Yasuo Chiba

Dye-Sensitized Solar Cells with Conversion Efficiency of 11.1%

Dye-sensitized solar cells (DSCs) using titanium dioxide (TiO2) electrodes with different haze were investigated. It was found that the incident photon to current efficiency (IPCE) of DSCs increases with increase in the haze of the TiO2 electrodes, especially in the near infrared wavelength region. Conversion efficiency of 11.1%, measured by a public test center, was achieved using high haze TiO2 electrodes. This indicates that raising the haze of TiO2 electrodes is an effective technique for improvement of conversion efficiency.

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.

Methods of Measuring Energy Conversion Efficiency in Dye-sensitized Solar Cells

The current–voltage characteristics of dye-sensitized solar cells (DSCs) were measured and compared with those of crystalline silicon solar cells. It was found that the energy conversion efficiency of DSCs is dependent on voltage sweep direction and sampling delay time (Td). Measurement of the transient photocurrent revealed that this dependence is due to the longer time constant of DSCs. This dependence was also confirmed in a simulation of current–voltage curves based on an equivalent circuit model of DSCs. Analysis of the current–voltage characteristics of polymer-based bulk heterojunction solar cells (BHSCs) and simulated measurements showed that the longer time constant is due to slow movement of ions in the electrolyte. To improve accuracy, the I–V measurement should be carried out from short circuit to open circuit with Td of 100 ms or longer.

STRUCTURAL DEPENDENCE OF GAN/AL2O3 ON ELECTRIC BIAS DURING ELECTRON-CYCLOTRON-RESONANCE PLASMA-EXCITED MOLECULAR BEAM EPITAXY (ECR-MBE)

GaN crystals were grown on a (0001) sapphire substrate by electron cyclotron resonance plasma excited molecular beam epitaxy (ECR-MBE). Grown crystals were analyzed by photoluminescence (PL) measurement at low temperatures and by pole figure mode X-ray diffraction (XRD) mapping. The GaN crystal structure changed from hexagonal dominant to cubic dominant as the V/III ratio decreased. It was also observed for the first time that a similar structural change occurred upon the application of a bias voltage to the substrate. This structural change was considered to be induced by the change in the effective V/III ratio on the growing surface introduced by the positive bias. These results imply the possibility of growing hexagonal and cubic heterostructures and supperlattices by simply changing the bias voltage during growth.

Time-dependent changes in copper indium gallium (di)selenide and cadmium telluride photovoltaic modules due to outdoor exposure

The performance of photovoltaic (PV) modules deteriorates with time due to outdoor exposure. We investigated the time-dependent changes in PV modules and evaluated the amount of power generated during their lifetime. Once a year, the exposed modules were removed and measured under standard test conditions using a solar simulator. Their outputs were measured indoors and normalized to nominal values. In addition, the relationship between the indoor measurement and the energy yield for thin-film PV modules will be reported. In CIGS PV modules, the normalized maximum power (P MAX) and performance ratio (PR) differ with the type of module. The P MAX and PR of CdTe PV modules significantly decrease after outdoor exposure for three years. These results help to determine the characteristics of the time-dependent changes in the P MAX of PV modules due to outdoor exposure.

Development of a practical method of estimating electric power from various photovoltaic technologies with high precision

The purpose of this study is to develop a method of estimating the electric power from various photovoltaic technologies with high precision. The actual outdoor performance of eight kinds (12 types) of photovoltaic (PV) modules has been measured since January 2012 in order to verify the precision of the method. Using ambient climatic datasets including solar irradiance, module temperature, and solar spectrum, the performance of these PV modules is corrected to the performance under standard test conditions (STC), which should be constant ideally. The results indicate that the performance of bulk crystalline silicon (c-Si) and copper indium gallium diselenide (CIGS) PV modules can be estimated with high precision (approximately less than ±2%). However, the estimation precision of thin-film Si and cadmium telluride (CdTe) PV modules is low because of the initial light-induced degradation and seasonal variation due to metastability.

On-Line System for Measuring Bubble Chamber Films with Manual Digitizer

An attempt has been made to develop an on-line measuring system which allows one to avoid troublesome processes of remeasurement in bubble chamber film analysis by using a conventional digitizer with the aid of an inexpensive minicomputer. The essential feature of the system is an on-line diagnosis of the measuring accuracy by means of a parabolic fitting test of individual tracks on each view and a spatial reconstruction test after the measurement on three views. This enables an operator to make a remeasturement, when necessary, immediately after the diagnosis. The rate of success in track-reconstruction amounts to 99% in the analysis of 597 tracks from two-prong events. The measuring speed has also been improved effectively by more than a factor of two over that of an off-line system owing to the use of an automatic sequence and interlocking of operation.