Proceedings of nanoGe September Meeting 2017 (NFM17)
Publication date: 20th June 2016
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 quantum dots coupled by organic ligands, and b) 2D percolative networks of PbSe quantum dots 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 enormous effects on the charge mobility and the yield of free charges resulting from CM. In 2D percolative PbSe networks CM occurs in a step-like fashion with threshold near the minimum energy of twice the band gap. In these 2D materials the CM efficiency and charge mobility are much higher than for films of QDs that are coupled by organic ligands. The effects of nanogeometry on the efficiency of CM and impact on power conversion in photovoltaic devices will be discussed.
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