Size and Energy Level Tuning of Quantum Dot Solids via a Hybrid Ligand Complex

Abstract

The performance of quantum dots (QDs) in optoelectronic devices suffers as a result of sub-bandgap states induced by the large fraction of atoms on the surface of QDs. Recent progress in passivating these surface states with thiol ligands and halide ions has led to competitive efficiencies. Here, we apply a hybrid ligand mixture to passivate PbSe QD sub-bandgap tail states via a low-temperature, solid-state ligand exchange. We show that this ligand mixture allows tuning of the energy levels and the physical QD size in the solid state during film formation. We hereby present a novel, postsynthetic path to tune the properties of QD films.

Multiple exciton generation in quantum dot-based solar cells

“…In these reports, the authors also find a close correlation between MEG th E and the MEG efficiency (η MEG ). While there seems to be general consensus that relaxed crystal momentum conservation rules in quantum-confined materials [47] contribute to MEG th E being close to 2E g , there is disagreement in the literature on how other quantum material properties such as discrete energy levels around the QD band gap [47] enhanced coulomb interaction in nanostructures of lower symmetry than spherical QDs [48] or the increased surface-tovolume ratio [49] contribute to the crucial MEG parameters MEG th E and η MEG [50,51]. What seems clear, however, is that materials with weak charge-phonon coupling (e.g.…”

“…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].…”

“…Furthermore, convenient ligand exchange methods have been established for these particles, giving control over properties of the QD superstructure (e.g. interparticle spacing [55], energy alignment [49], etc.). Together with slow hot carrier cooling, lead chalcogenide QDs are a valuable material platform to study MEG-related parameters not only on the process itself, but also on the PV device level.…”

“…Such surface states are generally referred to as tail or trap states and are thought to be responsible for some of the voltage losses associated with QD devices. As the wavefunctions of charge carriers are spatially confined in a smaller volume than in the parental bulk material [49], it follows that the probability of finding charge carriers at the particle surface is greatly increased. The consequence of both effects is that, in QDs, a significant proportion of generated charge carriers will get trapped at surface sites that are energetically removed from the semiconductor's charge transport level [72].…”

“…These strategies use for such surface passivation either a ligand sphere of organic [55,82,83] and/or inorganic molecules [49,80,84] [82,83,89]. While QDs passivated with dithiol ligands showed high charge-carrier mobilities [90,91] and competitive device performances in solar cells, no evidence for MEG could be found in devices [26,60].…”