Kunimoto Ninomiya

High-Quality Wide-Gap Hydrogenated Amorphous Silicon Fabricated Using Hydrogen Plasma Post-Treatment.

The hydrogen plasma post-treatment of hydrogenated amorphous silicon (a-Si:H) has been investigated to obtain high-quality wide-gap films. The hydrogen plasma treatment after film deposition substantially increases the hydrogen content and the optical gap of a-Si films without deteriorating their optoelectric properties within the range of treatment conditions in this study, where no microcrystallization of the films is observed. A photoconductivity of ~10-5 Ω-1 cm-1 and a photosensitivity (the ratio of photoconductivity to dark conductivity) of >106 are obtained for a-Si:H films with an optical gap of >1.7 eV from the (αhν)1/3 plot (>2.0 eV from Taucs plot) under AM-1, 100 mW/cm2 illumination. An extremely high open circuit voltage of >1 V is obtained for an a-Si single-junction cell whose i-layer was fabricated using the hydrogen plasma treatment.

Preparation and Properties of a-Si Films Deposited at a High Deposition Rate under a Magnetic Field

An rf plasma decomposition of SiH4 under a magnetic field was investigated. It was confirmed by the optical emission spectra that a high-electron-density plasma can be produced under a magnetic field. High-quality a-Si films with a photosensitivity of σph/σd of 7×105 were obtained at a high deposition rate of 10 A/s under the magnetic field. The a-Si solar cells with i-layers deposited at a high deposition rate under a magnetic field have a higher open-circuit voltage and a higher conversion efficiency than those without the magnetic field; a conversion efficiency of 10.1% under AM1(100mW/cm2) illumination was obtained at a deposition rate of 10 A/s. The rf plasma decomposition of SiH4 under a magnetic field is thought to be very suitable for fabricating a-Si solar cells with a high conversion efficiency at a high deposition rate.

Amorphous silicon-carbon alloy prepared by the CMP (controlled plasma magnetron) method

Hydrogenated amorphous silicon-carbon (a-SiC) films were fabricated using a novel plasma CVD method, called the CPM (Controlled Plasma Magnetron) method. The films obtained had a large Si-C bond density and a low Si-H bond density. The absorption coefficient at the photon energy level of 2.5 to 3.5 eV for these films was one order of magnitude lower than that for conventional a-SiC films. Furthermore, the dark conductivity of boron-doped films increased to more than 10-4 (Ωcm)-1, which is the highest value achieved for a-SiC with a carbon content of 55%.

High-Rate Deposition of a-Si:H Film with a Separated Plasma Triode Method

The separated plasma triode (SPT) method has been developed to deposit high-quality a-Si:H films at high deposition rates. Plasma properties were measured by a probe method and an optical emission spectroscopy (OES) method. It is possible to improve the electron density in a plasma without increasing the plasma potential near the substrate by the SPT method. High-quality a-Si:H films and high-performance a-Si solar cells were obtained at high deposition rates using the SPT method.

Transient Light-Induced ESR Investigations of the Role of Hydrogen in the Stability of a-Si:H

Transient light-induced electron spin resonance (LESR) at 120 K has been used to investigate the light-soaking behaviors and the role of hydrogen in the stability of a-Si:H through changes in the lineshape. Dramatic changes occur in the LESR lineshape upon prolonged light-soaking, and we suggest that hydrogen defects (Si-H2 or clustered Si-H bonds) play an important role in these changes. A microscopic explanation of the possible optical excitations and defect conversion processes leading to these changes during the LESR experiment is proposed.

Low-Hydrogen-Content, Stable Amorphous Silicon Thin Films Prepared by Ion-Assisted Method

Low-hydrogen (H)-content (≤10%) amorphous silicon (a-Si) films have been prepared by the hydrogen ion- and atom-assisted ionized cluster-based deposition method. In this technique, hydrogen content can be controlled independently and the optical gap (E opt3) of 1.5 to 1.15 eV can be obtained within an acceptable range of substrate temperature (180 to 230° C). The variation of optoelectronic properties with process parameters has been discussed. The stability against light exposure of these low-H-content films has been verified. The films are not susceptible to light exposure; however, for higher H content, film properties degrade, albeit slightly. The degradation behavior has been compared with that of a hydrogenated amorphous silicon germanium (a-SiGe:H) film with the same E opt3 (1.31 eV) and the superiority of low-H-content a-Si with regard to stability was indicated.

Approaches for stable multi-junction a-Si solar cells

Abstract Improvements in the stabilized efficiency of single-junction and multi-junction amorphous silicon (a-Si) solar cells have been studied. The stabilized efficiency, or efficiency after light exposure, of a-Si solar cells can be improved by suppressing impurities such as oxygen and nitrogen in the intrinsic (i-) layer of the cells. It has also been found that the defect density of a-Si films and conversion efficiency of a-Si solar cells both at the initial state and the stabilized state are strongly correlated with the content of hydrogen having an SiH2 configuration rather than with the total hydrogen content in a-Si i-layers. This strongly suggests that some measures are still available to reduce the stabilized Nd of the a-Si i-layer while conserving the suitable optical properties. A practical estimation method, which considers the variation of the I–V characteristics of each component cell, is also developed in order to optimize multi-junction cells. Stabilized efficiencies of 8.8% for a single-junction a-Si solar cell and 10.6% for an a-Si/a-SiGe double junction solar cell have been achieved for 1 cm2 cells.

High-Quality Microcrystalline SiC Films Fabricated by the Controlled Plasma Magnetron Method

This paper describes our investigations on high-efficiency a-Si solar cells and high-quality microcrystalline ( μc) SiC films fabricated by the CPM (Controlled Plasma Magnetron) method.

Control of a-SiGe:H film quality with regard to its composition

Abstract The quality of --SiGe:H films was systematically investigated with regard to their composition, including not only the Ge content ( C Ge ) but also the H content ( C H ), which influences the quality of the a-Si:H films. The optical gap of an a-SiGe:H film with device-quality is represented by a linear function of only C H and C Ge . An optimum composition of C H and C Ge exists for a certain optical gap from the viewpoint of both film properties and cell performance. Moreover, taking light-induced degradation into account, the optimum composition shifts to the low C H direction. In the case of an a-SiGe single-junction cell in which most of the i-layer had an optical gap ( E opt3 ) of 1.32 eV, the conversion efficiency under red light ( ψ > 650 nm) reached the maximum value with the composition of C Ge = 40 at.% and C H = 9 at.% for the initial state, and with the different composition of C Ge = 36 at.% and C H = 7 at.% for the degraded state.