Tadashi Saitoh

Optical and electrical properties of amorphous silicon films prepared by photochemical vapor deposition

Amorphous silicon films have been prepared through mercury‐photosensitized decomposition of monosilane gas at low temperatures. The films show optical and electrical properties comparable with those of the best films prepared by plasma chemical vapor deposition. The feasibility of amorphous solar cells with short‐circuit current densities of more than 10 mA/cm2 has been demonstrated by fabrication of a Schottky barrier structure.

Impurity gettering of polycrystalline solar cells fabricated from refined metallurgical-grade silicon

A damage-gettering technique is described which reduces the impurity content in grown crystals and enhances cell performance of diffused solar cells. Crystalline ingots were Czochralski-grown from an acid-leached metallurgical-grade source. Damage gettering was performed by preparing a mechanically damaged layer on the wafer back surface and subsequent annealing. Optimum annealing conditions were investigated as a function of ambient gas species, temperature, and time. In an O2ambient, the fill factor of the cells degraded to 0.25, while cell performance was greatly improved by annealing in N2. Conversion efficiency tends to increase with annealing time at higher temperatures. Maximum conversion efficiencies attained for mono- and polycrystalline solar cells fabricated from MG-Si are 9.8 and 7.7 percent, respectively. Light current-voltage characteristics and the leakage-current variations with depth were analyzed. It was found that impurity gettering begins at the wafer surfaces and proceeds gradually into the bulk regions.

Preparation and properties of microcrystalline silicon films using photochemical vapor deposition

Abstract microcrystalline silicon films have been prepared through mercury photosensitized decomposition of monosilane at low gas pressures. The dark and light conductivities of the silicon films tend to increase at reactant pressures lower than 65 Pa and become 10 −2 Ω −1 · cm −1 at 26 Pa. From the Raman scattering and x-ray diffraction, silicon films were found to consist of a mixed phase structure including both microcrystalline and amorphous regions.

Photochemical vapor deposition of amorphous silicon through 185 nm excitation of monosilane

Abstract Photochemical vapor deposition of a-Si films at a high rate using SiH 4 and a 185 nm low pressure mercury lamp is described. A maximum rate of 1 nm/sec was attained using the 185 nm lamp. This rate was about ten times higher than that using a 254 nm lamp. Assuming that there is no interaction between the effects of the two wavelengths, the deposition rate per light output power of 184.9 nm light is 160 times larger than that for 253.7 nm light. The absorption cross-section of the 184.9 nm light is ten times greater than that for the 253.7 nm light.

Optimization of High-Efficiency n + - p - p + Back-Surface-Field Silicon Solar Cells

High-efficiency silicon solar cells are fabricated with back-surface-field (BSF) structures using medium-resistivity, float-zone substrates. The cell processes are optimized using a new BSF process in order to maintain a high minority-carrier lifetime. In addition, they are improved with a passivated, V-grooved surface to enhance collection efficiency. The resultant cells exhibit conversion efficiencies of 19.1% with a short-circuit current density of 38.4 mA/cm2. The minority-carrier lifetime and surface recombination velocity are estimated to be 341 µs and 3×105 cm/s using a computer program.

Multicolor light-emitting diodes with double junction structure

A GaAs:Si-GaAsP heterostructure chip coated with NaYF 4 :Yb,Er phosphor has been fabricated to demonstrate multicolor operation. The one-chip light-emitting diodes (LEDs) with double junctions showed a centrosymmetrical distribution of green-and red-light intensities. An intermediate hue between green and red was achieved by adjusting the thickness of the phosphor. The luminance of each color was more than 100 fL at a current density of 30 A/cm2and the power efficiency was 2-3 × 10-4for green. The luminance of the green light was less than 250 fL at 20 A/cm2for a single GaAs:Si LED because of the refraction of infrared light in the GaAsP layer.

Growth and characterization of polycrystalline silicon ingots from metallurgical grade source material

Abstract Silicon crystals are grown from 98% pure metallurgical-grade source material by the Czochralski technique in an attempt to apply them to solar cells. Their crystallinity, impurity content and electrical properties are investigated. Inclusions, observed in the crystals grown at higher rates than 1 mm/min, or when the solidified fraction is over 0.4, are identified as Si-A1 alloys by electron probe micro-analysis. The impurity concentration in the crystals depends mainly on the growth rate. The growth rate of 0.5 mm/min is found to be the optimum for preparing relatively pure crystals. The impurity concentration cannot be decreased to the level expected from the segregation coefficient. This reason is considered to be due to the existence of small-sized inclusions and the formation of cellular structure.

Multi-color light emitting diodes with a double junction structure

A GaAs:Si-GaAsP heterostructure chip coated with NaYF4:Yb,Er phosphor has been fabricated to demonstrate multicolor operation. The one-chip light-emitting diodes (LEDs) with double junctions showed a centrosymmetrical distribution of green-and red-light intensities. An intermediate hue between green and red was achieved by adjusting the thickness of the phosphor. The luminance of each color was more than 100 fL at a current density of 30 A/cm2and the power efficiency was 2-3 × 10-4for green. The luminance of the green light was less than 250 fL at 20 A/cm2for a single GaAs:Si LED because of the refraction of infrared light in the GaAsP layer.

A new cell structure for very thin high-efficiency silicon solar cells

The cell has a corrugated structure, which is formed by aligned V grooves on both the front and back surfaces. The substrate thickness is reduced to 50 mu m while retaining high mechanical strength. This permits ease of handling during the fabrication process and subsequent procedures. This thin substrate promises a very high open-circuit voltage, and the structure is also beneficial to high optical performance. The surface reflectance is reduced in the same manner as that of V-grooved cells, but the optical path is lengthened by a minimum of four times the substrate thickness. Performance of experimental cells is also discussed. >

Highly efficient, large-area polycrystalline silicon solar cells fabricated using hydrogen passivation technology

Highly efficient, large area polycrystalline silicon solar cells have been fabricated using ion implantation as a hydrogen passivation technique. A new high-current ion implanter with a bucket-type ion source has been developed to hydrogenate crystal defects in cast polycrystalline cells. Effective hydrogen passivation of the defects has been realized by implanting hydrogen ions into cast cells from the back surfaces and increasing the hydrogen ion energy and dose. A scanning light beam-induced current image of the polycrystalline cells shows that lowering of the induced current distribution at linear grain boundaries is almost completely eliminated, but not at irregular grain boundaries. The resultant polycrystalline cells exhibit a high conversion efficiency of 15.2% for a large area of 100 cm2.