Charge transfer between quantum dots (QDs) and redox molecules is not well described by the Marcus theory, the hall-mark theory for charge transfer in molecular systems. The Marcus inverted region, where the rate decreases with increasing the free energy difference, has never been observed in QDs. The previously reported hypothesis for the absence of the Marcus inverted region in QDs is an Auger-assisted charge transfer pathway, which we refer to as the "Auger hypothesis". Here, we show that the Auger hypothesis does not hold to experimental tests. We measured the rate constants for two processes where Auger is either allowed or not: charge separation and charge recombination. We used ultrafast transient absorption spectroscopy to probe the rate of charge separation and recombination between PbS QDs and ferrocene derivatives ligands bound to their surface. We find that the rate constant for both charge separation and recombination increases by increasing the free energy difference, is temperature-independent, and increases with the number of molecular acceptors. All these results are against theoretical predictions for an Auger-assisted charge transfer pathway.

Indium phosphide (InP) quantum dots (QDs) are promising heavy-metal-free materials for optoelectronics, but their redox stability, trap-state landscape, and charge carrier dynamics are not well understood. Here we investigate InP and InP/ZnSe/ZnS QD films with different ligands by using spectroelectrochemistry. For both core-only and core/shell/shell QD films, the absorption spectra remain unchanged during charging, indicating that injected charges do not populate the conduction or valence bands. InP/ZnSe/ZnS QD films with original ligands exhibit reversible photoluminescence (PL) modulation: an increase at modest cathodic potentials, followed by quenching at more negative potentials. Solid-state ligand exchange using ethylenediamine (2DA) and sodium sulfide (Na₂S) enhances conductivity and induces stronger PL changes at both cathodic and anodic potentials. These results are in line with the population of electron traps at modest cathodic potentials (i.e., near the midbandgap), suppressing nonradiative recombination and increasing the PL. At more negative potentials, electrochemical reactions of surface species result in new trap states quenching the PL. Our findings provide insights into the stability and trap-state-mediated carrier dynamics during electrochemical charging of InP-based QDs.

Control over the electron density and conductivity is a cornerstone of semiconductor technology. Here, we report electrochemical control over electron density and conductivity in films of InAs colloidal quantum dots (QDs) capped with ethanedithiol ligands. The quantum-confined twofold degenerate 1S1/2(e) electron state can be reversibly and completely filled. Increasing the electron population yields four bleach features in the optical-absorption spectrum associated transitions to the 1S1/2(e) state, and state-resolved electronic conductivity which follows the 1S1/2(e) density of states, reaching a maximum of 0.45 S/m at 0.5 electrons per QD. The absence of 1P(e) bleach features and state-resolved conductivity imply a wide separation between the 1S1/2(e) and 1P(e) states resulting in electronic transport between 1S(e) states exclusively. The reversible electrochemistry of InAs QDs films allows determination of the absolute energy levels. InAs QDs of 4.2 nm in edge length and capped with ethanedithiol ligands are natively n-doped with the Fermi level at -4.6 eV, the 1S1/2(e) state at -4.28 eV and the 1S3/2(h) state at -5.54 eV vs vacuum. This work establishes a way to precisely control the charge carrier density and conductivity and gives insights into the charge transport properties and electronic structure of InAs QD films, opening the possibility of making devices with InAs QDs in which the charge carrier density is precisely controlled electrochemically.

The efficiency of quantum dot (QD) light-emitting diodes is limited by inefficient hole injection into the valence levels of the QDs. Electrochemical doping, where mobile ions form electrical double layers (EDLs) at electrodes, offers a route to removing injection barriers. While QD light-emitting electrochemical cells (QLECs) have shown promise, prior studies relied on additional charge injection layers, complicating the study of charge injection into QDs. In this work, devices with a simple ITO/QD active layer/Al structure were fabricated using highly photoluminescent ligand-exchanged CdSe/CdS/ZnS QDs, poly(ethylene oxide), and lithium trifluoromethanesulfonate as electrolyte. We show that the dense QD films in these QLECs can be electrochemically doped, transport charges, and exhibit electroluminescence. Symmetrical cyclic voltammograms and operando photoluminescence measurements prove that these devices function as electrochemically doped LECs. Spectroelectrochemical experiments on separately n- and p-doped QD films indicate that hole injection remains the primary limitation in QLEC performance. These findings demonstrate that using EDLs to facilitate charge injection in QD light-emitting devices is promising, but significant challenges remain to be solved before electron and hole injections are balanced.

Electrochemical charging of films of semiconductor nanocrystals (NCs) allows precise control over their Fermi level and opens up new possibilities for use of semiconductor NCs in optoelectronic devices. Unfortunately, charges added to the semiconductor NCs are often lost due to electrochemical side reactions. In this work, we examine which loss processes can occur in electrochemically charged semiconductor NC films by comparing numerical drift-diffusion simulations with experimental data. Both reactions with impurities in the electrolyte solution, as well as reactions occurring on the surface of the nanomaterials themselves, are considered. We show that the Gerischer kinetic model can be used to accurately model the one-electron transfer between charges in the semiconductor NC and oxidant or reductant species in solution. Simulations employing the Gerischer model are in agreement with experimental results of charging of semiconductor NC films with ideal one-electron acceptors ferrocene and cobaltocene. We show that reactions of charges in the semiconductor NC film with redox species in solution are reversible when the reduction potential is in the conduction band of the semiconductor NC material but are irreversible when the reduction potential is in the band gap. Experimental charging of semiconductor NC films in the presence of oxygen is always irreversible in our system, even when the reduction potential of oxygen is in the conduction band of the semiconductor NC material. We show that the Gerischer model in combination with a coupled reversible-irreversible reaction mechanism can be used to model oxygen reduction. Finally, we model irreversible reduction reactions with the semiconductor NC material itself, such as reduction of ligands or surface ions. Simulations of semiconductor NC cyclic voltammograms in the presence of material reduction reactions strongly resemble experimental cyclic voltammograms of InP and CdSe NC films. This marks material reduction reactions at the semiconductor NC surface as a likely candidate for the irreversible behavior of these materials in electrochemical experiments. These results show that all reduction reactions with redox potentials in the band gap of semiconductor NCs must be suppressed in order to achieve stable charging of these materials.

Indium phosphide (InP) quantum dots (QDs) are considered the most promising alternative for Cd and Pb-based QDs for lighting and display applications. However, while core-only QDs of CdSe and CdTe have been prepared with near-unity photoluminescence quantum yield (PLQY), this is not yet achieved for InP QDs. Treatments with HF have been used to boost the PLQY of InP core-only QDs up to 85%. However, HF etches the QDs, causing loss of material and broadening of the optical features. Here, we present a simple postsynthesis HF-free treatment that is based on passivating the surface of the InP QDs with InF_3. For optimized conditions, this results in a PLQY as high as 93% and nearly monoexponential photoluminescence decay. Etching of the particle surface is entirely avoided if the treatment is performed under stringent acid-free conditions. We show that this treatment is applicable to InP QDs with various sizes and InP QDs obtained via different synthesis routes. The optical properties of the resulting core-only InP QDs are on par with InP/ZnSe/ZnS core-shell QDs, with significantly higher absorption coefficients in the blue, and with potential for faster charge transport. These are important advantages when considering InP QDs for use in micro-LEDs or photodetectors.