Sung-Yeon Jang

Solution Chemistry of Self-Assembled Graphene Nanohybrids for High-Performance Flexible Biosensors

We report the preparation of free-standing flexible conductive reduced graphene oxide/Nafion (RGON) hybrid films by a solution chemistry that utilizes self-assembly and directional convective-assembly. The hydrophobic backbone of Nafion provided well-defined integrated structures, on micro- and macroscales, for the construction of hybrid materials through self-assembly, while the hydrophilic sulfonate groups enabled highly stable dispersibility ( approximately 0.5 mg/mL) and long-term stability (2 months) for graphene. The geometrically interlocked morphology of RGON produced a high degree of mechanical integrity in the hybrid films, while the interpenetrating network constructed favorable conduction pathways for charge transport. Importantly, the synergistic electrochemical characteristics of RGON were attributed to high conductivity (1176 S/m), facilitated electron transfer (ET), and low interfacial resistance. Consequently, RGON films obtained the excellent figure of merit as electrochemical biosensing platforms for organophosphate (OP) detection, that is, a sensitivity of 10.7 nA/microM, detection limit of 1.37 x 10(-7) M, and response time of <3 s. In addition, the reliability of RGON biosensors was confirmed by a fatigue test of 100 bending cycles. The strategy described here provides insight into the fabrication of graphene and hybrid nanomaterials from a material perspective, as well as the design of biosensor platforms for practical device applications.

Graphene mode-lockers for fiber lasers functioned with evanescent field interaction

Employing graphene as an intracavity passive power modulating element, we demonstrate the efficient laser pulsation in high pulse-energy regime with evanescent field interaction between the propagating light and graphene layer. Graphene is prepared by the solution based reduction of graphene oxide, and dispersed homogeneously into the water for spray onto an all-fiber substrate, side-polished fiber. With the intracavity power up to 21.41 dBm, we ensure the robust high-energy operation without any thermal damage of graphene. Resultant output pulses have center wavelength, spectral width, and repetition rate of 1561.6 nm, 1.96 nm, and 6.99 MHz, respectively.

Electrospray Preparation of Hierarchically-structured Mesoporous TiO2 Spheres for Use in Highly Efficient Dye-Sensitized Solar Cells

We report a simple method to prepare hierarchically structured TiO(2) spheres (HS-TiO(2)), using an electrostatic spray technique, that are utilized for photoelectrodes of highly efficient dye-sensitized solar cells (DSSCs). This method has an advantage to remove the synthesis steps in conventional sol-gel method to form nano-sized spheres of TiO(2) nanoclusters. The fine dispersion of commercially available nanocrystalline TiO(2) particles (P25, Degussa) in EtOH without surfactants and additives is electro-sprayed directly onto a fluorine-dopoed tin-oxide (FTO) substrate for DSSC photoelectrodes. The DSSCs of HS-TiO(2) photoelectrodes show high energy conversion efficiency over 10% under illumination of light at 100 mW cm(-2), AM1.5 global. It is concluded from frequency-dependent measurements that the faster electron diffusion coefficient and longer lifetime of HS-TiO(2) than those in nonstructured TiO(2) contribute to the enhanced efficiency in DSSCs.

Effect of Length and Contact Chemistry on the Electronic Structure and Thermoelectric Properties of Molecular Junctions

We present a combined experimental and computational study that probes the thermoelectric and electrical transport properties of molecular junctions. Experiments were performed on junctions created by trapping aromatic molecules between gold electrodes. The end groups (-SH, -NC) of the aromatic molecules were systematically varied to study the effect of contact coupling strength and contact chemistry. When the coupling of the molecule with one of the electrodes was reduced by switching the terminal chemistry from -SH to -H, the electrical conductance of molecular junctions decreased by an order of magnitude, whereas the thermopower varied by only a few percent. This has been predicted computationally in the past and is experimentally demonstrated for the first time. Further, our experiments and computational modeling indicate the prospect of tuning thermoelectric properties at the molecular scale. In particular, the thiol-terminated aromatic molecular junctions revealed a positive thermopower that increased linearly with length. This positive thermopower is associated with charge transport primarily through the highest occupied molecular orbital, as shown by our computational results. In contrast, a negative thermopower was observed for a corresponding molecular junction terminated by an isocyanide group due to charge transport primarily through the lowest unoccupied molecular orbital.

Electrodynamically sprayed thin films of aqueous dispersible graphene nanosheets: highly efficient cathodes for dye-sensitized solar cells.

Highly efficient cathodes for dye-sensitized solar cells (DSSCs) were developed using thin films of graphene nanosheets (GNS), which were fabricated by the electrospray method (e-spray) using aqueous dispersions of chemically driven GNS. The e-sprayed GNS films had the appropriate properties to be an efficient counter electrode (CE) for DSSCs; sufficient electrocatalytic activity for I(-)/I3(-) redox couples and low charge transfer resistance (RCT) at the CE/electrolyte interface as characterized by cyclic voltammetry and electrochemical impedance analysis. The performance of the GNS film based CEs was optimized by manipulating the density of surface chemical functional groups and plane conjugation of GNS via post thermal annealing (TA). Upon TA, the oxygen-containing surface functional groups, which have been shown to improve electrocatalytic activity of carbon based materials, were significantly reduced, while the electrical conductivity was enhanced by ∼40 times. The improvement of electrocatalytic activity and fill factor (FF) with reduced RCT of DSSCs after TA was primarily attributed to the increased charge transport within the GNS films, while the chemically prepared GNS typically contained sufficient defects, edges and surface functional groups for electrocatalysis. The performance of the DSSCs using our GNS-CEs was nearly identical (>95%) to the DSSCs using the state-of-the-art CE, thermolytically prepared Pt crystals. Our e-sprayed GNS-CE based DSSCs had a higher FF (69.7%) and cell efficiency (6.93%) when compared previously reported graphene based CEs for DSSCs, demonstrating the outstanding properties of graphene as the electrodes in electrochemical devices.

Deformation-immunized optical deposition of graphene for ultrafast pulsed lasers

We demonstrate deformation-suppressed optical deposition of graphene onto an optical fiber by forming a graphene/polyvinyl acetate (PVAc) composite for ultrafast nonlinear photonics. With pure graphene, its nonlinear operation threshold is elevated by the optical deposition that cannot guarantee the intact two-dimensional nanoshape of graphene. The role of PVAc that provides immunity to graphene against deleterious degradation of morphology and optical nonlinearity is elucidated via electron microscope analysis, Raman characterization, and realizing passive mode-locking operation of fiber lasers. Resultant center wavelength, spectral width, and repetition rate of the laser pulses are 1572.6 nm, 0.6 nm, and 91.5 MHz, respectively.

Performance optimization of low-temperature-annealed solution-processable ZnO buffer layers for inverted polymer solar cells

High-performance solution-processable ZnO thin films for use as electron-transporting layers (ETLs) of inverted-structured polymer solar cells (I-PSCs) are developed via a low-temperature annealing ( 20% higher power-conversion efficiency (PCE) than those employing the conventional L-ZnO films for a range of active materials including poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) and poly(thienothiophene-co-benzodithiophenes)7-F20 (PTB7-F20)/phenyl-C71-butyric acid methyl ester (PC71BM) blends. A PCE of 6.42% is achieved for the I-PSCs using the optimized L-ZnO films and PTB7-F20/PC71BM blends as the ETL and active materials, respectively. This study presents a universal method for optimizing sol–gel-driven ZnO-based ETLs, whilst the low-temperature processability and long-term stability of the developed ETLs are beneficial for the commercialization of I-PSCs.

High-Efficiency, Solid-State, Dye-Sensitized Solar Cells Using Hierarchically Structured TiO2 Nanofibers

High-performance, room-temperature (RT), solid-state dye-sensitized solar cells (DSSCs) were fabricated using hierarchically structured TiO₂ nanofiber (HS-NF) electrodes and plastic crystal (PC)-based solid-state electrolytes. The electrospun HS-NF photoelectrodes possessed a unique morphology in which submicrometer-scale core fibers are interconnected and the nanorods are dendrited onto the fibers. This nanorod-in-nanofiber morphology yielded porosity at both the mesopore and macropore level. The macropores, steming from the interfiber space, afforded high pore volumes to facilitate the infiltration of the PC electrolytes, whereas the mesoporous nanorod dendrites offered high surface area for enhanced dye loading. The solid-state DSSCs using HS-NFs (DSSC-NF) demonstrated improved power conversion efficiency (PCE) compared to conventional TiO₂ nanoparticle (NP) based DSSCs (DSSC-NP). The improved performance (>2-fold) of the DSSC-NFs was due to the reduced internal series resistance (R(s)) and the enhanced charge recombination lifetime (τ(r)) determined by electrochemical impedance spectroscopy and intensity modulated photocurrent/photovoltage spectroscopy. The easy penetration of the PC electrolytes into HS-NF layers via the macropores reduces R(s) significantly, improving the fill factor (FF) of the resulting DSSC-NFs. The τ(r) difference between the DSSC-NF and DSSC-NP in the PC electrolytes was extraordinary (~14 times) compared to reported results in conventional organic liquid electrolytes. The optimized PCE of DSSC-NF using the PC electrolytes was 6.54, 7.69, and 7.93% at the light intensity of 100, 50, and 30 mW cm⁻², respectively, with increased charge collection efficiency (>40%). This is the best performing RT solid-state DSSC using a PC electrolyte. Considering the fact that most reported quasi-solid state or nonvolatile electrolytes require higher iodine contents for efficient ion transport, our HS-NFs are a promising morphology for such electrolytes that have limited ion mass transport.

Aqueous Dispersible Graphene/Pt Nanohybrids by Green Chemistry: Application as Cathodes for Dye-Sensitized Solar Cells

Aqueous dispersible nanohybrids (NHBs) of graphene nanosheets (GNSs) and Pt nanoparticles (Pt-NPs) were synthesized through the one-pot reduction of their precursors using an environmentally benign chemical, vitamin C. The concurrent reduction of the precursors, which includes graphene oxide (GO) to GNS and H2PtCl6 to Pt(0), was facile and efficient to yield GNS/Pt-NHBs in which face-centered cubic (fcc) crystalline Pt-NPs with average diameters of ~5 nm were robustly attached on the surface of the GNSs. The conversion yield during Pt reduction was fairly high (∼90%) and the Pt content within the NHBs was easily controllable. The resulting stable aqueous colloidal dispersion of GNS/Pt-NHBs was successfully fabricated as thin films without using any binder by the electro-spray method at room temperature, and the fabricated samples were used as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The electrocatalytic activity of the NHBs for I(-)/I3(-) redox couples in conventional DSSCs was investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis. Doping of GNSs with small amounts of Pt-NPs (<10 wt %) could dramatically enhance the redox kinetics. The enhanced electrocatalytic activity of the GNS/Pt-NHBs was reflected in the performance of the DSSCs. The power conversion efficiency of optimized DSSCs using the NHB-CEs was 8.91% (VOC: 830 mV, JSC: 15.56 mAcm(-2), and FF: 69%), which is comparable to that of devices using the state-of-the-art Pt-based CEs (8.85%).

Facile external treatment for efficient nanoscale morphology control of polymer solar cells using a gas-assisted spray method

A facile and effective treatment method for controlling the morphology of bulk heterojunction (BHJ) structured polymer-based solar cells (PSCs) using a gas-assisted spray (g-spray) technique was demonstrated. High-efficiency BHJ-PSCs were fabricated using a g-spray method that can be adapted to large-scale high-throughput continuous production, and the bulk film morphology and internal nanomorphology of the active layers were well manipulated using a sprayed solvent overlayer (SSO) treatment. The efficient nanomorphology evolution, which is a prerequisite for obtaining high performance BHJ-PSCs, was confirmed by X-ray diffraction, UV-Vis, photoluminescence, and transmission electron microscopy analysis. The SSO treatment was a simple and rapid process that could be carried out at room temperature, unlike conventional external treatment (ET) methods such as solvent- or thermal-assisted treatment, which typically require a prolonged time (>1 h) or relatively high temperature (>110 °C). After SSO treatment, the PSC performance was enhanced remarkably. The power conversion efficiency (PCE) of the g-sprayed PSCs after SSO treatment was 2.99%, which is higher than that of a solvent vapor treated device (2.42%) and thermally annealed devices (2.61%). Further optimization of the nanomorphology was achieved by sequentially developing P3HT and PCBM. By combining thermal annealing with the SSO treatment, the P3HT/PCBM interfacial area could be enhanced; this enhancement was induced by the PCBM diffusion into the space among pre-assembled P3HT nanofibrils, which in turn promoted their bi-continuity. This means of sequential nanomorphology development further enhanced the PCE (3.35%), which was higher than the other reported values for PSCs using spray methods. Considering that the SSO treatment is a facile room temperature process that requires a short time, these results suggest that the g-spray method can be successfully applied to the continuous production of PSCs.