Absorption of energetic photons in a semiconductor leads to hot electrons and holes that usually cool to the band edge by thermal relaxation. 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.1
We studied charge carrier photogeneration, CM, charge mobility and decay in assemblies of PbSe quantum dots and a bulk perovskite material. The studies were performed using ultrafast pump-probe spectroscopy with optical or microwave/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.2 In this 2D material the CM efficiency and charge mobility are much higher than for films of quantum dots that are coupled by organic ligands. We also found efficient CM in a low band gap bulk Sn/Pb halide perovskite with onset just above twice the band gap.3
The effects of nanogeometry and electronic band structure on the efficiency of CM, as well as impact on power conversion in photovoltaic devices will be discussed.
1) S. ten Cate, C.S. Suchand Sandeep, Y. Liu, M. Law, S. Kinge, A.J. Houtepen, J.M. Schins, and L.D. A.Siebbeles, "Generating Free Charges by Carrier Multiplication in Quantum Dots for Highly Efficient Photovoltaics", Acc. Chem. Res. 48, 174-181 (2015).
2) A. Kulkarni, W.H. Evers, S. Tomić, M.C. Beard, D. Vanmaekelbergh, and L.D.A. Siebbeles, "Efficient Step-Like Carrier Multiplication in Percolative Networks of Epitaxially Connected PbSe Nanocrystals," ACS Nano 12, 378-384 (2018).
3) S. Maiti, S. Ferro, D. Poonia, B. Ehrler, S. Kinge, and L.D.A. Siebbeles, "Efficient Carrier Multiplication in Low Band Gap Mixed Sn/Pb Halide Perovskites," J. Phys. Chem. Lett. 11, 6146-6149 (2020).