Well, high school physics (courtesy Herr (Mr.) Albert Einstein) tells us that when a material is subjected to illumination, an electron in released from the valence to the conduction band, under the condition that the energy of the incoming photon is higher than the band gap of the material. This effect is known as the photoelectric effect. If, by any means, this electron is forced to remain in conduction band, then a persistent current can be drawn from the material. This is pretty much the underlying mechanism of solar cells, where such electrons are inhibited from recombining by creating an artificial potential junction.
But there is one problem!!!! The problem is us, the Physicist!!! Never ever content with what we have and always greedy for something more…..
Here comes this new material known as BFO (BiFeO3). The material has been known for some time but its true potential has only been recently revealed. It reacts to electric field and magnetic field, and more recently even to light. Therefore, it is often referred to as a Smart material. The material does not need any junction and results in a persistent current under light that can be equivalent to a voltage as high as 20 V! But just like any other smart person, this material is not so easy to be found and needs some good DNA from the parents (Bi2O3 and Fe2O3) and some nourishing (in this case high temperature processing and polishing).
The result is impressive and mind boggling at the same time. Just how does this material allow electrons to remain in the conduction band without any junction. The answer, as much as we hate, is a bit complicated (but so is life!). As it turns out, the band gap of this material is not really clean and contains levels. In the simplest of terms, the electron after being in the conduction band for a while tends to take rest in these levels, thus increasing its lifetime…..