When a semiconductor absorbs photons with energy in excess of the bandgap, the charge carriers generated will initially have a characteristic temperature that is higher than the lattice temperature. The carriers then proceed to cool down towards the bandgap through carrier-phonon scattering, which typically takes place in under 1 picosecond. That is much faster than the timescales involved in extracting the carriers in a solar cell device. This ultrafast dissipation of the excess photon energy before it may be extracted leads to what is known as the Shockley-Queisser limit for the efficiency of solar cells, which is around 30%. It is not a fundamental limit however: if carriers could be extracted before cooling, great gains in efficiency would be possible. A positive feature is that hot carriers typically diffuse faster than cold carriers, but the sub-picosecond timescales impose the largest challenge. Recently, it was shown that tin-based perovskites have carrier cooling times as long as nanoseconds, so there is immense promise that they can be used in new solar cell designs.

In this Master’s thesis, the student will learn and apply an innovative technique developed in our lab to study hot-carrier transport in tin-based halide perovskites in collaboration with the group of Maria Antonietta Loi (University of Groningen, Netherlands). The technique used is ultrafast holographic microscopy, a novel concept that enables ultrafast imaging of large sample areas simultaneously. Standard femtosecond microscopes image carrier diffusion around a single diffraction-limited excitation spot, lacking any ability to address sample heterogeneities and needing tedious sequences of measurements of different spots to acquire statistics. Thanks to the new holographic approach, we are able to study carrier diffusion around 100 diffraction limited spots in a single measurement, vastly outperforming standard techniques. The student will perform experiments to characterize the carrier diffusion in tin-based perovskites and analyze the data to extract the diffusion parameters.

If you want to have a chat and visit the lab, or more information on how the technique works, please email franco.camargo@cnr.it.

See also:

• H-H. Fang, S. Adjokatse, S. Shao, J. Even, and M. A. Loi, Nature Communications 9, 243 (2018)
• M. Liebel, F. V. A. Camargo, G. Cerullo, and N. F. van Hulst, Nano Letters 21, 4, 1666–1671 (2021)
• M. Hörmann, F. Visentin, …, G. Cerullo, and F. V. A. Camargo, Ultrafast Science 3, 0032 (2023)