@article{yu_kim_kim_barange_jiang_so_2020, title={Direct Acoustic Imaging Using a Piezoelectric Organic Light-Emitting Diode}, volume={12}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.0c05615}, abstractNote={Conventional ultrasonic imaging requires acoustic scanning over a target object using a piezoelectric transducer array, followed by signal processing to reconstruct the image. Here, we report a novel ultrasonic imaging device that can optically display an acoustic signal on the surface of a piezoelectric transducer. By fabricating an organic light-emitting diode (OLED) on top of a piezoelectric crystal (lead zirconate titanate, PZT), an acousto-optical piezoelectric OLED (p-OLED) transducer is realized, converting an acoustic wave profile directly to an optical image. Due to the integrated device architecture, the resulting p-OLED features a high acousto-optic conversion efficiency at the resonant ultrasound frequency, providing a piezoelectric field to drive the OLED. By incorporating an electrode array in the p-OLED, we demonstrate a novel tomographic ultrasound imaging device that is operated without a need for conventional signal processing.}, number={32}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Yu, Hyeonggeun and Kim, Jinwook and Kim, Howuk and Barange, Nilesh and Jiang, Xiaoning and So, Franky}, year={2020}, month={Aug}, pages={36409–36416} }
 @article{dong_ho_yu_salehi_so_2019, title={Defect Passivation by Fullerene Derivative in Perovskite Solar Cells with Aluminum-Doped Zinc Oxide as Electron Transporting Layer}, volume={31}, ISSN={["1520-5002"]}, DOI={10.1021/acs.chemmater.9b01292}, abstractNote={ZnO is a potential replacement for TiO2 as an electron transport layer (ETL) used in low-temperature processed hybrid perovskite solar cells. However, poor thermal stability of perovskites on ZnO and interfacial traps introduced during processing are obstacles to achieve a good device performance. Here, we demonstrate perovskite solar cells using aluminum doped zinc oxide (AZO) nanoparticles for the ETL having a better thermal stability compared with ZnO. However, the device shows a lower short circuit current density and a large photocurrent hysteresis, which are attributed to the poor interfacial properties between the ETL and the perovskite layer. To address this issue, a thin interfacial modification layer of phenyl-C61-butyric acid methyl ester (PCBM) was employed. The resulting device shows the efficiency is improved from 13 to 17% along with a significant reduction in hysteresis. Results from our thermal admittance spectroscopy show that the interface defect states are significantly reduced with th...}, number={17}, journal={CHEMISTRY OF MATERIALS}, author={Dong, Qi and Ho, Carr Hoi Yi and Yu, Hyeonggeun and Salehi, Amin and So, Franky}, year={2019}, month={Sep}, pages={6833–6840} }
 @article{yu_chung_shewmon_ho_carpenter_larrabee_sun_jones_ade_o'connor_et al._2017, title={Flexible inorganic ferroelectric thin films for nonvolatile memory devices}, volume={27}, number={21}, journal={Advanced Functional Materials}, author={Yu, H. and Chung, C. C. and Shewmon, N. and Ho, S. and Carpenter, J. H. and Larrabee, R. and Sun, T. L. and Jones, J. L. and Ade, H. and O'Connor, B. T. and et al.}, year={2017} }
 @article{ho_yu_so_2017, title={Transparent indium-tin oxide/indium-gallium-zinc oxide Schottky diodes formed by gradient oxygen doping}, volume={111}, ISSN={["1077-3118"]}, DOI={10.1063/1.4993430}, abstractNote={Amorphous InGaZnO (a-IGZO) is promising for transparent electronics due to its high carrier mobility and optical transparency. However, most metal/a-IGZO junctions are ohmic due to the Fermi-level pinning at the interface, restricting their device applications. Here, we report that indium-tin oxide/a-IGZO Schottky diodes can be formed by gradient oxygen doping in the a-IGZO layer that would otherwise form an ohmic contact. Making use of back-to-back a-IGZO Schottky junctions, a transparent IGZO permeable metal-base transistor is also demonstrated with a high common-base gain.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Ho, Szuheng and Yu, Hyeonggeun and So, Franky}, year={2017}, month={Nov} }
 @article{shewmon_yu_constantinou_klump_so_2016, title={Formation of Perovskite Heterostructures by Ion Exchange}, volume={8}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.6b10034}, abstractNote={Thin-film optoelectronic devices based on polycrystalline organolead-halide perovskites have recently become a topic of intense research. Single crystals of these materials have been grown from solution with electrical properties superior to those of polycrystalline films. In order to enable the development of more complex device architectures based on organolead-halide perovskite single crystals, we developed a process to form epitaxial layers of methylammonium lead iodide (MAPbI3) on methylammonium lead bromide (MAPbBr3) single crystals. The formation of the MAPbI3 layer is found to be dominated by the diffusion of halide ions, leading to a shift in the photoluminescence and absorption spectra. X-ray diffraction measurements confirm the single-crystal nature of the MAPbI3 layer, while carrier transport measurements show that the converted layer retains the high carrier mobility typical of single-crystal perovskite materials. Such heterostructures on perovskite single crystals open possibilities for new types of devices.}, number={48}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Shewmon, Nathan T. and Yu, Hyeonggeun and Constantinou, Iordania and Klump, Erik and So, Franky}, year={2016}, month={Dec}, pages={33273–33279} }
 @article{yu_kim_lee_baek_lee_singh_so_2016, title={High-gain infrared-to-visible upconversion light-emitting phototransistors}, volume={10}, ISSN={["1749-4893"]}, DOI={10.1038/nphoton.2015.270}, number={2}, journal={NATURE PHOTONICS}, author={Yu, Hyeonggeun and Kim, Doyoung and Lee, Jinhyung and Baek, Sujin and Lee, Jaewoong and Singh, Rajiv and So, Franky}, year={2016}, month={Feb}, pages={129-+} }
 @article{lee_kim_baek_yu_so_2016, title={Inorganic UV-Visible-SWIR Broadband Photodetector Based on Monodisperse PbS Nanocrystals}, volume={12}, ISSN={["1613-6829"]}, DOI={10.1002/smll.201503244}, abstractNote={Solution-processed inorganic UV-visible short-wave-infrared photodetectors with light sensitivity from 350 nm to 2000 nm are fabricated using highly monodispersed large PbS NCs. These devices showed detectivity values over 1 × 10(11) Jones from 350 nm to 2000 nm, and a maximum detectivity value of 1.2 × 10(12) Jones at 1800 nm.}, number={10}, journal={SMALL}, author={Lee, Jae Woong and Kim, Do Young and Baek, Sujin and Yu, Hyeonggeun and So, Franky}, year={2016}, month={Mar}, pages={1328–1333} }
 @article{yu_liu_baek_kim_dong_so_2016, title={Solution-processed copper oxide interlayers for broadband PbS quantum-dot photodiodes}, volume={4}, ISSN={["2050-7534"]}, DOI={10.1039/c6tc03531k}, abstractNote={Metal oxide interlayers are promising for optoelectronic applications due to solution processability, optical transparency, and excellent charge blocking properties.}, number={47}, journal={JOURNAL OF MATERIALS CHEMISTRY C}, author={Yu, Hyeonggeun and Liu, Shyui and Baek, Sujin and Kim, Do Young and Dong, Chen and So, Franky}, year={2016}, month={Dec}, pages={11205–11211} }
 @article{yu_dong_guo_kim_so_2016, title={Vertical Organic Field-Effect Transistors for Integrated Optoelectronic Applications}, volume={8}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.6b00182}, abstractNote={Direct integration of a vertical organic field-effect transistor (VOFET) and an optoelectronic device offers a single stacked, low power optoelectronic VOFET with high aperture ratios. However, a functional optoelectronic VOFET could not be realized because of the difficulty in fabricating transparent source and gate electrodes. Here, we report a VOFET with an on/off ratio up to 10(5) as well as output current saturation by fabricating a transparent gate capacitor consisting of a perforated indium tin oxide (ITO) source electrode, HfO2 gate dielectric, and ITO gate electrode. Effects of the pore size and the pore depth within the porous ITO electrodes on the on/off characteristic of a VOFET are systematically explained in this work. By combining a phosphorescent organic light-emitting diode with an optimized VOFET structure, a vertical organic light-emitting transistor with a luminance on/off ratio of 10(4) can be fabricated.}, number={16}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Yu, Hyeonggeun and Dong, Zhipeng and Guo, Jing and Kim, Doyoung and So, Franky}, year={2016}, month={Apr}, pages={10430–10435} }