Solar Cells To Have An Increased Voltage

Berkeley Lab scientists have found a way to increase the voltage obtained from a conventional solar cell. Researching upon new materials they have solved an old mystery as to why high voltages are produced in Ferroelectric materials. The result obtained by group are presented in a paper titled Efficient photovoltaic current generation at ferroelectric domain walls in the online journal Physical Review Letters.

At top, domains with opposite electrical polarization, averaging about 140 nanometers wide and separated by walls 2 nanometers thick, form a well-aligned array in a thin film of bismuth ferrite. When illuminated, electrons collect on one side of the walls and holes on the other, driving the current at right angles to the walls. Voltage increases as excess electrons accumulate stepwise from domain to domain | Image Credit: Berkeley Lab

The team was researching upon a strip of material Bismuth ferrite or BFO which was developed in the laboratory of Ramamurthy Ramesh who is a professor of materials sciences, engineering, and physics at UC Berkeley. The scientists observed that the material was divided in domains which ranged from 50 to 300 nano-meters across and 2 nano-meters thick. The peculiar feature which they noticed was the electrical polarization occurring in these domains was from opposite ends.

When a sample of BFO was subjected to light, it had an electrical voltage across it. The electrical efficiency was found to be greater in the regions near the domain walls while it was comparatively less at the center. A saturation of electrons and holes was observed near the walls. On one side of wall, electrons are accumulated and holes are repelled while on the opposite side holes are accumulated and electrons are repelled. The electrical activity does not seize to operate once the electrons and holes combine unlike conventional solar cells. This is because of the strong electric field produced by the oppositely charged walls.

Berkeley group tried to measure the voltage produced by this electrical movement by attaching platinum electrical contacts to the BFO film. A direct relationship was found between the distance and the voltage measured. The further apart were the probes of platinum contacts, the higher was the voltage measured. Thus it is proved that the electrons travel from one end to another and they are offered a high resistance at the domain walls which raises the voltage produced.

The researchers measured current and voltage with platinum contacts set at right angles to the current. In a typical set-up, the film was 100 nanometers thick, its width from back to front was 1 millimeter, and the distance between contacts was 200 micrometers. Voltage and current increase with the number of domain walls; with contacts 200 micrometers apart, voltage reached 16 volts, several times the 2.7 electron volt band gap of bismuth ferrite. | Image Credit: Berkeley Lab

However, the working range of these Ferroelectric material is highly limited because at present BFO only respond to ultraviolet or at the most Blue light. This eliminates most of the solar spectrum. One more disadvantage is that of course a high voltage is generated however the magnitude of current obtained is very small. This is not a convenient situation if BFO is to be used as a future solar cell material.

Berkeley lab researchers feel that similar properties will be observed in other Ferroelectric materials. It is hence essential to develop techniques so as to cover a wide range of spectrum and to increase the magnitude of current obtained from the electrical activities of electrons in the material.