All-Solution-Processed Thermally and Chemically Stable Copper-Nickel Core-Shell Nanowire-Based Composite Window Electrodes for Perovskite Solar Cells

Metal nanowire-based transparent electrode for flexible and stretchable optoelectronic devices

High-quality transparent electrodes are indispensable components of flexible optoelectronic devices as they guarantee sufficient light transparency and electrical conductivity. Compared to commercial indium tin oxide, metal nanowires are considered ideal candidates as flexible transparent electrodes (FTEs) owing to their superior optoelectronic properties, excellent mechanical flexibility, solution treatability, and higher compatibility with semiconductors. However, certain key challenges associated with material preparation and device fabrication remain for the practical application of metal nanowire-based electrodes. In this review, we discuss state-of-the-art solution-processed metal nanowire-based FTEs and their applications in flexible and stretchable optoelectronic devices. Specifically, the important properties of FTEs and a cost-benefit analysis of existing technologies are introduced, followed by a summary of the synthesis strategy, key properties, and fabrication technologies of the nanowires. Subsequently, we explore the applications of metal-nanowire-based FTEs in different optoelectronic devices including solar cells, photodetectors, and light-emitting diodes. Finally, the current status, future challenges, and emerging strategies in this field are presented.

Solid-State Reaction Synthesis of Nanoscale Materials: Strategies and Applications

Nanomaterials (NMs) with unique structures and compositions can give rise to exotic physicochemical properties and applications. Despite the advancement in solution-based methods, scalable access to a wide range of crystal phases and intricate compositions is still challenging. Solid-state reaction (SSR) syntheses have high potential owing to their flexibility toward multielemental phases under feasibly high temperatures and solvent-free conditions as well as their scalability and simplicity. Controlling the nanoscale features through SSRs demands a strategic nanospace-confinement approach due to the risk of heat-induced reshaping and sintering. Here, we describe advanced SSR strategies for NM synthesis, focusing on mechanistic insights, novel nanoscale phenomena, and underlying principles using a series of examples under different categories. After introducing the history of classical SSRs, key theories, and definitions central to the topic, we categorize various modern SSR strategies based on the surrounding solid-state media used for nanostructure growth, conversion, and migration under nanospace or dimensional confinement. This comprehensive review will advance the quest for new materials design, synthesis, and applications.

Advances in Flexible Metallic Transparent Electrodes

Transparent electrodes (TEs) are pivotal components in many modern devices such as solar cells, light-emitting diodes, touch screens, wearable electronic devices, smart windows, and transparent heaters. Recently, the high demand for flexibility and low cost in TEs requires a new class of transparent conductive materials (TCMs), serving as substitutes for the conventional indium tin oxide (ITO). So far, ITO has been the most used TCM despite its brittleness and high cost. Among the different emerging alternative materials to ITO, metallic nanomaterials have received much interest due to their remarkable optical-electrical properties, low cost, ease of manufacturing, flexibility, and widespread applicability. These involve metal grids, thin oxide/metal/oxide multilayers, metal nanowire percolating networks, or nanocomposites based on metallic nanostructures. In this review, a comparison between TCMs based on metallic nanomaterials and other TCM technologies is discussed. Next, the different types of metal-based TCMs developed so far and the fabrication technologies used are presented. Then, the challenges that these TCMs face toward integration in functional devices are discussed. Finally, the various fields in which metal-based TCMs have been successfully applied, as well as emerging and potential applications, are summarized.

Silver Nanowire Networks: Ways to Enhance Their Physical Properties and Stability

Silver nanowire (AgNW) networks have been intensively investigated in recent years. Thanks to their attractive physical properties in terms of optical transparency and electrical conductivity, as well as their mechanical performance, AgNW networks are promising transparent electrodes (TE) for several devices, such as solar cells, transparent heaters, touch screens or light-emitting devices. However, morphological instabilities, low adhesion to the substrate, surface roughness and ageing issues may limit their broader use and need to be tackled for a successful performance and long working lifetime. The aim of the present work is to highlight efficient strategies to optimize the physical properties of AgNW networks. In order to situate our work in relation to existing literature, we briefly reported recent studies which investigated physical properties of AgNW networks. First, we investigated the optimization of optical transparency and electrical conductivity by comparing two types of AgNWs with different morphologies, including PVP layer and AgNW dimensions. In addition, their response to thermal treatment was deeply investigated. Then, zinc oxide (ZnO) and tin oxide (SnO2) protective films deposited by Atmospheric Pressure Spatial Atomic Layer Deposition (AP-SALD) were compared for one type of AgNW. We clearly demonstrated that coating AgNW networks with these thin oxide layers is an efficient approach to enhance the morphological stability of AgNWs when subjected to thermal stress. Finally, we discussed the main future challenges linked with AgNW networks optimization processes.

Highly Conductive, Flexible, and Oxidation-Resistant Cu-Ni Electrodes Produced from Hybrid Inks at Low Temperatures

Recently, Ni and Ni-Cu nanoparticle-based inks have gained considerable research interest because of their high corrosion resistance as conductors in electronic devices. However, reported inks based on Cu-Ni nanoparticles need to be sintered at high temperatures above 300 °C to obtain electrodes with high conductivity on the order of 10^-5 Ω·cm. This study proposes a new conductive Cu-Ni-based hybrid ink that could be sintered at only 150-180 °C for producing Cu-Ni electrodes with low electrical resistance, high oxidation resistance, and flexibility. The hybrid ink contains Cu flakes and a complex of nickel formate and 1-amino-2-propanol (NiF-AmIP complex). At 150-180 °C, the Cu flakes catalyze the self-reduction of the NiF-AmIP complex, and Cu-Ni electrodes with high conductivity (on the order of 10^-5 Ω·cm) are formed on flexible polymer substrates at temperatures exceeding 150 °C. Analysis indicates that metallic Ni was decorated on the Cu flakes (especially on the edge) to improve the electrode's conductivity, oxidation resistance, and flexibility by forming bridging interconnections between the Cu flakes. The Cu-Ni electrodes demonstrated high stability against oxidation up to approximately 400 °C in air, as well as at 80 °C and 80% RH after 7 days. In addition to the excellent oxidation stability, the Cu-Ni electrode showed high durability under mechanical bending stress. Such sintered Cu-Ni electrodes obtained from hybrid inks have great potentials in printed/flexible devices due to their oxidation resistance and cost-effectiveness.

Energy Level-Graded Al-Doped ZnO Protection Layers for Copper Nanowire-Based Window Electrodes for Efficient Flexible Perovskite Solar Cells

Flexible perovskite solar cells (PSCs) have attracted significant interest as promising candidates for portable and wearable devices. Copper nanowires (CuNWs) are promising candidates for transparent conductive electrodes for flexible PSCs because of their excellent conductivity, flexibility, and cost-effectiveness. However, because of the thermal/chemical instability of CuNWs, they require a protective layer for application in PSCs. Previous PSCs with CuNW-based electrodes generally exhibited poor performances compared with their indium tin oxide-based counterparts because of the neglect of the interfacial energetics between the electron transport layer (ETL) and CuNWs. Herein, an Al-doped ZnO (AZO) protective layer fabricated using atomic layer deposition is introduced. The AZO/CuNW-based composite electrode exhibits improved thermal/chemical stability and favorable band alignment between the ETL and CuNWs, based on the Al dopant concentration tuning. As a result, the Al content gradient AZO (g-AZO), composed of three successively deposited AZO layers, leads to highly efficient flexible PSCs with a power conversion efficiency (PCE) of 14.18%, whereas the PCE of PSCs with a non-g-AZO layer is 12.34%. This improvement can be attributed to the efficient electron extraction and reduced charge recombination. Furthermore, flexible PSCs based on g-AZO-based composite electrodes retain their initial PCE, even after 600 bending cycles, demonstrating excellent mechanical stability.

Improved Stability of Metal Nanowires via Electron Beam Irradiation Induced Surface Passivation

Suppressing the corrosion of nanoscaled metal materials is a critical issue for various devices. Herein, we demonstrate the electron beam irradiation can be a simple and efficient method to realize silver/copper nanowires protection by transforming the original organic capping agents into dense carbonaceous shells. Single nanowire tests prove the significant stability improvement from 4 days to 20 days for silver nanowire and from 20 h to at least 1 week for copper nanowire. The comprehensive advantages such as solution/pollution-free and continuous process with high precision offer this method substantial potential applications in bottom-up assembled electronic and optoelectronic devices.

Enhanced Silver Nanowire Composite Window Electrode Protected by Large Size Graphene Oxide Sheets for Perovskite Solar Cells

Despite the outstanding features of high transmittance and low sheet resistance from silver nanowire (Ag NW) based transparent electrodes, their applications in perovskite solar cells (PVSCs) as window electrodes encounter significant obstacles due to the stability issue brought by the corrosion of halogen species from perovskite layer. In this study, we used large size graphene oxide (LGO) sheets as the protective barrier for bottom Ag NW nano-network. Contributed by the LGO with average size of 60 μm, less GO sheet was necessary for forming the fully covered protective barrier with fewer cracks, which consequently improved the optical transparency and anticorrosive ability of the composite electrode compared to the one from relatively small size GO. Our experiments demonstrated the composite electrode of Ag NW/LGO. The glass substrate exhibited transmittance of 83.8% and 81.8% at 550 nm before and after partial reduction, which maintained 98.4% and 95.1% average transmittance (AVT) of the pristine Ag NW electrode. Meanwhile, we utilized the steady hot airflow to assist the fast solvent evaporation and the uniform GO film formation on Ag NW electrode. Before the application of composite electrode in organic-inorganic hybrid perovskite solar cells, the operational stability of composite electrodes from different sizes of GO with perovskite film fabricated on top were characterized under continuing external bias and light irradiation. Experimental results indicate that the Ag NW electrode protected by LGO could maintain original resistance for more than 45 h. Finally, the PVSC fabricated on Ag NW/LGO based composite electrode yielded a power conversion efficiency (PCE) of 9.62%, i.e., nearly 85% of that of the reference device fabricated on the commercial indium-tin oxide (ITO) glass. Our proposed low temperature and solution processed bottom electrode with improved optical transparency and operational stability can serve as the very beginning layer of optoelectronic devices, to promote the development of low cost and large area fabrication perovskite solar cells.