Energy Focus
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ence of a sufficiently large bias electric field. But the energy landscape of a domain wall can be engineered and tuned so that under certain conditions the barrier to its motion is small enough to permit the domain wall to move back and forth, in resonance with even a weak incident microwave field,” says Jonathan E. Spanier, the leader of the research group from Drexel University. Essential to the large capacitance and frequency tunability are the abundance of different thermodynamic phases and a high density of domain walls. Engineering a film material to have many and more easily available phases allows the material to attain much higher capacitance tuning with the same voltage. “We first carried out theoretical simulations to predict the energy landscapes of domainrich films, and then optimized the substrate materials and their thickness to achieve the desired phases and density of domain walls,” says Zongquan Gu, a postdoctoral fellow from Spanier’s group and the lead author of Artist’s rendering of the oscillating domain walls. Credit: Felice Macera.
Energy Focus Continuous roll-to-roll system facilitates mass production of organic photovoltaic cells
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o promote the practical applications of organic photovoltaic (OPV) cells, manufacturing techniques allowing rapid and high-throughput production of highly uniform organic thin films are needed. Stephen R. Forrest of the University of Michigan and co-workers have now developed a continuous roll-to-roll vaporphase growth system for OPV cells.
This work was published in a recent issue of Applied Physics Letters (doi: 10.1063/1.5039701). According to Forrest, the motivation of this work was to “test whether vacuum deposition could be combined with organic vapor-phase deposition (OVPD) in a single integrated system to produce high-performance organic photovoltaics.” The core components of their apparatus include a low-pressure OVPD chamber upstream, and a high-vacuum vapor thermal evaporation (VTE) chamber downstream. Both chambers deposit active thin films from
the study. “Thus far we have identified how the abundance of different thermodynamic domain-wall-variant phases can be realized to produce the desired extrinsically driven properties,” he says. The domain-wall-enhanced ferroelectrics demonstrated a microwave tunability (1~8 GHz) of the loss minimum that is 100 times greater than the previous best intrinsically tunable material. The new approaches, including design of new engineered materials that have microwavefrequency solid-state ionic oscillators, may enable more facile access to the increasingly congested radio-frequency spectrum used in current telecommunications devices, as well as other novel applications. “This work nicely demonstrates that certain configurations of ferroelectric domain walls can really significantly enhance materials performance,” says Jiri Hlinka from the Institute of Physics of the Czech Academy of Sciences. It would be great news for the development of the next generation of tunable antennas and similar microwave devices if fu
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