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Posted: March 3, 2010

Thin-film ferroelectrics offer a fundamentally different route to photovoltaic devices

(Nanowerk News) The sun seems to be our only truly dependable source of renewable energy, and scientists are constantly on the hunt for innovative ways to convert sunlight into electricity. The potential of thin ferroelectric films for visible-light photovoltaic devices has now been demonstrated by researchers from the A*STAR Institute of Materials Research and Engineering and the National University of Singapore ("Bulk Photovoltaic Effect at Visible Wavelength in Epitaxial Ferroelectric BiFeO3 Thin Films").
Photovoltaic devices, including solar cells, produce electricity when the absorption of light provides electrons with enough energy to cross a barrier called the bandgap. To generate a useful electrical current, however, these charge carriers must be swept along in a particular direction rather than flowing randomly. In the most commonly used photovoltaic materials, such as silicon, scientists achieve this by constructing a device with a material interface referred to as a p–n junction.
Alternatively, scientists can exploit the ‘bulk photovoltaic effect’ of ferroelectric materials, by which photocurrent can flow in a uniform material without the need to form an interface. “In ferroelectric materials, the photovoltage magnitude is not limited by an energy barrier like it is in semiconductor photovoltaic materials,” explains senior A*STAR scientist Kui Yao. “The reason is that the underlying mechanism is fundamentally different.” Yao and his co-workers identified and studied bulk photovoltaic phenomena in a thin film made of bismuth ferrite (BFO).
The ferroelectric materials examined previously for photovoltaic effects had large bandgaps that absorbed ultraviolet radiation. The smaller bandgap of BFO has the advantage of generating a current from visible-light photons. Yao and his co-workers started by covering a substrate with an electrode. On this they deposited a film of BFO just 170 nm thick before laying down a second electrode of either gold or a transparent conductor. They induced a current through the device by exposing it to the output of a visible-light source. This photocurrent increased linearly with the intensity of the incident light, and researchers could control its direction by changing the polarization orientation of the BFO using a voltage applied across the electrodes.
In experimental and theoretical work on a thin film of lead zirconate titanate doped with lanthanum (PLZT), Yao’s research team has also shown ("Photovoltaic mechanisms in ferroelectric thin films with the effects of the electrodes and interfaces") that nanoscale thin films could achieve higher photovoltaic efficiency than much thicker samples.
Yao and his co-workers are now working to further clarify the photovoltaic mechanisms in ferroelectric thin films, which are less well understood than those in semiconductors.
Source: Reprinted with permission from A*STAR
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