Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots

TLDR

The authors aim to determine size‑dependent interband transition energies for PbSe quantum dots and introduce a coherent superposition model for multiple exciton generation. Transient absorption spectroscopy was employed to measure intraband and interband probe energies, and the new coherent superposition model for MEG was developed. Ultra‑efficient multiple exciton generation was observed in PbSe and PbS quantum dots, with quantum yields up to 300 % (≈three excitons per photon) at photon energies four times the bandgap, MEG onset at twice the lowest exciton energy, and a biexciton‑induced modulation of early transient absorption.

Abstract

We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent superposition of multiple excitonic states.