Mitsusuke Ikeda

Electrical and Optical Properties of CdS-MnS Single Crystals

Single crystals of Cd 1- x Mn x S ( x ≤0.4) were prepared by Bridgman-Stockbarger method under high gas pressure and electrical and optical properties of these crystals were measured. The mobility of electrons was found to be significantly influenced by the alloy scattering. In the optical absorption spectra, three absorption peaks due to Mn 2+ ions were observed at 455, 480, and 510 mµ, similar to the case of Zn 1- x Mn x S crystals. The band gap of Cd 1- x Mn x S varies anomalously with x ; it shows a minimum at x ∼0.03 and increases linearly with x for x ≥0.1. The reason for the appearance of this minimum at small x has not been understood.

Preparation of CuInSe2 thin films by selenization of CuInO precursors

Abstract Polycrystalline CuInSe 2 films were prepared by selenization of CuInO films. CuInO precursor were prepared from Cu 2 In 2 O 5 target by rf-magnetron sputtering and pulsed laser deposition. The CuInO films were converted into CuInSe 2 films by annealing in H 2 Se gas atmosphere. The CuInSe 2 films obtained by selenizing CuInO films prepared by a pulsed laser deposition, contained an impurity phase such as In 2 O 3 . But singlephase chalcopyrite CuInSe 2 films were obtained by selenizing the near stoichiometric CuInO film prepared by a rf-magnetron sputtering at a temperature of 450°C.

Photoinduced voltage‐decay characteristics of dye‐sensitized poly‐N‐vinylcarbazole

Measurements of the photoinduced voltage‐decay of dye‐sensitized polyvinylcarbazole (PVK) films are reported. A reaction product of a benzopyrylium salt and a benzopyrane is, as the sensitizing dye, uniformly dispersed in the PVK films. The voltage‐decay characteristics are analyzed on the basis of a model assuming that holes are generated by an electron transfer from PVK to photoexcited dyes. The model is confirmed by this analysis and by comparing the voltage‐decay rate of the uniformly sensitized films with that of a double‐layer film consisting of an undoped PVK layer and a very thin dye‐sensitized PVK layer. The quantum efficiency of photogeneration of holes is found to increase linearly with the electric field and is estimated to be 0.52 at 106 V/cm. The mean range of holes at 106 V/cm is longer than 11 μ for the films containing less than 1.6 weight parts of the dye relative to 100 weight parts of PVK. The range decreases with the increase in the dye concentration.

Dye Sensitization of Photoconductivity of Poly-N-Vinylcarbazole. I. Quantum Yield

The photocurrent of the dye-sensitized poly-N-vinylcarbazole (PNVC) film was measured. The dyes were used carbonium salts, benzopyrylium salts and crystal violet. These dyes were found to enhance the photocarriers in the film both at the first excitation band of PNVC and at the excitation band of the dyes. Since photocarriers were observed to be holes, it is concluded that the electron transfer occurs from PNVC to dyes with the absorption of photons and that the quantum yield of photocarriers is related to the rate of this electron transfer. The quantum yield was deduced from the combination of the photocurrent and the decay rate of the surface charge and was found to be more than 10-1 for the excitation of PNVC and 10-2~10-1 for the dye-excitation.

Preparation of CuInSe2 films on large grain CdS films for superstrate-type solar cells

Superstrate-type CuInSe2 solar cells were fabricated by preparation of CuInSe2 films on CdS films with large grains. The CdS films with large grains were obtained by annealing of evaporated films with CdS: CdCl2 powders. The horizontal grain size of the annealed CdS films was about 3 times as large as the film thickness. The orientation of the CuInSe2 films on the CdS films was evaluated by X-ray diffraction. The CuInSe2 films on the annealed CdS films had a preferred orientation with (102) or (110) plane parallel to the substrate, while that on the evaporated CdS films highly oriented along direction. The grains of the CuInSe2 films grew along several directions on the different CdS gains. As a result of the measurement of I–V characteristics under illumination, the cells using the large grain CdS films had higher Jsc than that using the evaporated CdS films. The best cell performance obtained was Voc=0.357 V, Jsc=32.2 mA/cm2 and FF=0.582 in 0.075 cm2 area. The conversion efficiency of the cell was 6.71%.

Fabrication of substrate-type CuInSe2 thin film solar cells

Heterojunctions based on CuInSe 2 films formed by a bilayer process are fabricated and their photovoltaic properties are investigated. The photovoltaic performances are enhanced by post-annealing in air at 200 o C. The fabricated heterojunctions of CdS/CuInSe 2 and ZnO/CuInSe 2 show efficiencies of 10,5% and 6.0%, respectively. The CdS/Cu(In,Ga)Se 2 heterojunction is also fabricated and shows an efficiency of 11.7%.

Formation of Ion Pairs and Carrier Transport in Undoped and Dye-Doped Poly (N-Vinylcarbazole) Films

Dark conductivities σ d of undoped and dye-doped poly(N-vinylcarbazole) (PVK) films have been measured at 298, 276, and 208 K over a wide range of static electric field E =10 4 ∼10 6 V/cm. The observed field dependence of σ d follows apparently the E 0.6 -power law, rather than the E 1/2 -power law of the well-known Poole-Frenkel (PF) type expression. Based on these experimental results, we have discussed the field-induced carrier generation processes from “ion pairs” (bound holes-charged acceptors) that are formed by an electron transfer between carbazole-rings and acceptors involved. Here we have taken into account the Onsager “dissociation-association” model to account for the observed electric field dependence of σ d , in which the dissociation of the ion pairs is enhanced by an applied field.

CdS/CdTe thin film solar cell fabricated by hot wall vacuum evaporation

Abstract CdS/CdTe heterojunctions have been made on glass/ITO or glass/ITO/ZnO substrate by the hot wall vacuum evaporation method, and their photovoltaic properties have been investigated. The cell performances have been improved by using a ZnO layer since the ZnO layer suppressed the interdiffusion of indium and tin from ITO to the CdS layer. In addition, the back side contacts to the CdTe film have been studied by doping acceptor impurities such as Cu, Ag, and P. The best photovoltaic cell shows an efficiency of 10.3% under the condition of AM 1.0 and 100 mW/cm 2 .

Hole Photoinjection from Gold into Poly (N-Vinylcarbazole)

Photoinjection currents from gold into poly (N-vinylcarbazole) were measured. The electric field and temperature dependence of the injection current are analyzed on the basis of Blosseys photoinjection model developed for metal-low mobility insulator systems, taking the mean free path x 0 of the injected hot holes as a parameter. The barrier height is estimated to be 1.10 eV from the linear extrapolation of the plot of x 0 2 as a function of photon energy.

Dye Sensitization of Photoconductivity of Poly-N-Vinylcarbazole II. Fluorescence Quenching and Charge Transfer

Fluorescence and excitation spectra of poly-N-vinylcarbazole (PNVC) and dyes were measured. The fluorescence of dyes decreased with the addition of PNVC and that of PNVC also decreased with the addition of dyes, with a peak at about 425 mµ. The quenching of PNVC fluorescence was found to be more remarkable for dyes having larger sensitization effects on the photocurrent in PNVC films. These results suggest that both the fluorescence quenching and the sensitization of the photocurrent are due to the electron transfer from PNVC to dyes under illumination.