Transparent conductors are essential elements in an array of optoelectronic devices. The most commonly used transparent conductor – indium tin oxide (ITO) suffers from issues including poor mechanical flexibility, rising cost, and the need for annealing to achieve high conductivity. Consequently, there has been intensive research effort in developing ITO‐free transparent conductors over the recent years. This article gives a comprehensive review on the development of an important ITO‐free transparent conductor, that is based on thin metal films. It starts with the background knowledge of material selection for thin‐metal‐film‐based transparent conductors and then surveys various techniques to fabricate high‐quality thin metal films. Then, it introduces the spectroscopic ellipsometry method for characterizing thin metal films with high accuracy, and discusses the optical design procedure for optimizing transmittance through thin‐metal‐film‐based conductors. The review also summarizes diverse applications of thin‐metal‐film‐based transparent conductors, ranging from solar cells and organic light emitting diodes, to optical spectrum filters, low‐emissivity windows, and transparent electromagnetic interference coatings.

Understanding of ultrathin metal film's electrical and optical properties at sub‐10 nm thickness may provide important engineering insight on its application as a transparent conductor. Here, a rapid change is observed in the ultrathin metal film's electrical and optical scaling properties as the thickness shrinks to below a certain critical thickness dc. Below this thickness, the metal film's electrical properties are shown to be strongly influenced by the inhomogeneity of the film which can be modeled via general effective media theory by incorporating size‐effect contribution. As a result, below dc, carrier's scattering time rapidly decreases with a reduced mean free path leading to a rapid rise in resistivity. Also, the film's optical loss increases while the optical transmission plateaus below dc. As one promising application of thin metal film is transparent conductor where the film's electrical and optical properties are equally important, its maximum theoretical figure‐of‐merit is shown, which is determined at this dc serving as an important engineering metric.

Conventional waveguide mode decoupling methods for organic light-emitting diodes (OLEDs) are typically not scalable and increase fabrication complexity/cost. Indium-tin-oxide–free transparent anode technologies showed efficiency improvement without affecting other device properties. However, previous works lack rigorous analysis to understand the efficiency improvement. Here, we introduced an ultrathin silver (Ag) film as transparent electrode and conducted systematic modal analysis of OLEDs and report that waveguide mode can be completely eliminated by designing an OLED structure that is below the cutoff thickness of waveguide modes. We also experimentally verified the waveguide mode removal in organic waveguides with the help of index-matching fluid and prism. The negative permittivity, extremely thin thickness (~5 nanometers), and highly conductive properties achieved by a uniform copper-seeded Ag film can suppress waveguide mode formation, enhancing external quantum efficiency without compromising any other characteristics of OLEDs, which paves the way for cost-effective high-efficiency OLEDs in current display industry.

We have demonstrated a well-ordered In-rich single crystalline InGaN nanoridge array grown on GaN/sapphire substrate using the integration of top-down etching and bottom-up molecular beam epitaxy. During the initial growth of InGaN on a patterned GaN/sapphire substrate, a (101̅1) r-plane predominantly forms, suppressing the growth in [101̅1] crystal direction and resulting in a triangular InGaN nanoprism. As the growth proceeds further, a narrow (∼50 nm) single-crystal fin-shaped InGaN nanoridge forms atop the InGaN nanoprism structure. The resulting narrow fin-shaped InGaN nanoridge structure shows extremely strong photoluminescence (PL) intensity with a center wavelength at 524–560 nm and narrow distribution compared to the epitaxially grown planar InGaN layer or InGaN nanowire. High-resolution scanning transmission electron microscopy (STEM) combined with an energy-dispersive X-ray (EDS) map reveals that a sharply faceted single-crystal InGaN nanoridge (∼50 nm width) forms along the top of each InGaN nanoprism and is composed of Ga, In, and N without phase segregation or dislocation. The single-crystalline In-rich InGaN nanoridge will pave the way to design a viable architecture for a broad range of III-nitride devices.