Mitsuru Ichikawa

Polycrystalline Cu(InGa)Se2 Thin-Film Solar Cells with ZnSe Buffer Layers

A ZnSe buffer layer has been applied as an attractive alternative to a CdS buffer layer in the development of polycrystalline Cu(InGa)Se2 (CIGS) thin-film solar cells, thus eliminating entirely the use of cadmium by employing the ZnO/ZnSe/CIGS structure. Moreover, we propose the use of a new deposition method for ZnSe buffer layers, the atomic-layer deposition (ALD) method. This method is basically the same as an atomic-layer epitaxy method but is applied to polycrystalline materials. Currently the best efficiency of CIGS thin-film solar cells with an about 10-nm-thick ZnSe buffer layer is 11.6%. Applying irradiation with a solar simulator under one-sun (AM-1.5, 100 mW/cm2) conditions, the efficiency of these cells was improved from about 5% to over 11% due to increased open-circuit voltage and fill factor with no change in short-circuit current density even after six-hour irradiation.

Spectral Effects of a Single-Junction Amorphous Silicon Solar Cell on Outdoor Performance

The device current–voltage (I–V) parameters of single-junction amorphous Si (a-Si) solar cells in natural sunlight at various locations have been investigated over three years. The effective efficiency was compared among testing locations from the latitude of 12.8 degrees to 43.8 degrees. The effective efficiency was varied periodically and the most advantageous performance was obtained in summer at all locations. The increase of output current dominantly contributed to the excellent performance. The output current of single-junction a-Si modules depended not only on module temperature and the annealing effect but also the seasonal variation of the air mass and the atmospheric conditions, such as turbidity and water vapor at each location. The annual performance ratio to initial efficiency measured under standard test conditions reached 82% in Hamamatsu, Japan.

Spectral Characteristics of Thin-Film Stacked-Tandem Solar Modules

The device current–voltage (I–V) parameters of thin-film silicon stacked-tandem solar modules consisting of amorphous and microcrystalline silicon have been investigated under various spectral irradiance distributions. Spectrometric characterization method by the current mismatch perturbation has been developed using a large-area multisource solar simulator consisting of Xe lamps and halogen lamps with adequate optical filters. This method enables us to reveal the current-limited component cell and to specify the most suitable spectral distribution for tandem modules to obtain the best performance. Practical procedures for determining the irradiance level for a given single-source solar simulator have been demonstrated in order to carry out I–V measurement for tandem cells under nearly standard test conditions (STC). We have determined the differential of I–V parameters at various spectral distributions and found that the procedure using a reference tandem cell and its efficiency at STC could minimize variation from the true value even if the spectral distribution deviates from the reference.

Optimization of Device Design for Thin-Film Stacked Tandem Solar Modules in Terms of Outdoor Performance

The device current-voltage (I-V) characteristics of thin film silicon stacked tandem solar modules (HYBRID modules) consisting of a hydrogenated amorphous silicon (a-Si:H) cell and a thin-film microcrystalline silicon solar cell (µc-Si) have been investigated under various spectral irradiance distributions. The spectrometric characterization had revealed that the performance of the HYBRID module exposed to sunlight tended to be limited by the top a-Si:H cell due to the photodegradation of the top cell, and the bottom µc-Si cell of the modules was stable as before exposure. The HYBRID module limited by the top cell exhibited a more efficient performance than the bottom-limited module in natural sunlight at noon. This behavior is well explained by the contour analysis of the performance as a function of spectral irradiance ratio and module temperature.

High Deposition Rate of Polycrystalline Silicon Thin Films Prepared by Hot Wire Cell Method

The hot wire (HW) cell method has been newly developed and successfully applied to grow polycrystalline silicon films at low temperatures with a relatively high growth rate of 0.9–1.1 nm/s. In the HW cell method, mono silane (SiH4) is decomposed by reacting with a heated tungsten wire placed near the substrate. It is found that polycrystalline silicon films can be obtained at substrate temperatures of 175–400°C without hydrogen dilution when the deposition pressure is 0.1 Torr.

Amorphous-to-Polycrystalline Silicon Transition in Hot Wire Cell Method

Hot Wire Cell method has been newly developed and successfully applied to grow polycrystalline silicon films at a low temperature with a relatively high growth rate. In the Hot Wire Cell method, reactant gases are decomposed as a result of reaction with a heated tungsten filament placed near a substrate and polycrystalline silicon films can be deposited at a growth rate of 0.9 nm/s without hydrogen dilution. The film crystallinity is changed from polycrystalline to amorphous by decreasing the total pressure. The model calculation of the Hot Wire Cell method is carried out and it is assumed that transition of crystallinity may be due to the shift in the preferential impinged radicals. X-ray analysis clearly showed that the films grown at the filament temperature of 1700°C have a very strong (220) preferential orientation. The films consist of large grains as well as small grains. These results suggest that the Hot Wire Cell method is a promising candidate to grow device-grade polycrystalline silicon films for photovoltaic application.

Advanced Light Trapping of High-Efficiency Thin Film Silicon Solar Cells

Kaneka has been proposing and developing the advanced super-light trapping (ASLT) structure for thin film silicon solar cells, which incorporates tailored light confinement structures in tandem thin film silicon solar cells to selectively enhance light trapping in top and bottom subcells. In this paper, we present the results of the development of intermediate reflectors with very low refractive index and the design and implementation of nanoimprinted substrates. We demonstrate that significant current gains are possible by reducing the refractive index of the interlayer beyond state-of-the-art values. In addition, we show that the transparent conductive oxide angular scattering properties correlate with the solar cell performance in the infrared part of the spectrum.

High-rate deposition of polycrystalline silicon thin films by hot wire cell method using disilane

Abstract A new process, hot wire cell method, was developed and successfully used to grow polycrystalline silicon thin films at a low-temperature and high deposition rate. In the hot wire cell method, reactant gases are decomposed by a heated tungsten filament. Polycrystalline silicon films can be deposited at deposition rates of 1.2xa0nm/s for mono-silane (SiH 4 ) and 2.8xa0nm/s for disilane (Si 2 H 6 ). By using disilane as a reactant gas, it is possible to achieve a high deposition rate without any change in the quality of the films.

Hydrogen Dilution Effect on the Crystallinity of Silicon Films Grow by Hot Wire Cell Method

A new process, the Hot Wire Cell method, was developed and successfully used to grow polycrystalline silicon thin films at a low temperature and high growth rate. In the Hot Wire Cell method, reactant gases are decomposed as a result of reacting with a heated tungsten filament placed near to a substrate and polycrystalline silicon films can be deposited at a growth rate of 1.2 nm/s without hydrogen dilution and 0.9 nm/s with the use hydrogen dilution. The film crystallinity changed from amorphous to polycrystalline due to the addition of hydrogen, thus hydrogen dilution was effective for improving film crystallinity. Furthermore, the authors obtained (220) oriented polycrystalline silicon thin films with a 90% crystal fraction by the use of hydrogen dilution. These results showed that the Hot Wire Cell method is promising for the deposition of device-grade polycrystalline silicon films for photovoltaic applications.

Comparison of gas-phase reactions in low-temperature growth of Si films by photochemical vapor deposition and the hot wire cell method

Abstract Mercury sensitized photochemical vapor deposition and hot wire cell method were studied as growth techniques of Si films at low temperature ( 1 nm/s, which was about 10 times larger than that of the films grown by photochemical vapor deposition. This method can generate a larger flux of SiH3 and the flux of H is also large enough to induce crystallization on glass substrate.