Nobuhiro Fuke

CdSe Quantum-Dot-Sensitized Solar Cell with ∼100% Internal Quantum Efficiency

We have constructed and studied photoelectrochemical solar cells (PECs) consisting of a photoanode prepared by direct deposition of independently synthesized CdSe nanocrystal quantum dots (NQDs) onto a nanocrystalline TiO(2) film (NQD/TiO(2)), aqueous Na(2)S or Li(2)S electrolyte, and a Pt counter electrode. We show that light harvesting efficiency (LHE) of the NQD/TiO(2) photoanode is significantly enhanced when the NQD surface passivation is changed from tri-n-octylphosphine oxide (TOPO) to 4-butylamine (BA). In the PEC the use of NQDs with a shorter passivating ligand, BA, leads to a significant enhancement in both the electron injection efficiency at the NQD/TiO(2) interface and charge collection efficiency at the NQD/electrolyte interface, with the latter attributed mostly to a more efficient diffusion of the electrolyte through the pores of the photoanode. We show that by utilizing BA-capped NQDs and aqueous Li(2)S as an electrolyte, it is possible to achieve ∼100% internal quantum efficiency of photon-to-electron conversion, matching the performance of dye-sensitized solar cells.

Integrated dye-sensitized solar cell module with conversion efficiency of 8.2%

We investigated an integrated dye-sensitized solar cell (DSC) module composed of a number of rectangular cells connected in series. Because neighboring cells are processed in reverse, the module is known as the W-contact module. It achieves a high active area of 85% by eliminating the interconnection between neighboring cells. By adjusting the transmittance of the Pt-coated counterelectrode, the TiO2 thickness, and the composition of the electrolyte, we achieved an overall energy conversion efficiency of 8.2% (aperture area of 25.45 cm2) as measured by a public test center under AM 1.5 irradiation. This sets a record for the confirmed efficiency of a DSC submodule.

Photoinduced electron injection in black dye sensitized nanocrystalline TiO2 films

Photoinduced electron injection in nanocrystalline TiO2 films sensitized with black dye [trithiocyanato(4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine)ruthenium(II), Ru(tcterpy)(NCS)3] was studied by means of transient absorption (TA) spectroscopy. We examined the effect of the additive 4-tert-butylpyridine (TBP) in acetonitrile, which is known to markedly improve the performance of solar cell devices. The efficiency of electron injection for the film immersed in acetonitrile was 50% higher than that for the film dried in air. Femtosecond TA measurements indicated that this efficiency enhancement was due to the opening of an additional injection pathway from a lower-lying energy level. Upon the addition of 1 M TBP, which is known to raise the level of the conduction band of TiO2, the enhancement induced by acetonitrile disappeared. We discuss the mechanism of electron injection by comparing our results with results previously reported for the N3/TiO2 system.

Back Contact Dye-Sensitized Solar Cells

A new structure for dye-sensitized solar cells (DSCs) was proposed in which the transparent conducting oxide (TCO) is omitted and the cathode is located on the opposite side of the titanium dioxide (TiO2) film surface to the side of light irradiation. DSCs with this structure are called back contact DSCs (BCDSCs). The structure and fabrication of the cathode of BCDSCs was investigated in comparison with silicon solar cells. BCDSCs with conversion efficiency of 7.1% were achieved by vacuum deposition of the cathode directly onto the TiO2 film, suggesting that BCDSCs can function as effective solar cells with potentially increased efficiency due to the omission of the TCO.

Influence of TiCl4 treatment on back contact dye-sensitized solar cells sensitized with black dye

To better understand why titanium tetrachloride (TiCl4) treatment improves short circuit current, we studied its effects on back contact dye-sensitized solar cells sensitized with black dye [tri(thiocyanato)(4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine)ruthenium(II), Ru(tcterpy)(NCS)3] using transient absorption spectroscopy and electrochemical impedance spectroscopy. We found that the TiCl4 treatment improved short circuit current and achieved an overall energy conversion efficiency of 8.9%. The transient absorption signals did not change as a result of the treatment, suggesting that electron injection efficiency is not affected by the treatment. The impedance related to electron transport between TiO2 particles decreased and the peak frequency of the imaginary part of the electrochemical impedance spectra assigned to electron transfer from TiO2 to the redox couple was shifted to lower frequency by the treatment. This clearly indicates that TiCl4 treatment improved electron transport in the nanocrystalline TiO2 film in back contact dye-sensitized solar cells.

Dye-Sensitized Photovoltaic Module with Conversion Efficiency of 8.4%

In this paper we investigate a dye-sensitized photovoltaic module (DSM) composed of many rectangular cells connected in series, known as a W-connected type module, where neighboring cells are processed in reverse. The characteristics of the serial connection of silicon solar cells have been investigated extensively, while, with dye-sensitized photovoltaic modules, the influence of each rectangular cell performance on the serially connected module has not been investigated as thoroughly. Here we investigate the performance of the rectangular cell in dye-sensitized photovoltaic modules, and we clarify the correlation between the conversion efficiency of the module and the uniformity of the TiO2 layer thicknesses. With these improvements, we achieved an overall energy conversion efficiency of 8.4% (aperture area: 26.47 cm2) as measured by a public test center under AM-1.5 irradiation. This sets a record for the confirmed efficiency of the DSM.

High Efficiency of Dye-Sensitized Solar Cell and Module

The improvement of cell performance of dye-sensitized solar cells (DSCs) was investigated by means of haze of TiO2 electrodes and series-internal resistance. It was found that the high haze of TiO 2 electrodes effectively improves the external quantum efficiency of DSCs. The series-internal resistance is successfully reduced by increasing the surface roughness of platinum counter electrodes and by decreasing the thickness of electrolyte layer. The highest single cell efficiency of 11.1% (aperture area: 0.219 cm2 ) was achieved and confirmed by a public test center (AIST). Furthermore, large-scaled cell was fabricated using current collecting metal grids, and the efficiency of 6.8% (aperture area: 101 cm2 ) was obtained. Moreover, integrated DSC module was investigated and the efficiency of 6.3% (aperture area: 26.50 cm2) was also confirmed by the public test center

Electron transport in back contact dye-sensitized solar cells

The electron transport properties of a back contact dye-sensitized solar cell (BCDSC) were investigated in comparison with a conventional DSC. It was found that the BCDSC had a lower short circuit current density (JSC) at the same thickness of TiO2 film and that JSC was not proportional to the thickness of the TiO2 film. Calculation of electron transport length in the TiO2 film suggested that the injected electron travels a longer distance to the electrode in the BCDSC than in the DSC. TiCl4 treatment of the TiO2 film produced a marked improvement of the JSC value in the BCDSC due to an increase in the electron diffusion coefficient of the TiO2 film, whereas the JSC of the DSC remained almost unchanged. It is clear that the value of JSC in the BCDSC is more dependent on the electron transport properties of the TiO2 film than in the DSC. Under standard AM 1.5 irradiation (100 mW cm−2), a BCDSC with N719 dye yielded an overall conversion efficiency of 8.0%.

Effect of organic passivation on photoinduced electron transfer across the quantum dot/TiO2 interface

We report a study of the internal quantum efficiency (IQE) of CdSe quantum-dot (QD)-sensitized solar cells prepared by direct adsorption of pre-synthesized QDs, passivated with either tri-n-octylphosphine oxide (TOPO) or n-butylamine (BA), onto a nanocrystalline TiO(2) film.

Improvement of efficiency of dye-sensitized solar cells by reduction of internal resistance

In order to elevate conversion efficiency, the series-internal resistance of dye-sensitized solar cells (DSCs) was investigated with electrochemical impedance spectroscopy measurement based on an equivalent circuit of DSCs. It was found that series-internal resistance correlates negatively with the roughness factor of the platinum counter electrode and positively with the thickness of the electrolyte layer. A cell sensitized with a black dye with series-internal resistance of 1.8 /spl Omega/cm/sup 2/ was fabricated and showed conversion efficiency of 10.2%, which was confirmed at the National Institute of Advanced Industrial Science and Technology (AIST, Japan).