Daesub Hwang

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.

Low-Temperature Fabrication of TiO2 Electrodes for Flexible Dye-Sensitized Solar Cells Using an Electrospray Process

Hierarchically structured TiO2 (HS-TiO2) was prepared on a flexible ITO-PEN (polyethylene naphthalate) substrate via electrospray deposition using a commercially available TiO2 nanocrystalline powder in order to fabricate flexible DSSCs under low-temperature (<150 °C) conditions. The cell efficiency increased when using flexible ITO-PEN substrates post-treated by either a mechanical compression treatment or a chemical sintering treatment using titanium n-tetrabutoxide (TTB). The mechanical compression treatment reduced the surface area and porosity of the HS-TiO2; however, this treatment improved the interparticle connectivity and physical adhesion between the HS-TiO2 and ITO-PEN substrate, which increased the photocurrent density of the as-pressed HS-TiO2 cells. The electron diffusion coefficients of the as-pressed HS-TiO2 improved upon compression treatment, whereas the recombination lifetimes remained unchanged. An additional chemical sintering post-treatment involving TTB was tested for its effects on DSSC efficiency. The freshly coated TiO2 submitted to TTB hydrolysis in water at 100 °C yielded an anatase phase. TTB treatment of the HS-TiO2 cell after compression treatment yielded faster electron diffusion, providing an efficiency of 5.57% under 100 mW cm(-2), AM 1.5 global illumination.

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.

Highly Efficient Plastic Crystal Ionic Conductors for Solid-state Dye-sensitized Solar Cells

We have developed highly efficient, ambient temperature, solid-state ionic conductors (SSICs) for dye-sensitized solar cells (DSSCs) by doping a molecular plastic crystal, succinonitrile (SN), with trialkyl-substituted imidazolium iodide salts. High performance SSICs with enhanced ionic conductivity (2–4 mScm−1) were obtained. High performance solid-state DSSCs with power conversion efficiency of 7.8% were fabricated using our SSICs combined with unique hierarchically nanostructured TiO2 sphere (TiO2-SP) photoelectrodes; these electrodes have significant macroporosity, which assists penetration of the solid electrolyte into the electrode. The performance of our solid-state DSSCs is, to the best of our knowledge, the highest reported thus far for cells using plastic crystal-based SSICs, and is comparable to that of the state-of-the-art DSSCs which use ionic liquid type electrolytes. This report provides a logical strategy for the development of efficient plastic crystal-based SSICs for DSSCs and other electrochemical devices.

Superior photoelectrodes for solid-state dye-sensitized solar cells using amphiphilic TiO2

Anatase amphiphilic TiO2 (am-TiO2) nanoparticles with an average diameter of ∼15 nm were synthesized using a sol–gel hydrothermal synthesis. The colloidal dispersion of am-TiO2 in alcoholic solvents had outstanding stability for several months at room temperature (RT). Hierarchically structured TiO2 nanospheres (TiO2-NSs) were then successfully prepared by an electrostatic spray (e-spray) method using the am-TiO2 dispersion at RT. The e-sprayed TiO2-NSs were employed as photoelectrodes for solid-state dye-sensitized solar cells (ss-DSSCs), in which a novel plastic crystal/polymer blend based solid-state electrolyte (PC-SSE), iodine-doped succinonitrile/polyacrylonitrile, was used. A photovoltaic power conversion efficiency (PCE) of ∼7.6% was reached under simulated AM 1.5 G (100 mW cm−2) illumination, while the incident photon-to-electrical current conversion was >60% upon excitation at 550 nm. The unique morphology of the TiO2-NSs, in which primary anatase TiO2 nanoparticles are interconnected within the submicron-scale spheres, exhibited superior characteristics as photoelectrodes for our ss-DSSCs compared to conventional TiO2 nanoparticles (TiO2-NPs). The nanoparticles-in-sphere shape offered enhanced surface area and pore volume for high dye-loading and beneficial solid-state electrolyte (SSE) infiltration, while the submicron-scale of the spheres induced efficient visible-light scattering. The efficient interconnection among the primary anatase TiO2 nanoparticles within the e-sprayed NSs offered enhanced charge collection efficiency and photoelectron density, which were confirmed by intensity-modulated photocurrent/photovoltage spectroscopy and electrochemical impedance spectroscopy. These superior properties of the TiO2-NSs were manifested in the cell performance. The PCE of the TiO2-NS-based ss-DSSCs was >2 times higher than that of TiO2-NP-based ss-DSSCs, with notable concurrent improvement in the short-circuit current (JSC = 13.9 mA cm−2), open circuit voltage (VOC = 765 mV), and fill factor (FF = 71.6%). The optimized cell efficiency of 7.6% is thus far the highest reported efficiency for PC-SSE-based DSSCs. Both the single-crystal-like anatase primary particles and the advantageous morphology effectively contributed to the high performance of the ss-DSSCs. Since the e-spray method is a high-throughput, scalable process, our hierarchically structured TiO2-NSs are a promising material for high-efficiency ss-DSSCs, possessing a range of advantages over conventional TiO2-NPs.

β-Functionalized Push-Pull Porphyrin Sensitizers in Dye-Sensitized Solar Cells: Effect of π-Conjugated Spacers.

A series of new β-functionalized push-pull-structured porphyrin dyes were synthesized so as to investigate the effect of the π-conjugated spacer on the performance of dye-sensitized solar cells (DSSCs). Suzuki- and Heck-type palladium-catalyzed coupling methodologies were used to obtain various β-functionalized porphyrins and β-benzoic acid (ZnPHn) and β-vinylbenzoic acid (ZnPVn) derivatives from β-borylated porphyrin precursors. Photophysical studies of the resulting porphyrins revealed a clear dependence on the nature of the β linker. In particular, it was found that a β-vinylene linkage perturbs the electronic structure of the porphyrin core; this is less true for a β-phenyl linkage. Theoretical analyses provided support for the intrinsic intramolecular charge-transfer character of the β-functionalized, push-pull porphyrins of this study. The extent of charge transfer depends on the nature of the β-conjugated linkage. The photovoltaic performances of the cells sensitized with β-phenylenevinylene ZnPVn exhibited higher power conversion efficiency values than those bearing β-phenyl linkages (ZnPHn). This was ascribed to differences in light-harvesting efficiency. Furthermore, compared to the use of a standard iodine-based electrolyte, the DSSC performance of cells made from the present porphyrins was improved by more than 1 % upon using a cobalt(II/III)-based electrolyte. Under standard AM 1.5 illumination, the highest efficiency, 8.2 %, was obtained by using cells made from the doubly β-butadiene-linked porphyrin.

Controlled Interfacial Electron Dynamics in Highly Efficient Zn2SnO4‐Based Dye‐Sensitized Solar Cells

Among ternary oxides, Zn2 SnO4 (ZSO) is considered for dye-sensitized solar cells (DSSCs) because of its wide bandgap, high optical transmittance, and high electrical conductivity. However, ZSO-based DSSCs have a poor performance record owing largely to the absence of systematic efforts to enhance their performance. Herein, general strategies are proposed to improve the performance of ZSO-based DSSCs involving interfacial engineering/modification of the photoanode. A conformal ZSO thin film (blocking layer) deposited at the fluorine-doped tin oxide-electrolyte interface by pulsed laser deposition suppressed the back-electron transfer effectively while maintaining a high optical transmittance, which resulted in a 22 % improvement in the short-circuit photocurrent density. Surface modification of ZSO nanoparticles (NPs) resulted in an ultrathin ZnO shell layer, a 9 % improvement in the open-circuit voltage, and a 4 % improvement in the fill factor because of the reduced electron recombination at the ZSO NPs-electrolyte interface. The ZSO-based DSSCs exhibited a faster charge injection and electron transport than their TiO2 -based counterparts, and their superior properties were not inhibited by the ZnO shell layer, which indicates their feasibility for highly efficient DSSCs. Each interfacial engineering strategy could be applied to the ZSO-based DSSC independently to lead to an improved conversion efficiency of 6 %, a very high conversion efficiency for a non-TiO2 based DSSC.

Hierarchically structured Zn2SnO4 nanobeads for high-efficiency dye-sensitized solar cells.

We developed a unique strategy for fabricating hierarchically structured (nanoparticles-in-beads) Zn2SnO4 beads (ZTO-Bs), which were then used to produce ternary metal oxide-based dye-sensitized solar cells (DSSCs). DSSCs were fabricated using the ZTO-Bs as the photoelectrodes and highly absorbable organic dyes as the sensitizers. The DSSCs based on the ZTO-Bs and the organic dyes (SJ-E1 and SJ-ET1) exhibited the highest performance ever reported for DSSCs with ternary metal oxide-based photoelectrodes. The optimized DSSCs exhibited a power conversion efficiency of 6.3% (VOC of 0.71 V, JSC of 12.2 mA cm−2, FF of 0.72), which was much higher than that for DSSCs with conventional ZTO-NPs-based photoelectrodes or those based on the popular ruthenium-based dye, N719. The unique morphology of the ZTO-Bs allowed for improvements in dye absorption, light scattering, electrolyte penetration, and the charge recombination lifetime, while the organic dyes resulted in high molar absorbability.

First‐Generation Subporphyrinatoboron(III) Sensitizers Surpass the 10 % Power Conversion Efficiency Threshold

Subporphyrinatoboron(III) (SubB) sensitizers were synthesized for use in dye-sensitized solar cells (DSSCs). The prototype, which comprises a sterically demanding 3,5-di-tert-butylphenyl scaffold, a meso-ethynylphenyl spacer, and a cyanoacrylic acid anchoring group, achieved an open-circuit voltage VOC of 836 mV, short-circuit current density JSC of 15.3 mA cm(-2) , fill factor of 0.786, and a photon-to-current conversion efficiency of 10.1 %. Such astonishing figures suggest that a bright future lies ahead for SubB in the realm of DSSCs.

Enhanced charge collection efficiency of dye-sensitized solar cells based on size-tunable hierarchically structured TiO2 beads

Hierarchically structured mesoporous TiO2 beads (HS-TBs), which are used as photoelectrodes in highly efficient dye-sensitized solar cells (DSCs), were prepared by an electrostatic spray (e-spray) technique. To prepare different sized HS-TBs, the electric field and the concentration of TiO2 particles were carefully controlled, because they are critical factors in preparing size-controlled TiO2 beads. Four different HS-TBs were formulated with average diameters of 250, 450, 700, and 1200 nm as high-quality photoelectrodes for use in DSCs. In this study, we found that the zero-dimensional HS-TBs were the most promising photoelectrode for DSCs due to enhanced charge collection efficiency and better penetration of electrolytes through relatively large pores among the HS-TBs. The HS-TBs were characterized by intensity modulated photocurrent spectroscopy (IMPS), the scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) method, Barrett–Joyner–Halenda (BJH) method, and electrochemical analysis. The conversion efficiency of HS-TB photoelectrodes improved with increasing bead size due to the enhanced electron transport through electrodes. The present HS-TB cells exhibit a noticeable improvement in the overall efficiency: maximum 9.54% (1200 nm) versus 5.83% for the reference cell made of a TiO2 nanocrystalline film.