Donghoon Song

Synergistic Catalytic Effect of a Composite (CoS/PEDOT:PSS) Counter Electrode on Triiodide Reduction in Dye-Sensitized Solar Cells

Inorganic/organic nanocomposite counter electrodes comprised of sheetlike CoS nanoparticles dispersed in polystyrenesulfonate-doped poly(3,4-ethylenedioxythiophene (CoS/PEDOT:PSS) offer a synergistic effect on catalytic performance toward the reduction of triiodide for dye-sensitized solar cells (DSSCs), yielding 5.4% power conversion efficiency, which is comparable to that of the conventional platinum counter electrode (6.1%). The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry measurements revealed that the composite counter electrodes exhibited better catalytic activity, fostering rate of triiodide reduction, than that of pristine PEDOT: PSS electrode. The simple preparation of composite (CoS/PEDOT:PSS) electrode at low temperature with improved electrocatalytic properties are feasible to apply in flexible substrates, which is at most urgency for developing novel counter electrodes for lightweight flexible solar cells.

Toward Higher Energy Conversion Efficiency for Solid Polymer Electrolyte Dye-Sensitized Solar Cells: Ionic Conductivity and TiO2 Pore-Filling.

Even though the solid polymer electrolyte has many intrinsic advantages over the liquid electrolyte, its ionic conductivity and mesopore-filling are much poorer than those of the liquid electrolyte, limiting its practical application to electrochemical devices such as dye-sensitized solar cells (DSCs). Two major shortcomings associated with utilizing solid polymer electrolytes in DSCs are first discussed, low ionic conductivity and poor pore-filling in mesoporous photoanodes for DSCs. In addition, future directions for the successful utilization of solid polymer electrolytes toward improving the performance of DSCs are proposed. For instance, the facilitated mass-transport concept could be applied to increase the ionic conductivity. Modified biphasic and triple-phasic structures for the photoanode are suggested to take advantage of both the liquid- and solid-state properties of electrolytes.

Ultrathin polypyrrole nanosheets doped with HCl as counter electrodes in dye-sensitized solar cells

Ultrathin polypyrrole nanosheets (UPNSs) were synthesized by chemical oxidation via organic single-crystal surface-induced polymerization (OCSP) using sodium decylsulfonate (SDSn) as a template. This process yields mass-producible, inexpensive materials that are suitable for flexible devices. UPNSs were deposited onto transparent, conductive oxide glasses for use as catalytic counter-electrodes (CEs) in dye-sensitized solar cells. These electrodes boast 94% transmittance against that of Pt CEs. Hydrochloric acid (HCl, 35 wt%), applied in the vapor state as a post-doping process, was used to improve the catalytic activity of the electrodes. This treatment enhanced the catalytic activity of UPNSs by increasing their conductivity by 8 S cm−1, with a 7.4% increase in the level of nitrogen doping. The Tafel polarization and impedance results support the enhancement of catalytic activity due to HCl doping. Dye-sensitized solar cells (DSSCs) employing the HCl-enhanced UPNS CEs showed a power conversion efficiency (AM 1.5, 100 mW cm−2) of 6.8%, which is 19.3% greater than the untreated case and comparable to that of Pt CE-based DSSCs.

Surface Modification of TiO2 Photoanodes with Fluorinated Self-Assembled Monolayers for Highly Efficient Dye-Sensitized Solar Cells.

Dye aggregation and electron recombination in TiO2 photoanodes are the two major phenomena lowering the energy conversion efficiency of dye-sensitized solar cells (DSCs). Herein, we introduce a novel surface modification strategy of TiO2 photoanodes by the fluorinated self-assembled monolayer (F-SAM) formation with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFTS), blocking the vacant sites of the TiO2 surface after dye adsorption. The F-SAM helps to efficiently lower the surface tension, resulting in efficient repelling ions, e.g., I3(-), in the electrolyte to decrease the electron recombination rate, and the role of F-SAM is characterized in detail by impedance spectroscopy using a diffusion-recombination model. In addition, the dye aggregates on the TiO2 surface are relaxed by the F-SAM with large conformational perturbation (i.e., helix structure) seemingly because of steric hindrance developed during the SAM formation. Such multifunctional effects suppress the electron recombination as well as the intermolecular interactions of dye aggregates without the loss of adsorbed dyes, enhancing both the photocurrent density (11.9 → 13.5 mA cm(-2)) and open-circuit voltage (0.67 → 0.72 V). Moreover, the combined surface modification with the F-SAM and the classical coadsorbent further improves the photovoltaic performance in DSCs.

Successful demonstration of an efficient I-/(SeCN)2 redox mediator for dye-sensitized solar cells

A new I(-)/(SeCN)(2) redox mediator has favorable properties for dye-sensitized solar cells (DSCs) such as less visible light absorption, higher ionic conductivity, and downward shift of redox potential than I(-)/I(3)(-). It was then applied for DSCs towards increasing energy conversion efficiency, giving a new potential for improving performance.

A facile and green synthesis of colloidal Cu2ZnSnS4 nanocrystals and their application in highly efficient solar water splitting

The development of solution-processable routes as well as compounds consisting of earth abundant elements is highly desirable to reduce the fabrication cost. Recently, kesterite Cu2ZnSnS4 (CZTS) nanocrystals (NCs) have attracted great attention for photoelectrochemical (PEC) water splitting owing to their suitable low-cost, earth-abundancy and suitable band gap energy. However, the environmentally benign synthesis of high-quality CZTS NCs without toxic solvents remains elusive. Here, a green chemistry approach employing vegetable oil as a non-toxic solvent for the synthesis of monodisperse and size-tunable CZTS NCs is introduced for the first time. Additionally, the relationship between the abnormal size behavior of the CZTS NCs and the degree of decomposition in the vegetable oil using electrospray ionization mass spectrometry (ESI-MS) measurements is elucidated for the first time. As a conceptual strategy, a ternary abundant compound based heterojunction nanostructure for efficient solar water splitting by introducing CZTS NCs onto 5 nm Zn(O,S) passivated layer/hydrothermally grown TiO2 nanorod arrays (TNRs) is designed and developed. Remarkably, this ternary CZTS NCs/Zn(O,S)/TNR photoelectrode shows a photocurrent density as high as 15.05 mA cm−2 at 1.23 V (vs. the NHE), which is the highest ever for previously reported CZTS NC-based photoelectrodes. The reasons for the enhanced PEC performance are discussed in detail based on different PEC characterizations. More importantly, this work reflects the sophistication of eco-friendly solution phase synthesized CZTS NCs without using any toxic chemicals as an earth abundant sensitizer and constitute a new paradigm towards the enhanced PEC performance with quantum dot based hetero-nanostructures.

Efficient binary organic thiolate/disulfide redox mediators in dye-sensitized solar cells based on a carbon black counter electrode

New thiolate/disulfide redox couples (M−/M2) were introduced and also combined with T−/T2, resulting in more efficient binary redox couples. The binary redox couples, in particular M−/T2, yielded a high energy conversion efficiency and reasonably stable performance with the carbon black counter electrode for dye-sensitized solar cells.

High-Resolution Transfer Printing of Graphene Lines for Fully Printed, Flexible Electronics

Pristine graphene inks show great promise for flexible printed electronics due to their high electrical conductivity and robust mechanical, chemical, and environmental stability. While traditional liquid-phase printing methods can produce graphene patterns with a resolution of ∼30 μm, more precise techniques are required for improved device performance and integration density. A high-resolution transfer printing method is developed here capable of printing conductive graphene patterns on plastic with line width and spacing as small as 3.2 and 1 μm, respectively. The core of this method lies in the design of a graphene ink and its integration with a thermally robust mold that enables annealing at up to ∼250 °C for precise, high-performance graphene patterns. These patterns exhibit excellent electrical and mechanical properties, enabling favorable operation as electrodes in fully printed electrolyte-gated transistors and inverters with stable performance even following cyclic bending to a strain of 1%. The high resolution coupled with excellent control over the line edge roughness to below 25 nm enables aggressive scaling of transistor dimensions, offering a compelling route for the scalable manufacturing of flexible nanoelectronic devices.

Promotion of strongly anchored dyes on the surface of titania by tetraethyl orthosilicate treatment for enhanced solar cell performance

Tetraethyl orthosilicate (TEOS) is employed as an equilibrium shifting agent for the esterification reaction between the carboxylic acid of the amphiphilic Z907 dye and Ti-OH on the mesoscopic TiO2 surface in dye-sensitized solar cells (DSCs). We demonstrate that TEOS treatment of the Z907-dyed TiO2 film increases the amount of strongly anchored dyes, and also improves the electron diffusion length (Ln). Consequently, the cell provides a remarkable short-circuit current density (Jsc) enhanced by 22% (9.0 → 11.0 mA cm−2) and the total power conversion efficiency increased by 14% (4.9 → 5.6%) under 1 sun (AM 1.5, 100 mW cm−2), even with less than half the amount of adsorbed dye as compared to the reference. Therefore, TEOS treatment seems to be effective in increasing the power conversion efficiency along with increasing the concentration of the strongly anchored dye, specifically when using an amphiphilic dye such as Z907.

Tetrathiafulvalene as an electron acceptor for positive charge induction on the surface of silver nanoparticles for facilitated olefin transport

Tetrathiafulvalene (TTF), a well-known electron donor, can also behave as an electron acceptor after being adsorbed on the surface of silver nanoparticles (Ag NPs), thereby inducing a partial positive charge on the Ag NPs surface. The Ag NPs activated by TTF help propylene transport much faster than propane, i.e., facilitated olefin transport, resulting in extremely high separation performance for propylene-propane mixtures.