Publications by authors named "Hyungchae Kim"

9 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

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

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

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

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

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

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
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