Graphene inks are becoming widely popular. However the vast majority of these inks are formulated in polar solvents with high-boiling points. Their slow evaporation is a bottleneck factor in roll-to-roll printing processes. Here, we developed a highly-conductive fast-drying graphene ink in isooctane, a non-polar and low-boiling solvent. For this purpose, a diblock copolymer containing pendant cholesterol groups was used during the exfoliation of natural graphite in isooctane. The polymer develops non-covalent supramolecular interactions with the graphene conjugated system, resulting in the formation of stable graphene dispersions (up to c = 4 mg mL). These dispersions were used for direct writing on a variety of substrates, and were shown to dry instantly after application. The influence of polymer concentration on graphene characteristics, on colloidal stability and on electrochemical characteristics has been studied. The lowest sheet resistance (80 Ω □) was obtained when 23% of the graphene surface was covered by the polymer. In this case, the flakes were constituted of 2-5 graphene layers. More extensive exfoliation, down to single-layer graphene, was achieved at greater surface coverage, but led to inks with higher sheet resistance. Thus, by combining a tailored polymeric dispersant, a smooth exfoliation process and a low-boiling non-polar ink solvent, we were able to prepare highly-conductive fast-drying graphene inks which should have a high potentital for the development of roll-to-roll printed electronics.
Titanium dioxide (TiO 2 ) is a remarkable metal-oxide semiconductor with unique optoelectronic properties ideal for photovoltaics and photocatalytic conversion. The principal crystalline phases for TiO 2 are anatase, rutile, and brookite. The combination of both anatase and rutile crystalline structures can positively impact the optoelectronic properties of TiO 2 films. With standard sol−gel processing, hightemperature conversion generally yields one dominant phase and limits the combined use of anatase and rutile TiO 2 for optoelectronic devices. We report on a singular route to controllably engineer hybrid nanocrystalline films of TiO 2 at room temperature to synergistically exploit both anatase and rutile TiO 2 phases. Relying on sol−gel chemistry, this approach starts from an amorphous film and uses photoinduced activation using a low-power laser to achieve specific spatially controlled pattern consisting of different TiO 2 crystalline phases within the same film. While achieving remarkable precision, reproducibility, and control, we also avoid costly high-temperature, ion-metalassisted, or specific atmospheric processing that currently prevents the integration of TiO 2 in several optoelectronic platforms. In the future, we believe this unprecedented level of control and the ability to engineer the TiO 2 crystalline structure at the microscopic scale will allow the design and fabrication of novel high-performance TiO 2 hybrids for energy conversion and environmental applications.
We demonstrate a fast and large area-scalable methodology for the fabrication of efficient dye sensitized solar cells (DSSCs) by simple addition of graphene micro-platelets to TiO2 nanoparticulate paste (graphene concentration in the range of 0 to 1.5 wt%). Two dimensional (2D) Raman spectroscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirm the presence of graphene after 500 C annealing for 30 minutes. Graphene addition increases the photocurrent density from 12.4 mA cm2 in bare TiO2 to 17.1 mA cm2 in an optimized photoanode (0.01 wt% graphene, much lower than those reported in previous studies), boosting the photoconversion efficiency (PCE) from 6.3 up to 8.8%. The investigation of the 2D graphene distribution showed that an optimized concentration is far below the percolation threshold, indicating that the increased PCE does not rely on the formation of an interconnected network, as inferred by prior investigations, but rather, on increased charge injection from TiO2 to the front electrode. These results give insights into the role of graphene in improving the functional properties of DSSCs and identifying a straightforward methodology for the synthesis of new photoanode
In recent years, additive manufacturing has been evolving towards flexible substrates for the fabrication of printable electronic devices and circuits. Generally polymer-based, these emerging substrates suffer from their heat sensitivity and low glass-transition temperatures. As such they require new highly-localized sintering processes to treat the electronic inks without damaging the polymer-based substrate. Laser-based sintering techniques have shown great promises to achieve high-quality sintering locally, while controlling the heat penetration to preserve the polymer substrates integrity. In this report, we explore new optimization pathways for dynamic laser-based sintering of conductive silver inks. Multiple passes of a pulsed laser are first performed while varying pulse train frequencies and pulse energies as an attempt to optimize the properties of the silver inks. Then, time-domain pulse shaping is performed to alter the properties of the conductive inks. Together, these pathways allow for the careful control of the time-domain laser energy distribution in order to achieve the best electronic performances while preserving the substrate’s integrity. Sheet resistance values as low as 0.024Ω/□ are achieved, which is comparable to conventional 1-hour oven annealing, with the processing time dramatically reduced to the milisecond range. These results are supported by finite element modeling of the laser-induced thermal dynamics.
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