Kyosemi Uses Ball Diodes in PV Modules
Kenji Tsuda, Asia Contributing Editor -- Semiconductor International, 3/10/2008
At the Second International Photovoltaic Power Generation Expo in Tokyo, Kyosemi Corp. (Kyoto, Japan) demonstrated a transparent photovoltaic (PV) cell made of an array of hundreds of small ball p-type/intrinsic/n-type (PIN) diodes, which deliver high efficiency.
| Kyosemi Corp. creates solar PIN diodes from spherical silicon balls. |
| Molten silicon is dripped in a gravity-less environment to form round silicon balls. |
The Sphelar diode consists of an n+- diffusion layer on the surface of the p-core sphere, with a diameter of ~1-1.5 mm. The company showed a wide variety of prototypes using the diode arrays, including a see-through solar cell made of an array with hundreds of ball diodes and a solar dome made of dozens of the balls.
A single PV cell shows a relatively high efficiency of 17% when positioned on a piece of reflective white paper, comparable with that of conventional silicon crystal solar cells. The ball diode easily absorbs light globally and reaches the 17% rate because light reflected to the backside can be processed. Efficiency is higher than amorphous silicon PV cells, according to the company.
The spherical silicon crystals are made in a gravity-less environment. In a furnace, molten silicon is dripped from a height of 14 m, and surface tension causes multiple drops to form. Under space-like gravity for 1.5 sec, crystallization occurs naturally. Several thousand crystalline spheres form each second and are then cooled. The solid balls roll out from the bottom of the chamber.
Kyosemi produced six different modules as engineering samples, with the diodes arranged 12 in series, 2-parallel/6-series, 3-parallel/4-series, 4-parallel/3-series, 6-parallel/2-series and 12-parallel connections. The engineering samples are presently shipping.
The diodes have an open voltage of 0.6 V, a current of 2.5 mA and output power of 1.5 mW. The module characteristics are based on 1.2 V and 15 mA specifications for the 6-parallel/2-series module, for example.
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