Japan Advanced Institute of Science and Technology (JAIST) announced Feb 7, 2011, that it has succeeded in making an amorphous silicon (Si) photovoltaic (PV) cell by applying a liquid Si for the first time in the world.
The conversion efficiency of the pin-type cell whose i-layer was made with the coating process was 1.79%, JAIST said. If the efficiency improves in the future, it might become possible to apply the technology to mass-produce Si PV cells by using a roll-to-roll method.
The amorphous Si PV cell was developed by a research group led by Tatsuya Shimoda, professor at the School of Materials Science of JAIST. The group used a liquid Si whose starting material is cyclopentasilane (CPS: Si5H10, hydrogen atoms are bonded with five membered rings of Si).
Though JAIST did not reveal the details of the liquid Si, it developed a liquid Si by emitting ultraviolet rays at CPS in a carbon hydrogen solvent for polymerizing part of the CPS to make polysilane (polymer molecules made by bonding SiH2 like a chain) during a period from 2006 to 2007. At that time, JAIST and others formed amorphous Si-TFTs by using this liquid Si.
This time, JAIST optimized the constituents of the liquid Si and developed the "Si ink," which has p-, i- and n-type semiconductor properties, by using boron (B) and phosphorus (P), which are used for Si doping. By applying the Si ink to a substrate, a polysilane film can be formed. When the film is heated, hydrogen is removed from the polysilane, forming solid amorphous Si.
Thus far, there has been a problem in the process of forming an uniform polysilane film. But this problem was solved by carefully examining controlled parameters. Furthermore, JAIST optimized the burning condition of the polysilane film and reduced the density of unattached hands to 1 x 1016/cm3.
This time, JAIST formed pin-type amorphous Si PV cells on a glass substrate by using the newly-developed process. The conversion efficiency of the cell whose i-layer was formed by using the new coating process and p- and n-layers were formed by using the CVD (chemical vapor deposition) method was 1.79%. On the other hand, the conversion efficiency of the cell whose p-, i- and n-layers were all formed by using the coating process was 0.51%.
However, the thickness of the new cell's i-layer is 120nm, which is thinner than that of existing amorphous Si PV cells (250nm). If this thickness can be increased, the efficiency will improve, JAIST said.