Researchers at the Lawrence Berkeley National Lab have found a new mechanism for the photovoltaic effect to take place in semiconductor thin-films. The newly developing route to photovoltaic cell production overcomes the “bandgap” voltage limitation that continues to bedevil conventional solid-state solar cell development.

Understanding the “bandgap” – in a conventional solid-state solar cell is a positive-negative junction (p-n junction) – or to visualize, it’s the interface between a semiconductor layer with an abundance of positively-charged “holes,” and a layer with an abundance of negatively charged electrons.  When photons arrive their energy creates electron-hole pairs that can be separated within a “depletion zone,” a microscopic region at the positive-negative junction measuring only a couple of micrometers across, then the energy is collected as electricity.  For this to work the photons have to penetrate the material to the depletion zone and their energy has to precisely match the energy of the semiconductor’s electronic “bandgap” – the gap between semiconductor’s valence and conduction energy bands where no electron states can exist.  More on this in a bit.

Jan Seidel, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and the UC Berkeley Physics Department has a newly discovered path for the conversion of sunlight to electricity that should brighten the future for photovoltaic technology.

Bismith Ferrite Thin Film in a Solar Cell. Click image for more info.

Working with bismuth ferrite, a ceramic made from bismuth, iron and oxygen that is multiferroic — meaning it simultaneously displays both ferroelectric and ferromagnetic properties — the Seidel and his team of researchers discovered that the photovoltaic effect can spontaneously arise at the nanoscale as a result of the ceramic’s rhombohedrally distorted crystal structure. Furthermore, they demonstrated that the application of an electric field makes it possible to manipulate this crystal structure and thereby control photovoltaic properties.

Seidel said, “We’re excited to find functionality that has not been seen before at the nanoscale in a multiferroic material. We’re now working on transferring this concept to higher efficiency energy-research related devices.”

The paper “Above-bandgap voltages from ferroelectric photovoltaic devices” has been published in the journal Nature Nanotechnology. It’s a big team including Seung-Yeul Yang, Steven Byrnes, Padraic Shafer,Chan-Ho Yang, Marta Rossell, Pu Yu, Ying-Hao Chu, James Scott, Joel Ager, Lane Martin and Ramamoorthy Ramesh.

Now back to the bandgap and the new technology.  Seidel explains, “The maximum voltage conventional solid-state photovoltaic devices can produce is equal to the energy of their electronic bandgap. Even for so called tandem-cells, in which several semiconductor p-n junctions are stacked, photovoltages are still limited because of the finite penetration depth of light into the material.”

Working through Berkeley Lab’s Helios Solar Energy Research Center, Seidel and the team discovered that by applying white light to bismuth ferrite, a material that is both ferroelectric and antiferromagnetic, they could generate photovoltages within submicroscopic areas between one and two nanometers across. These photovoltages were significantly higher than bismuth ferrite’s electronic bandgap.  Hold on to your seats –

“The bandgap energy of the bismuth ferrite is equivalent to 2.7 volts. From our measurements we know that with our mechanism we can get approximately 16 volts over a distance of 200 microns. Furthermore, this voltage is in principle linear scalable, which means that larger distances should lead to higher voltages,” said Seidel.

Now these are very big numbers.  If commercialized the whole system would need redesigned and with huge advantages.  Going up from 16 volts as an example is much less costly and more efficient than starting at 1 or 2 volts.  That’s why the “bandgap” thing is such a challenge or opportunity depending on how you see it.

The foundation of this new mechanism for photovoltage generation is domain walls -two-dimensional sheets that run through a multiferroic and serve as transition zones, separating regions of different ferromagnetic or ferroelectric properties. In their study, Seidel and the team found that these domain walls could serve the same electron-hole separation purpose as depletion zones only with distinct advantages.

The advantages Seidel says, “The much smaller scale of these domain walls enables a great many of them to be stacked laterally (sideways) and still be reached by light. This in turn makes it possible to increase the photovoltage values well above the electronic bandgap of the material.”

The photovoltaic effect (the energy event to electrical energy) arises because at the domain walls the polarization direction of the bismuth ferrite changes, which leads to steps in the electrostatic potential. Through annealing treatments of the substrate upon which bismuth ferrite is grown, the material’s rhombohedral crystals can be induced to form domain walls that change the direction of electric field polarization by either 71, 109 or 180 degrees. (See the image above.) Seidel and the team measured the photovoltages created by the 71 and 109 degree domain walls.

Seidel discusses the result saying, “The 71 degree domain walls showed unidirectional in-plane polarization alignment and produced an aligned series of potential voltage steps. Although the potential step at the 109 degree domain was higher than the 71 degree domain, it showed two variants of the in-plane polarization which ran in opposite directions.”

Now add this – Seidel and the team were also able to use a 200 volt electric pulse to either reverse the polarity of the photovoltaic effect or turn it off altogether. Such controllability of the photovoltaic effect has never been reported in conventional photovoltaic systems, and it paves the way for new applications in nano-optics and nano-electronics.  As a solar cell, turning off isn’t so interesting, but polarity reversals can be.

Seidel makes one of 2010 understatements of the year in the press release saying, “While we have not yet demonstrated these possible new applications and devices, we believe that our research will stimulate concepts and thoughts that are based on this new direction for the photovoltaic effect.”

While far from commercial or scalable this is sure to set off a flurry of new research with positively astonishing potential.


2 Comments so far

  1. Eric Connelly on July 29, 2010 12:44 PM

    Great post thanks.

  2. James Robert Seidel on July 26, 2011 9:04 PM

    Way to go Jan

    Now to convince Xunlight and Unisolar
    and Linuo and others like Solapower
    and even Mitsuibishi and Kyocera to start
    working with you.

    The sun is free, however remember
    that getting to utilize it is not.

    In time your progress will benefit mankind.

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