Laurens Siebbeles: Exciting multiple electrons by one photon in semiconductor nanomaterials for photovoltaics

Absorption of sufficiently energetic photons in a bulk semiconductor leads to hot electrons and holes that usually cool to the band edge by thermal relaxation. In semiconductor nanomaterials this cooling can be intercepted by excitation of additional electrons across the band gap. In this way, one photon generates multiple electron-hole pairs via a process known as Carrier Multiplication (CM), which is of interest for the development of highly efficient solar cells and photodetectors.

We studied charge carrier photogeneration, CM, charge mobility and decay in: a) films of PbSe nanocrystals coupled by organic ligands, and b) 2D percolative networks of PbSe nanocrystals directly connected by atomic bonds. The studies were performed using ultrafast pump-probe spectroscopy with optical or terahertz conductivity detection.

The nanogeometry of the material was found to have drastic impact on the efficiency of CM and the charge mobility. In percolative PbSe networks CM occurs in a step-like fashion with threshold near the minimum photon energy of twice the band gap. In these networks the CM efficiency and charge mobility are much higher than for films of PbSe nanocrystals that are coupled by organic ligands.

The factors governing the efficiency of CM and the impact on the efficiency of photovoltaic devices will be discussed.