Takashi Kado

Quasi-solid dye sensitised solar cells filled with ionic liquid—increase in efficiencies by specific interaction between conductive polymers and gelators

Photo-energy conversions for quasi-solid dye sensitised solar cells increased when gel electrolytes were combined with conductive polymers as counter electrodes and the conversion surpassed that for DSSCs equipped with conventional Pt counter electrodes.

Quasi-Solid Dye-Sensitized Solar Cells Containing Nanoparticles Modified with Ionic Liquid-Type Molecules

Gel electrolytes for quasi-solid dye-sensitized solar cells (QDSC) are reported. The gel electrolyte consists of ionic liquids and nanoparticles modified with imidazolium cations. Ionic liquid-type gel electrolytes containing nanoparticles are well-known. The difference between previous reports and this report is that imidazolium cations are bonded to nanoparticles through Ti-O-CO-bonds and long alkyl chains. The role of the long alkyl chains turns out to be critical for high performances. When unmodified nanoparticles are added into ionic liquids until the gel becomes of clay-like hardness, the photovoltaic performances decrease with an increase in the bare nanoparticle content. However, the decreases in photovoltaic performances are retarded when surface-modified nanoparticles are added. Photovoltaic performances increase with an increase in the chain lengths connecting nanoparticles and imidazolium cations. When the chain length becomes 12 and counter anion is I - , solidification occurs without losing the performance of DSCs having the parent liquid electrolytes. The photocurrents of QDSCs do not decrease even when the ratio (nanoparticles/ionic liquids) increases to 0.9 and the feature looks like hard clay. Ionic paths between nanoparticles are discussed.

Dye-Sensitized Solar Cells Fabricated by Electrospray Coating Using TiO2 Nanocrystal Dispersion Solution

Dye-sensitized solar cells (DSCs) were fabricated by an electrospray coating method using TiO 2 nanocrystal dispersion (P25) in water (DSC-ESC). They were compared with DSCs prepared by conventional coating methods (DSC-COAT). The former had higher short-circuit current (J sc ) than the latter. Electron lifetime and electron diffusion coefficient in TiO 2 layers did not vary largely between the two DSCs. Large differences were not observed between them in terms of the amount of dye molecules adsorbed on TiO 2 . Differences were observed only for limiting currents (I - 3 diffusion) through TiO 2 layers. The limiting current for DSC-ESC was higher than that of DSC-COAT. Considering the previous reports on fractal structures prepared by the ESC method and the fact that needle-like TiO 2 tends to grow on deposited TiO 2 needles by using the ESC method, we conclude that ionic paths are formed better in TiO 2 layers of DSC-ESC than in those of DSC-COAT.

Surface State Passivation Effect for Nanoporous TiO2 Electrode Evaluated by Thermally Stimulated Current and Application to All-Solid State Dye-Sensitized Solar Cells

The effect of surface state (trap) passivation of a nanoporous TiO2 electrode was examined by a thermally stimulated current method (TSC). TSC decreased after dye molecules were adsorbed or after Al2O3 thin layers were fabricated on the TiO2 electrode, suggesting that the surface trap of the TiO2 electrode was actually passivated following these surface modifications. Electron diffusion coefficient in the TiO2 electrode increased and electron recombination was retarded after the TSC peak decreased by the passivation. All-solid-type organic solar cells consisting of nanoporous TiO2/Al2O3/dye/polythiophene derivative (Cell 1) were prepared and the photovoltaic properties of Cell 1 were compared with those of Cell 2 consisting of TiO2/dye/polythiophene derivatives. By inserting the Al2O3 layer between the TiO2 layer and the dye molecule layer, we observed improvement of the photovoltaic properties.

Enhanced Electrochemiluminescence by Use of Nanoporous TiO2 Electrodes: Electrochemiluminescence Devices Operated with Alternating Current

It was found that electrochemiluminescence is enhanced by the use of nanoporous TiO2 electrodes. The nanoporous TiO2 electrodes were fabricated by cumulating nano-TiO2 crystals on the transparent conductive layers (SnO2/F). The device was composed of the porous TiO2 electrodes (10 µm) and SnO2/F electrodes as counterelectrodes. The gap was filled with electrolytes containing Ru(bpy)3(PF6)2 (bpy: bi-pyridyl) in various solvents. The electrochemiluminescence was much larger than that from the cell composed of two flat SnO2/F electrodes. It is likely that the increase in elecrochemiluminescence is associated with the nanopores as well as with the large surfaces of porous semiconductive oxide electrodes.

Additives for Increased Photoenergy Conversion Efficiencies of Quasi-Solid, Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are solidified with gelators containing polyvinylpyridine and 1,2,4,5-tetra(bromomethyl)benzene. The photoconversion efficiencies are improved by new additives. Lil and t-butylpyridine are commonly added in electrolytes for increasing short-circuit current (Jsc) and open-circuit voltage (Voc). These additives inhibit our gel electrolyte precursors from solidifying. We found that new additives, combinations of acetic acid, and methylpyrimidine or methylbenzimidazole, do not inhibit the solidification and are effective for increasing both Jsc and Voc. These mechanisms are discussed in terms of electron diffusion coefficients, 13 ion diffusion coefficients, and charge-transfer resistances between counter electrodes and gel electrolytes.

Increase in Intensity of Electrochemiluminescence from Cell Consisting of TiO2 Nanohole Array Film

The increase in the intensity of electrochemiluminescnece (ECL) from cells consisting of titania nanohole array film (T-NHA-AL) is reported. The T-NHA-AL with a thin alumina insulation layer on the edge was prepared by an improved liquid-phase deposition method. The T-NHA-AL was sandwiched between two transparent conductive glasses (F-doped SnO 2 , FTO). The space between these two FTO substrates was filled with a solution of Ru(bpy) 3 (PF 6 ) 2 in propylene carbonate. The ECL from the cell was observed by applying ac bias on the FTO glass electrode. The ECL from the cell consisting of the T-NHA-AL was larger than that without T-NHA-AL (the cell with flat FTO glasses). In addition, the ECL intensity from the cell consisting of T-NHA-AL was higher than that consisting of alumina nanohole array (anodically oxidized porous alumina). The increase in the intensity of the ECL from the cell consisting of T-NHA-AL was explained by the electron injection from the large surface of the T-NHA-AL and the confinement of the luminescence species [Ru(I) and Ru(III)] in the restricted area of nanopores in the T-NHA-AL film. If a TiO 2 nanohole array film without the thin Al 2 O 3 layer on the edge (T-NHA) was inserted between these two electrodes, short circuiting occurred through the T-NHA film. It is emphasized that the thin alumina insulation layer at the edge of the T-NHA-AL film is essential to realize an increase in ECL intensity.

Dye Sensitized Solar Cells with High Photo-Energy Conversion-Controll of Nano-Particle Surfaces

Some of factors affecting photo-conversion efficiency of dye sensitized solar cells (DSCs) are discussed in terms of TiO2 electrodes. The first topic is on the surface modification of TiO2 nano-particles, which is associated with electron traps on the surface of TiO2 nano-particles. The surface is modified with dye molecules under pressurized CO2 atmosphere to increase the surface coverage of TiO2 nano-particles with dye molecules. This increases Jsc because of an increase in the amount of dye molecules and a decrease in the amount of trapping sites on TiO2 nano-particles. In addition, the decrease in the amount of trap sites increases Voc because decreases in Voc are brought about by the recombination of I2 molecules with electrons trapped on the TiO2 surfaces. Selective staining for tandem cells is proposed. The second topic is on the contact between a SnO2/F transparent conductive layer (TCL) and nano-particles. Polishing the TCL surfaces with silica nano-particles increases the contact, resulting in Jsc increases

Quasi-solid dye-sensitized solar cells: control of TiO2-gel electrolyte interfaces

Quasi-solid dye sensitized solar cells (Q-DSSC) were fabricated by employing gel electrolytes containing ionic liquids and gelators. Sufficient physical contacts between nano-crystalline TiO2 particles and gel electrolytes in nano-porous TiO2 layers were achieved by solidifying gel electrolyte precursors after the cells are filled with the electrolytes. Photo-currents increased largely by embedding carboxylic acids among dye molecules on TiO2 crystals. The nano-porous TiO2 electrolytes were fabricated by dipping the dye anchored TiO2 substrates in dilute solutions of carboxylic acids. It was found that resistances in the TiO2 layers decreased by these treatments.

Quasi-Solid Dye Sensitized Solar Cells with Ionic Liquid Type Gel Electrolytes

(G1) for DSCs. (G1) is reactive liquids is inserted into solar cells. This can be solidified in the DSC directly. (G1) was reactive even at room temperature and the viscosity increases rapidly, which made it difficult for the precursors to be inserted in solar cells. We now report latent Cross-linked gel precursor (G2). (G2) is also reactive gel electrolyte precursors. However, the reactivity at room temperature is killed. This makes it possible to fabricate large cells. The Crosslinked gel precursors (G2) and the solidification mechanism are described in Figure 1. The chain length affected PV performance of quasi-solid DSC. Figure 2 show the relationship between the chain length and short circuit current (Jsc). The number of carbon 0 stands for the Jsc for a cell with the parent ionic liquid type electrolyte before gelation. The Jsc decreased first and then increased with the increase in the carbon chain length of the cross-linkers (dicarboxylic acids). Diffusion coefficient of I3 (rate determining-ionic species for DSCs) in gel electrolytes also decreased first and then increased, which is the same phenomena as those for Jsc. It is reasonable to explain that long alkyl groups should be phase-separated in hydrophilic ionic liquids. We have already reported that phase separation of gelator backbones in gel electrolytes is useful in order to maintain the high photo-energy conversion efficiencies during the solidification. It is likely that I3 can diffuse with less interaction with gelator backbones because of the phase separations.