Sadaji Tsuge

HITTM cells—high‐efficiency crystalline Si cells with novel structure

Our unique, high-efficiency c-Si solar cell, named the HIT cell, has shown considerable potential to improve junction properties and surface passivation since it was first developed. The improved properties in efficiency and temperature dependence compared to conventional p – n diffused c-Si solar cells are featured in HIT power 21TM solar cell modules and other applications which are now on the market. In the area of research, further improvement in the junction properties of the a-Si/c-Si heterojunction has been examined, and the highest efficiency to date of 20.1% has recently been achieved for a cell size of 101 cm2. The high open circuit voltage exceeding 700 mV, due to the excellent surface passivation of the HIT structure, is responsible for this efficiency. In this paper, recent progress in HIT cells by Sanyo will be introduced. Copyright

High-quality polycrystalline silicon thin film prepared by a solid phase crystallization method

Abstract We succeeded in fabricating high-quality polycrystalline silicon (poly-Si) thin films with no boundary from the bottom surface to the top, and achieved an extremely high electron mobility of 808 cm2/V s by a solid phase crystallization (SPC) method. This film was obtained by using a new nucleation layer with 1000 A wide single-crystalline grains embedded in a matrix of amorphous tissue. A poly-Si thin-film solar cell fabricated using this film as an active layer demonstrated a total area conversion efficiency of 9.2% (active area efficiency: 9.7%), which is the worlds highest value for crystalline silicon solar cells fabricated below 600°C on metal substrates.

Device‐quality wide‐gap hydrogenated amorphous silicon films deposited by plasma chemical vapor deposition at low substrate temperatures.

Hydrogenated amorphous silicon (a‐Si:H) films have been fabricated by a plasma chemical vapor deposition (plasma‐CVD) method at low substrate temperatures (Ts : 80 or 50 °C) to obtain wide‐gap films. Device‐quality wide‐gap films (photoconductivity under AM‐1.5, 100 mW/cm2 illumination ∼10−5 Ω−1 cm−1, ratio of photoconductivity and dark conductivity ∼106, and Tanc’s gap ∼2 eV) are obtained at low Ts, by optimizing the plasma parameters or by diluting the material gas (SiH4) with H2. Experimental results suggest that lowering the deposition rate of a‐Si:H films is an important factor in obtaining high‐quality films at low Ts using a plasma‐CVD method.

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.

Effect of substrates and film thickness on the structural, optical, and electrical properties of hydrogenated amorphous silicon films

Hydrogenated amorphous silicon (a‐Si:H) films have been deposited on different substrates by a plasma chemical vapor deposition method. Raman spectra of the a‐Si:H films are significantly dependent on the material of the substrate (glass, crystalline silicon, and stainless steel). The spectra are also dependent on the thickness of the films. The experimental results indicate that the silicon network structure of a‐Si:H films is dependent on the material of the substrate. The dependence of the optical absorption coefficient and electric conductivities on the thickness is also reported.

20.7% highest efficiency large area (100.5 cm2) HIT/sup TM/ cell

A world record total area conversion efficiency of 20.7% and high open circuit voltage (VOC) of 719 mV were achieved on a solar cell with HIT (heterojunction with intrinsic thin-layer) structures on both sides (wafer size: 100.5 cm/sup 2/, n-type solar-grade CZ-Si). This solar cell was fabricated with the same process as that used in our mass-production lines. The essence of this high performance is derived from the excellent passivation ability of the HIT structure on c-Si. This report discusses research for excess of 20% efficiency HIT cell (/spl sim/100 cm/sup 2/), focusing on the a-Si passivation effect estimated from the carrier lifetime, and describes product development for the industrialization of HIT cells.

Effect of heating SiH4 on the plasma chemical vapor deposition of hydrogenated amorphous silicon

The application of vibrational/rotational energy to material gas ( SiH4) and radicals by heating the material gas has been investigated in order to improve the properties of hydrogenated amorphous silicon (a-Si:H) for solar cells. It has been found that the optical gap (E opt) of a-Si:H films deposited from heated SiH4 (temperature of the gas heater: 280–380° C) is narrower by about 20–30 meV than those of conventional films where SiH4 is not heated, using the same substrate temperature (T s) and deposition rate (R d). This result suggests that the heated SiH4 molecules or related radicals promote reactions at or near the film-growing surface. In other words, applying vibrational, rotational, or translational energy to SiH4 gas has the same effect as raising T s in plasma chemical vapor deposition (plasma-CVD) of a-Si:H. It is demonstrated that gas heating can improve the conversion efficiency of a-Si solar cells by reducing the damage to underlying layers during i-layer deposition.

A unified relationship among the opto-electric properties of a-Si:H and approaches for further improvement

The optical, electric and structural properties of hydrogenated amorphous silicon (a-Si:H) films, deposited using the plasma-CVD method, are systematically investigated as functions of the substrate temperature (T s ) and plasma parameters such as the RF power, gas pressure, and the dimensions of the reactors in order to optimize their properties for solar cells. Those properties are successfully controlled over a wide range by the plasma parameters at a fixed T s . The decrease in the film deposition rate and the increase in the T s have practically identical effects on the properties of a-Si:H in this study. The experimental results suggest that the properties of a-Si:H films are governed by a competition between the rate of film growth and the rate of thermally-activated surface reactions at or near the film-growing surface during film deposition. Limitation in the controllability of ‘device-quality’ a-Si:H can be overcome by diluting SiH 4 with hydrogen or helium at low T s , or by hydrogen-plasma treatment of a-Si:H. The present results provide a guide for further improvement in the characteristics of a-Si:H for the photovoltaic layer of solar cells.

Fabrication of Polycrystalline Si Thin Film for Solar Cells

The a-Si/poly-Si thin film tandem solar cell is a promising candidate for low-cost solar cells. We have conducted R&D on poly-Si thin film using the Solid Phase Crystallization (SPC) method from amorphous silicon (a-Si). To improve the film quality of SPC poly-Si, we have developed a new SPC method called the partial doping method. This method features two stacked starting a-Si layers, a P-doped layer and a non-doped layer. Nucleation occurs in the P-doped layer, and the non-doped layer is the crystal growth layer. For the nucleation layer, we developed a Si film with a unique structure, which features relatively large crystallites (-1000A) embedded in a matrix of amorphous tissue. By combining these technologies, a conversion efficiency of 9.2% was obtained for poly-Si thin-film solar cells. For further improvement in the conversion efficiency, based on the concept of “independent control of nucleation and crystal growth”, it is necessary to combine the best fabrication methods for each layer. A high conversion efficiency of more than 12% was found possible by using the CVD method and a new back surface reflection structure.

Efficiency improvement in a-Si and a-SiGe solar cells using a super chamber method

A total area conversion efficiency of 11.1% was achieved for a 10*10 cm/sup 2/ integrated-type a-Si solar cell submodule. Approaches to improving conversion efficiency included the development of new materials, new fabrication methods, and a new device structure, specifically, the super chamber method, trimethylboron (B(CH/sub 3/)/sub 3/) p a-SiC:H, highly-textured TCO, an ultra-thin interface layer, and low-damage laser patterning process. New approaches to preparing a-Si:H and a-SiGe:H films are proposed. High-quality, wide-bandgap a-Si:H films have been fabricated at low substrate temperature by diluting silane with hydrogen. High-quality a-SiGe:H were fabricated using the super chamber method.<<ETX>>