2017 Henry Moseley Medal and Prize

Dr Akshay Rao of the University of Cambridge for groundbreaking studies in the electronic properties of organic semiconductors, particularly the roles of electron spin in the operation of solar cells.

Dr Akshay Rao is an outstanding physicist who has made a pivotal impact in the field of organic semiconductors. Rao’s principal concern has been the fission of spin-singlet photoexcitations in organic semiconductors into pairs of spin-triplet excitons. This process is spin-allowed and energetically allowed when the spin-triplet exciton matches half the singlet energy, as found in materials including pentacene.

Rao showed this process can be very fast and efficient, that the triplet exciton can be ionised to an electron–hole pair at a suitable heterojunction, and that the triplet exciton can be rapidly tunnelled into an inorganic semiconductor. He further showed how this process of fission is driven by the coupling of electronic and vibrational modes. This has established the scope to use this process of exciton doubling to couple to practical solar cells, which may allow for tandem-cell performance in a cheap single-junction cell.

Rao has also worked on the processes of electron-hole separation at the donor-acceptor heterojunctions used in organic solar cells. With Artem Bakulin he revealed the role of hot vibrational states in the process of charge separation. With Simon Gelinas he showed that the initial charge separation process can be extremely rapid, moving electron and hole more than 5 nm apart within 40 fsec and thereby escaping the electron–hole Coulomb attraction that would otherwise cause rapid recombination.

These observations indicate that there is quantum coherent electron motion at early times across multiple molecular units, and this paper has had a ground-breaking impact in this and closely related fields. Rao studied also electron–hole recombination and showed how kinetic processes can overcome the statistical formation of spin-triplet states.
This body of research has brought real international recognition to the UK activity in this field.