Ik Jae Park

Crystallographically preferred oriented TiO2 nanotube arrays for efficient photovoltaic energy conversion

We describe the fabrication of crystallographically preferred oriented TiO2 anatase nanotube arrays (p-NTAs) and the characterization of their photovoltaic properties. The preferred orientation to the (004) plane of the TiO2 nanotube array (NTA) was carefully controlled by adjusting the water content in the anodizing electrolyte; ∼2 wt% of water yielded a p-NTA, whereas other contents of water yielded randomly oriented NTAs (r-NTAs). A structural analysis using X-ray diffraction and a high-resolution transmission electron microscope revealed that the p-NTA showed a preferred orientation along the [001] direction of the anatase crystal structure. When the NTAs were employed to dye-sensitized solar cells (DSSCs) as photoelectrodes, the p-NTA showed a similar electron lifetime to the r-NTA, which was an order of magnitude higher than that for a TiO2 nanoparticle (NP) film. Moreover, the p-NTA exhibited faster electron transport than the NP film, and even faster than the r-NTA. These properties resulted in a longer electron diffusion length of the p-NTA, compared to the r-NTA and NP film, thereby improving the charge collection property of the photoelectrode. The p-NTA exhibited superior photovoltaic energy conversion performance in the DSSC system, and showed a higher thickness for the optimal photovoltaic performance compared to the NP film, which were attributed to the excellent charge collection properties. Our results address that the crystallographic orientation of NTAs improves their charge transport properties, which can be applied to various optoelectronics, especially to solar-driven energy conversion devices.

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.

A tree-like nanoporous WO3 photoanode with enhanced charge transport efficiency for photoelectrochemical water oxidation

We report a tree-like nanoporous tungsten trioxide (WO3) photoanode that largely improves the photoelectrochemical water oxidation performance. These novel WO3 photoanodes were prepared using a pulsed laser deposition method and their porosity was controlled by adjusting the oxygen partial pressure during the deposition process. The tree-like nanoporous WO3 photoanode has a nanoporous structure with a partially preferred alignment of the individual WO3 nanocrystals, which greatly improves the charge transport efficiency. Under simulated solar light illumination, the aforementioned features resulted in ∼9 times higher photocurrent density (1.8 mA cm−2 at 1.23 V vs. RHE) than a dense WO3 photoanode. An incident photon-to-current conversion efficiency of over 70% was also obtained at wavelengths of 350–400 nm.

300% Enhancement of Carrier Mobility in Uniaxial-Oriented Perovskite Films Formed by Topotactic-Oriented Attachment

Organic-inorganic perovskites with intriguing optical and electrical properties have attracted significant research interests due to their excellent performance in optoelectronic devices. Recent efforts on preparing uniform and large-grain polycrystalline perovskite films have led to enhanced carrier lifetime up to several microseconds. However, the mobility and trap densities of polycrystalline perovskite films are still significantly behind their single-crystal counterparts. Here, a facile topotactic-oriented attachment (TOA) process to grow highly oriented perovskite films, featuring strong uniaxial-crystallographic texture, micrometer-grain morphology, high crystallinity, low trap density (≈4 × 1014 cm-3 ), and unprecedented 9 GHz charge-carrier mobility (71 cm2 V-1 s-1 ), is demonstrated. TOA-perovskite-based n-i-p planar solar cells show minimal discrepancies between stabilized efficiency (19.0%) and reverse-scan efficiency (19.7%). The TOA process is also applicable for growing other state-of-the-art perovskite alloys, including triple-cation and mixed-halide perovskites.

Tailoring uniform γ-MnO2 nanosheets on highly conductive three-dimensional current collectors for high-performance supercapacitor electrodes

Recent efforts have focused on the fabrication and application of three-dimensional (3-D) nanoarchitecture electrodes, which can exhibit excellent electrochemical performance. Herein, a novel strategy towards the design and synthesis of size- and thickness-tunable two-dimensional (2-D) MnO2 nanosheets on highly conductive one-dimensional (1-D) backbone arrays has been developed via a facile, one-step enhanced chemical bath deposition (ECBD) method at a low temperature (∼50 °C). Inclusion of an oxidizing agent, BrO3−, in the solution was crucial in controlling the heterogeneous nucleation and growth of the nanosheets, and in inducing the formation of the tailored and uniformly arranged nanosheet arrays. We fabricated supercapacitor devices based on 3-D MnO2 nanosheets with conductive Sb-doped SnO2 nanobelts as the backbone. They achieved a specific capacitance of 162 F·g−1 at an extremely high current density of 20 A·g−1, and good cycling stability that shows a capacitance retention of ≈92% of its initial value, along with a coulombic efficiency of almost 100% after 5,000 cycles in an aqueous solution of 1 M Na2SO4. The results were attributed to the unique hierarchical structures, which provided a short diffusion path of electrolyte ions by means of the 2-D sheets and direct electrical connections to the current collector by 1-D arrays as well as the prevention of aggregation by virtue of the well-aligned 3-D structure.

Size-controlled synthesis of monodispersed mesoporous α-Alumina spheres by a template-free forced hydrolysis method

We demonstrated the formation of monodispersed spherical aluminum hydrous oxide precursors with tunable sizes by controlling the variables of a forced hydrolysis method. The particle sizes of aluminum hydrous oxide precursors were strongly dependent on the molar ratio of the Al(3+) reactants (sulfates and nitrates). In addition, the systematic phase and morphological evolutions from aluminum hydrous oxide to γ-alumina (Al(2)O(3)) and finally to α-Al(2)O(3) through thermal dehydrogenation were characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). After annealing the amorphous aluminum hydrous oxide in air at 900 °C and 1100 °C for 1 h, we observed complete conversion to phase-pure γ- and α-Al(2)O(3), respectively, while maintaining monodispersity (125 nm, 195 nm, 320 nm, and 430 nm diameters were observed). Furthermore, both γ- and α-Al(2)O(3) were found to be mesoporous in structure, providing enhanced specific surface areas of 102 and 76 m(2) g(-1), respectively, based on the Brunauer-Emmett-Teller (BET) measurement.

Template-free synthesis of monodispersed Y3Al5O12:Ce3+ nanosphere phosphor

Monodispersed Ce3+-doped Y3Al5O12 (YAG:Ce3+) nanospheres were synthesized through forced hydrolysis using a urea method, followed by thermal calcination processes, and their luminescence properties were examined. The crystallization of the YAG:Ce3+ phase and morphological evolutions after subsequent heat treatment were characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Energy dispersive spectroscopy (EDS) analysis revealed that the amorphous aluminum oxide layer played an important role in preventing necking between the particles during heat treatment. As a result, stand-alone YAG:Ce3+ nanospheres with an amorphous aluminum oxide layer shell were synthesized while maintaining monodispersity with an average particle diameter of about 33 nm. These nanospheres had a dense structure and smooth surface with relatively good crystallinity after annealing at 1075 °C. They absorbed light efficiently in the visible region of 400–500 nm, and showed a single broadband emission peak at 536 nm with a luminescence quantum efficiency (QE) of 33% and relatively good photostability.

Boosting the solar water oxidation performance of a BiVO4 photoanode by crystallographic orientation control

Materials with low crystal symmetry often exhibit anisotropic properties, allowing the tuning of their physical and chemical properties via crystallographic orientation and exposed facet control. Herein, for the first time, we have demonstrated that pristine BiVO4 with a preferred [001] growth orientation and exposed (001) facets exhibits excellent intrinsic charge transport properties and surface reactivity. Using preferentially [001]-oriented BiVO4 (p-BVO) as a photoanode for photoelectrochemical water splitting, an impressive photocurrent density at 1.23 V vs. the reversible hydrogen electrode (RHE) is achieved, which is approximately 16 times higher than that exhibited by a photoanode based on randomly oriented BiVO4. Importantly, when the surface of p-BVO is further roughened and decorated with an oxygen evolution electrocatalyst, photocurrent densities of ∼3.5 and ∼6.1 mA cm−2 are achieved at 0.6 and 1.23 VRHE, respectively; the latter value corresponds to ∼82% of the theoretically achievable photocurrent density for BiVO4 under 1 sun illumination. Our results demonstrate the effectiveness of crystal orientation and exposed facet control in optimizing materials for solar water-splitting applications.

Highly Efficient and Uniform 1 cm2 Perovskite Solar Cells with an Electrochemically Deposited NiOx Hole-Extraction Layer

Given that the highest certified conversion efficiency of the organic-inorganic perovskite solar cell (PSC) already exceeds 22 %, which is even higher than that of the polycrystalline silicon solar cell, the significance of new scalable processes that can be utilized for preparing large-area devices and their commercialization is rapidly increasing. From this perspective, the electrodeposition method is one of the most suitable processes for preparing large-area devices because it is an already commercialized process with proven controllability and scalability. Here, a highly uniform NiOx layer prepared by electrochemical deposition is reported as an efficient hole-extraction layer of a p-i-n-type planar PSC with a large active area of >1 cm2 . It is demonstrated that the increased surface roughness of the NiOx layer, achieved by controlling the deposition current density, facilitates the hole extraction at the interface between perovskite and NiOx , and thus increases the fill factor and the conversion efficiency. The electrochemically deposited NiOx layer also exhibits extremely uniform thickness and morphology, leading to highly efficient and uniform large-area PSCs. As a result, the p-i-n-type planar PSC with an area of 1.084 cm2 exhibits a stable conversion efficiency of 17.0 % (19.2 % for 0.1 cm2 ) without showing hysteresis effects.

Observation of anatase nanograins crystallizing from anodic amorphous TiO2 nanotubes

A mechanism underlying the appearance of a preferred orientation in anodized amorphous TiO2 nanotube arrays (NTAs) was studied. Transmission electron microscopic analyses of preferred-oriented nanotube arrays (p-NTAs) reveal that at an optimum water content (~2 wt%), large single crystalline domains oriented along grow from the outer wall to the inner wall to minimize the surface energy. In stark contrast, excessive water content (5 wt%) in the electrolyte leads to sporadic multiple nucleation of randomly oriented anatase crystallites in amorphous medium at the early stage of crystallization, which results in the formation of randomly oriented nanotube arrays (r-NTAs). During subsequent thermal annealing, multiple nucleation sites hindered the growth of the -oriented grains from the outer wall. When the water content in an ethylene glycol-based electrolyte is optimized by reducing the uncertainty of water content, the X-ray diffraction patterns of NTAs exhibited a 200 times increase in the intensity ratio of (004) to (200) peaks of the anatase phase. p-NTAs exhibit ~5 times lower electrical resistance than r-NTAs, which supports the idea that improving the preferred orientation of NTAs is a promising method for developing efficient electronic devices.