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.

With the aim of preparing a high performance conductive ink, we sought to control the surface chemistry of Cu nanoparticles so as to minimize surface oxidation. Specifically, the surface oxide layer on Cu nanoparticles synthesized in ambient atmosphere was minimized by adjusting the molecular weight of poly(N‐vinylpyrrolidone) capping molecules, as confirmed by high resolution transmission electron microscopy and X‐ray photoelectron spectroscopy analyses. In addition, we demonstrate that by minimizing the thickness of the surface oxide layer, Cu granular films with good conductivity could be obtained by sintering nanoparticle assembles. Finally, we fabricated highly conductive Cu patterns on a plastic substrate by ink‐jet printing.

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

Oxide semiconductors afford a promising alternative to organic semiconductors and amorphous silicon materials in applications requiring transparent thin film transistors (TFTs). We synthesized an aqueous inorganic precursor by a direct dissolution of zinc hydroxide in ammonium hydroxide solution from which a dense and uniform ZnO semiconducting layer is achieved. Solution-processed ZnO-TFTs prepared at 140 C by microwave irradiation have shown enhanced device characteristics of $1.7 cm 2 V À1 s À1 mobility and a $10 7 on/off current ratio, with good air stability. Spectroscopic analyses confirmed that such a device improvement originates from accelerated dehydroxylation and better crystallization at low temperature by microwave irradiation. Our results suggest that solutionprocessable oxide semiconductors have potential for low-temperature and high-performance applications in transparent devices.