Slow ‘hot electrons’ could improve solar cell efficiency

Photons with energy higher than the ‘band gap’ of the semiconductor absorbing them give rise to what are known as hot electrons. The extra energy in respect to the band gap is lost very fast, as it is converted into heat so it does not contribute to the voltage. University of Groningen Professor of Photophysics and Optoelectronics Maria Antonietta Loi has now found a material in which these hot electrons retain their high energy levels for much longer. This might make it possible to use more of their energy to obtain a higher voltage. 

The efficiency of solar panels is hampered by a Goldilocks problem: photons need to have just the right amount of energy to be converted into free electrons, which contribute to the voltage. Too little energy, and the photons pass right through the solar panel. Too much, and the excess energy disappears as heat.

Perovskites

The latter is due to the creation of hot (high-energy) electrons. Before they can be extracted from the solar cells, these hot electrons first give off their excess energy by causing vibrations in the crystalline material of the solar panel. ‘This energy loss puts a limit to the maximum efficiency of solar cells’, explains Loi.

She is working on a special type of solar cell that is made of organic-inorganic hybrid perovskites. Perovskites are named after a mineral that has the chemical formula ABX3. In the X position, anions form an octahedron, while in the A position cations form a cube around them, while a central cation takes the B position. Many materials in the perovskite family adopt this crystal structure. Hybrid perovskites contain organic cations in the A position.