Elisa M. Miller

Diffusion-Controlled Synthesis of PbS and PbSe Quantum Dots with in Situ Halide Passivation for Quantum Dot Solar Cells

We developed a simple non-hot-injection synthetic route that achieves in situ halide-passivated PbS and PbSe quantum dots (QDs) and simplifies the fabrication of Pb-chalcogenide QD solar cells. The synthesis mechanism follows a temperature-dependent diffusion growth model leading to strategies that can achieve narrow size distributions for a range of sizes. We show that PbS QDs can be produced with a diameter as small as 2.2 nm, corresponding to a 1.7 eV band gap, while the resulting size distribution (6-7%) is comparable to that of hot-injection syntheses. The in situ chloride surface passivation is demonstrated by X-ray photoelectron spectroscopy and an improved photostability of both PbS and PbSe QDs when stored under air. Additionally, the photoluminescence quantum yield of the PbS QDs is ∼30% higher compared to the traditional synthesis. We show that PbS QD solar cells with 6.5% power conversion efficiency (PCE) can be constructed. Finally, we fabricated PbSe QD solar cells in air (rather than in inert atmosphere), achieving a PCE of 2.65% using relatively large QDs with a corresponding band gap of 0.89 eV.

PbSe Quantum Dot Solar Cells with More than 6% Efficiency Fabricated in Ambient Atmosphere

Colloidal quantum dots (QDs) are promising candidates for the next generation of photovoltaic (PV) technologies. Much of the progress in QD PVs is based on using PbS QDs, partly because they are stable under ambient conditions. There is considerable interest in extending this work to PbSe QDs, which have shown an enhanced photocurrent due to multiple exciton generation (MEG). One problem complicating such device-based studies is a poor stability of PbSe QDs toward exposure to ambient air. Here we develop a direct cation exchange synthesis to produce PbSe QDs with a large range of sizes and with in situ chloride and cadmium passivation. The synthesized QDs have excellent air stability, maintaining their photoluminescence quantum yield under ambient conditions for more than 30 days. Using these QDs, we fabricate high-performance solar cells without any protection and demonstrate a power conversion efficiency exceeding 6%, which is a current record for PbSe QD solar cells.

Metal Halide Solid-State Surface Treatment for High Efficiency PbS and PbSe QD Solar Cells

We developed a layer-by-layer method of preparing PbE (E = S or Se) quantum dot (QD) solar cells using metal halide (PbI2, PbCl2, CdI2, or CdCl2) salts dissolved in dimethylformamide to displace oleate surface ligands and form conductive QD solids. The resulting QD solids have a significant reduction in the carbon content compared to films treated with thiols and organic halides. We find that the PbI2 treatment is the most successful in removing alkyl surface ligands and also replaces most surface bound Cl- with I-. The treatment protocol results in PbS QD films exhibiting a deeper work function and band positions than other ligand exchanges reported previously. The method developed here produces solar cells that perform well even at film thicknesses approaching a micron, indicating improved carrier transport in the QD films. We demonstrate QD solar cells based on PbI2 with power conversion efficiencies above 7%.

Air-Stable and Efficient PbSe Quantum-Dot Solar Cells Based upon ZnSe to PbSe Cation-Exchanged Quantum Dots.

We developed a single step, cation-exchange reaction that produces air-stable PbSe quantum dots (QDs) from ZnSe QDs and PbX2 (X = Cl, Br, or I) precursors. The resulting PbSe QDs are terminated with halide anions and contain residual Zn cations. We characterized the PbSe QDs using UV-vis-NIR absorption, photoluminescence quantum yield spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Solar cells fabricated from these PbSe QDs obtained an overall best power conversion efficiency of 6.47% at one sun illumination. The solar cell performance without encapsulation remains unchanged for over 50 days in ambient conditions; and after 50 days, the National Renewable Energy Laboratory certification team certified the device at 5.9%.

Defect Tolerance to Intolerance in the Vacancy-Ordered Double Perovskite Semiconductors Cs2SnI6 and Cs2TeI6

Vacancy-ordered double perovskites of the general formula A2BX6 are a family of perovskite derivatives composed of a face-centered lattice of nearly isolated [BX6] units with A-site cations occupying the cuboctahedral voids. Despite the presence of isolated octahedral units, the close-packed iodide lattice provides significant electronic dispersion, such that Cs2SnI6 has recently been explored for applications in photovoltaic devices. To elucidate the structure-property relationships of these materials, we have synthesized solid-solution Cs2Sn1-xTexI6. However, even though tellurium substitution increases electronic dispersion via closer I-I contact distances, the substitution experimentally yields insulating behavior from a significant decrease in carrier concentration and mobility. Density functional calculations of native defects in Cs2SnI6 reveal that iodine vacancies exhibit a low enthalpy of formation, and that the defect energy level is a shallow donor to the conduction band rendering the material tolerant to these defect states. The increased covalency of Te-I bonding renders the formation of iodine vacancy states unfavorable and is responsible for the reduction in conductivity upon Te substitution. Additionally, Cs2TeI6 is intolerant to the formation of these defects, because the defect level occurs deep within the band gap and thus localizes potential mobile charge carriers. In these vacancy-ordered double perovskites, the close-packed lattice of iodine provides significant electronic dispersion, while the interaction of the B- and X-site ions dictates the properties as they pertain to electronic structure and defect tolerance. This simplified perspective based on extensive experimental and theoretical analysis provides a platform from which to understand structure-property relationships in functional perovskite halides.

Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films

We use a high signal-to-noise X-ray photoelectron spectrum of bulk PbS, GW calculations, and a model assuming parabolic bands to unravel the various X-ray and ultraviolet photoelectron spectral features of bulk PbS as well as determine how to best analyze the valence band region of PbS quantum dot (QD) films. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) are commonly used to probe the difference between the Fermi level and valence band maximum (VBM) for crystalline and thin-film semiconductors. However, we find that when the standard XPS/UPS analysis is used for PbS, the results are often unrealistic due to the low density of states at the VBM. Instead, a parabolic band model is used to determine the VBM for the PbS QD films, which is based on the bulk PbS experimental spectrum and bulk GW calculations. Our analysis highlights the breakdown of the Brillioun zone representation of the band diagram for large band gap, highly quantum confined PbS QDs. We have also determined that in 1,2-ethanedithiol-treated PbS QD films the Fermi level position is dependent on the QD size; specifically, the smallest band gap QD films have the Fermi level near the conduction band minimum and the Fermi level moves away from the conduction band for larger band gap PbS QD films. This change in the Fermi level within the QD band gap could be due to changes in the Pb:S ratio. In addition, we use inverse photoelectron spectroscopy to measure the conduction band region, which has similar challenges in the analysis of PbS QD films due to a low density of states near the conduction band minimum.

Control of plasmonic and interband transitions in colloidal indium nitride nanocrystals.

We have developed a colloidal synthesis of 4-10 nm diameter indium nitride (InN) nanocrystals that exhibit both a visible absorption onset (∼1.8 eV) and a strong localized surface plasmon resonance absorption in the mid-infrared (∼3000 nm). Chemical oxidation and reduction reversibly modulate both the position and intensity of this plasmon feature as well as the band-to-band absorption onset. Chemical oxidation of InN nanocrystals with NOBF4 is found to red-shift the absorption onset to ∼1.3 eV and reduce the plasmon absorption energy (to 3550 nm) and intensity (by an order of magnitude at 2600 nm). Reduction of these oxidized species with Bu4NBH4 fully recovers the original optical properties. Calculations suggest that the carrier density in these InN nanocrystals decreases upon oxidation from 2.89 × 10(20) cm(-3) to 2.51 × 10(20) cm(-3), consistent with the removal of ∼4 electrons per nanocrystal. This study provides a unique example of the ability to tune the optical properties of colloidal nanomaterials, and in particular the LSPR absorption, with reversible redox reactions that do not affect the semiconductor chemical composition or phase.

Semiconductor Interfacial Carrier Dynamics via Photoinduced Electric Fields

Charge separation viewed in reflection When light strikes a semiconductor, excited electrons travel across the interface. Y. Yang et al. applied ultrafast reflection spectroscopy to probe this process in a gallium indium phosphide system used for hydrogen generation from water (see the Perspective by Hansen et al.). Platinum and titanium dioxide (TiO2) coatings enhanced charge separation of the excited electrons from the positive holes they left behind. TiO2, however, was more effective at suppressing the reverse process of unproductive recombination. Science, this issue p. 1061; see also p. 1030 Reflection spectroscopy offers insights into the boost to charge separation conferred by TiO2 coatings on photoelectrodes. [Also see Perspective by Hansen] Solar photoconversion in semiconductors is driven by charge separation at the interface of the semiconductor and contacting layers. Here we demonstrate that time-resolved photoinduced reflectance from a semiconductor captures interfacial carrier dynamics. We applied this transient photoreflectance method to study charge transfer at p-type gallium-indium phosphide (p-GaInP2) interfaces critically important to solar-driven water splitting. We monitored the formation and decay of transient electric fields that form upon photoexcitation within bare p-GaInP2, p-GaInP2/platinum (Pt), and p-GaInP2/amorphous titania (TiO2) interfaces. The data show that a field at both the p-GaInP2/Pt and p-GaInP2/TiO2 interfaces drives charge separation. Additionally, the charge recombination rate at the p-GaInP2/TiO2 interface is greatly reduced owing to its p-n nature, compared with the Schottky nature of the p-GaInP2/Pt interface.

Preparation of Cd/Pb Chalcogenide Heterostructured Janus Particles via Controllable Cation Exchange

We developed a strategy for producing quasi-spherical nanocrystals of anisotropic heterostructures of Cd/Pb chalcogenides. The nanostructures are fabricated via a controlled cation exchange reaction where the Cd(2+) cation is exchanged for the Pb(2+) cation. The cation exchange reaction is thermally activated and can be controlled by adjusting the reaction temperature or time. We characterized the particles using TEM, XPS, PL, and absorption spectroscopy. With complete exchange, high quality Pb-chalcogenide quantum dots are produced. In addition to Cd(2+), we also find suitable conditions for the exchange of Zn(2+) cations for Pb(2+) cations. The cation exchange is anisotropic starting at one edge of the nanocrystals and proceeds along the ⟨111⟩ direction producing a sharp interface at a (111) crystallographic plane. Instead of spherical core/shell structures, we produced and studied quasi-spherical CdS/PbS and CdSe/PbSe Janus-type heterostructures. Nontrivial PL behavior was observed from the CdS(e)/PbS(e) heterostructures as the Pb:Cd ratio is increased.

Solvent-Mediated Electron Hopping: Long-Range Charge Transfer in IBr−(CO2) Photodissociation

CO2 Lends a Hand Solvent plays a complex and multifaceted role in facilitating charge transfer events. One obstacle to understanding its influence is that solvent molecules are in constant motion; just teasing out their arrangement in space at the point in time when an electron hops from one substrate to another is often a great challenge. Sheps et al. (p. 220; published online 4 March) have studied a highly simplified prototype system, in which a single CO2 molecule coordinates, as a solvent might, to an IBr− ion in the gas phase. A combination of ultrafast photoelectron spectroscopy and theoretical simulations was applied that suggests that even this solitary interaction is sufficient to induce electron transfer from iodide to bromine during a dissociation reaction. Energy channeled through CO2-bending vibrations promoted formation of I(CO2) and Br−. The presence of an intervening carbondioxide molecule dramatically changes the electron transfer probability between two halogen atoms. Chemical bond breaking involves coupled electronic and nuclear dynamics that can take place on multiple electronic surfaces. Here we report a time-resolved experimental and theoretical investigation of nonadiabatic dynamics during photodissociation of a complex of iodine monobromide anion with carbon dioxide [IBr–(CO2)] on the second excited (A′) electronic state. Previous experimental work showed that the dissociation of bare IBr– yields only I– + Br products. However, in IBr–(CO2), time-resolved photoelectron spectroscopy reveals that a subset of the dissociating molecules undergoes an electron transfer from iodine to bromine 350 femtoseconds after the initial excitation. Ab initio calculations and molecular dynamics simulations elucidate the mechanism for this charge hop and highlight the crucial role of the carbon dioxide molecule. The charge transfer between two recoiling atoms, assisted by a single solvent-like molecule, provides a notable limiting case of solvent-driven electron transfer over a distance of 7 angstroms.