Abstract
The widespread use of transparent conductive films in modern display and solar technologies calls for engineering solutions with tunable light transmission and electrical characteristics. Currently, considerable effort is put into the optimization of indium tin oxide, carbon nanotube-based, metal grid, and nano-wire thin-films. The indium and carbon films do not match the chemical stability nor the electrical performance of the noble metals, and many metal films are not uniform in material distribution leading to significant surface roughness and randomized transmission haze. We demonstrate solution-processed masks for physical vapor-deposited metal electrodes consisting of hexagonally ordered aperture arrays with scalable aperture-size and spacing in an otherwise homogeneous noble metal thin-film that may exhibit better electrical performance than carbon nanotube-based thin-films for equivalent optical transparency. The fabricated electrodes are characterized optically and electrically by measuring transmittance and sheet resistance. The presented methods yield large-scale reproducible results. Experimentally realized thin-films with very low sheet resistance, Rsh = 2.01 ± 0.14 Ω/sq, and transmittance, T = 25.7 ± 0.08 %, show good agreement with finite-element method simulations and an analytical model of sheet resistance in thin-films with ordered apertures support the experimental results and also serve to aid the design of highly transparent conductive films. A maximum Haacke number for these 33 nm thin-films, ϕH = 10.7 × 10−3Ω−1 corresponding to T ≃ 80 % and Rsh ≃ 10 Ω/sq, is extrapolated from the theoretical results. Increased transparency may be realizable using thinner metal films trading off conductivity. Nevertheless, the findings of this article indicate that colloidal lithographic patterned transparent conductive films can serve as vital components in technologies with a demand for transparent electrodes with low sheet resistance.
Highlights
Many modern devices feature transparent conductive films (TCFs) as essential elements to their performance
In the fabrication process the diameter of the Polystyrene particles (PSP) is reduced by ICP-RIE prior to Ag physical vapor-deposited (PVD) and a subsequent lift-off process, Fig. 1(a), and an electron micrograph of a single nano-aperture after lift-off indicates that almost circular apertures are realized and that several small Ag nano-particles with diameters below 20 nm reside on the SiO2 substrate near the aperture edges, Fig. 1(b)
Transparent conductive films fabricated using the polystyrene particle method have low sheet resistance, low chemical reactivity, high ductility, low surface roughness relative to other noble metal solutions and transmission of the transparent conductive films is limited by plasmonic losses
Summary
Many modern devices feature transparent conductive films (TCFs) as essential elements to their performance. The significant diameter (50–200 nm) of the oftentimes stacked Ag nano-wires increases surface roughness, which combined with the randomized wire length and spacing as well as the required annealing process results in transmission haze, i.e. uneven light scattering, which is undesirable in most display applications, but can be advantageous in solar cells. Polystyrene particles (PSP) have previously been used as masks for TCFs [14,15,16,17] showing promising experimental results, e.g. a way to minimize the iridescence of these films by employing polydisperse particles [16] and a way to engineer highly efficient and wavelengthselective reflectors [17] In those works transmission is either solely experimentally determined or estimated by a simple geometric model not considering surface plasmon absorption and no theoretical analysis is presented for the TCF sheet resistance. The Haacke figure-of-merit [18], which has been shown to correlate well with current losses in photovoltaics [19], is used for evaluating the electro-optical performance of the TCF
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