parts apart from complex lithography-based approaches that can shape inorganic conductive materials or metals into buckled geometries, including island-bridge systems. [6][7][8][9] The first strategy is the inclusion of micro-or nano-scale conductive carbon materials into elastic polymer matrices. 1,10-13 For example, carbon-based materials, such as carbon nanotubes and graphene, have been extensively researched for stretchable/foldable conductors. Ma et al. 13 fabricated helical ribbon-structured composites composed of carbon nanotubes and Ag flakes using a shape-memory polymer, demonstrating stable resistivity up to a strain of 600%. However, the application of these materials in large-area, integrated devices may be restricted by their relatively poor electrical properties and their high cost. The second strategy is to use highly conductive metallic nanostructures, such as rectangular-patterned gold nanosheets 14 (strain (ε) = 100% on Ecoflex substrate) or silver nanoparticles embedded in composite materials 15 (ε = 140% on elastomeric rubber fiber).Recently, networks of one-dimensional metal nanowires, accompanied by a simple, scalable process (for example, solution-phase

Effective insertion of vertically aligned nanowires (NWs) into cells is critical for bioelectrical and biochemical devices, biological delivery systems, and photosynthetic bioenergy harvesting. However, accurate insertion of NWs into living cells using scalable processes has not yet been achieved. Here, NWs are inserted into living Chlamydomonas reinhardtii cells (Chlamy cells) via inkjet printing of the Chlamy cells, representing a low-cost and large-scale method for inserting NWs into living cells. Jetting conditions and printable bioink composed of living Chlamy cells are optimized to achieve stable jetting and precise ink deposition of bioink for indentation of NWs into Chlamy cells. Fluorescence confocal microscopy is used to verify the viability of Chlamy cells after inkjet printing. Simple mechanical considerations of the cell membrane and droplet kinetics are developed to control the jetting force to allow penetration of the NWs into cells. The results suggest that inkjet printing is an effective, controllable tool for stable insertion of NWs into cells with economic and scale-related advantages.

A composite transparent composite electrode using silver nanowire (AgNW) with sol–gel derived ZnO and AZO is proposed by J. Moon and co‐workers with careful analysis of the electrical conduction behavior using conductive‐atomic force microscopy. On page 2462, low temperature (≈200 °C) processes are achieved by a combustion reaction based sol‐gel method, demonstrating a comparable performance to the sputtered ITO based device.

Effective insertion of nanowires into cells is critical for monitoring subcellular activities, bio‐electrical devices, and photosynthetic bio‐energy harvesting. On page 1446, W. Ryu, J. Moon and co‐workers demonstrate the possibility of direct insertion of nanowires into living algal cells using inkjet printing technology. While most cell printing technologies aim to print cells on a flat substrate, this work investigates if living cells could be printed for insertion by nanowires at predetermined locations by control of the falling velocity of the bio‐ink droplet.