Jung Hoon Song

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.

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.

Lead Sulfide Nanocrystal Quantum Dot Solar Cells with Trenched ZnO Fabricated via Nanoimprinting

The improvement of power conversion efficiency, especially current density (Jsc), for nanocrystal quantum dot based heterojunction solar cells was realized by employing a trenched ZnO film fabricated using nanoimprint techniques. For an optimization of ZnO patterns, various patterned ZnO films were investigated using electrical and optical analysis methods by varying the line width, interpattern distance, pattern height, and residual layer. Analyzing the features of patterned ZnO films allowed us to simultaneously optimize both the pronounced electrical effects as well as optical properties. Consequently, we achieved an enhancement in Jsc from 7.82 to 12.5 mA cm(-2) by adopting the patterned ZnO with optimized trenched shape.

A Resonance‐Shifting Hybrid n‐Type Layer for Boosting Near‐Infrared Response in Highly Efficient Colloidal Quantum Dots Solar Cells

A new configuration of a plasmonic quantum dots solar structure is proposed. Gold-silver core-shell metal nanoparticles (Au@Ag NCs) are incorporated into the TiO2 layer (Au@Ag NCs-HL) of PbS-based solar cells. The TiO2 layer enables the Au@Ag NCs to have broad plasmonic responses and the external quantum efficiency and absorption of the plasmonic devices are significantly enhanced. The electrical performance of the solar cells is also improved.

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.

Inverted Schottky quantum dot solar cells with enhanced carrier extraction and air-stability

We introduce a novel colloidal quantum dot solar cell (CQD SC) architecture, defined as inverted Schottky CQD SCs, which consists of a thin film of PbS CQDs sandwiched between a low-work-function, transparent conducting oxide (Lϕ-TCO) and a high-work-function metal anode. On Lϕ-TCO substrates, which were generated by coating a thin layer of polyethylenimine (PEI) onto FTO, a series of inverted Schottky CQD SCs with varied PbS CQD sizes and QD layer thicknesses were fabricated and characterized using capacitance–voltage (C–V), current–voltage (J–V), and external quantum efficiency (EQE). A Schottky junction, of about 180 nm in width, was formed at the front TCO contact, resulting in an EQE of approximately 70% in the short-wavelength region. The champion device reached 3.8% AM1.5 in power conversion efficiency, and retained efficiency over several weeks of air-exposure. A record open-circuit voltage (VOC) of 0.75 V was achieved by employing PbS CQDs of 1.56 eV in the bandgap. Advantages including the simple device structure, efficient carrier extraction, and air-stability demonstrated in this study suggest that inverted Schottky CQD SCs can reduce the price per Watt ratio and facilitate the development of CQD tandem solar cells.

Charge Transport Characterization of PbS Quantum Dot Solids for High Efficiency Solar Cells

The PbS quantum dot is an emerging photovoltaic material, which may provide high efficiency breakthroughs. The most crucial element for the high efficiency solar cellss development is to understand charge transport characteristics of PbS quantum dot solids, which are also important in planning strategic research. We have investigated charge transport characteristics of PbS quantum dot solids thin films using space charge limited conduction analysis and assessed thickness dependent photovoltaic performances. The extracted carrier drift mobility was

PbS Colloidal Quantum Dot Solar Cells With Organic Hole Transport Layers for Enhanced Carrier Separation and Ambient Stability

low-10^{-2}cm^2/Vs

A Colloidal‐Quantum‐Dot‐Based Self‐Charging System via the Near‐Infrared Band

with the estimated diffusion length about 50 nm. These and recently reported values were compared with those from a commercial photovoltaic material, and we present an essential element in further development of PbS quantum dot solids materials.