The need for the development of transparent conductive electrodes (TCEs) supported on flexible polymer substrates has explosively increased in response to flexible polymer-based photovoltaic and display technologies; these TCEs replace conventional indium tin oxide (ITO) that exhibits poor performance on heat-sensitive polymers. An efficient, flexible TCE is required to exhibit high electrical conductance and high optical transmittance, as well as excellent mechanical flexibility and long-term stability, simultaneously. Recent advances in technologies utilizing an ultrathin noble-metal film in a dielectric/metal/dielectric structure, or its derivatives, have attracted attention as promising alternatives that can satisfy the requirements of flexible TCEs. This review will survey the background knowledge and recent updates of synthetic strategies and design rules toward highly efficient, flexible TCEs based on ultrathin metal films, with a special focus on the principal features and available methodologies involved in the fabrication of highly transparent, conductive, ultrathin noble-metal films. This survey will also cover the practical applications of TCEs to flexible organic solar cells and light-emitting diodes.

A series of low-pressure chemical vapor deposition experiments and gas-surface chemical kinetic simulations have been carried out to achieve significant reductions of twin and protrusion in 3C-SiC heteroepitaxial growth on Si(1 0 0) substrates. A two-step epitaxial process, consisting of a nucleation stage and a subsequent epitaxial stage, was newly proposed by comparisons between experimental results and numerical predictions. The twin formation was successfully suppressed under the growth conditions of the nucleation stage leading to a relative flux ratio of C to Si larger than 56 on the deposition surface. The surface protrusion density was decreased from 7.5×106 to 6.5×104 cm−2 when the conventional carbonization process was replaced with the proposed nucleation stage.

The chemical and morphological features of the interface between plasma-treated polymer substrates and oxide coatings are investigated to clarify the influence of the interfacial features on the adhesive and cohesive properties in the polymer-oxide system. It is found that one-dimensional polymer protrusions and polymer-oxide composite structures develop sequentially in the early growth stages of silicon oxide films on both acrylate hard coat and bare polyethylene terephthalate surfaces exposed to strong plasma-ion irradiation. These interfacial nanostructures cause a dramatic decrease in the wettability of the polymers with silicon oxide films, thus leading to a weak boundary layer, which results in adhesion failures at the polymer-oxide interface.

The chemical and morphological features of the interface between plasma-treated polymer substrates and oxide coatings are investigated to clarify the influence of the interfacial features on the adhesive and cohesive properties in the polymer–oxide system. It is found that one-dimensional polymer protrusions and polymer–oxide composite structures develop sequentially in the early growth stages of silicon oxide films on both acrylate hard coat and bare polyethylene terephthalate surfaces exposed to strong plasma-ion irradiation. These interfacial nanostructures cause a dramatic decrease in the wettability of the polymers with silicon oxide films, thus leading to a weak boundary layer, which results in adhesion failures at the polymer–oxide interface.

The chemical and morphological features of the interface between plasma-treated polymer substrates and oxide coatings are investigated to clarify the influence of the interfacial features on the adhesive and cohesive properties in the polymer–oxide system. It is found that one-dimensional polymer protrusions and polymer–oxide composite structures develop sequentially in the early growth stages of silicon oxide films on both acrylate hard coat and bare polyethylene terephthalate surfaces exposed to strong plasma-ion irradiation. These interfacial nanostructures cause a dramatic decrease in the wettability of the polymers with silicon oxide films, thus leading to a weak boundary layer, which results in adhesion failures at the polymer–oxide interface.

The crystallinity and morphology of single-crystal 3C-SiC homoepilayers grown on heteroepilayer (0 0 1) substrates by low-pressure chemical vapor deposition were investigated. The crystalline qualities of homoepilayers were critically dependent upon the defects of heteroepilayer substrates because planar defects, predominantly stacking faults and twins, and protrusions existing on the heteroepilayer surface propagated into the homoepilayers. A surface etching process using reactive ion etching (RIE) of the backside of free-standing heteroepilayers, the interface with Si substrates, was proposed to minimize the defect densities on the heteroepilayer surface. Analyses of high resolution X-ray diffraction (HRXRD) and atomic force microscope (AFM) reveal that simultaneous reductions of both surface roughness and defect densities on the heteroepilayer surface are achieved by an etching depth of 4 μm. Cross-sectional scanning electron microscopy (SEM) observation clearly shows that protrusions on the RIE-treated heteroepilayer surface are readily buried by growing homoepilayers. Furthermore, the results of cross-sectional transmission electron microscopy (XTEM) indicate that (i) significant amounts of planar defects are removed by the RIE process of the backside of heteroepilayers and (ii) most of planar defects propagating into the homoepilayers are terminated by coalescences between one another during early homoepitaxial growth stages.

The crystallinity and morphology of single-crystal 3C-SiC homoepilayers grown on heteroepilayer (0 0 1) substrates by low-pressure chemical vapor deposition were investigated. The crystalline qualities of homoepilayers were critically dependent upon the defects of heteroepilayer substrates because planar defects, predominantly stacking faults and twins, and protrusions existing on the heteroepilayer surface propagated into the homoepilayers. A surface etching process using reactive ion etching (RIE) of the backside of free-standing heteroepilayers, the interface with Si substrates, was proposed to minimize the defect densities on the heteroepilayer surface. Analyses of high resolution X-ray diffraction (HRXRD) and atomic force microscope (AFM) reveal that simultaneous reductions of both surface roughness and defect densities on the heteroepilayer surface are achieved by an etching depth of 4 μm. Cross-sectional scanning electron microscopy (SEM) observation clearly shows that protrusions on the RIE-treated heteroepilayer surface are readily buried by growing homoepilayers. Furthermore, the results of cross-sectional transmission electron microscopy (XTEM) indicate that (i) significant amounts of planar defects are removed by the RIE process of the backside of heteroepilayers and (ii) most of planar defects propagating into the homoepilayers are terminated by coalescences between one another during early homoepitaxial growth stages.

Single-crystal 3C-SiC epilayers were grown on on-axis Si(0 0 1) substrates by low-pressure chemical vapor deposition. The dependence of the densities of stacking faults and twins on epilayer thicknesses and growth conditions—including the reactor pressure, the substrate temperature, and the inlet gaseous composition—were investigated by a series of experiments and simulations. Simple indexes were developed to predict the planar defect densities in terms of the flux ratio of adatoms on the deposition surface. The planar defect densities were significantly reduced with increasing the epilayer thickness until continuous surfaces with {1 0 0} planes were formed at 0.7 μm. The stacking fault density was a function of the surface flux ratio of carbon adatom to atomic hydrogen, while the twin density was a function of that of silicon adatom to atomic hydrogen. Those densities were decreased almost linearly with increases in the surface flux of atomic hydrogen at fixed flux values of carbon and silicon adatoms. The surface flux of atomic hydrogen was increased as either the reactor pressure was decreased or the substrate temperature was increased.

Advances in flexible optoelectronic devices have led to an increasing need for developing highly efficient, low-cost, flexible transparent conducting electrodes. Copper-based electrodes have been unattainable due to the relatively low optical transmission and poor oxidation resistance of copper. Here, we report the synthesis of a completely continuous, smooth copper ultra-thin film via limited copper oxidation with a trace amount of oxygen. The weakly oxidized copper thin film sandwiched between zinc oxide films exhibits good optoelectrical performance (an average transmittance of 83% over the visible spectral range of 400–800 nm and a sheet resistance of 9 Ω sq−1) and strong oxidation resistance. These values surpass those previously reported for copper-based electrodes; further, the record power conversion efficiency of 7.5% makes it clear that the use of an oxidized copper-based transparent electrode on a polymer substrate can provide an effective solution for the fabrication of flexible organic solar cells.

A three-dimensional (3D) transparent conducting electrode, consisting of a quasi-periodic array of discrete indium-tin-oxide (ITO) nanoparticles superimposed on a highly conducting oxide-metal-oxide multilayer using ITO and silver oxide (AgOx) as oxide and metal layers, respectively, is synthesized on a polymer substrate and used as an anode in highly flexible organic solar cells (OSCs). The 3D electrode is fabricated using vacuum sputtering sequences to achieve self-assembly of distinct ITO nanoparticles on a continuous ITO-AgOx-ITO multilayer at room-temperature without applying conventional high-temperature vapour-liquid-solid growth, solution-based nanoparticle coating, or complicated nanopatterning techniques. Since the 3D electrode enhances the hole-extraction rate in OSCs owing to its high surface area and low effective series resistance for hole transport, OSCs based on this 3D electrode exhibit a power conversion efficiency that is 11-22% higher than that achievable in OSCs by means of conventional planar ITO film-type electrodes. A record high efficiency of 6.74% can be achieved in a bendable OSC fabricated on a poly(ethylene terephthalate) substrate.