Kicheon Yoo

Superprotonic Conductivity of a UiO‐66 Framework Functionalized with Sulfonic Acid Groups by Facile Postsynthetic Oxidation

Facile postsynthetic oxidation of the thiol-laced UiO-66-type framework UiO-66(SH)2 enabled the generation of UiO-66(SO3 H)2 with sulfonic acid groups covalently linked to the backbone of the system. The oxidized material exhibited a superprotonic conductivity of 8.4×10(-2)  S cm(-1) at 80 °C and 90 % relative humidity, and long-term stability of the conductivity was observed. This level of conductivity exceeds that of any proton-conducting MOF reported to date and is equivalent to the conductivity of the most effective known electrolyte, Nafion.

pH‐Dependent Proton Conducting Behavior in a Metal–Organic Framework Material

A porous metal-organic framework (MOF), [Ni2(dobdc)(H2O)2]⋅6 H2O (Ni2(dobdc) or Ni-MOF-74; dobdc(4-)=2,5-dioxido-1,4-benzenedicarboxylate) with hexagonal channels was synthesized using a microwave-assisted solvothermal reaction. Soaking Ni2(dobdc) in sulfuric acid solutions at different pH values afforded new proton-conducting frameworks, H(+)@Ni2(dobdc). At pH 1.8, the acidified MOF shows proton conductivity of 2.2×10(-2) S cm(-1) at 80 °C and 95% relative humidity (RH), approaching the highest values reported for MOFs. Proton conduction occurs via the Grotthuss mechanism with a significantly low activation energy as compared to other proton-conducting MOFs. Protonated water clusters within the pores of H(+)@Ni2(dobdc) play an important role in the conduction process.

Completely Transparent Conducting Oxide-Free and Flexible Dye-Sensitized Solar Cells Fabricated on Plastic Substrates

To achieve commercialization and widespread application of next-generation photovoltaics, it is important to develop flexible and cost-effective devices. Given this, the elimination of expensive transparent conducting oxides (TCO) and replacement of conventional glass substrates with flexible plastic substrates presents a viable strategy to realize extremely low-cost photovoltaics with a potentially wide applicability. To this end, we report a completely TCO-free and flexible dye-sensitized solar cell (DSSC) fabricated on a plastic substrate using a unique transfer method and back-contact architecture. By adopting unique transfer techniques, the working and counter electrodes were fabricated by transferring high-temperature-annealed TiO2 and Pt/carbon films, respectively, onto flexible plastic substrates without any exfoliation. The fabricated working electrode with the conventional counter electrode exhibited a record efficiency for flexible DSSCs of 8.10%, despite its TCO-free structure. In addition, the completely TCO-free and flexible DSSC exhibited a remarkable efficiency of 7.27%. Furthermore, by using an organic hole-transporting material (spiro-MeOTAD) with the same transfer method, solid-state flexible TCO-free DSSCs were also successfully fabricated, yielding a promising efficiency of 3.36%.

Rapid Dye Adsorption via Surface Modification of TiO2 Photoanodes for Dye-Sensitized Solar Cells

A facile method for increasing the reaction rate of dye adsorption, which is the most time-consuming step in the production of dye-sensitized solar cells (DSSCs), was developed. Treatment of a TiO2 photoanode with aqueous nitric acid solution (pH 1) remarkably reduced the reaction time required to anchor a carboxylate anion of the dye onto the TiO2 nanoparticle surface. After optimization of the reaction conditions, the dye adsorption process became 18 times faster than that of the conventional adsorption method. We studied the influence of the nitric acid treatment on the properties of TiO2 nanostructures, binding modes of the dye, and adsorption kinetics, and found that the reaction rate improved via the synergistic effects of the following: (1) electrostatic attraction between the positively charged TiO2 surface and ruthenium anion increases the collision frequency between the adsorbent and the anchoring group of the dye; (2) the weak anchoring affinity of NO3(-) in nitric acid with metal oxides enables the rapid coordination of an anionic dye with the metal oxide; and (3) sufficient acidity of the nitric acid solution effectively increases the positive charge density on the TiO2 surface without degrading or transforming the TiO2 nanostructure. These results demonstrate the developed method is effective for reducing the overall fabrication time without sacrificing the performance and long-term stability of DSSCs.

Effect of the thickness of the Ru-coating on a counter electrode on the performance of a dye-sensitized solar cell

A ruthenium (Ru) catalytic layer was assessed as the counter electrode (CE) in dye sensitized solar cells (DSSCs) by examining the effect of the Ru thickness on the DSSC performance. Ru films with different thicknesses (34, 46, 69 and 90 nm) were deposited on glass/fluorine-doped tin oxide (FTO) substrates as the CE by atomic layer deposition (ALD) at 250 °C using RuDi as the precursor and O2 as the reaction gas. Finally, a 0.45 cm2 DSSC of glass/FTO/TiO2/dye(N719)/electrolyte(C6DMII, GSCN)/Ru CE structure was prepared. The properties of the DSSCs were examined by field emission scanning electron microscopy (FESEM), four-point-probe, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), current-voltage (I–V), incident photon-to-current conversion efficiency (IPCE), and dark current measurements. FESEM showed that the crystallized Ru films had been deposited quite uniformly and conformally on the glass/FTO surface. The sheet resistance of the Ru film decreased with increasing Ru thickness. CV profiling revealed an increase in catalytic activity with increasing film thickness. The charge transfer resistance at the interface between the Ru-coated CE and electrolyte decreased with increasing Ru thickness. I–V profiling showed that the energy conversion efficiency was increased up to 3.40 % by increasing the Ru thickness. Moreover, the IPCE and dark current results showed the efficiency of the Ru-coated CE was comparable to that of a conventional platinum (Pt) CE.

Rapid sintering of TiO2 photoelectrodes using intense pulsed white light for flexible dye-sensitized solar cells

Intense pulsed white light (IPWL) sintering was carried out at room temperature, which is suitable dye-sensitized solar cells (DSSCs) fabrication process on plastic substrates for the mass production. Five seconds irradiation of IPWL on TiO2 electrode significantly improves the photocurrent density and power conversion efficiency of DSSCs by more than 110% and 115%, respectively, compared to the DSSCs without IPWL treatment. These improvements were mainly attributed to the enhanced interconnection between the TiO2 nanoparticles induced by IPWL illumination, which is confirmed by the impedance spectra analysis.

Selective CO2 adsorption and proton conductivity in the two-dimensional Zn(II) framework with protruded water molecules and flexible ether linkers

A two-dimensional (2D) Zn(II) metal-organic framework with flexible aryl ether linkers and water molecules exposed to the pores was prepared. The supramolecular three-dimensional (3D) network is generated by the presence of extensive π-π contacts, which could be responsible for gas uptake. The water molecules and oxygen atoms from the flexible linkers create a polar environment within the integrated framework, leading to simultaneous selective CO2 adsorption and proton conductivity in the two-dimensional Zn(II) framework.

Novel π-extended porphyrin derivatives for use in dye-sensitized solar cells

Two novel donor-π-acceptor (D-π-A type) porphyrin dyes were successfully synthesized and use in a dye-sensitized solar cell (DSSC). The molecular structures of both porphyrins are composed of the same dialkyl-substituted diphenylamino unit acting as the donor part, and two bisalkoxyphenyl substituents at the 5,15-meso positions. The acceptor part is composed of different ethyne-linked π-extended bridges, and a cyanoacrylic acid (Dye I) or carboxyphenyl (Dye II) moiety acting as anchoring groups. In order to investigate the effects of including the π-extended bridge between the porphyrin and acceptor unit, two different π-extended bridges such as 2,2′-bithiophene and 2-(phenylethynyl)-thiophene, were employed. In particular, Dye II contains two triple bonds between donor substituted porphyrin and carboxylic acid group. These modifications could potentially reduce dye aggregation on the TiO2 surface. The charge recombination resistance and diffusion length for the cells with Dye II were relatively higher for all the measured ranges of bias potentials, implying that electron recombination loss from injected electrons was highly suppressed when Dye II molecules were adsorbed on the TiO2 surface. Eventually, Dye II containing a 2,2′-bithiophene π-spacer and anchored trough a carboxyphenyl group exhibited a superior power conversion efficiency of 6.7% under AM 1.5 illumination (100 mW.cm-2) in a photoactive area of 0.46 cm2 than Dye I with a 2-(phenylethynyl)thiophene (PCE = 3.5%) anchored through a cyanoacrylic group.

Synergistic strategies for the preparation of highly efficient dye-sensitized solar cells on plastic substrates: combination of chemical and physical sintering

Preparation of well-interconnected TiO2 electrodes at low temperature is critical for the fabrication of highly efficient dye-sensitized solar cells (DSCs) on plastic substrates. Herein we explore a synergistic approach using a combination of chemical and physical sintering. We formulate a binder-free TiO2 paste based on “nanoglue” as the chemical sintering agent, and use it to construct a photoelectrode on plastic by low-temperature physical compression to further improve the connectivity of TiO2 films. We systematically investigated the factors affecting the photovoltaic performance and found the conditions to achieve electron diffusion lengths as long as 25 μm and charge collection efficiencies as high as 95%, as electrochemical impedance spectroscopy measurements indicate. We apply this approach to obtain a DSC deposited on plastic displaying 6.4% power conversion efficiency based on commercial P25 titania particles.

Optimized light harvesting and charge collection properties of plastic dye-sensitized solar cells with theoretical modeling and synergistic approach

Highly efficient dye-sensitized solar cells were prepared on plastic substrates by a synergistic approach and reached an efficiency of 8.6%, which optimized the light trapping and charge collection properties based on a theoretical modeling.