Multiple Exciton Generation in Colloidal Nanocrystals

Abstract

In a conventional solar cell, the energy of an absorbed photon in excess of the band gap is rapidly lost as heat, and this is one of the main reasons that the theoretical efficiency is limited to ~33%. However, an alternative process, multiple exciton generation (MEG), can occur in colloidal quantum dots. Here, some or all of the excess energy is instead used to promote one or more additional electrons to the conduction band, potentially increasing the photocurrent of a solar cell and thereby its output efficiency. This review will describe the development of this field over the decade since the first experimental demonstration of multiple exciton generation, including the controversies over experimental artefacts, comparison with similar effects in bulk materials, and the underlying mechanisms. We will also describe the current state-of-the-art and outline promising directions for further development.

Multiple exciton generation in quantum dot-based solar cells

“…in an operational solar cell), strengthening the quantum confinement also weakens the interparticle coupling within the QD film [54] and is thus detrimental for efficient charge extraction. It follows that changing the quantum confinement has an effect not only on the MEG process itself, but also alters device parameters that are crucial for PV performance [29,49,[54][55][56].…”

“…tuning of ligand chemistry, QD shape and material) [26][27][28]. Besides the QD film designs presented here, there are other promising quantum-confined materials including indium phosphide QDs [7], (6,5) single-walled carbon nanotubes [128] and silicon QDs [58], which show high CM rates at comparably low photon energies and η MEG in a solar cell [29,31,32]. For further improvements in MEG-enhanced PV performance, a deeper understanding of the MEG photophysics is therefore necessary.…”

“…A different scenario is presented when the semiconductor is confined to the nanoscale where quantum confinement effects can reduce the aforementioned material restrictions for the generation of additional charge-carrier pairs [29,40]. Quantum confinement effects emerge when the generated excitons are restricted to smaller spatial volumes than they would occupy in the parental bulk material.…”

“…For MEG in particular, recent spectroscopic findings suggest a combination of high MEG threshold energies and photon energy-dependent MEG efficiency (i.e. MEG efficiency increases slowly after the threshold energy rather than as a step function) as the predominant reasons for the modest contribution to PV power conversion [29,30].…”

“…There are several comprehensive review articles summarising the current understanding of the MEG mechanism itself provided in the literature [29,31,32]. In this review, we will only briefly discuss mechanistic details and will focus mainly on recent progress towards an efficient integration of MEG in operational solar cells.…”