Exciton Dissociation within Quantum Dot–Organic Complexes: Mechanisms, Use as a Probe of Interfacial Structure, and Applications

Abstract

This article reviews the structural and electronic features of colloidal quantum dot (QD)–organic complexes that influence the rate of photoinduced charge separation (PCS) across the interface between the inorganic core of the QD and its organic surface ligands. While Marcus theory can be used to describe the rate of PCS in QD–organic complexes, uncertainties in the exact atomic configuration of the inorganic–organic interface and heterogeneities in this interfacial structure within an ensemble of QDs complicate the determination of the most fundamental Marcus parameterselectronic coupling, reorganization energy, and driving force. This article discusses strategies for accounting for uncertainties and heterogeneities when using Marcus theory to interpret rates of PCS in QD–organic complexes and highlights how measurement of PCS rates can provide information about the interfacial structure of the QD surface. Recent progress in the application of mechanistic knowledge of PCS to harvest multiple charge carriers from QDs containing multiple excitons and extend the lifetime of the charge-separated state is also discussed.

Efficiency of Hole Transfer from Photoexcited Quantum Dots to Covalently Linked Molecular Species

“…Charge transfer studies in the literature have been mostly done on low PLQY, trap-heavy particles. (8) In these trap-heavy particles, charge transfer must compete with the native highly fluctuating nonradiative pathways. In contrast, we demonstrate through these studies that one can use the design of QD heterostructures and acceptor ligands to mitigate undesirable and ill-defined traps, while still being able to extract the charge efficiently to desirable traps with specificity in energy level, physical and electronic interaction, and quantity.…”

“…(7) A growing body of spectroscopic work has examined charge transfer rates from QDs to molecular charge acceptors typically physisorbed onto the QD surface, exploring the parameter space in the Marcus equation. (8) Electron transfer studies (8)(9)(10)(11)(12)(13) outnumber hole studies, (14)(15)(16)(17)(18)(19) despite hole transfer being the limiting factor in the efficiencies of QD sensitized solar cells and in QD-based colloidal photocatalytic hydrogen evolving systems. (20,21) To establish a sound model for charge transfer from nanocrystals to molecular acceptors, we must address the features of this system that make the process more difficult to characterize than that of the pure molecular case.…”

Hole Transfer from Photoexcited Quantum Dots: The Relationship between Driving Force and Rate

“…However, clear relationships between driving force and rate have been difficult to chart due to inherent heterogeneities in ensembles of QD−molecular conjugates, a limited set of tools for accurately probing the QD surface, and a lack of control in varying the driving force without also affecting other key parameters. 31 Many QD− molecular charge transfer studies employ molecular species that weakly bind to the QD surface in an ill-defined manner, thus eliminating the ability to accurately determine a reliable charge transfer rate per molecule. A recent review by Knowles et al 31 highlights much of this work and the associated difficulties in performing mechanistic studies on QD−molecular charge transfer systems.…”

“…31 Many QD− molecular charge transfer studies employ molecular species that weakly bind to the QD surface in an ill-defined manner, thus eliminating the ability to accurately determine a reliable charge transfer rate per molecule. A recent review by Knowles et al 31 highlights much of this work and the associated difficulties in performing mechanistic studies on QD−molecular charge transfer systems.…”

Photoinduced Charge Accumulation in Molecular Systems

“…so-called protoncoupled electron transfer (PCET), [3,[5][6][7] or which are followed by a secondary photoinduced electron transfer leading to the accumulation of two redox equivalents on a given molecular unit. In nanoparticles the light-driven accumulation of multiple charge carriers is readily possible, [8] and such materials are of course highly promising for application purposes. In purely molecular systems, photoinduced charge accumulation is more difficult to achieve, and until now there have been comparatively few studies which have succeeded in this regard.…”