Hui Joon Park

Depth-resolved band alignments of perovskite solar cells with significant interfacial effects

The band alignment in heterojunction solar cells, including perovskite solar cells (PSCs), is critically related to power conversion efficiency (PCE) improvement as it has a significant effect on the control of photocarrier transport. In the present study, in an attempt to identify the correlation between the energy band structure of PSCs and their PCEs, we analyzed the energy-band structures of both p–i–n type and n–i–p type PSC devices, especially those having the most widely utilized planar and mesoporous (mp) structures, by using spectroscopic approaches. Two types (planar and mp) of perovskite solar cells were fabricated. The PCEs were measured and found to be 12.5% for the planar structure and 14.7% for the mp structure. To investigate the physicochemical origin of such PCE differences depending on the cell type, comprehensive band alignment and band structure analyses at the actual cell stacks were performed using UV-vis and XPS depth measurements. Both planar and mp structures showed negative conduction band offset (CBO) for the efficient electron transfer and positive valence band offset for hole transfer. Especially, the perovskite/mp-TiO2 interface with a negative CBO has a widely distributed 3-dimensional junction with a graded electronic band structure. In the present study, we identify the electron transport mechanism and its relevance to the band alignment and its effect on PCE by comparing two different perovskite solar cell structures and, furthermore, examine the unique junction formation caused by the mp-TiO2 nanostructure.

Optical and Structural Properties of Solvent Free Synthesized Starch/Chitosan-ZnO Nanocomposites

The objective of this work is to develop an environmentally friendly method for preparation of ZnO nanocomposites. ZnO nanocomposites were prepared by three natural fibers such as coir, sawdust, and chitosan using aqueous solution of zinc chloride and sodium hydroxide. The functional groups of ZnO, C=O for polysaccharide, and N-H bending vibration of amine were confirmed by FTIR spectroscopy. A new high intensity absorption band has been observed at 424u2009cm−1 which corresponds to the E2 mode of hexagonal ZnO. The crystallinity and phase formation of coir, chitosan, and sawdust combined ZnO nanocomposites were confirmed by X-ray diffraction patterns. XRD patterns revealed the polycrystalline nature of ZnO composites belonging to the hexagonal phase with (101) preferential lattice orientation. The microstructural parameters were calculated for coir, chitosan, and saw wood combined ZnO composites. Also texture coefficients were estimated for all the diffraction lines of ZnO based nanocomposites. SEM and TEM analyses confirmed evenly distributed nanosized grains in the ZnO composites. The UV-Vis absorption spectra were observed where the blue shift absorption peak was at 334u2009nm. The optical band gap values were estimated in the range of 3.18–3.26u2009eV. The emission peak was observed at ~388u2009nm and ~463u2009nm by photoluminescence spectroscopy.

Nano-Geometry Dependent Electrical Property of Organic Semiconductor

AbstractIn this work, we manipulated the crystallinity and geometry of P3HT nanodomains and investigated their effects on carrier mobility of organic thin film transistor (OTFT). Furthermore, it was confirmed that the reproducibility of OTFT devices could be improved with nanostructure-based active layer. P3HT nanostructures, nanodot and nanowire, were prepared through the control of polymer solution concentration and ultrasonic treatment. Those nanostructures produced well-organized crystalline morphology, which resulted in outperforming field-effect mobility, compared to bulk P3HT film, thermally annealed after spin-casting. OTFT devices fabricated with P3HT nanowires showed slightly higher electrical mobility than those with nanodots, due to their geometric advantage for inter-connection between electrodes. In terms of device-to-device uniformity, nanodot-based devices have lower performance variation among OTFT arrays due to their symmetric geometry.n

All-Optical Ultrasound Transducer Using CNT-PDMS and Etalon Thin-Film Structure

Compared with conventional piezoelectric transductions, an all-optical high-frequency ultrasound (HFUS) transducer is a promising modality for high-resolution ultrasound imaging. We demonstrate an all-optical HFUS transducer by integrating a carbon nanotube-polydimethylsiloxane composite film with an etalon thin-film structure incorporating SiO2/TiO2 distributed Bragg reflectors and an SU-8 resonator. The optical and acoustic characteristics are investigated for two different configurations (forward and backward operation modes). The maximum amplitude of the pulse echo in backward mode is approximately twofold higher than that of the forward mode. This difference is contributed by the increased reflectance and the absorptive loss of the incident pulsed laser in the forward mode. The pulse echo from the transducer exhibits a broad frequency bandwidth of 27 MHz. Furthermore, the scalability of the 2-D all-optical transducer array is also evaluated by characterizing the optical properties of the etalon across an area of 0.1 × 0.2 mm2. Our experimental results show that the proposed transducer is a promising candidate for high-resolution ultrasound imaging systems.

Facile Synthesis of Molybdenum Diselenide Layers for High-Performance Hydrogen Evolution Electrocatalysts

A cost-effective solution-based synthesis route to grow MoSe2 thin films with vertically aligned atomic layers, thereby maximally exposing the edge sites on the film surface as well as enhancing charge transport to the electrode, is demonstrated for hydrogen evolution reaction. The surface morphologies of thin films are investigated by scanning electron microscopy and atomic force microscopy, and transmission electron microscopy analyses confirm the formation of the vertically aligned layered structure of MoSe2 in those films, with supporting evidences obtained by Raman. Additionally, their optical and compositional properties are investigated by photoluminescence and X-ray photoelectron spectroscopy, and their electrical properties are evaluated using bottom-gate field-effect transistors. The resultant pristine MoSe2 thin film exhibited low overpotential of 88 mV (at 10 mA·cm–2) and a noticeably high exchange current density of 0.845 mA·cm–2 with excellent stability, which is superior to most of other reported MoS2 or MoSe2-based catalysts, even without any other strategies such as doping, phase transformation, and integration with other materials.