Masao Isomura

The Influence of the Si-H2 Bond on the Light-Induced Effect in a-Si Films and a-Si Solar Cells

The influence of the Si-H2 bond on light-induced degradation and the thermal recovery of a-Si films and a-Si solar cells were studied. The influence of the Si-H2 bond on light-induced degradation depends on the impurity content in a-Si films, and light-induced degradation can be reduced by decreasing the Si-H2 bond density in a-Si films with impurity content of 1018 cm-3. The activation energy of the thermal recovery process was about 1.0 eV, and it did not depend on the Si-H2 bond density. However, an irreversible phenomenon was observed in film properties and solar cell characteristics with high Si-H2 bond density. It is thought that the structural flexibility of the Si-H2 bond is related to this irreversible phenomenon.

Preparation and Properties of High-Quality a-Si Films with a Super Chamber (Separated Ultra-High Vacuum Reaction Chamber)

A separated ultra-high vacuum (UHV) reaction chamber system, called the super chamber, has been newly developed. A background pressure of 10-9 Torr was obtained, and the impurity concentrations of oxygen, nitrogen and carbon in an a-Si film fabricated in the super chamber were 2×1018 cm-3, 1×1017 cm-3, and 2×1018 cm-3, respectively. The space charge density and the ESR spin density of the a-Si film were 5×1014 cm-3 and 2×1015 cm-3, respectively. These values were much lower than those for films fabricated in a conventional chamber. The ratio of the light-induced degradation in the photoconductivity of the a-Si film was also small compared with that of conventional a-Si films. A conversion efficiency of 11.7% was obtained for a glass/textured TCO/pin/Ag a-Si solar cell, whose i-layer was fabricated in the super chamber.

Influence of alloy composition on carrier transport and solar cell properties of hydrogenated microcrystalline silicon-germanium thin films

The authors report on carrier transport properties and spectral sensitivities of hydrogenated microcrystalline silicon-germanium (μc-Si1−xGex:H) alloys fabricated by low-temperature (∼200°C) plasma-enhanced chemical vapor deposition over the wide compositional range. Hall-effect and conductivity measurements reveal a change from weak n-type to strong p-type conduction for x>0.75 and a monotonic decrease in photoconductivity upon Ge incorporation. In a p-i-n diode structure, the Ge incorporation into i layer reduces quantum efficiencies in the short wavelengths, indicating an increased photocarrier recombination at p∕i interface. Nevertheless, under reverse biased condition, a 0.9-μm-thick μc-Si0.6Ge0.4:H absorber yields a large photocurrent of >27mA∕cm2 (air mass 1.5 global) with spectral sensitivities extending into infrared wavelengths, offering a potential advantage over conventional microcrystalline silicon solar cells.

Microcrystalline Si1-xGex Solar Cells Exhibiting Enhanced Infrared Response with Reduced Absorber Thickness

Thin film p–i–n junction solar cells incorporating hydrogenated microcrystalline Si1-xGex (µc-Si1-xGex:H) absorber i layers (1 µm) have been fabricated by plasma-enhanced chemical vapor deposition in the composition range of 0≤x≤0.35. By increasing Ge content from x=0 to 0.15–0.2, short-circuit current density increases by ~5 mA/cm2 with spectral sensitivities extending into the infrared wavelengths (>600 nm). However, solar cell parameters for larger Ge contents (x>0.2) are lowered by the increased charge carrier recombination in the µc-Si1-xGex:H i layer. As a result, a 6.3% efficient solar cell is obtained at x=0.2, exhibiting infrared response even higher than that of double-thickness µc-Si:H solar cells. The solar cell shows excellent performance stability under prolonged light soaking.

Influence of Oxygen and Nitrogen in the Intrinsic Layer of a-Si:H Solar Cells

The effects of oxygen and nitrogen on hydrogenated amorphous silicon (a-Si:H) films and solar cells are systematically studied in the initial and light-soaked states. The oxygen and nitrogen concentrations were varied from 1018 to 1021 cm-3 and 1016 to 1019 cm-3, respectively. In the initial state, dark conductivity and photoconductivity increase, and the activation energy of dark conductivity decreases with increase in the oxygen and nitrogen concentrations, because of the creation of donorlike states. These film properties after light-soaking, however, are independent of the oxygen and nitrogen concentrations, probably because they are dominated by light-induced states. On the other hand, the conversion efficiency of solar cells in both the initial and light-soaked states drops as the oxygen and nitrogen concentrations increase. Collection efficiency measurements show that the electric field distribution in the i-layer is affected by the donorlike states even after light-soaking. The photovoltaic properties of solar cells more sensitively reflect the effect of oxygen and nitrogen incorporation than the film properties.

Device-Grade Amorphous Silicon Prepared by High-Pressure Plasma

The high-pressure plasma regime (~5 Torr) was investigated in order to break through the growth rate limit for device-grade hydrogenated amorphous silicon (a-Si:H) without increasing the substrate temperature. The growth rate was successfully increased up to ~7 A/s while preserving quality. Controlling the electrode gap is important for utilizing high-pressure plasma, and the narrowest possible gap, below which plasma becomes unstable, is necessary to obtain high growth rates with the same high quality that is obtained at lower growth rates. The combination of high pressure and the narrowest possible electrode gap reduces the higher silane formation and enhances the generation of proper growth radicals such as SiH3. It is essential to suppress the influence of higher silane related radicals in the surface reactions for the film growth. This method allows device-grade a-Si:H to be produced at practical growth rates without increasing the substrate temperature, and contributes to the effective production of a-Si:H devices.

Dependence of Open Circuit Voltage of Amorphous Silicon Solar Cells on Thickness and Doping Level of the p-Layer

We have focused on the thickness and the boron-doping concentration of the p-layer of amorphous silicon solar cells and systematically obtained data for open circuit voltage (Voc) and the built-in potential to reveal the mechanism causing a high Voc. A highly doped p-layer gives a higher built-in potential in the entire thickness range, but Voc is limited by the carrier recombination caused by the doping-induced defects. A low-doped p-layer causes a higher Voc in a sufficiently thick film because less carrier recombination occurs due to the lower density of the doping-induced defects. After light-soaking, significant Voc degradation occurs with the low-doped p-layer. The light-induced defects are not negligible compared with the initial defects in the low-doped p-layer and thus more carrier recombination occurs. Moreover, some of the acceptors are compensated by light-induced defects. The highly doped p-layer, however, does not cause much Voc degradation because the light-induced defects are negligible compared with the large number of doping-induced defects and acceptors. The experimental data show that the midgap defects induced by doping or light-soaking near the p/i interface cause the Voc limitation.

A New Analytical Method of Amorphous Silicon Solar Cells

A new analytical method for amorphous silicon solar cells, called DICE (dynamic inner collection efficiency), has been developed. The depth profile of the photovoltaic characteristics of solar cells can be obtained by using the DICE method under any operating condition in a non-destructive manner for the first time. The DICE value is defined as the probability that an electron-hole pair generated at a certain depth in the generated region of an a-Si solar cell becomes an output current. In this paper the theory and the calculation method of DICE are described, and the results of applications to practical solar cells are reported. By using the DICE method it was found that carrier recombination at the p/i interface affects the open-circuit voltage.

Effect of illumination-induced space charge on photocarrier transport in hydrogenated microcrystalline Si1−xGex p-i-n solar cells

Photocarrier transport in hydrogenated microcrystalline Si1−xGex (μc-Si1−xGex:H) p-i-n solar cells (0 0.35, particularly in the wavelength range of <650nm, induces a strong carrier recombination near the p-i interface and a weak collection enhancement in the bulk, indicative of field distortion by the negative space charge generated near the p-i interface. This finding is consistently explained by the increased acceptorlike states in undoped μc-Si1−xGex:H for large Ge contents.

Effects of the i-layer properties and impurity on the performance of a-Si solar cells

Abstract The relation between the opto-electric properties of an a-Si : H i-layer and the performance of a-Si solar cells are extensively investigated, paying careful attention to the controllable range of the opto-electric properties and the effects of the impurities. It is shown that even trace amounts (10 19 cm −3 or less) of oxygen impurity in the i-layer affect the film properties and the solar cell performance. When the impurity concentration and other undesirable factors are suppressed, the mutual relationship among the i-layer properties becomes clear. Although there is a limitation in the controllable range of the properties of device-quality a-Si : H, techniques such as a hydrogen plasma treatment can improve the controllable range. Guidelines to design and optimize the i-layer for solar cells are discussed on the basis of these experimental results. A total-area conversion efficiency of 12.0% is achieved for a 10 cm × 10 cm integrated a-Si solar cell submodule.