Energy Focus: Field-effect transistor is powered by solar energy
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he invention of transistors in the middle of the 20th century set the course for the electronics revolution. Transistors use a “gate” voltage to switch a semiconductor between ON and OFF states. Although a disruptive technology, owing to their reliance on an external gate, transistors are, by nature, externally powered and energyintensive, especially with the increasing packing densities of these devices. A selfpowered transistor utilizing a renewable source of energy would therefore be a potential game-changing technology. Now a solar-powered field-effect transistor or “solaristor” has been demonstrated by the research groups of Mónica Lira-Cantú and Gustau Catalán at the Catalan Institute of Nanoscience and Nanotechnology (ICN2), Spain. Employing a ferroelectric film in the most commonly used solarcell architecture of organic photovoltaics (bulk heterojunction), the researchers were able to switch the photocurrent of the device between ON and OFF states, utilizing absorbed photons as the gate. Importantly, the solaristor was realized as a two-electrode vertical stack architecture, significantly reducing the interconnection complexity of the conventional in-plane three-electrode transistor architecture. Previous attempts used ferroelectric films as the absorber layer in solar cells. However, since these films are usually wide-bandgap semiconductors and, therefore, poor absorbers of the visible spectrum, photocurrents have been severely limited, leading to overall poor power-conversion efficiencies. Lira-Cantú’s group turned to the concept of an organic bulk heterojunction to absorb photons. These absorbers are solution-processed, have an optimal bandgap in the visible region, and are currently attracting intense research interest for applications in thin-film organic photovoltaics (OPVs). Utilization of these organic absorbers resulted in high photocurrents in the range of 5–10 mA cm–2. A conventional OPV device sandwiches these absorbers between electron- and holeextracting layers. In the present study, the researchers also included a wide-bandgap
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n-type ferroelectric semiconducting film at the electronextracting electrode. The high bandgap (3.6 eV) of the ferroelectric film means that this layer is transparent to most of the solar spectrum, ensuring that maximal photon flux reaches the sandwiched organic absorber. Poling the ferroelectric by applying a DC Schematic showing the active ingredients of the solaristor, organic bulk voltage to one of heterojunction placed atop the ferroelectric film. Credit: Mónica Lira-Cantú. the electrodes effectively tuned the band offset at the electron-collecting studying complex functional materials and junction, resulting in efficient collection nanostructures, feel that the work holds (ON) or blocking (OFF) of photoelectrons. promise for the future of the photovolta As reported in a recent issue of Advanced ics community and market. “This report Functional Materials (doi:10.1002/ represents a new device architecture for adfm.201707099), a transparent conducphotoswitchin
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