Transparent conducting electrodes (TCEs) have been made on flat, flexible, and curved surfaces, following a crack template method in which a desired surface was uniformly spray-coated with a crackle precursor (CP) and metal (Ag) was deposited by vacuum evaporation. An acrylic resin (CP1) and a SiO2 nanoparticle-based dispersion (CP2) derived from commercial products served as CPs to produce U-shaped cracks in highly interconnected networks. The crack width and the density could be controlled by varying the spray conditions, resulting in varying template thicknesses. By depositing Ag in the crack regions of the templates, we have successfully produced Ag wire network TCEs on flat-flexible PET sheets, cylindrical glass tube, flask and lens surface with transmittance up to 86%, sheet resistance below 11 Ω/□ for electrothermal application. When used as a transparent heater by joule heating of the Ag network, AgCP1 and AgCP2 on PET showed high thermal resistance values of 515 and 409 °C cm(2)/W, respectively, with fast response (<20 s), requiring only low voltages (<5 V) to achieve uniform temperatures of ∼100 °C across large areas. Similar was the performance of the transparent heater on curved glass surfaces. Spray coating in the context of crack template is a powerful method for producing transparent heaters, which is shown for the first time in this work. AgCP1 with an invisible wire network is suited for use in proximity while AgCP2 wire network is ideal for use in large area displays viewed from a distance. Both exhibited excellent defrosting performance, even at cryogenic temperatures.

Conducting nanowire networks find diverse applications in solar cells, touch-screens, transparent heaters, sensors, and various related transparent conducting electrode (TCE) devices. The performances of these devices depend on effective resistance, transmittance, and local current distribution in these networks. Although, there have been rigorous studies addressing resistance and transmittance in TCE, not much attention is paid on studying the distribution of current. Present work addresses this compelling issue of understanding current distribution in TCE networks using analytical as well as Monte-Carlo approaches. We quantified the current carrying backbone region against isolated and dangling regions as a function of wire density (ranging from percolation threshold to many multiples of threshold) and compared the wired connectivity with those obtained from template-based methods. Further, the current distribution in the obtained backbone is studied using Kirchhoff's law, which reveals that a significant fraction of the backbone (which is believed to be an active current component) may not be active for end-to-end current transport due to the formation of intervening circular loops. The study shows that conducting wire based networks possess hot spots (extremely high current carrying regions) which can be potential sources of failure. The fraction of these hot spots is found to decrease with increase in wire density, while they are completely absent in template based networks. Thus, the present work discusses unexplored issues related to current distribution in conducting networks, which are necessary to choose the optimum network for best TCE applications.

Conducting nanowire networks have been developed as viable alternative to existing indium tin oxide based transparent electrode (TE). The nature of electrical conduction and process optimization for electrodes have gained much from the theoretical models based on percolation transport using Monte Carlo approach and applying Kirchhoff's law on individual junctions and loops. While most of the literature work pertaining to theoretical analysis is focussed on networks obtained from conducting rods (mostly considering only junction resistance), hardly any attention has been paid to those made using template based methods, wherein the structure of network is neither similar to network obtained from conducting rods nor similar to well periodic geometry. Here, we have attempted an analytical treatment based on geometrical arguments and applied image analysis on practical networks to gain deeper insight into conducting networked structure particularly in relation to sheet resistance and transmittance. Many literature examples reporting networks with straight or curvilinear wires with distributions in wire width and length have been analysed by treating the networks as two dimensional graphs and evaluating the sheet resistance based on wire density and wire width. The sheet resistance values from our analysis compare well with the experimental values. Our analysis on various examples has revealed that low sheet resistance is achieved with high wire density and compactness with straight rather than curvilinear wires and with narrower wire width distribution. Similarly, higher transmittance for given sheet resistance is possible with narrower wire width but of higher thickness, minimal curvilinearity, and maximum connectivity. For the purpose of evaluating active fraction of the network, the algorithm was made to distinguish and quantify current carrying backbone regions as against regions containing only dangling or isolated wires. The treatment can be helpful in predicting the properties of a network simply from image analysis and will be helpful in improvisation and comparison of various TEs and better understanding of electrical percolation.

No abstract

No abstract