Absorption of energetic photons in a semiconductor leads to hot electrons and holes that usually cool to the band edge by thermal relaxation. In 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 Carrier Multiplication (CM), which is of interest for highly efficient photovoltaics.
We studied cooling and CM in: a) films of PbSe nanocrystals coupled by organic ligands, and b) 2D superlattices of directly coupled PbSe nanocrystals. The studies were performed using pump-probe spectroscopy with optical or terahertz/microwave conductivity detection.
Reducing the size of ligands between nanocrystals strongly increases the charge mobility. Removal of ligands and direct coupling of nanocrystals in a square or honeycomb superlattice allowed us to further tune the mobility and to realize values as high as 150 cm^2/Vs. We found that a high mobility is essential for multiple electron-hole pairs formed via CM to escape from recombination.
The coupling between the nanocrystals was found to strongly affect the competition between cooling of hot charges by phonon emission and CM. In square 2D superlattices of nanocrystals CM is much more efficient than in films with ligands between the nanocrystals. In square superlattices CM occurs in a step-like fashion with threshold near the minimum energy of twice the band gap.
The factors governing the charge mobility and the efficiency of CM, as well as the impact on the efficiency of photovoltaic devices, will be discussed on the basis of theoretical modeling.
Laurens D. A. Siebbeles, "Carrier cooling and multiplication in semiconductor nanocrystals and superlattices (Conference Presentation)," Proc. SPIE 10916, Ultrafast Phenomena and Nanophotonics XXIII, 109160T (Presented at SPIE OPTO: February 04, 2019; Published: 5 March 2019); https://doi.org/10.1117/12.2506483.6010442174001.
Conference Presentations are recordings of oral presentations given at SPIE conferences and published as part of the proceedings. They include the speaker's narration with video of the slides and animations. Most include full-text papers. Interactive, searchable transcripts and closed captioning are now available for most presentations.
Search our growing collection of more than 22,000 conference presentations, including many plenaries and keynotes.