Publications by authors named "Changsoon Kim"

25 Publications

  • Page 1 of 1

Chiral polymer hosts for circularly polarized electroluminescence devices.

Chem Sci 2021 Jul 21;12(25):8668-8681. Epub 2021 May 21.

Division of Chemical Engineering and Materials Science, Graduate Program in System Health Science and Engineering, Ewha Womans University Seoul 03760 Republic of Korea

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications. The fabrication of a CP PLED requires a polymer host that provides the appropriate chiral environment around the emitting dopant. However, chemical strategies for the design of chiral polymer hosts remain underdeveloped. We have developed new polymer hosts for CP PLED applications. These polymers were prepared through a free-radical polymerization of 3-vinylcarbazole with a chiral -alkyl unit. This chiral unit forces the carbazole repeat units to form mutually helical half-sandwich conformers with preferred ()-helical sense along the polymer main chain. Electronic circular dichroism measurements demonstrate the occurrence of chirality transfer from chiral monomers to achiral monomers during chain growth. The ()-helical-sense-enriched polymer interacts diastereoselectively with an enantiomeric pair of new phosphorescent ()- and ()-dopants. The magnitude of the Kuhn dissymmetry factor ( ) for the ()-helically-enriched polymer film doped with the ()-dopant was found to be one order of magnitude higher than that of the film doped with the ()-dopant. Photoluminescence dissymmetry factors ( ) of the order of 10 were recorded for the doped films, but the magnitude of diastereomeric enhancement decreased to that of . The chiral polymer host permits faster energy transfer to the phosphorescent dopants than the achiral polymer host. Our photophysical and morphological investigations indicate that the acceleration in the chiral polymer host is due to its longer Förster radius and improved compatibility with the dopants. Finally, multilayer CP PLEDs were fabricated and evaluated. Devices based on the chiral polymer host with the ()- and ()-dopants exhibit electroluminescence dissymmetry factors ( ) of 1.09 × 10 and -1.02 × 10 at a wavelength of 540 nm, respectively. Although challenges remain in the development of polymer hosts for CP PLEDs, our research demonstrates that chiroptical performances can be amplified by using chiral polymer hosts.
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http://dx.doi.org/10.1039/d1sc02095aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8246120PMC
July 2021

Twist to Boost: Circumventing Quantum Yield and Dissymmetry Factor Trade-Off in Circularly Polarized Luminescence.

Inorg Chem 2021 Jun 24;60(11):7738-7752. Epub 2021 Mar 24.

Division of Chemical Engineering and Materials Science, and Graduate Program in System Health Science and Engineering, Ewha Womans University, Seoul 03760, Republic of Korea.

Circularly polarized luminescence (CPL) enables promising applications in asymmetric photonics. However, the performances of CPL molecules do not yet meet the requirements of these applications. The shortcoming originates from the trade-off in CPL between the photoluminescence quantum yield (PLQY) and the photoluminescence dissymmetry factor (). In this study, we developed a molecular strategy to circumvent this trade-off. Our approach takes advantage of the strong propensity of [Pt(N^C^N)Cl], where the N^C^N ligand is 1-(2-oxazoline)-3-(2-pyridyl)phenylate, to form face-to-face stacks. We introduced chiral substituents, including ()-methyl, ()- and ()-isopropyl, and ()-indanyl groups, into the ligand framework. This asymmetric control induces torsional displacements that give homohelical stacks of the Pt(II) complexes. X-ray single-crystal structure analyses for the ()-isopropyl Pt(II) complex reveal the formation of a homohelical dimer with a Pt···Pt distance of 3.48 Å, which is less than the sum of the van der Waals radii of Pt. This helical stack elicits the metal-metal-to-ligand charge-transfer (MMLCT) transition that exhibits strong chiroptical activity due to the electric transition moment making an acute angle to the magnetic transition moment. The PLQY and values of the MMLCT phosphorescence emission of the ()-isopropyl Pt(II) complex are 0.49 and 8.4 × 10, which are improved by factors of ca. 6 and 4, respectively, relative to the values of the unimolecular emission (PLQY, 0.078; , 2.4 × 10). Our photophysical measurements for the systematically controlled Pt(II) complexes reveal that the CPL amplifications depend on the chiral substituent. Our investigations also indicate that excimers are not responsible for the enhanced chiroptical activity. To demonstrate the effectiveness of our approach, organic electroluminescence devices were fabricated. The MMLCT emission devices were found to exhibit simultaneous enhancements in the external quantum efficiency (EQE, 9.7%) and the electroluminescence dissymmetry factor (, 1.2 × 10) over the unimolecular emission devices (EQE, 5.8%; , 0.3 × 10). These results demonstrate the usefulness of using the chiroptically active MMLCT emission for achieving an amplified CPL.
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http://dx.doi.org/10.1021/acs.inorgchem.1c00070DOI Listing
June 2021

Europium-Diethylenetriaminepentaacetic Acid Loaded Radioluminescence Liposome Nanoplatform for Effective Radioisotope-Mediated Photodynamic Therapy.

ACS Nano 2020 10 27;14(10):13004-13015. Epub 2020 Aug 27.

Department of Applied Bioengineering, Graduate School of Convergence Science and Technology, Seoul National University, Seoul 08826, Republic of Korea.

Photodynamic therapy (PDT) is an effective anticancer strategy with a higher selectivity and fewer adverse effects than conventional therapies; however, shallow tissue penetration depth of light has hampered the clinical utility of PDT. Recently, reports have indicated that Cerenkov luminescence-induced PDT may overcome the tissue penetration limitation of conventional PDT. However, the effectiveness of this method is controversial because of its low luminescence intensity. Herein, we developed a radiolabeled diethylenetriaminepentaacetic acid chelated Eu (Eu-DTPA)/photosensitizer (PS) loaded liposome (Eu/PS-lipo) that utilizes ionizing radiation from radioisotopes for effective imaging and radioluminescence-induced PDT. We utilized Victoria blue-BO (VBBO) as a PS and observed an efficient luminescence resonance energy transfer between Eu-DTPA and VBBO. Furthermore, Cu-labeled Eu lipo demonstrated a strong radioluminescence with a 2-fold higher intensity than Cerenkov luminescence from free Cu. In our radioluminescence liposome, radioluminescence energy transfer showed a 6-fold higher energy transfer efficiency to VBBO than Cerenkov luminescence energy transfer (CLET). Cu-labeled Eu/VBBO lipo (Cu-Eu/VBBO lipo) showed a substantial tumor uptake of up to 19.3%ID/g by enhanced permeability and retention effects, as revealed by positron emission tomography. Finally, the PDT using Cu-Eu/VBBO lipo demonstrated significantly higher and therapeutic effects than Cerenkov luminescence-induced PDT using Cu-VBBO lipo. This study envisions a great opportunity for clinical PDT application by establishing the radioluminescence liposome which has high tumor targeting and efficient energy transfer capability from radioisotopes.
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http://dx.doi.org/10.1021/acsnano.0c04324DOI Listing
October 2020

Computational wrapping: A universal method to wrap 3D-curved surfaces with nonstretchable materials for conformal devices.

Sci Adv 2020 Apr 10;6(15):eaax6212. Epub 2020 Apr 10.

Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea.

This study starts from the counterintuitive question of how we can render conventional stiff, nonstretchable, and even brittle materials sufficiently conformable to fully wrap curved surfaces, such as spheres, without failure. Here, we extend the geometrical design method of computational origami to wrapping. Our computational wrapping approach provides a robust and reliable method for fabricating conformal devices for arbitrary curved surfaces with a computationally designed nonpolyhedral developable net. This computer-aided design transforms two-dimensional (2D)-based materials, such as Si wafers and steel sheets, into various targeted conformal structures that can fully wrap desired 3D structures without fracture or severe plastic deformation. We further demonstrate that our computational wrapping approach enables a design platform that can transform conventional nonstretchable 2D-based devices, such as electroluminescent lighting and flexible batteries, into conformal 3D curved devices.
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http://dx.doi.org/10.1126/sciadv.aax6212DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7148111PMC
April 2020

High-performance circular-polarization-sensitive organic photodetectors based on a chiral plasmonic nanocavity.

Opt Express 2020 Jan;28(2):1805-1816

Chiral photodetectors, optoelectronic devices that can detect circularly polarized light (CPL), have attracted much attention as building blocks of next-generation information technology. However, their performance has been severely limited by the tradeoff between the external quantum efficiency (η) and the dissymmetry factor of photocurrent, the latter typically being limited by the small dissymmetry factor of absorption (g). This work numerically demonstrates that a circular polarization-sensitive organic photodetector (CP-OPD) based on a chiral plasmonic nanocavity can achieve both high η and g. The design of the chiral nanocavity, featuring a circular dichroic plasmonic mode with a high photonic density of states in the subwavelength thick photoactive layer, is decoupled with that of the photoactive layer, which enables the independent control of the circular dichroic and photon-to-charge conversion properties. By investigating the interaction between CPL and the molecules constituting the photoactive layer, a design principle of the plasmonic CP-OPD is established, resulting in superior performance with η = 23.8 % and g = 1.6.
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http://dx.doi.org/10.1364/OE.383206DOI Listing
January 2020

Selective crack suppression during deformation in metal films on polymer substrates using electron beam irradiation.

Nat Commun 2019 10 1;10(1):4454. Epub 2019 Oct 1.

Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.

While cracks are usually considered detrimental, crack generation can be harnessed for various applications, for example in ceramic materials, via directing crack propagation and crack opening. Here, we find that electron beam irradiation prompts a crack suppression phenomenon in a copper (Cu) thin film on a polyimide substrate, allowing for the control of crack formation in terms of both location and shape. Under tensile strain, cracks form on the unirradiated region of the Cu film whereas cracks are prevented on the irradiated region. We attribute this to the enhancement of the adhesion at the Cu-polyimide interface by electrons transmitted through the Cu film. Finally, we selectively form conductive regions in a Cu film on a polyimide substrate under tension and fabricate a strain-responsive organic light-emitting device.
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http://dx.doi.org/10.1038/s41467-019-12451-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6773782PMC
October 2019

Amplified circularly polarized phosphorescence from co-assemblies of platinum(ii) complexes.

Chem Sci 2019 Feb 29;10(5):1294-1301. Epub 2018 Nov 29.

Division of Chemical Engineering and Materials Science , Ewha Womans University , Seoul 03760 , The Republic of Korea . Email:

Molecules capable of producing zero-field circularly polarized phosphorescence (CPP) are highly valuable for chiroptoelectronic applications that rely on triplet exciton. However, the paucity of tractable molecular design rules for obtaining CPP emission has inhibited full utilization. We report amplification of CPP by the formation of helical co-assemblies consisting of achiral square planar cycloplatinated complexes and small fractions of homochiral cycloplatinated complexes. The latter has a unique Pfeiffer effect during the formation of superhelical co-assemblies, enabling versatile chiroptical control. Large dissymmetry factors in electronic absorption ( , 0.020) and phosphorescence emission ( , 0.064) are observed from the co-assemblies. These values are two orders of magnitude improved relative to those of individual molecules. In addition, photoluminescence quantum yields (PLQY) also increase by a factor of ten. Our structural, photophysical, and quantum chemical investigations reveal that the chiroptical amplification is attributable to utilization of both the magnetically allowed electronic transition and asymmetric coupling of excitons. The strategy overcomes the trade-off between and PLQY which has frequently been found for previous molecular emitters of circularly polarized luminescence. It is anticipated that our study will provide new insight into the future research for the exploitation of the full potential of CPP.
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http://dx.doi.org/10.1039/c8sc04509gDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6357861PMC
February 2019

Controlled Enhancement in Hole Injection at Gold-Nanoparticle-on-Organic Electrical Contacts Fabricated by Spark-Discharge Aerosol Technique.

ACS Appl Mater Interfaces 2019 Feb 30;11(6):6276-6282. Epub 2019 Jan 30.

Graduate School of Convergence Science and Technology, and Inter-University Semiconductor Research Center , Seoul National University , 1 Gwanak-ro , Gwanak-gu, Seoul 08826 , Republic of Korea.

We demonstrate that hole injection from a top electrode composed of Au nanoparticles (AuNPs) capped with a thick Au layer into an underlying organic semiconductor, N, N'-diphenyl- N, N'-bis-[4-(phenyl- m-tolyl-amino)-phenyl]-biphenyl-4,4'-diamine (DNTPD), is significantly enhanced compared to that in a control device whose top electrode is composed entirely of a thick Au layer. The fabrication of this organic hole-only device with the AuNP electrode is made possible by dry, room-temperature distribution of AuNPs onto DNTPD using a spark-discharge aerosol technique capable of varying the average diameter ( D̅) of the AuNPs. The enhancement in hole injection is found to increase with decreasing D̅, with the current density of a device with D̅ = 1.1 nm being more than 3 orders of magnitude larger than that of the control device. Intensity-modulated photocurrent measurements show that the built-in potentials of the devices with the AuNP electrode are smaller than that of the control device by as much as 0.68 V, indicating that the enhanced hole injection originates from the increased work functions of these devices, which in turn decreases the hole injection barrier heights. X-ray photoelectron spectroscopy reveals that the increased work functions of the AuNP electrodes are due to surface oxidation of the AuNPs resulting in AuN and AuN. The degree of oxidation of the AuNPs increases with decreasing D̅, consistent with the D̅-dependencies of the hole injection enhancement and the built-in potential reduction.
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http://dx.doi.org/10.1021/acsami.8b16303DOI Listing
February 2019

Poly(dimethylsiloxane) Stamp Coated with a Low-Surface-Energy, Diffusion-Blocking, Covalently Bonded Perfluoropolyether Layer and Its Application to the Fabrication of Organic Electronic Devices by Layer Transfer.

ACS Appl Mater Interfaces 2018 Jul 6;10(28):24003-24012. Epub 2018 Jul 6.

Graduate School of Convergence Science and Technology and Inter-University Semiconductor Research Center , Seoul National University , Seoul 08826 , Republic of Korea.

It is demonstrated that a stamp composed of a poly(dimethylsiloxane) (PDMS) bulk and perfluoropolyether (PFPE) coating fabricated by a simple dip-coating method has the following properties that are ideal for the transfer patterning of various materials. Deposited by a condensation reaction between PDMS and PFPE molecules as well as the adjacent PFPE molecules, the PFPE coating has a strong adhesion to the PDMS surface and strong internal cohesion, while providing a low energy surface. Furthermore, it is found to function as a bidirectional diffusion barrier: it effectively prevents organic small molecules deposited on the stamp from being absorbed into free volumes of PDMS; it also prevents PDMS oligomers from migrating onto the layer to be transferred, thereby avoiding the contamination of that layer. Morphological and elemental characterization of the surfaces of the transferred organic semiconductor and graphene layers confirms a successful transfer with a high degree of surface cleanliness. The quality of interfaces mechanically bonded using the PFPE-coated stamps and the cleanliness of the transferred layers are remarkably high that the electronic functions of a transfer-bonded organic heterojunction are comparable to those of the same interface formed by vacuum deposition, and that the charge transport across the transfer-bonded graphene-graphene and graphene-MoO interfaces is efficient. Our results demonstrate that the PFPE-coated stamp enables patterned depositions of materials with high quality interfaces while avoiding a high temperature or wet process.
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http://dx.doi.org/10.1021/acsami.8b03811DOI Listing
July 2018

Eliminating the solvent blocking requirement of interconnection layers in polymer tandem solar cells by thin-film transfer technique.

Nanoscale 2018 Jul;10(26):12588-12594

Graduate School of Convergence Science and Technology, and Inter-University Semiconductor Research Center, Seoul National University, Seoul 08826, Republic of Korea.

Interconnection layers (ICLs) for polymer tandem solar cells reported so far are limited in materials' choice and layer structure, because of a requirement that the ICLs must prevent the penetration of solvents used for the top cells. In this research, it is demonstrated that depositing the active layer of the top subcell using a dry thin-film transfer technique allows for incorporation of an ICL composed of vacuum deposited materials in a polymer tandem cell, providing a large degree of freedom in ICL design. Specifically, a polymer tandem solar cell was fabricated using an ICL composed of bathocuproine:silver/silver islands/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (BCP:Ag/Ag islands/HAT-CN), where the thicknesses of the BCP:Ag and Ag island layers are precisely controlled at the nanoscale to facilitate the transport of electrons generated in the bottom subcell and to ensure their efficient recombination with holes generated in the top subcell. Consequently, the tandem device featuring the optimized ICL, whose active layers are composed of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCPDTBT:PC71BM), exhibits an open-circuit voltage of 1.20 V, which is equal to the sum of the open-circuit voltages of the two subcells, with a fill factor (FF) of 0.60 almost identical to the FFs of the subcells.
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http://dx.doi.org/10.1039/c8nr00292dDOI Listing
July 2018

Multifunctional Bilayer Template for Near-Infrared-Sensitive Organic Solar Cells.

ACS Appl Mater Interfaces 2018 May 3;10(19):16681-16689. Epub 2018 May 3.

Graduate School of Convergence Science and Technology, and Inter-University Semiconductor Research Center , Seoul National University , Seoul 08826 , Republic of Korea.

For organic solar cells (OSCs) based on nonplanar phthalocyanines, it has previously been reported that a thin film composed of triclinic crystals with face-on (or flat-lying)-oriented molecules, typically obtained with a CuI template layer, is desired for optical absorption in the near-infrared (NIR) spectral region. However, this work demonstrates that for a PbPc-C donor-acceptor pair, less face-on orientation with a broader orientation distribution obtained with a new template layer consisting of a ZnPc/CuI bilayer is more desirable in terms of solar cell efficiency than the face-on orientation. A NIR-sensitive PbPc-C OSC employing this bilayer-templated PbPc film is found to increase the internal quantum efficiency (IQE) by 36% on average in the NIR spectral region compared to a device using a CuI-templated PbPc film. Analyses of the change in IQE using the exciton diffusion model and the entropy- and disorder-driven charge-separation model suggest that the improved IQE is attributed to the facilitated dissociation of charge-transfer excitons as well as the reduction in exciton quenching near the indium tin oxide surface.
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http://dx.doi.org/10.1021/acsami.8b03468DOI Listing
May 2018

Carbon-Nanosheet Based Large-Area Electrochemical Capacitor that is Flexible, Foldable, Twistable, and Stretchable.

Small 2018 10 24;14(43):e1702145. Epub 2018 Mar 24.

Advanced Functional Thin Films Department, Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon, Gyeongnam, 51508, Republic of Korea.

With the growing demand for wearable electronics, developing new compatible energy systems is a prominent topic of research. Energy systems mounted on wearable electronics should exhibit high cost efficiency, mechanical robustness, and high electrochemical activity. Herein, all-carbon-based large-area nanocomposites for freely deformable electrochemical capacitors are suggested to address these requirements. The three-dimensionally integrated, self-supported nanocomposites consist of activated carbons (ACs) distributed in direct spinning-derived carbon nanotube (DS-CNT) sheets without any additives, including conducting agents or binders. Owing to synergetic effects of the highly porous AC particles, high electron transport kinetics of CNTs, and facile ion accessibility resulting from acid treatment, the nanocomposites show a greatly improved specific capacitance of 128 F g , compared to that of pristine ACs (62 F g ), based on the total mass of the electrodes. The exceptional mechanical stability of the nanocomposites, which are attached on prestretched elastomer substrates, is confirmed; only a ≈15% increase in the electrical resistance is observed under a tensile strain of 100%, and the initial resistance is fully recovered after releasing. Finally, the outstanding durability and electrochemical performance of the deformable all-carbon-based symmetric capacitors under various mechanical deformations of bending, folding, twisting, and stretching are successfully demonstrated.
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http://dx.doi.org/10.1002/smll.201702145DOI Listing
October 2018

Efficient Planar-Heterojunction Perovskite Solar Cells Fabricated by High-Throughput Sheath-Gas-Assisted Electrospray.

ACS Appl Mater Interfaces 2018 Feb 15;10(8):7281-7288. Epub 2018 Feb 15.

Graduate School of Convergence Science and Technology, and Inter-University Semiconductor Research Center, Seoul National University , Seoul 08826, Republic of Korea.

When a perovskite precursor solution is electrosprayed using the conventional method where the nebulization of the solution is primarily governed by electrostatics, its high electrical conductivity tends to cause electrospray instabilities and thus makes high-quality perovskite films very difficult to obtain. Here, we report high-throughput fabrication of efficient perovskite solar cells (PSCs) whose CHNHPbICl films are deposited using a sheath-gas-assisted electrospray system. Our system, based on strong pneumatic nebulization as well as high-voltage electrostatic charging of droplets, enables very stable high-flow electrospray of small charged droplets, even for the highly conductive perovskite precursor solution. Consequently, with the control of the drying rate of the droplets deposited on substrates by adjusting the substrate temperature during deposition, crystalline, void-free CHNHPbICl films with nearly 100% surface coverage and high thickness uniformity are obtained. Inverted planar-heterojunction PSCs employing these films have a maximum power conversion efficiency of 14.2% with a small standard deviation of 0.9%, comparable to that of the spin-coated device.
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http://dx.doi.org/10.1021/acsami.7b18643DOI Listing
February 2018

Hybrid Organic Tandem Solar Cell Comprising Small-Molecule Bottom and Polymer:Fullerene Top Subcells Fabricated by Thin-Film Transfer.

Sci Rep 2017 05 16;7(1):1942. Epub 2017 May 16.

Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul, 08826, Republic of Korea.

Multilayer structures involving solution-deposited polymer films are difficult to fabricate, not allowing for unrestricted designs of polymer-based optoelectronic devices required for maximizing their performance. Here, we fabricate a hybrid organic tandem solar cell whose top and bottom subcells have polymer:fullerene and small molecules active layers, respectively, by a solvent-free process based on transferring the polymer:fullerene layer from an elastomeric stamp onto a vacuum-deposited bottom subcell. The interface between small-molecule and transferred polymer:fullerene layers is void-free at the nanoscale, allowing for efficient charge transport across the interface. Consequently, the transfer-fabricated tandem cell has an open-circuit voltage (V ) almost identical to the sum of V values for the single-junction devices. The short-circuit current density (J ) of the tandem cell is maximized by current matching achieved by varying the thickness of the small-molecule active layer in the bottom subcell, which is verified by numerical simulations. The optimized transfer-fabricated tandem cell, whose active layers are composed of poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl]]:[6,6]-Phenyl-C-butyric acid methyl ester and Di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane:C, has V  = 1.46 V, J  = 8.48 mA/cm, a fill factor of 0.51, leading to the power-conversion efficiency of 6.26%, the highest among small molecule-polymer:fullerene hybrid tandem solar cells demonstrated so far.
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http://dx.doi.org/10.1038/s41598-017-02181-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5434005PMC
May 2017

Scattering of surface plasmon polaritons at a planar interface by an embedded dielectric nanocube.

Opt Express 2017 Apr;25(8):9105-9115

We investigate scattering of surface plasmon polaritons (SPPs) at a planar metal-dielectric interface by a dielectric nanocube embedded in the metal layer using finite element method-based simulations. The scattering characteristics of the embedded nanocube, such as the scattering and absorption cross sections, far-field scattering patterns, reflectance, and transmittance, are calculated as functions of the wavelength of the incident SPP waves in the visible range. The main features of the characteristics are explained in connection with the excitation of plasmonic eigenmodes of the embedded nanocube. The most efficient scattering into waves propagating away from the metal surface, i.e., the radiating modes, occurs when a dipolar-like plasmonic mode is excited, whose eigenfrequency can be tuned by changing the edge length of the nanocube.
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http://dx.doi.org/10.1364/OE.25.009105DOI Listing
April 2017

A dual-scale metal nanowire network transparent conductor for highly efficient and flexible organic light emitting diodes.

Nanoscale 2017 Feb;9(5):1978-1985

Applied Nano and Thermal Science Lab, Department of Mechanical Engineering, Seoul National University, Gwanak-gu, Seoul, South Korea.

Although solution processed metal nanowire (NW) percolation networks are a strong candidate to replace commercial indium tin oxide, their performance is limited in thin film device applications due to reduced effective electrical areas arising from the dimple structure and percolative voids that single size metal NW percolation networks inevitably possess. Here, we present a transparent electrode based on a dual-scale silver nanowire (AgNW) percolation network embedded in a flexible substrate to demonstrate a significant enhancement in the effective electrical area by filling the large percolative voids present in a long/thick AgNW network with short/thin AgNWs. As a proof of concept, the performance enhancement of a flexible phosphorescent OLED is demonstrated with the dual-scale AgNW percolation network compared to the previous mono-scale AgNWs. Moreover, we report that mechanical and oxidative robustness, which are critical for flexible OLEDs, are greatly increased by embedding the dual-scale AgNW network in a resin layer.
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http://dx.doi.org/10.1039/c6nr09902eDOI Listing
February 2017

Low-Temperature, Dry Transfer-Printing of a Patterned Graphene Monolayer.

Sci Rep 2015 Dec 9;5:17877. Epub 2015 Dec 9.

Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul 151-742, Republic of Korea.

Graphene has recently attracted much interest as a material for flexible, transparent electrodes or active layers in electronic and photonic devices. However, realization of such graphene-based devices is limited due to difficulties in obtaining patterned graphene monolayers on top of materials that are degraded when exposed to a high-temperature or wet process. We demonstrate a low-temperature, dry process capable of transfer-printing a patterned graphene monolayer grown on Cu foil onto a target substrate using an elastomeric stamp. A challenge in realizing this is to obtain a high-quality graphene layer on a hydrophobic stamp made of poly(dimethylsiloxane), which is overcome by introducing two crucial modifications to the conventional wet-transfer method - the use of a support layer composed of Au and the decrease in surface tension of the liquid bath. Using this technique, patterns of a graphene monolayer were transfer-printed on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate and MoO3, both of which are easily degraded when exposed to an aqueous or aggressive patterning process. We discuss the range of application of this technique, which is currently limited by oligomer contaminants, and possible means to expand it by eliminating the contamination problem.
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http://dx.doi.org/10.1038/srep17877DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4673461PMC
December 2015

Triple-junction hybrid tandem solar cells with amorphous silicon and polymer-fullerene blends.

Sci Rep 2014 Nov 21;4:7154. Epub 2014 Nov 21.

Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea.

Organic-inorganic hybrid tandem solar cells attract a considerable amount of attention due to their potential for realizing high efficiency photovoltaic devices at a low cost. Here, highly efficient triple-junction (TJ) hybrid tandem solar cells consisting of a double-junction (DJ) amorphous silicon (a-Si) cell and an organic photovoltaic (OPV) rear cell were developed. In order to design the TJ device in a logical manner, a simulation was carried out based on optical absorption and internal quantum efficiency. In the TJ architecture, the high-energy photons were utilized in a more efficient way than in the previously reported a-Si/OPV DJ devices, leading to a significant improvement in the overall efficiency by means of a voltage gain. The interface engineering such as tin-doped In2O3 deposition as an interlayer and its UV-ozone treatment resulted in the further improvement in the performance of the TJ solar cells. As a result, a power conversion efficiency of 7.81% was achieved with an open-circuit voltage of 2.35 V. The wavelength-resolved absorption profile provides deeper insight into the detailed optical response of the TJ hybrid solar cells.
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http://dx.doi.org/10.1038/srep07154DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4239567PMC
November 2014

Design and numerical analysis of surface plasmon-enhanced fin Ge-Si light-emitting diode.

Opt Express 2014 Mar;22(5):5927-36

Heavily-doped strained germanium (Ge) can emit light efficiently thanks to its pseudo direct band gap characteristic. This makes Ge a good candidate for on-chip monolithic light sources in silicon (Si) photonics systems. We propose fin-shaped Ge-Si heterojunction light-emitting diode (LED) with metal gates, which can enhance light emission by coupling with surface plasmon resonant modes and modulate light emission from the LED. We verify these two aspects through numerical analysis and device simulations. We develop the method to find the optimal device structure and specific device dimensions to maximize the spontaneous emission rate enhancement. Also we find that the LED can be modulated by a gate voltage bias.
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http://dx.doi.org/10.1364/OE.22.005927DOI Listing
March 2014

Analysis and optimization of surface plasmon-enhanced organic solar cells with a metallic crossed grating electrode.

Opt Express 2012 Sep;20 Suppl 5:A740-53

Department of Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul, South Korea.

We perform a systematic analysis of enhanced short-circuit current density (J(sc) in organic solar cells (OSCs) where one metallic electrode is optically thick and the other consists of a two-dimensional metallic crossed grating. By examining a model device representative of such surface plasmon (SP)-enhanced OSCs by the Fourier modal and finite-element methods for electromagnetic and exciton diffusion calculations, respectively, we provide general guidelines to maximize J(sc) of the SP-enhanced OSCs. Based on this study, we optimize the performance of a small-molecule OSC employing a copper phthalocyanine-fullerene donor-acceptor pair, demonstrating that the optimized SP-enhanced device has J(sc) that is 75 % larger than that of the optimized device with an ITO-based conventional structure.
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http://dx.doi.org/10.1364/OE.20.00A740DOI Listing
September 2012

Multiplexed broadband beam steering system utilizing high speed MEMS mirrors.

Opt Express 2009 Apr;17(9):7233-44

Fitzpatrick Institute for Photonics, Electrical and Computer Engineering Department, Duke University, Durham, NC 27708, USA.

We present a beam steering system based on micro-electromechanical systems technology that features high speed steering of multiple laser beams over a broad wavelength range. By utilizing high speed micromirrors with a broadband metallic coating, our system has the flexibility to simultaneously incorporate a wide range of wavelengths and multiple beams. We demonstrate reconfiguration of two independent beams at different wavelengths (780 and 635 nm) across a common 5x5 array with 4 micros settling time. Full simulation of the optical system provides insights on the scalability of the system. Such a system can provide a versatile tool for applications where fast laser multiplexing is necessary.
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http://dx.doi.org/10.1364/oe.17.007233DOI Listing
April 2009

Optically multiplexed imaging with superposition space tracking.

Opt Express 2009 Feb;17(3):1691-713

Department of Electrical and Computer Engineering, University of Arizona, Arizona, USA.

We describe a novel method to track targets in a large field of view. This method simultaneously images multiple, encoded sub-fields of view onto a common focal plane. Sub-field encoding enables target tracking by creating a unique connection between target characteristics in superposition space and the target's true position in real space. This is accomplished without reconstructing a conventional image of the large field of view. Potential encoding schemes include spatial shift, rotation, and magnification. We discuss each of these encoding schemes, but the main emphasis of the paper and all examples are based on one-dimensional spatial shift encoding. System performance is evaluated in terms of two criteria: average decoding time and probability of decoding error. We study these performance criteria as a function of resolution in the encoding scheme and signal-to-noise ratio. Finally, we include simulation and experimental results demonstrating our novel tracking method.
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http://dx.doi.org/10.1364/oe.17.001691DOI Listing
February 2009

Organic photovoltaic cell in lateral-tandem configuration employing continuously-tuned microcavity sub-cells.

Opt Express 2008 Nov;16(24):19987-94

Fitzpatrick Institute for Photonics, Department of Electrical and Computer Engineering, Duke University, Durham, NC 27708, USA.

We propose a lateral-tandem organic photovoltaic system consisting of a dispersive-focusing element and continuously-tuned, series-connected sub-cells. The proposed system overcomes the efficiency limitation of organic photovoltaic devices by spectral re-distribution of incoming solar photons and their delivery to the wavelength-matched, resonant sub-cells. By numerical simulations, we demonstrate that optical resonance in a microcavity sub-cell with a metal/organic multilayer/metal structure can be tuned over a broad spectrum by varying the thickness of the organic multilayer. We show that the power-conversion efficiency exceeding 18% can be obtained in a lateral-tandem system employing an ideal dispersive-focusing element and the microcavity sub-cells.
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http://dx.doi.org/10.1364/oe.16.019987DOI Listing
November 2008

Superimposed video disambiguation for increased field of view.

Opt Express 2008 Oct;16(21):16352-63

Department of Electrical and Computer Engineering, Duke University, Durham, NC 27708, USA.

Many infrared optical systems in wide-ranging applications such as surveillance and security frequently require large fields of view (FOVs). Often this necessitates a focal plane array (FPA) with a large number of pixels, which, in general, is very expensive. In a previous paper, we proposed a method for increasing the FOV without increasing the pixel resolution of the FPA by superimposing multiple sub-images within a static scene and disambiguating the observed data to reconstruct the original scene. This technique, in effect, allows each sub-image of the scene to share a single FPA, thereby increasing the FOV without compromising resolution. In this paper, we demonstrate the increase of FOVs in a realistic setting by physically generating a superimposed video from a single scene using an optical system employing a beamsplitter and a movable mirror. Without prior knowledge of the contents of the scene, we are able to disambiguate the two sub-images, successfully capturing both large-scale features and fine details in each sub-image. We improve upon our previous reconstruction approach by allowing each sub-image to have slowly changing components, carefully exploiting correlations between sequential video frames to achieve small mean errors and to reduce run times. We show the effectiveness of this improved approach by reconstructing the constituent images of a surveillance camera video.
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http://dx.doi.org/10.1364/oe.16.016352DOI Listing
October 2008

MEMS-based optical beam steering system for quantum information processing in two-dimensional atomic systems.

Opt Lett 2008 Feb;33(3):273-5

Fitzpatrick Institute for Photonics, Electrical and Computer Engineering Department, Duke University, Durham, NC 27708, USA.

To provide scalability to quantum information processors utilizing trapped atoms or ions as quantum bits (qubits), the capability to address multiple individual qubits in a large array is needed. Microelectromechanical systems (MEMS) technology can be used to create a flexible and scalable optical system to direct the necessary laser beams to multiple qubit locations. We developed beam steering optics using controllable MEMS mirrors that enable one laser beam to address multiple qubit locations in a two-dimensional trap lattice. MEMS mirror settling times of approximately 10 micros were demonstrated, which allow for fast access time between qubits.
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http://dx.doi.org/10.1364/ol.33.000273DOI Listing
February 2008
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