Metal nanogrids, nanowires, and nanofibers for transparent electrodes
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Introduction Transparent electrodes are ubiquitously used in optoelectronic devices where light must pass through a section, and current or voltage needs to be applied. These devices include thin-film solar cells (organic, inorganic, hybrid), displays (liquid-crystal display, plasma display, organic light-emitting diode [OLED]), touch screens (resistive, capacitive), electronic books, smart windows, fl exible lighting, and passive devices (such as transparent heating, transparent speakers).1 The properties of transparent electrodes are extremely important for device performance, and the specifications vary based on types of devices. For example, while source roughness and work function are insignificant for resistive touch screens, they are crucial for OLEDs and organic solar cells.1 Moreover, the optical haze of particular transparent electrodes is beneficial for high-efficiency solar cells but problematic for certain types of displays. The mechanical properties of the electrode are crucial for flexible devices but not important for devices on rigid substrates. Across all devices, however, the optical transparency (T ) and electrical sheet resistance (Rs) are the two most important parameters. Unfortunately, these two parameters are in constant competition with one another. Since Rs values are often reported at different transparencies, it is hard to compare two transparent electrodes directly, and hence a figure of merit (σdc/σop, where σdc is the dc conductivity, and σop is the optical conductivity)
is widely used to evaluate the performance of transparent electrodes.2,3 Depending on the application, Rs can range from 10 Ω/sq to 106 Ω/sq, but typically the transparency needs to be ≥90%. 106 Ω/sq is sufficient for antistatic applications, 400–1000 Ω/sq is sufficient for many touch screen applications, and ≤10 Ω/sq is needed for OLEDs and solar cells.1 Recently, Rowell and McGehee conducted a theoretical study on Rs for thin-film solar cell modules. Their conclusion was that a competitive transparent electrode must have an optical transparency of at least 90% at an Rs of less than 10 Ω/sq for monolithically integrated modules.3 Koishiyev and Sites conducted a similar study on the impact of Rs in 2D modeling of thin-film solar cells.4 Metal busbar fingers conduct current globally, and transparent electrodes conduct current locally. Assuming the current is distributed uniformly across the transparent electrode, the power loss associated with the sheet resistance of the transparent electrode is P = RsL2J 2/3, where P is the power loss, J is the current density, and L is sub-cell length.4 For high-efficiency solar cells, Rs needs to be as low as possible to reduce power loss from transparent electrodes. Over the past eight years, various types of transparent electrodes based on nanoscale materials have emerged. Percolative networks with randomly distributed carbon nanotubes (CNTs) have been extensively studied, and startups such as Unidym
Liangbing Hu, Department of Materials Science and Engineering at the University of Maryland at
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