Hyekyoung Choi

Steric-Hindrance-Driven Shape Transition in PbS Quantum Dots: Understanding Size-Dependent Stability

Ambient stability of colloidal nanocrystal quantum dots (QDs) is imperative for low-cost, high-efficiency QD photovoltaics. We synthesized air-stable, ultrasmall PbS QDs with diameter (D) down to 1.5 nm, and found an abrupt transition at D ≈ 4 nm in the air stability as the QD size was varied from 1.5 to 7.5 nm. X-ray photoemission spectroscopy measurements and density functional theory calculations reveal that the stability transition is closely associated with the shape transition of oleate-capped QDs from octahedron to cuboctahedron, driven by steric hindrance and thus size-dependent surface energy of oleate-passivated Pb-rich QD facets. This microscopic understanding of the surface chemistry on ultrasmall QDs, up to a few nanometers, should be very useful for precisely and accurately controlling physicochemical properties of colloidal QDs such as doping polarity, carrier mobility, air stability, and hot-carrier dynamics for solar cell applications.

Ultrastable PbSe Nanocrystal Quantum Dots via in Situ Formation of Atomically Thin Halide Adlayers on PbSe(100)

The fast degradation of lead selenide (PbSe) nanocrystal quantum dots (NQDs) in ambient conditions impedes widespread deployment of the highly excitonic, thus versatile, colloidal NQDs. Here we report a simple in situ post-synthetic halide salt treatment that results in size-independent air stability of PbSe NQDs without significantly altering their optoelectronic characteristics. From TEM, NMR, and XPS results and DFT calculations, we propose that the unprecedented size-independent air stability of the PbSe NQDs can be attributed to the successful passivation of under-coordinated PbSe(100) facets with atomically thin PbX2 (X = Cl, Br, I) adlayers. Conductive films made of halide-treated ultrastable PbSe NQDs exhibit markedly improved air stability and behave as an n-type channel in a field-effect transistor. Our simple in situ wet-chemical passivation scheme will enable broader utilization of PbSe NQDs in ambient conditions in many optoelectronic applications.

One-Step Deposition of Photovoltaic Layers Using Iodide Terminated PbS Quantum Dots.

We present a one-step layer deposition procedure employing ammonium iodide (NH4I) to achieve photovoltaic quality PbS quantum dot (QD) layers. Ammonium iodide is used to replace the long alkyl organic native ligands binding to the QD surface resulting in iodide terminated QDs that are stabilized in polar solvents such as N,N-dimethylformamide without particle aggregation. We extensively characterized the iodide terminated PbS QD via UV-vis absorption, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), FT-IR transmission spectroscopy, and X-ray photoelectron spectroscopy (XPS). Finally, we fabricated PbS QD photovoltaic cells that employ the iodide terminated PbS QDs. The resulting QD-PV devices achieved a best power conversion efficiency of 2.36% under ambient conditions that is limited by the layer thickness. The PV characteristics compare favorably to similar devices that were prepared using the standard layer-by-layer ethandithiol (EDT) treatment that had a similar layer thickness.

Halide–Amine Co-Passivated Indium Phosphide Colloidal Quantum Dots in Tetrahedral Shape

Wet chemical synthesis of covalent III-V colloidal quantum dots (CQDs) has been challenging because of uncontrolled surfaces and a poor understanding of surface-ligand interactions. We report a simple acid-free approach to synthesize highly crystalline indium phosphide CQDs in the unique tetrahedral shape by using tris(dimethylamino) phosphine and indium trichloride as the phosphorus and indium precursors, dissolved in oleylamine. Our chemical analyses indicate that both the oleylamine and chloride ligands participate in the stabilization of tetrahedral-shaped InP CQDs covered with cation-rich (111) facets. Based on density functional theory calculations, we propose that fractional dangling electrons of the In-rich (111) surface could be completely passivated by three halide and one primary amine ligands per the (2×2) surface unit, satisfying the 8-electron rule. This halide-amine co-passivation strategy will benefit the synthesis of stable III-V CQDs with controlled surfaces.

Analysis and characterization of iron pyrite nanocrystals and nanocrystalline thin films derived from bromide anion synthesis

We use a solution-based hot injection method to synthesize stable, phase pure and highly crystalline cubic iron pyrite (FeS2) nanocrystals, with size varying from ∼70 to 150 nm. We use iron(II) bromide as an iron precursor, elemental sulfur as the sulfur source, trioctylphosphine oxide (TOPO) and 1,2-hexanediol as capping ligands, and oleylamine (OLA) as a non-coordinating solvent during the synthesis. We report on the influence of hydrazine treatment, and of thermal sintering, on the morphological, electronic, optical, and surface chemical properties of FeS2 films. Four point probe and Hall measurements indicate that these iron pyrite films are highly conductive. Although they are unsuitable as an effective photovoltaic light-absorbing layer, they offer clear potential as a conducting contact layer in photovoltaic and other optoelectronic devices.

Increased open-circuit voltage in a Schottky device using PbS quantum dots with extreme confinement

We fabricated the PbS nanocrystal quantum dots (NQDs) based Schottky structure device (ITO/PbS/LiF/Al) with varying bandgap of NQDs from 0.8 to 2.2 eV. The open-circuit voltage increased monotonically with NQDs bandgap until 0.67 V, achieved using extremely confined, 1.5 nm sized-PbS NQDs. The power conversion efficiency reached the maximum value over 3% under AM 1.5 with NQDs bandgap of about 1.3 eV. Size-dependent photovoltaic evaluation in extreme confinement regime provides basis for efficient multi-junction solar cells composed of PbS NQDs of different sizes.

Space charge limited conduction in ultrathin PbS quantum dot solid diodes

As a simple and direct characterization of carrier transport in nanocrystal quantum dot (NQD) solids, current-voltage characterization of ultrathin diodes is proposed. We found the space charge limited conduction (SCLC) behavior in ultrathin PbS NQD diodes with active layer thickness half of the full depletion width; and extracted hole concentrations in the order of 1015 cm−3, hole mobilities from 10−4 to 10−5 cm2/Vs, trap energy depths varying from 140 meV to 200 meV, and volume trap density around 1017 cm−3 for thin films with NQDs of diameters 3.3 and 3.6 nm, respectively. We further discuss the validity of applying SCLC to the NQD solids based diodes and the implications of the extracted parameters extensively. Proposed characterization method here is a direct measure of carrier transport in solar cell structures which could provide exact directions in NQD solids based solar cell fabrication and modeling.

Characterization of two by two electron-beam microcolumn array aligned with field emission array

A two by two electron microcolumn array aligned with field emission array (FEA) was fabricated based on our electron-beam simulation. The spherical and chromatic aberrations, that affect the spot size of the e-beam, are highly dependent on the alignment of the electrostatic microlenses. A laser micromachining technique was used for making a self-aligned microcolumn. A FEA with a designed size and spacing was aligned and bonded to the microcolumn. The I–V and current stabilities of the microcolumn were measured and the field emission pattern of highly focused e-beam was obtained. The application of focused electron beam or ion beam for lithography and miniaturized scanning electron microscopy are suggested.

Determination of heterojunction band offsets between CdS bulk and PbS quantum dots using photoelectron spectroscopy

Photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (ΔEV) of a CdS/PbS quantum dot (QD) heterojunction for which the PbS QD layer was deposited using solution based layer-by-layer dip coating method on top of RF magnetron sputtered CdS. A value of ΔEV = 1.73 eV was obtained using the Cd 3d and Pb 4f energy levels as references. Given the band gap energies of the CdS and PbS-QD layers, the conduction band offset ΔEC was determined to be 0.71 eV.

Supersonically Spray-Coated Colloidal Quantum Dot Ink Solar Cells

Controlling the thickness of quantum dot (QD) films is difficult using existing film formation techniques, which employ pre-ligand-exchanged PbS QD inks, because of several issues: 1) poor colloidal stability, 2) use of high-boiling-point solvents for QD dispersion, and 3) limitations associated with one-step deposition. Herein, we suggest a new protocol for QD film deposition using electrical double-layered PbS QD inks, prepared by solution-phase ligand exchange using methyl ammonium lead iodide (MAPbI3). The films are deposited by the supersonic spraying technique, which facilitates the rapid evaporation of the solvent and the subsequent deposition of the PbS QD ink without requiring a post-deposition annealing treatment for solvent removal. The film thickness could be readily controlled by varying the number of spraying sweeps made across the substrate. This spray deposition process yields high-quality n-type QD films quickly (within 1 min) while minimizing the amount of the PbS QD ink used to less than 5 mg for one device (300-nm-thick absorbing layer, 2.5 × 2.5 cm2). Further, the formation of an additional p-layer by treatment with mercaptopropionic acid allows for facile hole extraction from the QD films, resulting in a power conversion efficiency of 3.7% under 1.5 AM illumination.