Seon Joo Lee

Fabrication of Efficient Formamidinium Tin Iodide Perovskite Solar Cells through SnF2–Pyrazine Complex

To fabricate efficient formamidinium tin iodide (FASnI3) perovskite solar cells (PSCs), it is essential to deposit uniform and dense perovskite layers and reduce Sn(4+) content. Here we used solvent-engineering and nonsolvent dripping process with SnF2 as an inhibitor of Sn(4+). However, excess SnF2 induces phase separation on the surface of the perovskite film. In this work, we report the homogeneous dispersion of SnF2 via the formation of the SnF2-pyrazine complex. Consequently, we fabricated FASnI3 PSCs with high reproducibility, achieving a high power conversion efficiency of 4.8%. Furthermore, the encapsulated device showed a stable performance for over 100 days, maintaining 98% of its initial efficiency.

Hot Carrier-Driven Catalytic Reactions on Pt–CdSe–Pt Nanodumbbells and Pt/GaN under Light Irradiation

Hybrid nanocatalysts consisting of metal nanoparticle-semiconductor junctions offer an interesting platform to study the role of metal-oxide interfaces and hot electron flows in heterogeneous catalysis. Here, we report that hot carriers generated upon photon absorption significantly impact the catalytic activity of CO oxidation. We found that Pt-CdSe-Pt nanodumbbells exhibit a higher turnover frequency by a factor of 2 during irradiation by light with energy higher than the bandgap of CdSe, while the turnover rate on bare Pt nanoparticles did not depend on light irradiation. We found that Pt nanoparticles deposited on a GaN substrate under light irradiation exhibit changes in catalytic activity of CO oxidation that depends on the type of doping of the GaN. We suppose that hot electrons are generated upon the absorption of photons by the semiconducting nanorods or substrates, whereafter the hot electrons are injected into the Pt nanoparticles, resulting in the change in catalytic activity. The results imply that hot carrier flows generated during light irradiation significantly influence the catalytic activity of CO oxidation, leading to potential applications as a hot electron-based catalytic actuator.

Geometric Effect of Single or Double Metal-Tipped CdSe Nanorods on Photocatalytic H2 Generation

In the present work, we focused on geometrical (single- or double-tipped) and compositional (Pt or Au) variations of active metal components in a well-defined CdSe nanorod system. These colloidal nanostructures were employed for photocatalytic hydrogen generation from water under the identical reaction conditions with visible light irradiation. The catalysts exhibited significant dependency of the catalytic activity, specifically on the catalyst geometry and the choice of the metal tips, determined by the energetic consideration of electron transfer to the metal tips and hole transfer to the sacrificial reagents on the CdSe nanorods.

Tailoring of Electron-Collecting Oxide Nanoparticulate Layer for Flexible Perovskite Solar Cells

Low-temperature-processed perovskite solar cells (PSCs), especially those fabricated on flexible substrates, exhibit device performance that is worse than that of high-temperature-processed PSCs. One of the main reasons for the inferior performance of low-temperature-processed PSCs is the loss of photogenerated electrons in the electron collection layer (ECL) or related interfaces, i.e., indium tin oxide/ECL and ECL/perovskite. Here, we report that tailoring of the energy level and electron transporting ability in oxide ECLs using Zn2SnO4 nanoparticles and quantum dots notably minimizes the loss of photogenerated electrons in the low-temperature-fabricated flexible PSC. The proposed ECL with methylammonium lead halide [MAPb(I0.9Br0.1)3] leads to fabrication of significantly improved flexible PSCs with steady-state power conversion efficiency of 16.0% under AM 1.5G illumination of 100 mW cm(-2) intensity. These results provide an effective method for fabricating high-performance, low-temperature solution-processed flexible PSCs.

Coordination Power Adjustment of Surface‐Regulating Polymers for Shaping Gold Polyhedral Nanocrystals

PVP (poly(vinyl pyrrolidone)) is a common polymer that behaves as a surface-regulating agent that shapes metal nanocrystals in the polyol process. We have used different polymers containing tertiary amide groups, namely PVCL (poly(vinyl caprolactam)) and PDMAm (poly(N,N-dimethyl acrylamide)), for the synthesis of gold polyhedrons, including octahedrons, cuboctahedrons, cubes, and higher polygons, under the present polyol reaction conditions. The basicity and surface coordination power of the polymers are in the order of PVCL, PVP, and PDMAm. A correlation is observed between the coordination power of the polymers and the resulting gold nanocrystal size. Strong coordination and electron donation from the polymer functional groups to the gold surface restrict particle growth rates, which leads to small nanocrystals. The use of PVCL can yield gold polyhedral structures with small sizes, which cannot be achieved in the reactions with PVP. Simultaneous hydrolysis of the amide group in PDMAm leads to carboxylate functionality, which is very useful for generating chemical and bioconjugates through the formation of ester and amide bonds.

Controllable synthesis of single crystalline Sn-based oxides and their application in perovskite solar cells

We synthesized single-crystalline Sn-based oxides for use as electron-transporting layers (ETLs) in perovskite solar cells (PSCs). The control of the Zn-to-Sn cation ratio (Zn/Sn = 0–2) in a fixed concentration of hydrazine solution leads to the formation of various types of Sn-based oxides, i.e., spherical SnO2 and Zn2SnO4 nanoparticles (NPs), SnO2 nanorods, and Zn2SnO4 nanocubes. In particular, a ratio of Zn/Sn = 1 results in nanocomposites of single-crystalline SnO2 nanorods and Zn2SnO4 nanocubes. This is related to the concentration of free hydrazine unreacted with Zn and Sn ions in the reaction solution, because the resulting OH− concentration affects the growth rate of intermediate phases such as ZnSn(OH)6, Zn(OH)42− and Sn(OH)62−. Additionally, we propose plausible pathways for the formation of Sn-based oxides in hydrazine solution. The Sn-based oxides are applied as ETLs and annealed at a low temperature below 150 °C in PSCs. The PSCs fabricated by using the nanocomposite ETLs consisting of single-crystalline SnO2 nanorods and Zn2SnO4 nanocubes exhibit superior device performance to TiO2-based PSCs due to their excellent charge collection ability and optical properties, achieving a power conversion efficiency of ≥17%.

Air-stable CuInSe2 nanoparticles formed through partial cation exchange in methanol at room temperature

Spherical CuInSe2 nanoparticles were synthesized through a partial cation exchange reaction of Cu2−xSe nanoparticles in methanol at room temperature. An additive, tributylphosphine, rapidly reduced copper and selenium ions, and facilitated the dissolution of Cu ions. Poly(vinylpyrrolidone) retained the spherical morphology of the nanoparticles by anchoring on the surface. The fraction of In3+ ions increased as the reaction progressed, which led to a decrease of the optical bandgap from 2.06 eV of Cu2−xSe to 1.03 eV of CuInSe2. The resulting CuInSe2 particles exhibited high air and thermal stability, and can be applied as colloidal inks for thin film fabrication in optoelectronic devices.

Engineering Reaction Kinetics by Tailoring the Metal Tips of Metal–Semiconductor Nanodumbbells

Semiconductor-metal hybrid nanostructures are one of the best model catalysts for understanding photocatalytic hydrogen generation. To investigate the optimal structure of metal cocatalysts, metal-CdSe-metal nanodumbbells were synthesized with three distinct sets of metal tips, Pt-CdSe-Pt, Au-CdSe-Au, and Au-CdSe-Pt. Photoelectrochemical responses and transient absorption spectra showed that the competition between the charge recombination at the metal-CdSe interface and the water reduction on the metal surface is a detrimental factor for the apparent hydrogen evolution rate. For instance, a large recombination rate (krec) at the Pt-CdSe interface limits the quantum yield of hydrogen generation despite a superior water reduction rate (kWR) on the Pt surface. To suppress the recombination process, Pt was selectively deposited onto the Au tips of Au-CdSe-Au nanodumbbells in which the krec was diminished at the Au-CdSe interface, and the large kWR was maintained on the Pt surface. As a result, the optimal structure of the Pt-coated Au-CdSe-Au nanodumbbells reached a quantum yield of 4.84%. These findings successfully demonstrate that the rational design of a metal cocatalyst and metal-semiconductor interface can additionally enhance the catalytic performance of the photochemical hydrogen generation reactions.