Genshiro Nakamura

Tandem type amorphous solar cells

Abstract An efficiency of 8.6 % by the three-stacked tandem type amorphous solar cell which consists of a-Si:H and a-SiGe:H with a size of 10 by 10 cm was obtained. This tandem type solar cell showed stable characteristics for the prolonged light exposure.

Study of Tin Diffusion into Silicon by Backscattering Analysis

Diffusion of tin into Si from tin-doped oxide is studied by the backscattering and channeling analyses. The depth distribution of tin in Si can be fitted to complementary error functions. The diffusion coefficient can be expressed as D=0.054exp (-3.5eV/kT). The solubility limit in Si and the segregation constant between Si and SiO2 are estimated to be 6~8×1019/cm3 and 0.066 at 1100°C~1200°C, respectively. The lattice location of tin in Si is also determined by angular scan, and it is found that more than 90% of tin atoms occupy substitutional sites. Uniformity of the concentration over a silicon wafer is also checked with the microanalysis system by using backscattering and is found to be good. Effect of post-diffusion of phosphorous or boron on the depth profile of tin is also studied.

Amorphous SiGe: H for High Performance Solar Cells

A preliminary study has been carried out on electrical and optical properties of amorphous silicon-germanium mixed films (a-SiGe: H) deposited by a glow discharge plasma reaction. As a germanium content in the film increases, an optical absorption coefficient increases and an optical gap energy Egopt. decreases. Growth rate of a-SiGe: H films increases with the germanium content in the film and reaches to the value for pure a-Ge: H films about 3 times larger than that of a pure a-Si: H film. Film properties were examined by dark- and photoconductivity, MOS FET transistor characteristics, and photoluminescence. Photoconductivity and carrier mobilities decreases drastically when the mole ratio GeH4/(GeH4+SiH4) exceeds 25%. Solar cells of various structures using a-SiGe: H and a-Si: H films were fabricated and compared their photovoltaic properties with that of a simple p-i-n type a-Si: H solar cell. Tandem structure with a-Si: H p-i-n and a-SiGe: H p-i-n cells shows a broadened spectral response at long wave length region and an open circuit voltage of 1.13 V, about twice of a single cell.

Staebler-Wronski effects in hydrogenated amorphous Si1−xGex

Abstract Photo-induced defects in hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous Si1−xGex (a-Si1−xGex:H) created by prolonged light exposure and their effecton opto-electronic properties are studied. The nearly constant carrier lifetime in a-Si1−xGex:H with increasing germanium content shows that the states created by germanium dangling bonds in as-deposited a-Si1−xGex:H are situated near midband and that the spread of their distribution within the band gap may be small compared with that of the photo-induced metastable defect states. A relatively small Staebler-Wronski effect is observed in a-Si1−xGex:H. However, the density of photo-induced states in a-Si1−xGex:H is of about the same order of magnitude as that in a-Si:H. The reason for this seems to be that the large density of states created by germanium dangling bonds masks the effect of light-induced defects.

Arsenic-Implanted Emitter and Its Application to UHF Power Transistors

The basic feature of heavy dose arsenic implantation is studied with an aim of the application to shallow emitters of bipolar devices. Secondary defect caused by heavy dose arsenic implantation is significantly affected by an annealing atmosphere. A TEM observation and an electrical measurement show that the high temperature annealing in an N2 atmosphere results in a high quality crystalline layer. The arsenic concentration profile measured by a He+ backscattering method is well predicted by the computer simulation with the concentration dependent diffusivity. The transistor with arsenic-implanted emitter and boron-implanted base has shown to have negligible emitter-push effects. A UHF power transistor developed by using arsenic implantation is characterized by a power gain of 5 dB with an associated collector efficiency of 60%, and a saturated power of 22W, at 900 MHz with 12.5 V dc supply voltage.

Amorphous Solar Cells Using a-Si: H and a-SiGe: H Films

The electric properties of optimized a-SiGe: H films deposited by an R.F. glow discharge in (SiH4+GeH4) and application of these films to the tandem type solar cells were investigated. The dark-conductivity of a-SiGe: H films increase with increasing Ge content and follows quite well to the Meyer-Neldel rule with measured activation energies. Fermi level which was deduced from the activation energy shifts toward mid-band gap under the optimized growth condition. The ratio σph/µn which is related to the lifetime of photogenerated carriers was found nearly constant for various Ge contents in the a-SiGe: H films. Inverted P-I-N type amorphous solar cell with an intrinsic a-SiGe: H film shows higher spectral response in the red region than other kind of a-Si: H aolar cells. The highest conversion efficiency of 8.5% has been obtained by the three junctions stacked cell composed of two inverted a-Si: H P-I-N cells and one inverted P-I-N cell with an intrinsic a-SiGe: H film.

High reliability tandem type amorphous solar cells fabricated by multi reaction chamber system

Abstract Mechanical stress and hydrogen bonding states are investigated in hydrogenated amorphous silicon binary alloys used for tandem type amorphous solar cells. Magnitude of compressive stress is found to be proportional to the concentration of SiH bonds, content of germanium, and volume fraction of μc-Si in the amorphous network, and inversely proportional to the concentration of SiH 2 bonds, (SiH 2 ) n bonds, phosphorus atoms and boron atoms.

Amorphous Si1_xGex for High Performance Solar Cell

Amorphous silicon germanium alloy (a-SiGe:H) films have been studied to obtain a lower optical gap material for a tandem cell structure. The optical gap energy can be controlled in the range between 1.9 5 eV and 1.14 eV by addition of Ge in to a-Si:H during deposition process. However, as the Ge content in the film increases, photoconductivity and carrier mobilities decrease.

Broadening of Spectral Response Width by a-Si/a-SiGe p-i-n type Solar Cells

We report the MIS type a-Si:H solar cells, plasma analysis using quadrupole mass analyzer and tandem type solar cell which improved spectral response in the long wavelength region. MIS type a-Si:H solar cells inserting TiOx layer as an insulator show the conversion efficiency of 4.4%. The improvement of the collection efficiency in the short wavelength region were accomplished by decrease in interface state density with addition of TiOx film. Plasma polymerized species, (SiHm+)2, increased with the rf power up to 40–50 W and decreased as a result of the decomposition to (SiHm+) species, but in the case of GeH4 reaction, (GeHm+)2 species decreased monotonously with rf power. Suitable condition of a-SiGe:H films deposition are related to the behavior of the polymerization and decomposition of SiH4 and GeH4 gases, and high quality films are obtained at relatively low rf power condition before (GeHm+)2 decomposition. Tandem type amorphous solar cell shows 3.2% coversion efficiency and the collection efficiency in the long wavelength region are clearly improved.

Photo induced effects on a-Si1−xGex:H films

Abstract Mechanical stresses on two types of a-SiGe:H films prepared from (SiH4GeH4) and from (Si2H6GeH4) are compared. We observed some correlation between the magnitude of stresses and the light induced degradation in these films. Comparing IR spectra in these films, it was found that the sign and magnitude of stress has a strong dependence on the chemical bonding structure between Si and H.