Researchers at Lawrence Berkeley National Laboratory have found a new material that overcomes the bandgap limitations of conventional solid-state solar cells as used in thin-film photovoltaic applications.

The band gap is a sort of “free zone” (between the top of a valence band and the bottom of a conduction band) in solids (materials) where no electrons travel. In photovoltaics, the band gap of a material determines what part (or how much) of the solar spectrum a cell absorbs.

For photovoltaics to work, photons have to collide with electrons to “excite” them. For it to work better than it currently does, the material used has to have a small enough bandgap width to allow most of the energized electrons to reach the conduction band of the semiconductor’s electronic bandgap.

Basically, in human speak, this means that photons can only penetrate so far into a given material given its atomic structure. For non-techies like me, think of trying to stack perforated pizza pans with the holes lined up to let light pass through. For techies, this is the quantum mechanical state of matter that permits or prevents electrons at specific levels in periodic crystalline lattice structures.

For example, silicon has a bandgap of 1.11 (measured in electron volts, or eV), which means it permits the movement of photons rather well. Gallium and indium, at 0.7, perform even better.

Berkeley Lab researchers now find that bismuth ferrite (a multiferroic ceramic made from bismuth, iron and oxygen), which has both electric and magnetic properties, can produce spontaneous photovoltaic effects at nanoscale levels because of the ceramic’s crystal structure.

This, rather than being a three-dimensional cube, is a rhombehedron, or “leaning” cube; a crystalline structure which – by applying an electric field – can be manipulated to control photovoltaic properties.

The discovery was made when Berkeley Lab physicist Jan Seidel and colleagues applied white light to bismuth ferrite and discovered that they were generating photon-driven voltages at the one- to two-nanometer level. Surprisingly, the voltages were higher than bismuth ferrite’s bandgap (2.7).

The discovery is part of the exciting new field of materials science at the nano level, and, according to Seidel, is the first step in an ongoing process to deliver better energy efficiencies at the developmental level of photovoltaic energy.

The effect is, according to Seidel, due to domain walls (transition zones that differentiate ferromagnetic and ferroelectric zones) which serve as electron-hole separators just like p-n junctions but at such nano levels that many layers can be stacked sideways and still receive light waves. In addition, the domain walls can change their polarization, adding unsuspected voltage “steps” (intensities) for photovoltaic energy.

Seidel and colleagues also demonstrated that an electric pulse could either reverse polarity or turn it off completely, which implies new applications in both optics and electronics (at the nano scale) as well.