Katsunobu Sayama

Optimization of a-SiGe:H Alloy Composition for Stable Solar Cells

The film properties and solar cell performance of amorphous SiGe:H (a-SiGe:H) samples have been systematically investigated, using constant optical gap and various compositions of hydrogen and germanium. It was found that the hydrogen content and bonding configurations play important roles in determining both the initial properties and stability. The optimum compositions were clarified for the minimum Urbach tail characteristic energy and defect density in the as-deposited film, and for the maximum conversion efficiency of the solar cells. The stability of a-SiGe single and a-Si/a-SiGe tandem solar cells becomes higher as the hydrogen content of the photovoltaic layer becomes lower. As a result, the optimum composition after light soaking shifts to the region of lower hydrogen content. Applying the above findings to the design of devices, the highest stabilized conversion efficiencies of 3.3% (initial 3.7%) under red light (λ>650 nm) for an a-SiGe single-junction solar cell and 10.6% (initial 11.6%) for an a-Si/a-SiGe tandem solar cell have been achieved (area: 1 cm2).

Film Property Control of Hydrogenated Amorphous Silicon Germanium for Solar Cells

The optoelectric properties of a-SiGe:H alloys, deposited by the plasma chemical vapor deposition (plasma-CVD) method, were investigated with precise measurement of their germanium content (CGe) and hydrogen content (CH). This investigation revealed that the optical gap of a-SiGe:H alloys can be approximated by a linear function of CH and CGe and various combinations of CH and CGe resulted in identical optical gaps. For each optical gap, the optimum composition for the lowest defect density was derived by comparison with the subgap absorptions measured by the constant photocurrent method (CPM). Based on these, the highest conversion efficiency of 3.7% under red light illumination (>650 nm) for a 1 cm2 a-SiGe single-junction solar cell 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.

Controlling hydrogen contents and bond configurations for high-quality and high-reliability a-Si films

Abstract We previously reported that a reduction of SiH 2 bond density in the i-layer effectively prevents light-induced degradation of a-Si solar cells. For further reduction of SiH 2 bond density, we fabricate a-Si films at higher substrate temperature (Ts:250 ∼ 400°C) with a glow discharge method by using super chamber. SiH 2 bond density can be reduced to about 10 20 cm −3 , and highly photoconductive and stable a-Si films are obtained. As a new attempt to reduce SiH 2 bond density, we have investigated an ion-gun CVD method. Hydrogen bond configurations are found to be strongly affected by the accelerating voltage of ions.

Approach from a-Ge Films for Development of High-Quality a-SiGe Films

a-Ge:H films fabricated by means of a separated ultrahigh-vacuum reaction chamber, called the super chamber, were systematically studied. In the conventional glow-discharge method, there is a very narrow substrate-temperature region for fabricating high-density a-Ge:H films; that is, a minimum deposition rate and a maximum refractive index were obtained at about 250°C. From the point of view of optoelectrical properties, it was clear that not only rigidity of the film network but also total hydrogen content are important. In order to satisfy the two above-mentioned factors simultaneously, a low substrate-temperature high hydrogen-dilution method was effective, and film properties of a-Ge:H were largely improved; δd~4.0×10-5 Ω-1 cm-1, δph~1.5×10-4 Ω-1 cm-1, B value ~803 (eVcm)-1/2, and the ESR spin density ~1.5×1017 cm-3.

Preparation and Properties of a-SiGe:H Films Fabricated with a Super Chamber (Separated Ultra-High Vacuum Reaction Chamber)

High-quality a-SiGe:H films with low impurity concentrations were studied using a separated ultra-high vacuum reaction chamber system called the super chamber. The ESR spin density and the tail characteristic energy of the a-SiGe films (Eopt 1.5 eV) were 6.5×1015 cm-3 and 46 meV, respectively. These values were much lower than those for films fabricated in a conventional chamber as well as those for a-Si films. Structural properties, such as the refractive indices and thermal effusion of hydrogen, were also measured. The results suggest that impurity reduction contributed not only to an improvement in the optoelectrical properties, but also the formation of a rigid a-SiGe network.

Hydrogenated Amorphous Silicon Germanium Alloy for Stable Solar Cells

The film properties and solar cell performance of a-SiGe:H samples with the same optical gap and different combinations of hydrogen content (C H ) and germanium content (C Ge ) have been compared. The optimum composition for the initial properties, such as the tail characteristic energy, defect density and conversion efficiency of the solar cell, was determined, and the differences could be explained by the difference in H bonding configuration. The degradation ratio of the conversion efficiency becomes larger in higher C H samples. This suggests that hydrogen or Si-H 2 participates in light-induced degradation. As a result, the optimum C H for an efficient solar cell is believed to shift to the lower C H region after light soaking. Based on these findings, the stabilized conversion efficiency of 3.3% under red light (γ>650nm) for an a-SiGe:H single-junction solar cell (1cm 2 ) and 10.6% under lsun light for an a-Si/a-SiGe double-junction stacked solar cell (1cm 2 ) have been achieved. The degradation ratio is only 8.6% for the double-junction solar cell.

High-Performance a-SiGe Solar Cells Using a Super Chamber Method

The influence of impurities on the a-SiGe film properties was investigated using a super chamber. It was found that a-SiGe films with a low impurity concentration can maintain a high photosensitivity of about 106 for Ge content up to 13%, and impurity incorporation deteriorates film rigidity, which was first deduced from the STM (scanning tunneling microscope) measurement. Using a super chamber method, the highest conversion efficiency of 3.34% was obtained for an a-SiGe single-junction cell under red light (long-wavelength light (>~650 nm) by filtering AM-1.5, 100 mW/cm2 light). A conversion efficiency of 11.9% was also achieved for a stacked cell of a-Si/a-Si/a-SiGe.

Development of high-efficiency a-Si solar cell submodule with a size of 30 cm × 40 cm

We have achieved a stabilized conversion efficiency of 8.9% in a single-junction a-Si solar cell and 10.6% in a double-junction a-Sia-SiGe solar cell for a size of 1 cm2, which are the worlds highest values achieved so far for this size and structure. We have been investigating the improvement of stability in a-SiGe film with regard to the bottom cell i-layer, and the control of Eopt in a-SiGe film in order to confirm the tandem cell design. On the other hand, uniformity of ± 1% has been obtained in conversion efficiency for many small cells fabricated in a size of 30 cm × 40 cm, evaluated by using a-Si single-junction structure. As a result, we have achieved the stabilized high-effective area conversion efficiency of 8.64% in a 30 cm × 40 cm a-Si/a-Si tandem submodule. The combination of the above techniques and further optimization can be expected to achieve a stabilized conversion efficiency of more than 10% for a 30 cm × 40 cm double-junction a-Sia-SiGe submodule.

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.