207 results on '"So, Franky"'
Search Results
2. Understanding the Role of Ion Migration in the Operation of Perovskite Light-Emitting Diodes by Transient Measurements
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Siliang He, Lei Lei, Franky So, Liping Zhu, Juliana Mendes, Dovletgeldi Seyitliyev, Kenan Gundogdu, and Qi Dong
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Materials science ,business.industry ,Ion migration ,Transient current ,law.invention ,law ,Optoelectronics ,General Materials Science ,Transient (oscillation) ,Charge injection ,business ,Transient electroluminescence ,Diode ,Light-emitting diode ,Perovskite (structure) - Abstract
Perovskite light-emitting diodes have been gaining attention in recent years due to their high efficiencies. Despite of the recent progress made in device efficiency, the operation mechanisms of these devices are still not well understood, especially the effects of ion migration. In this work, the role of ion migration is investigated by measuring the transient electroluminescence and current responses, with both the current and efficiency showing a slow response in a time scale of tens of milliseconds. The results of the charge injection dynamics show that the slow response of the current is attributed to the migration and accumulation of halide ions at the anode interface, facilitating hole injection and leading to a strong charge imbalance. Further, the results of the charge recombination dynamics show that the slow response of the efficiency is attributed to enhanced charge injection facilitated by ion migration, which leads to an increased carrier density favoring bimolecular radiative recombination. Through a combined analysis of both charge injection and recombination dynamics, we finally present a comprehensive picture of the role of ion migration in device operation.
- Published
- 2020
3. Mode Dispersion in Photonic Crystal Organic Light-Emitting Diodes
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Franky So, Adam Rozelle, Yash Mehta, Yi-An Chen, Chih-Hao Chang, Nilesh Barange, Dong-Hun Shin, Xiangyu Fu, Cheng Peng, and Monica Samal
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Materials science ,Photon ,Physics::Instrumentation and Detectors ,business.industry ,Physics::Optics ,Electron ,Electronic, Optical and Magnetic Materials ,Materials Chemistry ,Electrochemistry ,OLED ,Optoelectronics ,Emission spectrum ,Photonics ,business ,Common emitter ,Photonic crystal ,Diode - Abstract
Similar to an electronic lattice determining the motion of electrons in solids, photonic crystals (PhCs) are periodic photonic nanostructures that determine the propagation of photons. By incorporating PhCs into organic light-emitting diodes (OLEDs), the device efficiency and emission spectra can be modified, which can be explained and predicted by the mode dispersion. In this work, we experimentally measure the mode dispersion of 1-D and 2-D PhC OLEDs at different azimuthal angles with angle-resolved electroluminescence spectra. The results are explained using an intuitive geometry approach, which shifts and slices the cone-shaped optical modes to obtain the mode dispersion of PhC OLEDs. We note that the weak cavity mode and a narrow photonic band gap are visible only after eliminating the intrinsic emitter spectrum in the air mode dispersion. In the end, we discuss the implication of mode dispersion on the OLED light extraction.
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- 2020
4. En route to a unified model for photoelectrochemical reactor optimization. II–geometric optimization of perforated photoelectrodes
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Franky E. Bedoya-Lora, Anna Hankin, and Geoff Kelsall
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Photocurrent ,Technology ,solar hydrogen ,Materials science ,Hydrogen ,business.industry ,Chemical technology ,Multiphysics ,Gas evolution reaction ,current density distribution ,Perforation (oil well) ,Photoelectrochemistry ,chemistry.chemical_element ,TP1-1185 ,Electrolyte ,photoelectrochemistry ,electrochemical reactor ,chemistry ,photoelectrode ,Electrode ,Optoelectronics ,business - Abstract
Results have been reported previously of a model describing the performance of photoelectrochemical reactors, which utilize semiconductor | liquid junctions. This model was developed and verified using SnIV-doped α-Fe2O3 as photoanodes. Hematite films were fully characterized to obtain parameter inputs to a model predicting photocurrent densities. Thus, measured photocurrents were described and validated by the model in terms of measurable quantities. The complete reactor model, developed in COMSOL Multiphysics, accounted for gas evolution and desorption in the system. Hydrogen fluxes, charge yields and gas collection efficiencies in a photoelectrochemical reactor were estimated, revealing a critical need for geometric optimization to minimize H2-O2 product recombination as well as undesirable spatial distributions of current densities and “overpotentials” across the electrodes. Herein, the model was implemented in a 3D geometry and validated using solid and perforated 0.1 × 0.1 m2 planar photoanodes in an up-scaled photoelectrochemical reactor of 2 dm3. The same model was then applied to a set of simulated electrode geometries and electrode configurations to identify the electrode design that would maximize current densities and H2 fluxes. The electrode geometry was modified by introducing circular perforations of different sizes, relative separations and arrangements into an otherwise solid planar sheet for the purpose of providing ionic shortcuts. We report the simulated effects of electrode thickness and the presence or absence of a membrane to separate oxygen and hydrogen gases. In a reactor incorporating a membrane and a photoanode at 1.51 V vs RHE and pH 13.6, an optimized hydrogen flux was predicted for a perforation geometry with a separation-to-diameter ratio of 4.5 ± 0.5; the optimal perforation diameter was 50 µm. For reactors without a membrane, this ratio was 6.5 and 8.5 for a photoanode in a “wired” (monopolar) and “wireless” (photo-bipolar) design, respectively. The results and methodologies presented here will serve as a framework to optimize composite photoelectrodes (semiconductor | membrane | electrolyte), and photoelectrochemical reactors in general, for the production of hydrogen (and oxygen) from water using solar energy.
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- 2021
5. Light Manipulation of Photonic-Structured OLEDs
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Franky So
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Materials science ,Stack (abstract data type) ,business.industry ,Electrode ,OLED ,Physics::Optics ,Optoelectronics ,Beam shaping ,Photonics ,Thin film ,Polarization (waves) ,business ,Refractive index - Abstract
OLEDs are thin-film devices consisting of multilayers of organic thin films sandwiched between a metal and an ITO electrodes. Because of the different refractive indices of the layers in the thin film stack, different optical modes are trapped in the device. In this talk, we will first discuss the physics of these optical modes and describe techniques to characterize them. We will then describe how to use various photonic structures to maximize the light output and manipulate these optical modes to control the polarization as well as directionality to achieve beam shaping.
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- 2021
6. Enhanced lead sulfide quantum dots infrared photodetector performance through ligand exchange
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Shuo Ding, Qi Dong, Liping Zhu, Shichen Yin, Carr Hoi Yi Ho, and Franky So
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Materials science ,business.industry ,Band gap ,Layer by layer ,Photodetector ,chemistry.chemical_compound ,Responsivity ,chemistry ,Quantum dot ,Optoelectronics ,Quantum efficiency ,Lead sulfide ,business ,Quantum well infrared photodetector - Abstract
Narrow bandgap lead sulfide (PbS) quantum dots (QDs) are solution-processed materials used for optoelectronic applications in the short-wavelength infrared (SWIR) range (1400 - 3000 nm). The PbS QDs based photodetector has achieved comparable detectivity with current commercial SWIR sensors. However, there are still obstacles towards commercialization in commonly used layer by layer (LbL) deposition, such as high material consumption and low reproducibility. Here, we developed a new ligand exchange strategy to prepare ligand exchanged QD inks for single-step PbS film deposition. Compared with LbL deposition, the EQE of PbS QD photodetector made by single-step deposition has improved from 31% to 53%. The EQE and responsivity can be further improved to 95% with IR transparent electrode.
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- 2021
7. World Scientific Handbook of Organic Optoelectronic Devices
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Franky So, Dongge Ma, and Tae-Woo Lee
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Bioelectronics ,Engineering ,business.industry ,Optoelectronics ,business ,Volume (compression) - Published
- 2021
8. World Scientific Handbook of Organic Optoelectronic Devices
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Dongge Ma, Franky So, and Tae-Woo Lee
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Materials science ,Volume (thermodynamics) ,business.industry ,OLED ,Optoelectronics ,business - Published
- 2021
9. 12‐1: Invited Paper: Directional SPP Emission in OLEDs Using Diffractive Optical Elements
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Franky So, Xiangyu Fu, Yi-An Chen, Renuka Gogusetti, Chih-Hao Chang, and Dong-Hun Shin
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Materials science ,business.industry ,OLED ,Optoelectronics ,business - Published
- 2020
10. Eliminate angular color shift in top‐emitting OLEDs through cavity design
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Amin Salehi, Dong-Hun Shin, Stephen Amoah, Franky So, Juliana Mendes, Chen Dong, Adam Rozelle, and Xiangyu Fu
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Materials science ,business.industry ,OLED ,Color shift ,Optoelectronics ,Electrical and Electronic Engineering ,business ,Atomic and Molecular Physics, and Optics ,Electronic, Optical and Magnetic Materials - Published
- 2019
11. 5‐1: Distinguished Paper: Eliminate Angular Color Shift in Top Emitting OLEDs through Cavity Design
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Chen Dong, Stephen Amoah, Dong-Hun Shin, Franky So, Amin Salehi, Adam Rozelle, and Xiangyu Fu
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Materials science ,business.industry ,OLED ,Optoelectronics ,Color shift ,business - Published
- 2019
12. Flat band potential determination: avoiding the pitfalls
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John Callum Alexander, Anna Hankin, Geoff Kelsall, Franky E. Bedoya-Lora, and Anna Regoutz
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Technology ,Energy & Fuels ,SURFACE ,Materials Science ,Photoelectrochemistry ,DOPED HEMATITE NANOSTRUCTURES ,Materials Science, Multidisciplinary ,02 engineering and technology ,0915 Interdisciplinary Engineering ,IMPEDANCE ,ELECTROCHEMISTRY ,General Materials Science ,MOTT-SCHOTTKY PLOTS ,0912 Materials Engineering ,Photocurrent ,Science & Technology ,Chemistry, Physical ,Renewable Energy, Sustainability and the Environment ,business.industry ,Open-circuit voltage ,PHOTOANODES ,0303 Macromolecular and Materials Chemistry ,General Chemistry ,PHOTOELECTROCHEMISTRY ,021001 nanoscience & nanotechnology ,SEMICONDUCTOR ELECTRODES ,WATER OXIDATION ,Dielectric spectroscopy ,Anode ,Chemistry ,Physical Sciences ,TIO2 ELECTRODES ,Electrode ,Optoelectronics ,Charge carrier ,0210 nano-technology ,business ,Electrode potential - Abstract
The flat band potential is one of the key characteristics of photoelectrode performance. However, its determination on nanostructured materials is associated with considerable uncertainty. The complexity, applicability and pitfalls associated with the four most common experimental techniques used for evaluating flat band potentials, are illustrated using nanostructured synthetic hematite (α-Fe2O3) in strongly alkaline solutions as a case study. The motivation for this study was the large variance in flat band potential values reported for synthetic hematite electrodes that could not be justified by differences in experimental conditions, or by differences in their charge carrier densities. We demonstrate through theory and experiments that different flat band potential determination methods can yield widely different results, so could mislead the analysis of the photoelectrode performance. We have examined: (a) application of the Mott–Schottky (MS) equation to the interfacial capacitance, determined by electrochemical impedance spectroscopy as a function of electrode potential and potential perturbation frequency; (b) Gärtner–Butler (GB) analysis of the square of the photocurrent as a function of electrode potential; (c) determination of the potential of transition between cathodic and anodic photocurrents during slow potentiodynamic scans under chopped illumination (CI); (d) open circuit electrode potential (OCP) under high irradiance. Methods GB, CI and OCP were explored in absence and presence of H2O2 as hole scavenger. The CI method was found to give reproducible and the most accurate results on hematite but our overall conclusion and recommendation is that multiple methods should be employed for verifying a reported flat band potential.
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- 2019
13. Optical Losses at Gas Evolving Photoelectrodes: Implications for Photoelectrochemical Water Splitting
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Faye Alhersh, Klaus Hellgardt, Isaac Holmes-Gentle, and Franky E. Bedoya-Lora
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Photon ,Materials science ,Hydrogen ,Population ,chemistry.chemical_element ,02 engineering and technology ,Electrolyte ,010402 general chemistry ,01 natural sciences ,Physical and Theoretical Chemistry ,education ,Electrical conductor ,education.field_of_study ,business.industry ,021001 nanoscience & nanotechnology ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,General Energy ,Integrating sphere ,chemistry ,Electrode ,Water splitting ,Optoelectronics ,0210 nano-technology ,business - Abstract
Many photoelectrodes produce a gaseous product, such as hydrogen or oxygen, from a liquid electrolyte and require light transmission directly through the two-phase mixture forming at the semiconductor–electrolyte interface. Consequently, incidence solar photons will be scattered and reflected from the bubbly mixture leading to an additional optical loss. In this work, these optical losses are quantified for a population of bubbles that evolved from the vertical surface of a transparent conductive electrode (F-SnO2) by measuring the amount of light transmitted. The transmitted photons were collected in an integrating sphere placed directly behind the 15 mm × 15 mm electrode to capture the forward scattered light. The empirical results were compared with a simple dimensionless model. Finally, mitigation strategies are suggested and critically discussed. With progress in the development of large scale prototype photoelectrochemical devices comes the need to understand, quantify, and potentially resolve the i...
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- 2018
14. Structural and electrical properties analysis of InAlGaN/GaN heterostructures grown at elevated temperatures by MOCVD
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Jian Hao Huang, Firman Mangasa, Yung Yi Tu, Edward Yi Chang, Ren Yao Huang, Xia Xi Zheng, Ching-Ting Lee, and Franky Lumbantoruan
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010302 applied physics ,Materials science ,business.industry ,chemistry.chemical_element ,Heterojunction ,02 engineering and technology ,High-electron-mobility transistor ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Inorganic Chemistry ,X-ray photoelectron spectroscopy ,chemistry ,0103 physical sciences ,Materials Chemistry ,Surface roughness ,Sapphire ,Optoelectronics ,Metalorganic vapour phase epitaxy ,Gallium ,0210 nano-technology ,business ,Sheet resistance - Abstract
We report the effects of the growth temperature on the structural and electrical properties of the InAlGaN/GaN heterostructures grown on c-plane sapphire substrates. X-ray Photoelectron Spectroscopy and Atomic Force Microscopy measurements results indicate that the InAlGaN layer properties is strongly dependent on the growth temperature. It is observed that the Gallium incorporation increases with the increase of the growth temperature. Meanwhile, the surface roughness decreases from 0.49 nm to 0.34 nm with the increase of growth temperature. The variation of structural properties, composition and surface morphology influences the transport properties of the InAlGaN/GaN heterostructures. High 2DEG electron density, low sheet resistance and good C-V response with a steep slope for InAlGaN/GaN HEMT were achieved at an optimized growth temperature window between 900 °C and 950 °C.
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- 2018
15. Electrochemical techniques for photoelectrode characterisation
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Isaac Holmes-Gentle, Anna Hankin, and Franky E. Bedoya-Lora
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Materials science ,business.industry ,Process Chemistry and Technology ,010501 environmental sciences ,Management, Monitoring, Policy and Law ,Chronoamperometry ,Electrochemistry ,01 natural sciences ,Catalysis ,010406 physical chemistry ,0104 chemical sciences ,Dielectric spectroscopy ,Chemistry (miscellaneous) ,Ultrafast laser spectroscopy ,Optoelectronics ,Water splitting ,business ,Spectroscopy ,Waste Management and Disposal ,Voltammetry ,0105 earth and related environmental sciences ,Surface states - Abstract
Photoelectrodes enable simultaneous light absorption and catalysis of water splitting reactions. Their performance is established using electrochemical characterisation methods. Besides basic characterisation techniques such as voltammetry and chronoamperometry, employed in the dark or under illumination, more advanced techniques, including (photo-)electrochemical impedance spectroscopy, intensity-modulated impedance spectroscopy and transient absorption spectroscopy, can be used to evaluate key parameters and processes. For some of these techniques, data is often interpreted using over-simplified models, leading to the calculation of unreliable parameters. The values of the flat band potential and charge transfer efficiency depend heavily on the methods used to determine them, and it is recommended that the values are corroborated using multiple techniques. Lastly, certain ‘efficiencies’ defined in the literature for electrically biased systems should be revised.
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- 2021
16. Efficient Double- And Triple-Junction Nonfullerene Organic Photovoltaics and Design Guidelines for Optimal Cell Performance
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Emre Yengel, Mohamad Insan Nugraha, Thomas D. Anthopoulos, Vincent M. Le Corre, Yuanbao Lin, Yuliar Firdaus, Franky So, Carr Hoi Yi Ho, Emre Yarali, and Photophysics and OptoElectronics
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Materials science ,Organic solar cell ,Renewable Energy, Sustainability and the Environment ,business.industry ,Triple junction ,Energy Engineering and Power Technology ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Bottleneck ,0104 chemical sciences ,Fuel Technology ,Chemistry (miscellaneous) ,Materials Chemistry ,Optoelectronics ,0210 nano-technology ,business - Abstract
The performance of multijunction devices lags behind single-junction organic photovoltaics (OPVs) mainly because of the lack of suitable subcells. Here, we attempt to address this bottleneck and demonstrate efficient nonfullerene-based multijunction OPVs while at the same time highlighting the remaining challenges. We first demonstrate double-junction OPVs with power conversion efficiency (PCE) of 16.5%. Going a step further, we developed triple-junction OPVs with a PCE of 14.9%, the highest value reported to date for this triple-junction cells. Device simulations suggest that improving the front-cell's carrier mobility to >5 × 10-4 cm2 V-1 s-1 is needed to boost the efficiency of double- and triple-junction OPVs. Analysis of the efficiency limit of triple-junction devices predicts that PCE values of close to 26% are possible. To achieve this, however, the optical absorption and charge transport within the subcells would need to be optimized. The work is an important step toward next-generation multijunction OPVs.
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- 2020
17. Recovering cavity effects in corrugated organic light emitting diodes
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I-Te Chen, Yi-An Chen, Franky So, Chih-Hao Chang, Xiangyu Fu, Yash Mehta, Liping Zhu, Nilesh Barange, Stephen Amoah, and Dong-Hun Shin
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Waveguide (electromagnetism) ,Total internal reflection ,Materials science ,business.industry ,02 engineering and technology ,Substrate (electronics) ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,Anode ,010309 optics ,Optics ,0103 physical sciences ,Dispersion (optics) ,Electrode ,OLED ,Optoelectronics ,Quantum efficiency ,0210 nano-technology ,business - Abstract
Cavity effects play an important role in determining the out-coupling efficiency of an OLED. By fabricating OLEDs on corrugated substrates, the waveguide and SPP modes can be extracted by diffraction. However, corrugation does not always lead to an enhancement in out-coupling efficiency due to the reduction of the electrode reflectance and hence the cavity effects. Based on the results of our rigorous couple-wave analysis (RCWA) simulation, we found that the cavity effects can be partially recovered using a low index Teflon layer inserted between the ITO anode and the substrate due to the enhancement of the reflectance of the corrugated electrodes. To verify the simulation results, we fabricated corrugated OLEDs having a low-index Teflon interlayer with an EQE of 36%, which is 29% higher than an optimized planar OLED. By experimentally measuring the OLED air mode dispersion, we confirm the cavity emission of a corrugated OLED is enhanced by the low index layer.
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- 2020
18. Directional Polarized Light Emission from Thin-Film Light-Emitting Diodes
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Lei Lei, Franky So, Juliana Mendes, Siliang He, Renuka Gogusetti, Shichen Yin, Xiangyu Fu, Yi-An Chen, Yash Mehta, Nilesh Barange, Liping Zhu, Chih-Hao Chang, and Qi Dong
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Materials science ,Extinction ratio ,business.industry ,Mechanical Engineering ,Physics::Optics ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Polarization (waves) ,01 natural sciences ,0104 chemical sciences ,law.invention ,Mechanics of Materials ,law ,Optoelectronics ,General Materials Science ,Light emission ,Photonics ,0210 nano-technology ,business ,Waveguide ,Diode ,Perovskite (structure) ,Light-emitting diode - Abstract
Light-emitting diodes (LEDs) with directional and polarized light emission have many photonic applications, and beam shaping of these devices is fundamentally challenging because they are Lambertian light sources. In this work, using organic and perovskite LEDs (PeLEDs) for demonstrations, by selectively diffracting the transverse electric (TE) waveguide mode while suppressing other optical modes in a nanostructured LED, the authors first demonstrate highly directional light emission from a full-area organic LED with a small divergence angle less than 3° and a TE to transverse magnetic (TM) polarization extinction ratio of 13. The highly selective diffraction of only the TE waveguide mode is possible due to the planarization of the device stack by thermal evaporation and solution processing. Using this strategy, directional and polarized emission from a perovskite LED having a current efficiency 2.6 times compared to the reference planar device is further demonstrated. This large enhancement in efficiency in the PeLED is attributed to a larger contribution from the TE waveguide mode resulting from the high refractive index in perovskite materials.
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- 2020
19. An all-solution-processed interconnecting layer for highly reproducible and efficient tandem organic solar cells
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Taesoo Kim, Carr Hoi Yi Ho, Franky So, Thomas D. Anthopoulos, Yuliar Firdaus, Shuo Ding, Harald Ade, Aram Amassian, and Yuan Xiong
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Organic semiconductor ,Materials science ,Fabrication ,Tandem ,Organic solar cell ,PEDOT:PSS ,business.industry ,Photovoltaic system ,Energy conversion efficiency ,Optoelectronics ,business ,Absorption (electromagnetic radiation) - Abstract
Tandem structure provides a practical way to realize high efficiency organic photovoltaic (OPV) cells due to the limited optical absorption in organic semiconductors and tandem cells can be used to extend the wavelength coverage of the solar spectrum for light harvesting. The interconnecting layer (ICL) between subcells in a tandem solar cell plays a critical role in the reproducibility and the performance of tandem devices, and the processability of the ICL in a tandem cell has been a challenge. In this work we report on the fabrication of highly reproducible high efficiency tandem cells by employing a commercially available material, PEDOT:PSS HTL Solar (HSolar), as the hole transporting material used for the ICL. Comparing with the conventional PEDOT:PSS Al 4083, HSolar offers a better wettability on the underlying non-fullerene photoactive layers, resulting in better charge extraction properties of the ICL. When FTAZ:IT-M and PTB7-Th:IEICO-4F are used as the front cell and the back cells to fabricate the tandem solar cells, a power conversion efficiency (PCE) of 14.7% is achieved. To validate the processability of these tandem cells, three other research groups have successfully fabricated tandem cells using the same recipe and the highest PCE obtained is 16.1%. With further development of donor polymers and device optimization, our device simulation results show that a PCE < 22% can be realized in tandem cells in the near future.
- Published
- 2020
20. Low-threshold ASE and DFB lasers using quasi-2D perovskites with highly oriented nanocrystals
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Siliang He, Harald Ade, Kenan Gundogdu, Lei Lei, Franky So, Qi Dong, Juliana Mendes, Dovletgeldi Seyitliyev, and Samuel J. Stuard
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Amplified spontaneous emission ,Materials science ,business.industry ,Halide ,Population inversion ,Laser ,law.invention ,Nanocrystal ,law ,Optoelectronics ,Thin film ,Crystallization ,business ,Perovskite (structure) - Abstract
In this work, we demonstrated that through the addition of large organic cation PEABr, the quality of cesium lead halide thin films has been enhanced by controlling the crystallization process and thus the domain size and morphology. As a result, by increasing the PEABr concentration to 40%, the perovskite thin films with a small domain size, highly oriented crystal structure and low defect density demonstrated a lowest amplified spontaneous emission threshold of 7 µJ/cm2. However, further increasing PEABr concentration will result in a slower energy transfer, which limits the population inversion process and thus showed a higher ASE threshold. Using this strategy, by tuning the halide ratio, the ASE emissions were observed over the broad visible range (from 490 nm to 680 nm), demonstrating its wide wavelength-tunability.
- Published
- 2020
21. Precursor engineering enhanced responsivity of lead sulfide quantum dots shortwave infrared photodiode
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Franky So, Carr Hoi Yi Ho, Shichen Yin, and Julian Gullett
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Materials science ,business.industry ,Heterojunction ,Photodiode ,law.invention ,chemistry.chemical_compound ,Responsivity ,chemistry ,law ,Quantum dot ,Optoelectronics ,Quantum efficiency ,Lead sulfide ,Quantum well infrared photodetector ,business ,Lead oxide - Abstract
Lead sulfide (PbS) quantum dots (QDs) exhibiting the narrow bandgap (Eg < 0.9 eV) provide a promising avenue to high performance, inexpensive shortwave infrared (SWIR) photodetector. However, most PbS-QD based SWIR photodiodes suffers from low responsivity and low external quantum efficiency (EQE) in the IR range due to insufficient ligand exchange. Here, we report a precursor engineering strategy that facilitates the commonly used Tetrabutylammonium iodide (TBAI) ligand exchange. We synthesized the PbS QDs from Lead Oxide (PbO) and lead acetate trihydrate (PbAc2·3H2O). Compared with PbS QDs photodiode based on PbO, the responsivity and EQE of PbS QDs photodiode based on PbAc2·3H2O have been improved from 0.15 to 0.586A/W and 12.35 to 47.27%.
- Published
- 2020
22. Multi-mode Organic Light-Emitting Diode to Suppress the Viewing Angle Dependence
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Chen Dong, Xiangyu Fu, Jian Li, Linyu Cao, Franky So, Kenan Gundogdu, and Stephen Amoah
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010302 applied physics ,Materials science ,Physics::Instrumentation and Detectors ,business.industry ,Mode (statistics) ,Color shift ,Viewing angle ,01 natural sciences ,010309 optics ,0103 physical sciences ,Electrode ,OLED ,Optoelectronics ,General Materials Science ,Chromaticity ,business ,Common emitter ,Diode - Abstract
A typical top-emitting organic light-emitting diode (OLED) has a strong microcavity effect because of the two reflective electrodes. The cavity effect causes a serious color shift with the viewing angles and restricts the organic layer thickness. To overcome these drawbacks, we design a multi-mode OLED structure with dual-dielectric spacer layers, which extend the cavity length by more than 10 times. This design completely eliminates the intrinsic cavity effect caused by the top and bottom boundaries and provides freedom for the organic layer thickness. We demonstrate these effects in a white multi-mode OLED using a white emitter, which shows a negligible angular chromaticity shift of Δuv = 0.006 from 0 to 70° and a Lambertian emission profile. The simple design and the perfect angular color profiles make the multi-mode OLED structure promising in large-area displays and solid-state lighting applications.
- Published
- 2020
23. Long-Wavelength Lead Sulfide Quantum Dots Sensing up to 2600 nm for Short-Wavelength Infrared Photodetectors
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Jae Woong Lee, Chen Dong, Shuyi Liu, Shichen Yin, Tyler N. Pardue, Xueping Yi, Franky So, and Nilesh Barange
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Materials science ,business.industry ,Band gap ,Photodetector ,Heterojunction ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,chemistry.chemical_compound ,Responsivity ,chemistry ,Quantum dot ,Optoelectronics ,General Materials Science ,Quantum efficiency ,Lead sulfide ,0210 nano-technology ,business ,Absorption (electromagnetic radiation) - Abstract
Lead sulfide nanoparticles (PbS NPs) are used in the short-wavelength infrared photodetectors because of their excellent photosensitivity, band gap tunability, and solution processability. It has been a challenge to synthesize high-quality PbS NPs with an absorption peak beyond 2000 nm. In this work, using PbS seed crystals with an absorption peak at 1960 nm, we report a successful synthesis of very large monodispersed PbS NPs having a diameter up to 16 nm by multiple injections. The resulting NPs have an absorption peak over 2500 nm with a small full width at half-maximum of 24 meV. To demonstrate the applications of such large quantum dots (QDs), broadband heterojunction photodetectors are fabricated with the large PbS QDs of an absorption peak at 2100 nm. The resulting devices have an external quantum efficiency (EQE) of 25% (over 50% internal quantum efficiency) at 2100 nm corresponding to a responsivity of 0.385 A/W and an EQE of ∼60% in the visible range.
- Published
- 2019
24. Semi-transparent vertical organic light-emitting transistors
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Franky So, Nilesh Barange, Hyeonggeun Yu, Ryan Larrabee, and Szuheng Ho
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Fabrication ,Materials science ,Oxide ,02 engineering and technology ,01 natural sciences ,Luminance ,law.invention ,Biomaterials ,chemistry.chemical_compound ,law ,0103 physical sciences ,Materials Chemistry ,Electrical and Electronic Engineering ,Porosity ,Diode ,010302 applied physics ,business.industry ,Transistor ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,chemistry ,Modulation ,Electrode ,Optoelectronics ,0210 nano-technology ,business - Abstract
Vertical organic light-emitting transistor (VOLET) having an organic light-emitting diode integrated with a vertical thin-film transistor is promising for transparent electronics because the vertical device structure potentially offers a display with a large aperture ratio and a low power consumption. However, making a transparent VOLET has been challenging due to the requirements for all transparent electrodes including fabrication of a porous source electrode for current modulation in the device. Here, we report a semi-transparent VOLET with a large modulation of light emitted through the top and bottom electrodes using a nano-porous indium-tin oxide (ITO) source electrode, a Mg:Ag drain electrode, and an ITO gate electrode. The porous ITO source electrode is not only important for luminance modulation, but the nano-textured film morphology also enhances light extraction from the device. Finally, we show that the off current of the VOLET can be suppressed with an electron transporting layer (C60), leading to a large luminance on/off ratio of 104.
- Published
- 2018
25. High-Performance LPCVD-SiNx/InAlGaN/GaN MIS-HEMTs With 850-V 0.98-$\text{m}{\Omega} \cdot$ cm2 for Power Device Applications
- Author
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Ting-En Hsieh, Yueh-Chin Lin, Chia-Hsun Wu, Edward Yi Chang, Franky Lumbantoruan, and Huan-Chung Wang
- Subjects
Materials science ,Passivation ,LPCVD ,Gate dielectric ,02 engineering and technology ,figure of merit ,01 natural sciences ,MIS-HEMT ,Plasma-enhanced chemical vapor deposition ,0103 physical sciences ,Figure of merit ,Breakdown voltage ,Electrical and Electronic Engineering ,010302 applied physics ,business.industry ,021001 nanoscience & nanotechnology ,Subthreshold slope ,Electronic, Optical and Magnetic Materials ,Threshold voltage ,Optoelectronics ,lcsh:Electrical engineering. Electronics. Nuclear engineering ,0210 nano-technology ,business ,InAlGaN/GaN ,SiNₓ ,lcsh:TK1-9971 ,Current density ,Biotechnology - Abstract
We demonstrate the electrical performances of the quaternary InAlGaN/GaN MIS-HEMTs with high quality SiNx gate dielectric and surface passivation layer deposited by low pressure chemical vapor deposition (LPCVD) at 780 °C. Excellent LPCVD-SiNx/InAlGaN interface and SiNx film quality were obtained, resulting in very high output current density, a very small threshold voltage hysteresis and steep subthreshold slope. The LPCVD-SiNx/InAlGaN/GaN MIS-HEMT device exhibited high on/off current ratio, large gate voltage swing, high breakdown voltage, and very low dynamic on-resistance (RON) degradation, meaning effective current collapse suppression compared to the plasma enhanced chemical vapor deposition -SiNx/InAlGaN/GaN MIS-HEMTs. The corresponding specific on-resistance (RON,sp) for LPCVD-SiNx device was as low as 0.98 mΩ·cm2, yielding a high figure of merit of 737 MW/cm2. These results demonstrate a great potential of the LPCVD-SiNx/InAlGaN/GaN MIS-HEMTs for high-power switching applications.
- Published
- 2018
26. Probing the Emission Zone Length in Organic Light Emitting Diodes via Photoluminescence and Electroluminescence Degradation Analysis
- Author
-
Cheng Peng, Franky So, Amin Salehi, Ying Chen, Georgios Liaptsis, and Michael Danz
- Subjects
Materials science ,Photoluminescence ,Dopant ,business.industry ,Concentration effect ,02 engineering and technology ,Electroluminescence ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Fluorescence ,0104 chemical sciences ,OLED ,Optoelectronics ,Degradation (geology) ,General Materials Science ,0210 nano-technology ,business ,Common emitter - Abstract
The understanding and control of the emission zone in organic light emitting diodes (OLEDs) is crucial to the device operational stability. Using the photoluminescence and electroluminescence degradation data, we have developed a modeling methodology to quantitatively determine the length of the emission zone and correlate that with the degradation mechanism. We first validate the modeling results by studying the emitter concentration effect on operational stability of devices using the well-studied thermal activated delayed fluorescent (TADF) emitter (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN), and our results are consistent with previous published data. We further applied this methodology to study the emitter concentration effect using another TADF emitter, 4-carbazolyl-2-methylisoindole-1,3-dione (dopant 1). The results show that the emission zone of the dopant 1 devices is narrower than the 4CzIPN device, leading to faster degradation. While a higher emitter concentration does not result in widening of the emission zone, we were able to widen the emission zone and hence extend the device lifetime using a mixed host.
- Published
- 2017
27. Interconnecting layers for tandem organic solar cells
- Author
-
J. Kothari, Carr Hoi Yi Ho, X. Fu, and Franky So
- Subjects
Materials science ,Organic solar cell ,Tandem ,Renewable Energy, Sustainability and the Environment ,business.industry ,Materials Science (miscellaneous) ,Transmission loss ,Energy Engineering and Power Technology ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Fuel Technology ,Nuclear Energy and Engineering ,Optoelectronics ,0210 nano-technology ,business ,Layer (electronics) - Abstract
The tandem structure provides a practical way to realize high-efficiency organic solar cells (OSCs). It overcomes several bottlenecks in single-junction OSCs such as thermalization loss, transmission loss, and inefficient light absorption. In tandem OSCs, the interconnecting layer (ICL) electrically connects two subcells and it plays a critical role in determining the device performance and reproducibility. In this review paper, we first give a brief introduction to the working mechanism and the requirements for ICLs. We then compare three classes of ICLs based on their optical, mechanical, and chemical properties. Finally, the possible paths forward for tandem OSCs are discussed.
- Published
- 2021
28. Light extraction in tandem organic light emitting diodes
- Author
-
Y. L. Chen, Xiangyu Fu, Shichen Yin, Liping Zhu, Franky So, Qi Dong, and Chih-Hao Chang
- Subjects
Charge generation ,Brightness ,Materials science ,Key factors ,Physics and Astronomy (miscellaneous) ,Tandem ,business.industry ,Extraction (chemistry) ,OLED ,Optoelectronics ,Waveguide mode ,business - Abstract
Since the invention of organic light emitting diodes (OLEDs), great research efforts have been dedicated to improving their efficiency and lifetime. For high-brightness applications, tandem OLED structures have advantages because of the lower current densities required to achieve high brightness. With the successful development of highly efficient charge generation layers, high brightness tandem OLEDs are used in displays and lighting. However, the major challenge for tandem OLEDs is the low light extraction efficiency, because about 50% of the light is trapped inside the device as waveguide modes. In this Perspective, we first review the recent works done on light extraction, analyze different waveguide mode extraction structures, and then identify the key factors determining the extraction efficiencies in tandem OLEDs.
- Published
- 2021
29. Band Edge Control of Quasi‐2D Metal Halide Perovskites for Blue Light‐Emitting Diodes with Enhanced Performance
- Author
-
Shichen Yin, Jordan Frick, Kenneth Hanson, Franky So, Michael Worku, Daniel B. Dougherty, Alex J. Robb, He Liu, Biwu Ma, J. S. Raaj Vellore Winfred, Azza Ben-Akacha, Samanvitha Sridhar, Qingquan He, and Maya Chaaban
- Subjects
Materials science ,business.industry ,Halide ,Edge (geometry) ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Biomaterials ,Metal ,visual_art ,Electrochemistry ,visual_art.visual_art_medium ,Optoelectronics ,business ,Blue light ,Diode - Published
- 2021
30. Effects of NH3 Flow Rate During AlGaN Barrier Layer Growth on the Material Properties of AlGaN/GaN HEMT Heterostructure
- Author
-
Wei Ching Huang, Edward Yi Chang, Yuen Yee Wong, Franky Lumbantoruan, and Hung Wei Yu
- Subjects
010302 applied physics ,Electron density ,Materials science ,business.industry ,chemistry.chemical_element ,Heterojunction ,02 engineering and technology ,High-electron-mobility transistor ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Electronic, Optical and Magnetic Materials ,Volumetric flow rate ,Barrier layer ,chemistry ,Impurity ,0103 physical sciences ,Atom ,Materials Chemistry ,Optoelectronics ,Electrical and Electronic Engineering ,0210 nano-technology ,business ,Carbon - Abstract
NH3 flow rate during AlGaN barrier layer growth not only affects the growth efficiency and surface morphology as a result of parasitic reactions but also influences the concentration of carbon impurity in the AlGaN barrier. Carbon, which decomposes from metal precursors, plays a role in electron compensation for AlGaN/GaN HEMT. No 2-dimensional electron gas (2-DEG) was detected in the AlGaN/GaN structure if grown with 0.5 slm of NH3 due to the presence of higher carbon impurity (2.6 × 1019 cm−2). When the NH3 flow rate increased to 6.0 slm, the carbon impurity reduced to 2.10 × 1018 atom cm−3 and the 2 DEG electron density recovered to 9.57 × 1012 cm−2.
- Published
- 2017
31. Utilizing Forster resonance energy transfer to extend spectral response of PCDTBT:PCBM solar cells
- Author
-
Erik Klump, Tzung-Han Lai, Franky So, and Iordania Constantinou
- Subjects
Materials science ,Fullerene ,Organic solar cell ,Infrared ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,Polymer solar cell ,Biomaterials ,chemistry.chemical_compound ,Materials Chemistry ,Electrical and Electronic Engineering ,chemistry.chemical_classification ,Squaraine dye ,business.industry ,Energy conversion efficiency ,General Chemistry ,Polymer ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,0104 chemical sciences ,Electronic, Optical and Magnetic Materials ,chemistry ,Optoelectronics ,0210 nano-technology ,Ternary operation ,business - Abstract
Light harvesting in the near-infrared part of the solar spectrum is important to achieve high efficiency polymer solar cells (PSCs). In this work, we demonstrate that we take an existing polymer:fullerene blend and extend its spectral response into the near-IR region by adding a small amount near-IR absorbing dye in the blend. The polymer studied in this work is Poly[ N -9′-heptadecanyl-2,7-carbazole- alt -5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT). By adding only 2.5% squaraine dye to the blend, we were able to extend the spectral response of the resulting devices 100 nm into the near-IR spectral region. We show that the enhanced light harvesting is due to efficient Forster resonance energy transfer (FRET) between PCDTBT and the squaraine dye, resulting in an increase in power conversion efficiency. This type of ternary polymer solar cells is unique in that it allows the use of a small amount of selected dyes to extend light harvesting in infrared region.
- Published
- 2017
32. Comparing non-fullerene acceptors with fullerene in polymer solar cells: a case study with FTAZ and PyCNTAZ
- Author
-
Jingbo Zhao, He Henry Yan, Harald Ade, Wei You, Liang Yan, Nicole Bauer, Qianqian Zhang, Joo-Hyun Kim, Franky So, Iordania Constantinou, and Long Ye
- Subjects
chemistry.chemical_classification ,Electron mobility ,Materials science ,Fullerene ,Organic solar cell ,Renewable Energy, Sustainability and the Environment ,business.industry ,Nanotechnology ,02 engineering and technology ,General Chemistry ,Electron ,Hybrid solar cell ,Electron acceptor ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Acceptor ,Polymer solar cell ,0104 chemical sciences ,chemistry ,Optoelectronics ,General Materials Science ,0210 nano-technology ,business - Abstract
Non-fullerene acceptors (NFAs) are becoming a serious contender to fullerene-based electron acceptors in organic photovoltaics, due to their structural versatility and easily tunable optical and electronic properties. However, NFA-based solar cells often have a decreased short-circuit current (Jsc) and fill factor (FF) compared to their fullerene-based counterparts. Here, we investigate the fundamental causes of this decrease in the performance of solar cells using a non-fullerene acceptor (SF-PDI2) paired with two polymer donors, FTAZ and PyCNTAZ, compared with their fullerene-based counterparts. Through a number of experimental techniques and morphological studies, we show that the SF-PDI2-based solar cells suffer from insufficient charge generation, transport, and collection when compared with the PCBM-based solar cells. The SF-PDI2-based solar cells show increased bimolecular recombination, which, together with other recombination loss mechanisms in these cells, causes a significant decrease in their Jsc and FF. Notably, the less pure domains, low electron mobility (on the order of 10−5 cm2 V−1 s−1), and imbalanced mobility (in regard to the hole mobility) further explain the low FF. On the other hand, the higher open-circuit voltage (Voc) in the SF-PDI2 devices is mainly due to the increase in the CT state energy. It is worth mentioning that the PyCNTAZ-based devices show an ultralow charge separation energy (ΔECS), close to 0 eV. Our results demonstrate that further increasing the mobility (both of electrons and holes) in these NFA-based solar cells would be a viable approach to further enhance the efficiency of these new types of solar cells, ideally, without losing the high Voc of such cells.
- Published
- 2017
33. Efficient Energy funneling in hybrid quasi-2D perovskites: from light emitting diodes to lasers (Conference Presentation)
- Author
-
Lei Lei, Qi Dong, Harald Ade, Franky So, Samual Stuard, Dovletgeldi Seyitliyev, Chih-Hao Chang, Xueping Yi, Kenan Gundogdu, Y. L. Chen, Juliana Mendes, Liping Zhu, and Xiangyu Fu
- Subjects
Amplified spontaneous emission ,Distributed feedback laser ,Materials science ,Band gap ,business.industry ,law.invention ,law ,Optoelectronics ,Charge carrier ,Thin film ,business ,Quantum well ,Light-emitting diode ,Perovskite (structure) - Abstract
Quasi-2D Ruddlesden-Popper halide perovskites with a large exciton binding energy, self-assembled quantum wells and high quantum yield draw attention for optoelectronic device applications. Thin films of these quasi-2D perovskites consist of a mixture of domains having different dimensionality, allowing energy funneling from lower-dimensional nanosheets (narrower bandgap domains) to three-dimensional nanocrystals (wide bandgap domains). Here, the quality of quasi-2D perovskite (PEA)2(FA)3Pb4Br13 is controlled by solution engineering. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were conducted to study the crystal orientation, and transient absorption spectroscopy (TAS) measurements were conducted to study the charge carrier dynamics. Our data show that the highly oriented 2D crystals have a faster energy transfer to the low bandgap regime (< 0.5 ps) compared to less oriented films. High-performance LEDs can be realized with these 2D films. Finally, amplified spontaneous emission is achieved with an ultralow threshold of 4.16 μJ/cm2 and distributed feedback lasers are realized. Our results show that it is important to control the morphology and spatial domain distribution of the quasi-2D films to achieve efficient energy transfer, which is a critical requirement for optoelectronic devices.
- Published
- 2019
34. Realization of high-efficiency fluorescent organic light-emitting diodes with low driving voltage
- Author
-
Chen Dong, Amin Salehi, Christopher M. Papa, Dong-Hun Shin, Felix N. Castellano, Liping Zhu, Franky So, and Anh Thy Bui
- Subjects
0301 basic medicine ,Materials science ,Band gap ,Science ,Physics::Optics ,General Physics and Astronomy ,02 engineering and technology ,Electroluminescence ,Article ,General Biochemistry, Genetics and Molecular Biology ,03 medical and health sciences ,Electronic devices ,OLED ,Organic LEDs ,lcsh:Science ,Diode ,Multidisciplinary ,business.industry ,General Chemistry ,021001 nanoscience & nanotechnology ,Fluorescence ,030104 developmental biology ,Optoelectronics ,lcsh:Q ,Quantum efficiency ,0210 nano-technology ,business ,Phosphorescence ,Electrical efficiency - Abstract
It is commonly accepted that a full bandgap voltage is required to achieving efficient electroluminescence (EL) in organic light-emitting diodes. In this work, we demonstrated organic molecules with a large singlet-triplet splitting can achieve efficient EL at voltages below the bandgap voltage. The EL originates from delayed fluorescence due to triplet fusion. Finally, in spite of a lower quantum efficiency, a blue fluorescent organic light-emitting diode having a power efficiency higher than some of the best thermally activated delayed fluorescent and phosphorescent blue organic light-emitting diodes is demonstrated. The current findings suggest that leveraging triplet fusion from purely organic molecules in organic light-emitting diode materials offers an alternative route to achieve stable and high efficiency blue organic light-emitting diodes., Though organic light-emitting diodes (OLEDs) with electroluminescence at sub-bandgap voltages have been reported, realizing high efficiency in such a device is difficult. Here, the authors report high efficiency sub-bandgap OLEDs featuring organic molecules with large singlet-triplet splitting.
- Published
- 2019
35. Low Cost Corrugated Substrates for High Efficiency OLEDs (Final Report)
- Author
-
Franky So
- Subjects
Materials science ,business.industry ,OLED ,Optoelectronics ,business - Published
- 2018
36. Metal Halide Perovskites for Laser Applications
- Author
-
Lei Lei, Franky So, Qi Dong, and Kenan Gundogdu
- Subjects
Amplified spontaneous emission ,Materials science ,business.industry ,Halide ,Electron phonon coupling ,Condensed Matter Physics ,Laser ,Electronic, Optical and Magnetic Materials ,law.invention ,Biomaterials ,Metal ,law ,visual_art ,Electrochemistry ,visual_art.visual_art_medium ,Optoelectronics ,business - Published
- 2021
37. Corrugated Organic Light Emitting Diodes Using Low Tg Electron Transporting Materials
- Author
-
Shuyi Liu, Chen Ying, Kirk S. Schanze, Franky So, Xiangyu Fu, Zhenxing Pan, and Cheng Peng
- Subjects
Materials science ,Annealing (metallurgy) ,business.industry ,Respiratory electron transport ,Composite number ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Planar ,OLED ,Optoelectronics ,General Materials Science ,Quantum efficiency ,0210 nano-technology ,business ,Operational stability ,Glass transition - Abstract
A corrugated organic light emitting diode (OLED) with enhanced light extraction is realized by incorporating a corrugated composite electron transport layer (ETL) consisting of two ETLs with different glass transition temperatures. The morphology of the corrugated structure is characterized with atomic force microscopy. The results show that the corrugation can be controlled by the layer thicknesses and annealing temperature. Compared with the control planar device, the corrugated OLED shows a more than 35% enhancement in current efficiency from 31 cd/A to 43 cd/A and a 20% enhancement in external quantum efficiency from 10% to 12% at 100 cd/m(2). In addition, the corrugated OLED also has a greatly improved operational stability. The LT90 lifetime of a device operated at 1000 cd/m(2) is improved greater than 100-fold in the corrugated OLED.
- Published
- 2016
38. Novel Patterning Method for Silver Nanowire Electrodes for Thermal-Evaporated Organic Light Emitting Diodes
- Author
-
Shuyi Liu, Szuheng Ho, and Franky So
- Subjects
Materials science ,Polydimethylsiloxane ,business.industry ,Nanowire ,Nanotechnology ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Light scattering ,0104 chemical sciences ,chemistry.chemical_compound ,chemistry ,PEDOT:PSS ,Electrode ,OLED ,Optoelectronics ,General Materials Science ,Wetting ,0210 nano-technology ,business ,Diode - Abstract
Silver nanowires (AgNWs) mesh has been used as transparent electrodes in optoelectronic devices. However, the lack of practical patterning techniques for the random percolating nanowire network has limited its applications in devices where a well-defined pixel is required. Here, by controlling the surface wetting properties of a polydimethylsiloxane (PDMS) release template, we are able to pattern the random AgNWs network with uniform conducting property; and due to the hydrophobic recovery nature of PDMS, a multilayer patterning and transferring process can be realized, resulting in a fine-patterned, smooth, and uniform AgNWs mesh/poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PSS) composite electrode. A thermal-evaporated organic light-emitting diode (OLED) is directly fabricated onto the patterned AgNWs/PSS composite electrode. The device shows well-defined pixel edges and a uniformly lighted pixel area. A uniform OLED with very low leakage current is realized. The enhanced efficiency compared to the controlled device with prepatterned indium tin oxide (ITO) electrode is attributed to the scattering effects of the AgNWs electrode.
- Published
- 2016
39. Efficiency Roll-Off in Blue Emitting Phosphorescent Organic Light Emitting Diodes with Carbazole Host Materials
- Author
-
Xiang Chaoyu, Bryce Nelson, Yong Zhang, Wei Wei, Xiangyu Fu, Franky So, Rui Liu, and Viktor P. Balema
- Subjects
Materials science ,Photoluminescence ,02 engineering and technology ,Electroluminescence ,010402 general chemistry ,Photochemistry ,01 natural sciences ,law.invention ,Biomaterials ,chemistry.chemical_compound ,law ,Electrochemistry ,OLED ,Quenching ,Carbazole ,business.industry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,0104 chemical sciences ,Electronic, Optical and Magnetic Materials ,chemistry ,Optoelectronics ,Phosphorescent organic light-emitting diode ,0210 nano-technology ,Glass transition ,business ,Phosphorescence - Abstract
The efficiency roll-off in blue phosphorescent organic light emitting diodes (OLEDs) using different carbazole compounds as the host is systematically studied. While there is no significant difference in device efficiency, OLEDs using ter-carbazole as the host show a reduction in efficiency roll-off at high luminance. Data from transient photoluminescence and electroluminescence measurements show that the lower triplet–triplet annihilation (TTA) and triplet–polaron quenching (TPQ) rates in devices with the ter-carbazole host compared with other carbazole hosts are the reasons for this reduced efficiency roll-off. It is also found that the host materials with low glass transition temperatures are more susceptible to the efficiency roll-off problem.
- Published
- 2016
40. High-gain infrared-to-visible upconversion light-emitting phototransistors
- Author
-
Jae Woong Lee, Hyeonggeun Yu, Franky So, Sujin Baek, Rajiv K. Singh, Jinhyung Lee, and Do Young Kim
- Subjects
Materials science ,Infrared ,business.industry ,Energy conversion efficiency ,Photodetector ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,Photon upconversion ,0104 chemical sciences ,Electronic, Optical and Magnetic Materials ,Photodiode ,law.invention ,law ,OLED ,Optoelectronics ,0210 nano-technology ,business ,Visible spectrum ,Diode - Abstract
Infrared-to-visible upconversion devices made by integrating an infrared quantum dot photodetector with an organic light-emitting diode potentially offer a route to low-cost, pixel-free infrared imaging. However, making such devices sufficiently efficient for practical use is a challenge. Here, we report a high-gain vertical phototransistor with a perforated metallic source electrode having an EQE up to 1 × 105% and a detectivity of 1.2 × 1013 Jones. By incorporating a phosphorescent organic light-emitting diode in this phototransistor, an infrared-to-visible upconversion LEPT with a photon-to-photon conversion efficiency of over 1,000% is demonstrated. An optoelectronic device that efficiently converts infrared light to visible light could prove useful for imaging applications.
- Published
- 2016
41. Inorganic UV-Visible-SWIR Broadband Photodetector Based on Monodisperse PbS Nanocrystals
- Author
-
Franky So, Hyeonggeun Yu, Sujin Baek, Do Young Kim, and Jae Woong Lee
- Subjects
Materials science ,Light sensitivity ,business.industry ,Dispersity ,Photodetector ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Biomaterials ,Nanocrystal ,Quantum dot ,Optoelectronics ,General Materials Science ,0210 nano-technology ,business ,Biotechnology - Abstract
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.
- Published
- 2016
42. Degradation study of organic light-emitting diodes with solution-processed small molecule phosphorescent emitting layers
- Author
-
Chen Ying, Franky So, Shuyi Liu, Cheng Peng, and Alexandria Cruz
- Subjects
Materials science ,Chemical substance ,business.industry ,Nanotechnology ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Solvent ,Impurity ,Excited state ,Materials Chemistry ,OLED ,Optoelectronics ,Degradation (geology) ,0210 nano-technology ,business ,Phosphorescence ,Layer (electronics) - Abstract
The degradation mechanisms of solution processed small molecule phosphorescent OLEDs are investigated along with thermal evaporated devices. Hole-induced degradation in the presence of excited states is found to be a major culprit for the fast degradation in solution-processed devices. Degradation in the solution-processed emitting layer (EML) is found to be strongly dependent on the initial hole injection/transport properties of the EML. Gas chromatography-mass spectrometry (GC-MS) studies on solvents used to prepare the solution-processed EMLs reveal that solvent impurities are the dominant reason for their substantially shorter operational lifetime compared with thermal evaporated devices.
- Published
- 2016
43. Solution-processed copper oxide interlayers for broadband PbS quantum-dot photodiodes
- Author
-
Chen Dong, Hyeonggeun Yu, Shyui Liu, Franky So, Do Young Kim, and Sujin Baek
- Subjects
Copper oxide ,Materials science ,business.industry ,Oxide ,Photodetector ,02 engineering and technology ,General Chemistry ,Substrate (electronics) ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Photodiode ,law.invention ,chemistry.chemical_compound ,chemistry ,Quantum dot ,law ,Materials Chemistry ,Polyethylene terephthalate ,Optoelectronics ,0210 nano-technology ,business ,Dark current - Abstract
Metal oxide interlayers are promising for optoelectronic applications due to solution processability, optical transparency, and excellent charge blocking properties. Highly efficient, air-stable quantum dot photodetectors have been reported using solution-processed metal oxide interlayers. However, the processing temperatures are high, significantly limiting their potential for roll-to-roll processing. Here, we report low temperature-processed broadband PbS quantum dot photodiodes by employing a solution-processed CuOx interlayer. The resulting photodiodes exhibit a low dark current of 10 nA cm−2 with a detectivity over 1013 Jones. Finally, we demonstrate a flexible inorganic photodiode on a polyethylene terephthalate substrate.
- Published
- 2016
44. Organic Solar Cells: High‐Performance Tandem Organic Solar Cells Using HSolar as the Interconnecting Layer (Adv. Energy Mater. 25/2020)
- Author
-
Taesoo Kim, Jeromy James Rech, Carr Hoi Yi Ho, Wei You, Ronald E. Booth, Thomas D. Anthopoulos, Aram Amassian, Yuan Xiong, Abay Gadisa, Qi Dong, Yuliar Firdaus, Xueping Yi, Franky So, Brendan O'Connor, and Harald Ade
- Subjects
Materials science ,Organic solar cell ,Tandem ,Renewable Energy, Sustainability and the Environment ,business.industry ,Optoelectronics ,General Materials Science ,business ,Layer (electronics) ,Energy (signal processing) - Published
- 2020
45. Efficient Energy Funneling in Quasi‐2D Perovskites: From Light Emission to Lasing
- Author
-
Chih-Hao Chang, Lei Lei, Y. L. Chen, Franky So, Liping Zhu, Qi Dong, Kenan Gundogdu, Dovletgeldi Seyitliyev, Siliang He, Harald Ade, Juliana Mendes, Xiangyu Fu, Xueping Yi, and Samuel J. Stuard
- Subjects
Amplified spontaneous emission ,Materials science ,business.industry ,Mechanical Engineering ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,7. Clean energy ,01 natural sciences ,0104 chemical sciences ,law.invention ,Crystal ,Mechanics of Materials ,law ,Optoelectronics ,General Materials Science ,Light emission ,Thin film ,0210 nano-technology ,business ,Lasing threshold ,Quantum well ,Light-emitting diode ,Perovskite (structure) - Abstract
Quasi-2D Ruddlesden-Popper halide perovskites with a large exciton binding energy, self-assembled quantum wells, and high quantum yield draw attention for optoelectronic device applications. Thin films of these quasi-2D perovskites consist of a mixture of domains having different dimensionality, allowing energy funneling from lower-dimensional nanosheets (high-bandgap domains) to 3D nanocrystals (low-bandgap domains). High-quality quasi-2D perovskite (PEA)2 (FA)3 Pb4 Br13 films are fabricated by solution engineering. Grazing-incidence wide-angle X-ray scattering measurements are conducted to study the crystal orientation, and transient absorption spectroscopy measurements are conducted to study the charge-carrier dynamics. These data show that highly oriented 2D crystal films have a faster energy transfer from the high-bandgap domains to the low-bandgap domains (
- Published
- 2020
46. Operational stability of perovskite light emitting diodes
- Author
-
Juliana Mendes, Lei Lei, Qi Dong, and Franky So
- Subjects
Materials science ,business.industry ,law ,Ion migration ,Optoelectronics ,General Materials Science ,Condensed Matter Physics ,business ,Operational stability ,Atomic and Molecular Physics, and Optics ,Perovskite (structure) ,Light-emitting diode ,law.invention - Abstract
Organometal halide perovskite light emitting diodes (LEDs) have attracted a lot of attention in recent years, owing to the rapid progress in device efficiency. However, their short operational lifetime severely impedes the practical uses of these devices. The operating stability of perovskite LEDs are due to degradation due to ambient environment and degradation during operation. The former can be suppressed by encapsulation while the latter one is the intrinsic degradation due to the electrochemical stability of the perovskite materials. In addition, perovskites also suffer from ion migration which is a major degradation mechanism in perovskite LEDs. In this review, we specifically focus on the operational stability of perovskite LEDs. The review is divided into two parts: the first part contains a summary of various degradation mechanisms and some insight on the degradation behavior and the second part is the strategies how to improve the operational stability, especially the strategies to suppress ion migration. Based on the current advances in the literature, we finally present our perspectives to improve the device stability.
- Published
- 2020
47. A Novel Digital Etch Technique for p-GaN Gate HEMT
- Author
-
Edward Yi Chang, Yueh Chin Lin, Burhanuddin Yeop Majilis, Yuan Lin, Franky Lumbantoruan, and Chang Fu Dec
- Subjects
010302 applied physics ,Plasma etching ,Materials science ,business.industry ,chemistry.chemical_element ,02 engineering and technology ,Plasma ,High-electron-mobility transistor ,021001 nanoscience & nanotechnology ,01 natural sciences ,Boron trichloride ,Oxygen ,Threshold voltage ,Metal ,chemistry.chemical_compound ,chemistry ,visual_art ,0103 physical sciences ,visual_art.visual_art_medium ,Optoelectronics ,Breakdown voltage ,0210 nano-technology ,business - Abstract
We demonstrate the digital etching (DE) process to fabricated E-mode p-GaN/AIGaN/GaN HEMT. DE process comprising low power oxygen (02) plasma oxidizing and low power boron trichloride (BCl 3 ) plasma etching to selectively remove p-GaN layer. The atomic layer etching (ALE) has an etching rate of 1.62 nm/cycle to achieved depth of 70nm. The 5-µm source-drain offset length (L SD ) device with Ni/Au gate metal demonstrated 365 mAlmm drain current density with threshold voltage (V TH ) of +1.8V, on/off current ratio of 1.6×106, breakdown voltage (BV) of 154V, and static on-resistance (R ON ) of 8.47 Ω.mm. The 20-µm L SD device with Ni/Au gate metal demonstrated 211 mA/mm drain current density with V TH of +2V, and on/off current ratio of 1. 2×106, BV of 426V, and static R ON of 17.3 Ω.mm.
- Published
- 2018
48. World Scientific Handbook of Organic Optoelectronic Devices
- Author
-
Franky So
- Subjects
Materials science ,Volume (thermodynamics) ,business.industry ,Optoelectronics ,Electronics ,business ,Perovskite (structure) - Published
- 2018
49. Sub-Band Gap Turn-On Near-Infrared-to-Visible Up-Conversion Device Enabled by an Organic-Inorganic Hybrid Perovskite Photovoltaic Absorber
- Author
-
Franky So, Yuanhang Cheng, Sai-Wing Tsang, Menglin Li, and Hyeonggeun Yu
- Subjects
Materials science ,business.industry ,Infrared ,Band gap ,Near-infrared spectroscopy ,Photodetector ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Formamidinium ,OLED ,Optoelectronics ,General Materials Science ,0210 nano-technology ,business ,Diode ,Perovskite (structure) - Abstract
Direct integration of an infrared (IR) photodetector with an organic light-emitting diode (OLED) enables low-cost, pixel-free IR imaging. However, the operation voltage of the resulting IR-to-visible up-conversion is large because of the series device architecture. Here, we report a low-voltage near-IR (NIR)-to-visible up-conversion device using formamidinium lead iodide as a NIR absorber integrated with a phosphorescent OLED. Because of the efficient photocarrier injection from the hybrid perovskite layer to the OLED, we observed a sub-band gap turn-on of the OLED under NIR illumination. The device showed a NIR-to-visible up-conversion efficiency of 3% and a luminance on/off ratio of 103 at only 5 V. Finally, we demonstrate pixel-free NIR imaging using the up-conversion device.
- Published
- 2018
50. Origin of Sub-Bandgap Electroluminescence in Organic Light-Emitting Diodes
- Author
-
Cheng Peng, Franky So, Xiang Chaoyu, and Ying Chen
- Subjects
Photoluminescence ,Materials science ,Physics::Instrumentation and Detectors ,Condensed Matter::Other ,Band gap ,business.industry ,Exciton ,Physics::Optics ,General Chemistry ,Electroluminescence ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Biomaterials ,Condensed Matter::Materials Science ,chemistry.chemical_compound ,chemistry ,OLED ,Optoelectronics ,General Materials Science ,Rubrene ,business ,Biotechnology ,Common emitter ,Voltage - Abstract
Sub-bandgap electroluminescence in organic light emitting diodes is a phenomenon in which the electroluminescence turn-on voltage is lower than the bandgap voltage of the emitter. Based on the results of transient electroluminescence (EL) and photoluminescence and electroabsorption spectroscopy measurements, it is concluded that in rubrene/C60 devices, charge transfer excitons are generated at the rubrene/C60 interface under sub-bandgap driving conditions, leading to the formation of triplet excitons, and sub-bandgap EL is the result of the subsequent triplet-triplet annihilation process.
- Published
- 2015
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