Hidetaka Takato

Quinhydrone/methanol treatment for the measurement of carrier lifetime in silicon substrates

Quinhydrone/methanol treatment for the measurement of carrier lifetime in crystalline silicon substrates has been investigated. To estimate the surface passivation effect, the lifetimes of the silicon substrates were measured using the microwave photoconductive decay method. The measured lifetime is dependent on quinhydrone concentration and passivation time. The 0.01 mol/dm3 quinhydrone/methanol treatment exhibited a good passivation effect, and a very low surface recombination velocity was obtained. The quinhydrone/methanol treatment can provide a reliable lifetime map of silicon wafers since a constant lifetime value without degradation can be obtained. Therefore, the quinhydrone/methanol treatment can be used for estimating the bulk lifetime of silicon substrates.

Surface Passivation Effect of Silicon Substrates due to Quinhydrone/Ethanol Treatment

A new technique of surface passivation of silicon substrates by quinhydrone/ethanol treatment has been investigated. To estimate the surface passivation effect, the lifetimes of the silicon substrates were measured using the microwave photoconductive decay method. The measured lifetimes were dependent on quinhydrone concentration and passivation time. The 0.01 mol/dm3 quinhydrone/ethanol treatment showed a good passivation effect, and a very low surface recombination velocity was obtained. The quinhydrone/ethanol treatment was a more effective passivation technique than the iodine/ethanol treatment. Therefore, the quinhydrone/ethanol passivation can be widely used for lifetime measurement.

Effects of Optical Confinement in Textured Antireflection Coating using ZnO Films for Solar Cells

The effect of optical confinement in textured antireflection coating (AR coating) was investigated. Textured ZnO films prepared by metalorganic chemical vapor deposition were applied to solar cells as AR coating. The reflectance of the cells decreased as the grain size increased with film thickness, especially at long wavelength. This reduction in the reflectance caused an increase in short-circuit current and spectral response of the cells at near-infrared. The effect of optical confinement in the cell by this film was shown as far as long-wavelength range was concerned.

Wet Chemical Surface Passivation of Germanium Wafers by Quinhydrone--Methanol Treatment for Minority Carrier Lifetime Measurements

We have applied quinhydrone/methanol (Q/M) treatment to germanium (Ge) surfaces and shown that this treatment is also effective for passivating Ge surfaces for minority carrier lifetime measurements. Surface recombination velocity (S) of less than 20 cm/s has been obtained, which enables us to accurately evaluate the bulk lifetime of minority carriers, τb, in Ge wafers. To the best of our knowledge, this is the first report on wet chemical treatment successfully applied to Ge surfaces achieving low values of S. The effects of quinhydrone concentration and passivation time on the results of lifetime measurements are described and discussed.

Oxygen-atmosphere heat treatment in spin-on doping process for improving the performance of crystalline silicon solar cells

Spin-on doping of phosphorus has been investigated and applied for the emitter fabrication of crystalline Si solar cells.Heat treatment in oxygen atmosphere at relatively low temperature of 550 ° C prior to phosphorus diffusion is proved effective for improving solar cell performance, showing a conversion efficiency enhancement of more than 0.2% absolute. Internal quantum efficiency measurements show obvious enhancements at both short and long-wavelength regions. Secondary ion mass spectroscopy and Infrared absorption analysis reveal reduced C impurities after the heat treatment, possibly caused by burning the organic residues in the coated dopant source layer.

Thin-film silicon solar cells using an adhesive bonding technique

Thin-film silicon solar cells using an adhesive bonding technique have been investigated. Surface passivation effect due to the developed adhesive is observed and a very low surface recombination velocity is obtained. A 10-/spl mu/m-thick single-crystalline silicon solar cell is fabricated by adhesive bonding of a near-Lambertian alumina ceramic substrate. Open circuit voltage of 602 mV and short circuit current of 25.8 mA/cm/sup 2/ are obtained. Incident light is effectively confined by the adhesive bonding technique. The results obtained indicate that the adhesive bonding technique is suitable for realizing high-efficiency, low-cost, thin-film cell.

Evaluation of the Si-SiO2 Interface by the Measurement of the Surface Recombination Velocity S by the Dual-Mercury Probe Method

A novel evaluation technique of the surface recombination velocity S at the Si-SiO2 interface based on the dual-mercury probe method is proposed. It is shown that the experimentally obtainable S takes an extremely low value of less than 10 cm/s under both accumulation and inversion surface conditions whether high Dit exists or not, and the S has a pyramidlike peak from the weak inversion condition to the weak accumulation condition. These experimental results indicate that strong band bending at the interface and a well-prepared interface are quite effective in reducing the surface carrier recombination.

Palladium nanoparticle array-mediated semiconductor bonding that enables high-efficiency multi-junction solar cells

A detailed study on the application of Pd nanoparticle arrays, produced by self-assembled block copolymer templates, in bonding of III–V-based solar cell materials was carried out. The Pd nanoparticle array-mediated bonding (mechanical stacking) of GaAs-based thin-films (cells) was readily performed on the surface of GaAs or InP-based substrates (cells) to form multi-junction device architectures. Using the optimized Pd NP array, a 30.4%-efficiency four-junction two-terminal cell, consisting of an InGaP/GaAs top cell and an InGaAsP/InGaAs bottom cell, was achieved owing to the excellent electrical and optical bonding properties (bonding resistance, 1.81 Ω cm2; optical loss, 2.9%). Together with the verification of the long-term reliability of the Pd nanoparticle array-mediated bonding, our approach would become practically attractive for producing high-efficiency multi-junction solar cells.

Surface Passivation of Thin Silicon Solar Cells Using Silicon-on-Insulator Wafer

Solar cells with various cell thicknesses were fabricated using silicon-on-insulator (SOI) wafers, and effects of applied bias voltages (V B) at the SOI layer/SiO2 back interface on cell performance were investigated. A surface passivation effect was obtained by applying negative V B to accumulate holes at the interface. Both open-circuit voltage (V oc) and short-circuit current were improved by applying negative V B. This effect becomes more dominant for thinner cells. V oc of 20- and 50-µ m-thick cells at V B=-15 V increased compared to that of a 100-µ m-thick cell. Surface recombination velocity (S) was estimated from the dependence of internal quantum efficiency on cell thickness. It was found that S decreased from about 106 to 104 cms-1 when V B changed from 0 to -15 V. The improvement of cell performance by V B was due to this reduction in S. Therefore, the surface passivation by applying bias voltages is noted as an important technique to realize high-efficiency thin silicon solar cells.

Optical confinement in thin-film crystalline silicon solar cells by adhesive bonding of ceramic substrate

Thin film (5-/spl mu/m-thick) silicon solar cells by adhesive bonding of a near-Lambertian Al/sub 2/O/sub 3/ ceramic substrate have been fabricated, and the electrical and optical performance of the cells have been investigated. This cell structure shows a good optical confinement effect. From quantum efficiency measurements, it is found that a short circuit current that is as high as that of water-based cells can be obtained by reducing the surface reflectance. Cell and module fabrication processes can be simplified using this technique.