K. D. M. Rao

Spray coating of crack templates for the fabrication of transparent conductors and heaters on flat and curved surfaces.

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.

Large area solution processed transparent conducting electrode based on highly interconnected Cu wire network

Virtually unlimited and highly interconnected Cu wire networks have been fabricated on polyethylene terephthalate (PET) substrates with sheet resistance of <5 Ω □−1 and transmittance of ∼75%, as alternatives to the commonly used tin doped indium oxide (ITO) based electrodes. This is a four step process involving deposition of commercially available colloidal dispersions onto PET, drying to induce crackle network formation, nucleating Au or Pd seed nanoparticles inside the crackle regions, washing away the sacrificial layer and finally, depositing Cu electrolessly or by electroplating. The formed Cu wire network is continuous and seamless, and devoid of crossbar junctions, a property which brings high stability to the electrode towards oxidation in air even at 130 °C. The flexible property of the PET substrate is easily carried over to the TCE. The sheet resistance remained unaltered even after a thousand bending cycles. The as-prepared Cu wire network TCE is hydrophobic (contact angle, 80°) which, upon UV–ozone treatment, turned to hydrophilic (∼40°).

Visibly Transparent Heaters.

Heater plates or sheets that are visibly transparent have many interesting applications in optoelectronic devices such as displays, as well as in defrosting, defogging, gas sensing and point-of-care disposable devices. In recent years, there have been many advances in this area with the advent of next generation transparent conducting electrodes (TCE) based on a wide range of materials such as oxide nanoparticles, CNTs, graphene, metal nanowires, metal meshes and their hybrids. The challenge has been to obtain uniform and stable temperature distribution over large areas, fast heating and cooling rates at low enough input power yet not sacrificing the visible transmittance. This review provides topical coverage of this important research field paying due attention to all the issues mentioned above.

Metal wire network based transparent conducting electrodes fabricated using interconnected crackled layer as template

A metal (Au) wire network, nearly invisible to the naked eye, has been realized on common substrates such as glass, to serve as a transparent conducting electrode (TCE). The process involves coating a TiO2 nanoparticle dispersion to a film thickness of ~10 μm, which following solvent evaporation, spontaneously forms a crackle network; the film is then used as a sacrificial template for metal deposition. The TCE thus formed exhibited visible transmittance of ~82% and sheet resistance of 3–6 Ω/square for a metal fill factor of 7.5%. With polyethylene terephthalate substrate, flexible and robust TCE could be produced and with quartz, the spectral range could be widened to cover UV and IR regions.

Transparent and flexible capacitor fabricated using a metal wire network as a transparent conducting electrode

As transparency is becoming a desirable property even in non-optoelectronic devices, there is great impetus given to alternate materials and strategies replacing the conventional ITO. Here we have used transparent Au wire networks as electrodes, obtained from a crackle template method, in a capacitor where an ion gel serves as the dielectric. The transparent capacitor thus fabricated with flexible PET as substrate showed a capacitance of 20 μF cm−2 at 1 Hz with average transmittance of ∼68% and was stable after many bending cycles. Such a performance is made possible due to high transmittance (∼86%) and low sheet resistance (∼4.2 Ω sq−1) of the Au wire network. An ITO-free, flexible organic solar cell has also been fabricated using the Ag wire network.

Highly Conformal Ni Micromesh as a Current Collecting Front Electrode for Reduced Cost Si Solar Cell

Despite relatively high manufacturing cost, crystalline-Si solar cell continues to hold promising future due to its high energy conversion efficiency and long life. As regards cost, one pertinent issue is the top electrode metallization of textured cell surface, which typically involves screen printing of silver paste. The associated disadvantages call for alternative methods that can lower the cost without compromising the solar cell efficiency. In the present work, a highly interconnected one-dimensional (1D) metal wire network has been employed as front electrode on conventional Si wafers. Here, for the first time, we report an innovative solution based crackle templating method for conformal metal wire network patterning over large textured surfaces. Laser beam induced current mapping showed uniform photocurrent collection by the electrodes without any shadow losses. With electroless deposition of Ni wire network on corrugated solar cell, a short circuit current of 33.28 mA/cm2 was obtained in comparison to 20.53 mA/cm2 without the network electrode. On comparing the efficiency with the conventional cells with screen printed electrodes, a 20% increment in efficiency has been observed. Importantly, the estimated manufacturing cost is at least two orders lower.

Transparent Pd Wire Network-Based Areal Hydrogen Sensor with Inherent Joule Heater

A high degree of transparency in devices is considered highly desirable for futuristic technology. This demands that both the active material and the electrodes are made of transparent materials. In this work, a transparent Pd wire network (∼1 cm(2)), fabricated using crackle lithography technique with sheet resistance and transmittance of ∼200 Ohm per square and ∼80%, respectively, serves multiple roles; besides being an electrode, it acts as an active material for H2 sensing as well as an in-built electrothermal heater. The sensor works over a wide range of hydrogen (H2) concentration down to 0.02% with a response time of ∼41 s, which could be improved to ∼13 s by in situ Joule heating to ∼75 °C. Importantly, the device has the potential of scale-up to a window size transparent panel and to be flexible when desired.

Solution-processed soldering of carbon nanotubes for flexible electronics

We report a simple lithography-free, solution-based method of soldering of carbon nanotubes with Ohmic contacts, by taking specific examples of multi-walled carbon nanotubes (MWNTs). This is achieved by self-assembling a monolayer of soldering precursor, Pd(2+) anchored to 1,10 decanedithiol, onto which MWNTs could be aligned across the gap electrodes via solvent evaporation. The nanosoldering was realized by thermal/electrical activation or by both in sequence. Electrical activation and the following step of washing ensure selective retention of MWNTs spanning across the gap electrodes. The soldered joints were robust enough to sustain strain caused during the bending of flexible substrates as well as during ultrasonication. The estimated temperature generated at the MWNT-Au interface using an electro-thermal model is ∼150 °C, suggesting Joule heating as the primary mechanism of electrical activation. Further, the specific contact resistance is estimated from the transmission line model.