Mikio Taguchi

24.7% Record Efficiency HIT Solar Cell on Thin Silicon Wafer

A new record conversion efficiency of 24.7% was attained at the research level by using a heterojunction with intrinsic thin-layer structure of practical size (101.8 cm2, total area) at a 98-μm thickness. This is a world height record for any crystalline silicon-based solar cell of practical size (100 cm2 and above). Since we announced our former record of 23.7%, we have continued to reduce recombination losses at the hetero interface between a-Si and c-Si along with cutting down resistive losses by improving the silver paste with lower resistivity and optimization of the thicknesses in a-Si layers. Using a new technology that enables the formation of a-Si layer of even higher quality on the c-Si substrate, while limiting damage to the surface of the substrate, the Voc has been improved from 0.745 to 0.750 V. We also succeeded in improving the fill factor from 0.809 to 0.832.

Development of New a-Si/c-Si Heterojunction Solar Cells: ACJ-HIT (Artificially Constructed Junction-Heterojunction with Intrinsic Thin-Layer)

A new type of a-Si/c-Si heterojunction solar cell, called the HIT (Heterojunction with Intrinsic Thin-layer) solar cell, has been developed based on ACJ (Artificially Constructed Junction) technology. A conversion efficiency of more than 18% has been achieved, which is the highest ever value for solar cells in which the junction was fabricated at a low temperature (<200°C).

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

Temperature Dependence of Amorphous/Crystalline Silicon Heterojunction Solar Cells

We evaluated the conduction mechanisms and temperature dependence of HIT (heterojunction with intrinsic thin layer) structure solar cells while changing the thickness of the undoped amorphous silicon layer. It was confirmed that the diffusion model determined the carrier transport property of this device at the high-forward-bias region (0.4<V<0.8 V), whereas the multistep tunneling model determined the current transport at the low-bias region (0.1<V<0.4 V). The insertion of the high-quality hydrogenated amorphous silicon (a-Si:H) i-layer is very important for suppressing the probabilities of tunneling through the localized states in a-Si:H and surface recombination velocity at the heterointerface. The better temperature dependence of output power of HIT structure solar cells than that of the crystalline silicon (c-Si) homojunction solar cell is caused mainly by the high-open circuit voltage that originates in the effectively suppressed saturation current with HIT structure, and also by the fill factor (F.F.), which is affected by the change in conductivity in the a-Si:H i-layer.

Sanyo's Challenges to the Development of High-efficiency HIT Solar Cells and the Expansion of HIT Business

The worlds highest conversion efficiency levels of 21.8% (Voc: 0.718 V, Isc: 3.852 A, FF: 79.0%, confirmed by AIST) with a practical size of 100.4 cm2 has been achieved by using the HIT (hetero-junction with intrinsic thin layer) structure. This high efficiency has been mainly realized by the excellent c-Si/a-Si hetero-interface property obtained by our optimized surface cleaning process and high-quality and low-damage a-Si deposition technologies. This excellent c-Si/a-Si hetero-interface of the HIT structure results in a relatively high open circuit voltage (Voc) over 710 mV. Recently, we have succeeded in achieving an outstanding Voc of 730 mV for other efficient HIT solar cells. This result indicates the possibility of further improvement in the conversion efficiency of HIT solar cells. The higher Voc results in not only a higher conversion efficiency but also an improved temperature coefficient, which is another practical advantage for outdoor use

An approach for the higher efficiency in the HIT cells

The highest conversion efficiency to date of 21.5% (confirmed by AIST) with a size of 100.3 cm/sup 2/ has been achieved in an HIT cell. Because of this high efficiency and the cells superior temperature characteristics, HIT cells are highly regarded by consumers. Sanyo will increase the production volume of cells and modules to meet the demand both inside and outside of Japan. We have been investigating suitable materials based on Sanyos technology for fabricating high-quality a-Si solar cells to obtain higher build-in potential and control the junction properties, and have been studying how to treat the surface to create a good interface without introducing any damage. We will continue our efforts to obtain even higher levels of conversion efficiency by using the high potential that this structure has.

More than 16% solar cells with a new 'HIT' (doped a-Si/nondoped a-Si/crystalline Si) structure

A HIT (heterojunction with intrinsic thin-layer) structure solar cell has been developed. In this structure, a nondoped a-Si thin layer was inserted between a p-type a-Si layer and an n-layer c-Si substrate. The open-circuit voltage and fill factor (FF) were significantly improved in these HIT structure solar cells compared with conventional p/n heterojunction solar cells. The improvement seems to originate in the reduction of backward current density. For higher efficiency, this HIT structure has been applied to textured substrates and achieved an efficiency of 18.1% (1 cm/sup 2/ cell). This efficiency is the highest value reported for a solar cell in which the junction was fabricated at a low temperature (120 degrees C). Application of this structure to the poly-Si thin film will yield a-Si/poly-Si thin-film solar cells of high efficiency.<<ETX>>

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.

Excellent power-generating properties by using the HIT structure

We are developing HIT solar cells with high conversion efficiency, which was achieved the worlds highest conversion efficiency of 22.3% in a practical size solar cell in July 2007. We have four main approaches to reducing power-generating cost: improve the conversion efficiency, apply the HIT structure to a thin wafer, improve the temperature coefficient, and apply HIT solar cells to a bifacial solar module. Using these approaches, we have achieved the remarkably high conversion efficiency of 21.4% due to a high Voc of 0.739 V with an 85-μm cell, which was measured at SANYO. A thinner Si wafer brings not only high Voc but also generating more output power at high temperature for a better temperature coefficient. We have confirmed that the HIT structure is suitable for use in thinner wafers, allowing us to reduce power-generating cost.

High-efficiency HIT solar cells with a very thin structure enabling a high Voc

To increase the competitiveness of HIT (Heterojunction with Intrinsic Thin-layer) solar cells, we have been working on the enhancing their conversion efficiency. This time, we improved the heterojunction of the HIT solar cell, which made it possible to enhance the cell conversion efficiency. In addition, we have developed module technologies such as a new tab design and anti-reflection coated glass. By combining these technologies, we have achieved 240-W model with module conversion efficiency of 19.0%. Those HIT solar cells have the worlds highest level of cell conversion efficiency 21.6 % at the mass-production stage. We have also been investigating the performance of thinner HIT solar cell using crystalline silicon (c-Si) wafers less than 100 μm in thickness. To minimize optical losses, such as the ultraviolet light absorption in the front transparent conductive oxide (TCO) layer and amorphous Si (a-Si) layers, and the near-infrared light absorption in the rear TCO layer, we have improved the deposition conditions of a-Si, and developed the TCO material respectively. To improve the surface passivation quality of the a-Si/c-Si heterointerface, we have examined our fabrication process from these three viewpoints: (1) the cleanliness of the c-Si surface, (2) the damage in the deposition process, and (3) the quality of the deposited a-Si layer. As a result, we have achieved an excellent Voc of 0.747 V with 58- and 75-μm-thick cells.