The silicon heterojunction [SHJ] structure adopted in Panasonic’s photovoltaic module HIT® is recognized as one of the highest efficiency solar cells that has following advantages over a conventional c-Si solar cell, (1) a high efficiency over 22% with high open circuit voltage [Voc] of more than 720 mV, (2) higher power generation in practical use due to the world's leading output temperature coefficient of -0.258%/°C, (3) a symmetrical cell structure essentially suitable for bifacial module.
This unique structure features a very thin, non-doped amorphous silicon [a-Si:H(i)] layer inserted between p or n-type doped a-Si layer and n-type c-Si substrate. With the insertion of a-Si:H(i) layer at the a-Si/c-Si heterojunction, surface recombination is drastically suppressed. We started our R&D activity on SHJ from 1990, and achieved a conversion efficiency of 20% with a small (1 cm2) cell in 1994. The deposition of the high quality a-Si:H film is the key technology that has been cultivated through our long time experience on amorphous silicon solar cells started from 1975.
Since then, we have continued to improve the conversion efficiencies of the SHJ solar cell with a practical size (> 100 cm2) and have broken the world record for several times. Especially, we have proved that the well surface-passivated solar cell can maintain the high efficiency with reducing the thickness of Si substrate down to less than 100 µm. We obtained the efficiency of 24.7% with 98-µm-thick Si in 2013. Where, the surface recombination velocity at the a-Si/c-Si heterointerface was estimated as low as 1.4 cm/s and the excellent Voc of 750 mV was achieved. In addition, an interdigitated back contact structure was adopted with our SHJ cells to eliminate the shadow loss of front grid electrode and absorption losses of TCO and a-Si on front surface in our standard structure SHJ cell. With this structure, we obtained a new world record efficiency of 25.6% in 2014 (designated area, 143.7 cm2, confirmed by AIST). This broke the previous world record of 25.0% (4 cm2) after interval of 15 years and gathered much attention. We always led the high efficiency Si solar cell technology in this way.
Panasonic built its first production line of the HIT® in 1997, and has produced billions of cells with accumulating the experience of mass production for 20 years. The module technologies suitable for SHJ cells to obtain high efficiency and high reliability have been developed. We have adopted non-soldered, grooved tabbing wire for cell interconnection to utilize the light that strikes the tabbing wire from 2010. Further, we introduced the bifacial module to the market in 2000 and did pioneered work from the early stage of our business. In 2012, Panasonic was commended "for developing and commercializing high-performance heterojunction solar technology (HIT®)" by the IEEE.
Furthermore, to expand the use of the HIT®, we have developed technologies to laminate three-dimensional curved glass to match the automotive body design, achieving the installation of modules on the roof of the new Prius PHV released by Toyota Motor Corporation. Electrical energy generated from solar cells can be applied to the charging of driving power sources, thereby extending an EV's travel distance per charge.
Now, Tesla and Panasonic are preparing to start manufacturing solar cells and modules at the Buffalo, New York factory.
We will continue to pursue technology development of our SHJ solar cells, aimed at realizing higher efficiency, lower costs and the more efficient use of resources, and will work towards mass production of the industry’s top level efficiency and energy saving solar panels on the market.
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