Yasuyoshi Kurokawa

Preparation of Nanocrystalline Silicon in Amorphous Silicon Carbide Matrix

We have successfully prepared silicon quantum dots/amorphous silicon carbide multilayers by the thermal annealing of stoichiometric hydrogenated amorphous silicon carbide (a-SiC:H)/silicon-rich hydrogenated amorphous silicon carbide (a-Si1-xCx) multilayers. Raman scattering spectroscopy and transmission electron microscopy (TEM) revealed that silicon quantum dots were formed in only a-Si1-xCx layers. We also found that the size of silicon quantum dots can be controlled by the thickness of a-Si1-xCx layers.

Photoluminescence from Silicon Quantum Dots in Si Quantum Dots/Amorphous SiC Superlattice

We prepared size-controlled silicon quantum dots superlattices (Si-QDSLs) by thermal annealing of stoichiometric hydrogenated amorphous silicon carbide (a-SiC:H)/silicon-rich hydrogenated amorphous silicon carbide (a-Si1+xC:H) multilayers. Transmission electron microscope (TEM) observation revealed that silicon quantum dots were formed in only a-Si1+xC:H layers. The size of silicon quantum dots can be controlled by the thickness of the a-Si1+xC:H layers. It was found that hydrogen plasma treatment (HPT) significantly enhanced the photoluminescence of the Si-QDSLs. The luminescence peaks shifted to shorter wavelength with decreasing the diameter of the silicon quantum dots in the Si-QDSL.

Numerical study of Cu(In,Ga)Se2 solar cell performance toward 23% conversion efficiency

The effects of conduction band grading in a Cu(In,Ga)Se2 (CIGS) thin film with an average bandgap of 1.4 eV on solar cell performance were investigated by changing the minimum bandgap (Egmin) and its position, employing the software wxAMPS. The calculation was carried out, taking CdS/CIGS heterointerface recombination into account, by incorporating a thin defective layer into the interface. For CIGS with a flat conduction band profile, i.e., without conduction band grading, the effects of the valence band offset (ΔEV) between a CdS layer and a CIGS layer with bandgaps from 1.05 to 1.6 eV were investigated. It was found that efficiency was increased by up to 3% by changing the conduction band profile from flat to double-graded, with a deep notch located in the vicinity of the CdS/CIGS interface. On the other hand, efficiency was increased by over 6% and reached 22% by increasing ΔEV up to 0.3 eV in the case of CIGS with a bandgap of 1.35 eV. Finally, an efficiency of 23.4% was achieved by combining a single-graded conduction band profile with a ΔEV of 0.3 eV. This result shows that a single-graded conduction band profile is required for high-efficiency wide-bandgap CIGS solar cells if the recombination at the CdS/CIGS heterointerface can be suppressed.

Improvement of carrier diffusion length in silicon nanowire arrays using atomic layer deposition

To achieve a high-efficiency silicon nanowire (SiNW) solar cell, surface passivation technique is very important because a SiNW array has a large surface area. We successfully prepared by atomic layer deposition (ALD) high-quality aluminum oxide (Al2O3) film for passivation on the whole surface of the SiNW arrays. The minority carrier lifetime of the Al2O3-depositedSiNW arrays with bulk silicon substrate was improved to 27 μs at the optimum annealing condition. To remove the effect of bulk silicon, the effective diffusion length of minority carriers in the SiNW array was estimated by simple equations and a device simulator. As a result, it was revealed that the effective diffusion length in the SiNW arrays increased from 3.25 to 13.5 μm by depositing Al2O3 and post-annealing at 400°C. This improvement of the diffusion length is very important for application to solar cells, and Al2O3 deposited by ALD is a promising passivation material for a structure with high aspect ratio such as SiNW arrays.

Optical assessment of silicon nanowire arrays fabricated by metal-assisted chemical etching.

Silicon nanowire (SiNW) arrays were prepared on silicon substrates by metal-assisted chemical etching and peeled from the substrates, and their optical properties were measured. The absorption coefficient of the SiNW arrays was higher than that for the bulk silicon over the entire region. The absorption coefficient of a SiNW array composed of 10-μm-long nanowires was much higher than the theoretical absorptance of a 10-μm-thick flat Si wafer, suggesting that SiNW arrays exhibit strong optical confinement. To reveal the reason for this strong optical confinement demonstrated by SiNW arrays, angular distribution functions of their transmittance were experimentally determined. The results suggest that Mie-related scattering plays a significant role in the strong optical confinement of SiNW arrays.

TiO2-Coated Transparent Conductive Oxide (SnO2:F) Films Prepared by Atmospheric Pressure Chemical Vapor Deposition with High Durability against Atomic Hydrogen

The durability of textured transparent conductive oxide (TCO) thin films against atomic hydrogen was investigated. An ultrathin TiO2 layer of 2 nm thickness was deposited on textured fluorine-doped tin oxide (SnO2:F) films, successively by atmospheric pressure chemical vapor deposition (AP-CVD). TCO films with a TiO2 layer showed a higher optical transmittance and a lower resistivity after exposure to atomic hydrogen excited by very high frequency (VHF) plasma, while TCO films without a TiO2 layer showed a lower optical transmittance and a higher resistivity after the exposure. These TCO films were characterized by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) before and after the exposure to atomic hydrogen.

Electrophoretic deposition of high quality transparent conductive graphene films on insulating glass substrates

Graphene is a promising material for transparent conductive films (TCFs) because of its high electrical conductivity and excellent optical transparency over a wide spectral range. We have previously reported on an inexpensive means of producing graphene-based TCFs using chemically derived graphene flakes. However, the deposition of chemically derived graphene can yield poor stacking of graphene flakes, which degrades the electrical conductivity of the resulting films. Here, we describe an alternative approach for producing large areas of TCFs based on electrophoretic deposition of graphene onto glass substrates using charged graphene oxide flakes. This method enabled the deposition of highly stacked graphene films onto insulating glass substrates with potential for TCFs.

Experimental and Theoretical Evaluation of Cu(In,Ga)Se2 Concentrator Solar Cells

Cu(In,Ga)Se2 (CIGS) solar cells which have yielded high performance devices under 1-sun were experimentally evaluated under various concentrated lights. The open-circuit voltage, fill factor, and efficiency of the fabricated solar cell under 6.6-suns were 728 mV, 0.770, and 20.3%, respectively. It was found that the efficiency of low performance CIGS solar cells was increased by the irradiation of concentrated light and was comparable to the efficiency of high performance CIGS solar cells. Theoretical simulation revealed that the increment of the recombination velocity toward the defect density in CIGS thin films were reduced under concentrated light, due to the compensation of defects by the large amount of carriers generated by irradiating concentrated light.

Deposition of Ag(In,Ga)Se2 Solar Cells by a Modified Three-Stage Method Using a Low-Temperature-Deposited Ag–Se Cap Layer

Ag(In,Ga)Se2 (AIGS) films with a uniform Ag depth profile were successfully deposited by a modified three-stage method in which a Ag–Se layer was pre deposited at a low temperature (350 °C) before a high-temperature process at around 600 °C. The Ag–Se layer acted as a cap layer and effectively prevented the desorption of In from the films during the high-temperature process. The In/(In+Ga) ratio of the AIGS films was found to be about 0.15. The best AIGS solar cell deposited by this method showed an active area conversion efficiency of 10.7%.

Control of valence band offset at CdS/Cu(In,Ga)Se2 interface by inserting wide-bandgap materials for suppression of interfacial recombination in Cu(In,Ga)Se2 solar cells

We inserted Cu(In,Ga)3Se5 into the CdS/Cu(In,Ga)Se2 interface of Cu(In,Ga)Se2 solar cells with a flat band profile and energy bandgaps (Eg) of 1.2 and 1.4 eV in order to investigate the repelling of holes by the effect of valence band offset (ΔEv). We found that open circuit voltage (VOC) was clearly improved from 0.66 to 0.75 V with Eg of 1.4 eV, although VOC was only increased from 0.63 to 0.64 V with Eg of 1.2 eV. For high efficiency, we fabricated Cu(In,Ga)Se2 solar cells with a single-graded band profile and an average Eg of 1.4 eV. Eventually, a conversion efficiency of 14.4% was obtained when Cu(In,Ga)3Se5 with a thickness of 30 nm was inserted, although the conversion efficiency was 10.5% without Cu(In,Ga)3Se5. These results suggest the importance of ΔEv in the suppression of interfacial recombination by repelling holes and possibility that the highest efficiency of Cu(In,Ga)Se2 solar cells with an average Eg of 1.4 eV could be achieved.