Kenichiro Wakisaka

High-quality polycrystalline silicon thin film prepared by a solid phase crystallization method

Abstract We succeeded in fabricating high-quality polycrystalline silicon (poly-Si) thin films with no boundary from the bottom surface to the top, and achieved an extremely high electron mobility of 808 cm2/V s by a solid phase crystallization (SPC) method. This film was obtained by using a new nucleation layer with 1000 A wide single-crystalline grains embedded in a matrix of amorphous tissue. A poly-Si thin-film solar cell fabricated using this film as an active layer demonstrated a total area conversion efficiency of 9.2% (active area efficiency: 9.7%), which is the worlds highest value for crystalline silicon solar cells fabricated below 600°C on metal substrates.

Comprehensive study of lateral grain growth in poly-Si films by excimer laser annealing and its application to thin film transistors

Lateral grain growth in nondoped poly-Si films was studied by using Si thin films (500 A) with different structures as a starting material for excimer laser crystallization. It was clarified that the lateral grain growth phenomenon (micron-size grains with strong (111) orientation) upon excimer laser annealing was strongly affected by both the microstructure and the orientation of the initial Si thin films. This result supports our previous speculation that the principal driving force of the lateral grain growth phenomenon is surface energy anisotropy. Poly-Si thin-film transistors using these films show a high field effect mobility of 440 cm2/Vs, achieved through a low-temperature process below 600° C. This excellent electrical characteristic is thought to be due to the large grain size of poly-Si thin film with controlled orientation, good crystallinity, and a smooth surface.

Principles for controlling the optical and electrical properties of hydrogenated amorphous silicon deposited from a silane plasma

The optical, electrical, and structural properties of hydrogenated amorphous silicon (a‐Si:H) films are systematically investigated as functions of the substrate temperature (Ts) and plasma parameters, such as the rf power, gas pressure, and electrode dimensions. The films are deposited by the plasma chemical vapor deposition method. The properties of a‐Si:H can be controlled over a wide range by varying the plasma parameters at fixed Ts. Reducing the film deposition rate and raising Ts have the same effect on the properties of a‐Si:H. A unified relationship is found to exist among those properties of a‐Si:H in the range of deposition conditions in this study, which includes ‘‘device‐quality’’ conditions. No apparent effects of gas‐phase polymerization or ion bombardment are observed. The experimental results suggest that during device‐quality a‐Si:H film deposition under conventional plasma conditions, the film properties are governed by a competition between the rate of film growth and the rate of therm...

Preparation of high-quality n-type poly-Si films by the solid phase crystallization (SPC) method

For further improvement of conversion efficiency in a-Si solar cells, it is necessary to develop materials with high photosensitivity in the long-wavelength region. A new solid phase crystallization (SPC) method was developed to grow a Si crystal at temperatures as low as 600°C. Using this method, high-quality thin-film polycrystalline silicon (poly-Si) with a Hall mobility of 70 cm2/Vs was obtained. Quantum efficiency in the range of 800 nm ~ 1000 nm was achieved up to 80% in an experimental solar cell using the n-type poly-Si with a grain size of about 1.5 µm. Therefore, it was found that our SPC method was suitable as a new technique to prepare high-quality solar cell materials.

More than 16% solar cells with a new 'HIT' (doped a-Si/nondoped a-Si/crystalline Si) structure

A HIT (heterojunction with intrinsic thin-layer) structure solar cell has been developed. In this structure, a nondoped a-Si thin layer was inserted between a p-type a-Si layer and an n-layer c-Si substrate. The open-circuit voltage and fill factor (FF) were significantly improved in these HIT structure solar cells compared with conventional p/n heterojunction solar cells. The improvement seems to originate in the reduction of backward current density. For higher efficiency, this HIT structure has been applied to textured substrates and achieved an efficiency of 18.1% (1 cm/sup 2/ cell). This efficiency is the highest value reported for a solar cell in which the junction was fabricated at a low temperature (120 degrees C). Application of this structure to the poly-Si thin film will yield a-Si/poly-Si thin-film solar cells of high efficiency.<<ETX>>

Low-Temperature Crystallization of Poly-SiGe Thin-Films by Solid Phase Crystallization

We investigated the low-temperature solid phase crystallizat ion of a-SiGe films and the hydrogen treatment effect on solid phase crystallized SiGe fil ms. We achieved a reduction in the crystallization temperature of a-Si 0.3Ge0.7 from 600°C to 500°C in an a-Ge/a-Si 0.3Ge0.7 bilayer structure where a-Ge film was used as a seed layer. This result will leads to the use of a low-cost substrate. P-type state density increased with increasing Ge cont nt for solid phase crystallized SiGe films. By performing hydrogen plasma treatment, non-doped SiGe fil ms neared the state of intrinsic film and P-doped SiGe film was changed from a p-type to an n-type semiconductor. Therefore, the p-type states of SiGe can be reduced by hydrogen plasma treat ment and are Ge related dangling bonds. These results suggest that solid phase crystallized SiGe fi lm is a preferable material for low-cost and large electronic devices. Introduction Polycrystalline silicon (poly-Si) thin films for large-area lectronic devices are usually fabricated by an excimer laser annealing (ELA) method. However, ELA is a re lativ ly high-cost method and is not suitable for fabricating large-area devices because of the difficulty of precise laser beam scanning and energy control for large areas. On the other hand, the solid-phase c rystallization (SPC) method is more easily employed for large-area devices [1], but conventional SPC for a-Si requires a high temperature (>600°C), which means that a low-cost substrate cannot b e used. In this report, first, we tried to reduce the SPC temperature by the use of silicon-ger manium (SiGe) thin films. The crystallization temperature of a-SiGe usually decreases w ith an increase in the Ge content [2]. We tried to achieve further reduction of the crystallization temperat ure of a-SiGe films by using an a-Ge seed layer. Second, we investigated the hydrogen treatment effe ct on SPC-SiGe in order to improve the quality of poly-SiGe films. Experimental a-SiGe films were deposited onto glass substrates by plasma enhance d chemical vapor deposition at 300°C. The pressure and RF power were 40 Pa and 5 W, respectively. P hosphorous (P)-doped films were also fabricated by using PH 3, and the PH3/(SiH4+GeH4) ratios were 20 ppm and 200 ppm. The Ge content of the SiGe was controlled by changing the SiH 4 and GeH4 flow rate ratio. The Solid State Phenomena Online: 2003-06-10 ISSN: 1662-9779, Vol. 93, pp 231-236 doi:10.4028/www.scientific.net/SSP.93.231

Solid-State Dye-Sensitized Solar Cells Using Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] as a Hole-Transporting Material

Solid-state dye-sensitized solar cells (DSSCs), consisting of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEHPPV) as a hole-transporting material, an indium tin oxide (ITO) electrode, and a titanium dioxide (TiO2) electrode, were studied with respect to the factors affecting their properties. The morphology of TiO2 electrodes, which was controlled by changing preparation conditions, affected the performance of solid-state DSSCs, and cells with a TiO2 electrode having a smooth surface showed better properties. The carrier transportation between the TiO2 layer and the MEHPPV layer was one of the most important factors that determined the overall efficiency of the solar cells. This carrier transportation process was improved by the addition of an interlayer consisting of potassium iodide (KI) and iodine (I2). In addition to improving the carrier transportation, this KI/I2 blend interlayer also improved the pore filling of the solid-state DSSC. By controlling these parameters, a solid-state DSSC was obtained, with a short-circuit current density of about 1.51 mA/cm2, an open circuit voltage of about 0.65 V, a fill factor of about 0.5, and an energy conversion efficiency of about 0.51%.

Effects of very high hydrogen dilution at low temperature on hydrogenated amorphous silicon germanium

The effects of hydrogen dilution of up to 54:1 (=H2: SiH4) on hydrogenated amorphous silicon germanium (a-SiGe:H) were investigated at substrate temperatures 10−5 Ω−1 cm−1 and silicon dihydride content (<2 at.%) of a-SiGe:H can be maintained with a high hydrogen dilution ratio of 54:1, although these properties deteriorate with our conventional low hydrogen dilution conditions at a substrate temperature range <200°C. And this high-quality a-SiGe:H film was applied to the bottom photovoltaic layer of a glass superstrate type a-Si/a-SiGe tandem solar cell submodule (30 cm×40 cm), and a stabilized efficiency of 9.5% (light-soaked and measured at Japan Quality Assurance organization (JQA)) was achieved.

Efficiency evaluation of a-Si and c-Si solar cells for outdoor use

We have achieved the worlds highest stabilized efficiency of 8.9% for an a-Si single-junction solar cell (1 cm/sup 2/) and 10.6% for an a-Si/a-SiGe tandem solar cell (1 cm/sup 2/). To apply these results to practical outdoor power use, the annual output power of a-Si single-junction (a-Si) and tandem (a-Si/a-Si) solar cells and a crystalline silicon (c-Si) solar cell was calculated based on annual meteorological data and the solar spectrum in Tokyo, Japan, including the effect of the output power dependence on temperature, incident irradiance and solar spectrum. As a result, this simulation revealed that the annual change of efficiency for the c-Si solar cell is most affected by the solar spectrum among the three types of solar cells, and the annual fluctuations of the a-Si and a-Si/a-Si solar cells are mostly caused by recovery by the annealing effect.

Investigation of Hydrogenated Amorphous Silicon Germanium Fabricated under High Hydrogen Dilution and Low Deposition Temperature Conditions for Stable Solar Cells

The effects of hydrogen dilution of up to 54:1 (=H2:SiH4) on hydrogenated amorphous silicon germanium (a-SiGe:H) were investigated while keeping the optical gap (Eopt) constant. It was found that deterioration of the film properties of a-SiGe:H due to a decrease in substrate temperature (Ts) can be compensated by the high hydrogen dilution method. As Ts decreases from 230°C to 180°C, the high photoconductivity [~1×10-5 (Ωcm)-1] and low silicon dihydride content (~1 at.%) of a-SiGe:H can be maintained with a high hydrogen dilution ratio of 54:1, although these properties deteriorate with the conventional low hydrogen dilution ratio of 2.5:1. Probably, hydrogen radicals supply the energy required for the surface reaction during a-SiGe:H deposition which is lost when Ts is decreased. This tendency is useful for solar cell fabrication, especially for superstrate-type a-Si/a-SiGe tandem solar cells, because the decrease in the deposition temperature of a-SiGe:H for the bottom photovoltaic layer can reduce damage to the underlying layers caused by a high deposition temperature. As a result of applying this technique to the fabrication process of an a-Si/a-SiGe stacked solar cell submodule (area: 1200 cm2), the worlds highest stabilized efficiency of 9.5% (light-soaked and measured at JQA) was achieved.