Bong-Gi Kim

Solution-Processible Crystalline NiO Nanoparticles for High-Performance Planar Perovskite Photovoltaic Cells

In this work, we report on solution-based p-i-n-type planar-structured CH3NH3PbI3 perovskite photovoltaic (PV) cells, in which precrystallized NiO nanoparticles (NPs) without post-treatment are used to form a hole transport layer (HTL). X-ray diffraction and high-resolution transmission electron microscopy showed the crystallinity of the NPs, and atomic force microscopy and scanning electron microscopy confirmed the uniform surfaces of the resultant NiO thin film and the subsequent perovskite photoactive layer. Compared to the conventional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL, the NiO HTL had excellent energy-level alignment with that of CH3NH3PbI3 and improved electron-blocking capability, as analyzed by photoelectron spectroscopy and diode modeling, resulting in Voc ~0.13 V higher than conventional PEDOT:PSS-based devices. Consequently, a power conversion efficiency (PCE) of 15.4% with a high fill factor (FF, 0.74), short-circuit current density (Jsc, 20.2 mA·cm−2), and open circuit voltage (Voc, 1.04 V) having negligible hysteresis and superior air stability has been achieved.

Inverted planar perovskite solar cells with dopant free hole transporting material: Lewis base-assisted passivation and reduced charge recombination

Three novel triarylamine derivatives (TPACs: TPAC0M, TPAC2M and TPAC3M), sharing the same conjugation scaffold but possessing different numbers of methoxy units, were designed for high performance hole transport materials (HTMs) of p–i–n planar perovskite solar cells (PSCs). Cyclic voltammetry and absorption results showed that their energy levels would be beneficial to work as HTMs of PSCs, and time-resolved photoluminescence and transient photo-voltage studies proved that TPAC-based PSCs had better charge extraction and more suppressed non-radiative recombination properties than PEDOT:PSS-based PSCs, leading to superior power conversion efficiency (PCE). We confirmed that the methoxy units introduced to the triarylamine derivatives did not cause noticeable changes in their optical properties, such as absorption and bandgap, electrical conductance, measured by conductive atomic force microscopy, and mobility, but the charge extraction and recombination behaviors of TPAC-based PSCs could be improved by increasing the number of methoxy units in the arylamine moiety. X-ray photoelectron spectroscopy revealed that methoxy units could act as a Lewis base passivating the defect sites at the interface between HTM and perovskite, consequently resulting in an increase of their PCE to 17.54% without any dopants.

A polydiacetylene-based colorimetric chemosensor for malondialdehyde detection: a food spoilage indicator

A colorimetric chemosensor adopting polydiacetylene derivatives was rationally designed to detect malondialdehyde (MDA) as a potential food spoilage indicator. A newly developed diacetylene monomer containing an active methylene group effectively reacted with the aldehyde units of the MDA molecule via Knoevenagel-type condensation and resulted in a CC bond, judging from the results of FT-IR spectroscopy. When the monomer was incorporated into polydiacetylene liposomes, they exhibited a strong change in color from blue to red in the presence of MDA molecules, even though their sensitivity was partly affected by the feed ratio of the applied diacetylene monomers during liposome preparation. In addition, it was verified that the devised MDA chemosensor selectively responded to the divalent MDA, rather than the monovalent butanal. The detection limit of the optimized MDA chemosensor was about 250 μM in solution, but when a film-type MDA chemosensor was fabricated through the accumulation of the liposomes on top of a nylon membrane and then applied, the detection limit improved dramatically, reaching down to 10 μM at room temperature, even by judging the change in color with the naked eye. To confirm that the change of the resulting Hue angle can reflect the detection sensitivity of the MDA chemosensor under different experimental conditions, CIELAB analysis was conducted with the film-type MDA sensor. The results indicated that the Hue angle changed according to the MDA concentration, and the resulting slope can be quantitatively correlated with the sensitivity of the colorimetric detection system.

Nano-Geometry Dependent Electrical Property of Organic Semiconductor

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

Manipulation of Chain Conformation for Optimum Charge-Transport Pathways in Conjugated Polymers

A pair of different diketopyrrolopyrrole-based conjugated polymers (CPs) were designed and synthesized to investigate the effect of chain conformation on their molecular assembly. Conformation management was achieved by the incorporation of different linkers during polymerization. Through the use of computational calculations and UV-vis absorption measurements, the resulting CPs (PDPP-T and PDPP-BT) were found to exhibit partly modulated chain geometry. Grazing incident X-ray diffraction experiments with a two-dimensional detector revealed that PDPP-T having a planar chain conformation exhibited an edge-on type molecular arrangement, which evolved to a face-on type chain assembly when the planar geometry was altered to a slightly twisted one as in PDPP-BT. In addition, it was verified that the directional electric carrier mobility of CPs was critically distinguished by the distinctive chain arrangement in spite of their similar chemical structure. Concentration-dependent absorption measurements could provide an improved understanding of the assembly mechanism of CP chains: the planar conformation of PDPP-T facilitates the formation of preassembled chains in a concentrated solution and further directs the edge-on stacking, while the twisted dihedral angle along the benzothiophene in PDPP-BT prevents chain assembly, resulting in the face-on stacking. Because CP chain conformation is inevitably connected with the generation of preassembled chains, manipulating CP geometry could be an efficient tool for extracting an optimum chain assembly that is connected with the principal charge-transport pathway in CPs.

Ladder-Structured Polysilsesquioxane/Al 2 O 3 Nanocomposites for Transparent Wear-Resistant Windows

As a protective layer for deformable displays, we synthesized ladder-type polysilsesquioxanes (LPSQs) containing cyclic epoxy as a curable unit. The mechanical properties after photo- and thermal-curing of LPSQs with a small amount of added Al2O3 nanoparticles were compared with those of the pure LPSQs. The prepared LPSQ-Al2O3 nanocomposites and the pure LPSQs exhibited comparable optical transparencies and thermal stabilities. In addition, the degree of conversion of the applied epoxy units in LPSQs and the resulting mechanical properties, as monitored by Fourier transform infrared spectroscopy and nanoindentation tests, indicated that the addition of nanoparticles to LPSQs moderately enhanced the epoxy conversion rate and remarkably improved the wear resistance, including hardness, after photo-/thermal-curing processes. The LPSQ-Al2O3 nanocomposites achieved higher wear resistance than epoxy-silica nanocomposites containing similar curable functional groups and reinforcing fillers (silica). The excellent mechanical properties of the LPSQ-Al2O3 nanocomposites could be attributed to three-dimensionally interconnected networks of organic-inorganic hybrid-type chemical structures in the LPSQ as well as additional reinforcement from amine-functionalized Al2O3 nanoparticles covalently interconnected with the LPSQ. We believe that the devised LPSQ-Al2O3 nanocomposites could serve effectively as a wear-resistant platform for deformable display windows.

Facile In-situ Polymerization of Thermotropic Liquid Crystalline Polymers as Thermally Conductive Matrix Materials

Although thermally conductive composites that can efficiently dissipate the heat generated from electronic devices are in high demand, most neat polymers used as matrix materials are problematic because they have poor thermal conductivities. The low thermal conductivity of polymeric materials is caused by structural defects; therefore, it can be improved by increasing the orientational regularity of the polymer chains. Here, main-chain liquid crystalline polymers (LCPs) were designed and synthesized to investigate the effects of liquid crystallinity-induced structural regularity on the thermal conductivity of the polymers. In addition, an in-situ polymerization method was devised for commercial applicability, and the thermal conductivity of the obtained polymer was compared to that of a conventionally polymerized polymer having the same structure. The designed polymers exhibited thermotropic liquid crystalline characteristics, and the polymer with longer spacers between the rigid segments showed relatively higher thermal conductivity exceeding 0.5 W·m-1· K-1 after sample preparation by injection molding. In addition, X-ray diffraction analysis revealed that the differences in the thermal conductivity, depending on the molding temperature during specimen preparation, were caused by variations in chain orientation within the same polymer. Based on the obtained results, it was concluded that LCPs are strong candidate matrix materials for thermally conductive composites; the suggested in-situ polymerization method could be applied practically to the polymerization of polyester-type LCPs.

Instantaneous Detection of Trichlorinated Carbon via Photo-Induced Electron Transfer toward Chemosensor for Toxic Organochlorides

Despite the usefulness of organochlorides as raw materials for organic synthesis, they cause several issues in the human body, such as hepatic dysfunction, tumor, and heavy damage to the central nervous system. Especially when organochlorides contain three or more chlorinated carbons, they tend to be more toxic to the human body possibly owing to relatively high reactivity. Several electron donors (TPCAs) are designed to devise a novel detection system for toxic organochlorides containing trichlorinated carbons, and the detection mechanism of the devised sensor system is systematically identified by EPR measurement and the analysis of the solution after the detection of chloroform, which is used as a model compound. Since the detection system simultaneously utilizes the radical-generation capability and the low LUMO level of the trichlorinated carbon, it provides high selectivity against most of the common organic compounds including other organochlorides containing mono- or dichlorinated carbons, and the outstanding selectivity of the designed sensor has been verified with Mirex composed of numerous chlorinated carbons. In addition, the detection system exhibits immediate sensing capability because only electron transfer and radical reaction are involved in the detection process. Finally, when diphosgene is detected with the devised sensing platform, a noticeable change in fluorescence intensities can be identified within 5 s even for a diphosgene concentration of less than 1 ppm.