A world record photoelectric conversion efficiency rate of 18.6 percent in a 150-millimeter square practical use multi-crystalline silicon solar cell was claimed by Mitsubishi Electric Corporation, Japan last week - an improvement of 0.6 percent over the company's previous record.
Silicon is a key element in the wafers used to make solar cells, but because the supply of silicon has not been able to keep up with demand this has created a drive into the research and development of thinner wafers that use less silicon, have improved efficiency and increased electrical output.
Mitsubishi stated that in a 150-millimeter square practical-use multi-crystalline silicon solar cell, they achieved the world's highest conversion efficiency rate of 18.6 percent by adding a low reflectivity surface texture on the multi-crystalline silicon wafer. This was accomplished by optimizing the p-n junction to increase electric current generation and by developing a process to print electrodes on the surface of the silicon (metallization) to reduce shade loss of front grid electrodes. They say that this technology contributes to higher efficiency in small installations such as narrow roofs.
To create highly efficient PV cells, it is necessary to reduce light reflectivity on the surface. While a honeycomb-textured structure is recognized as suitable for reducing surface reflectivity, it was a challenge for Mitsubishi Electric to develop a way to apply this technology in production lines. The company states that they have developed the world's first method for fabricating a honeycomb structure on the surface of a 150-millimeter square multi-crystalline silicon by combining laser patterning and wet etching.
Power generation also relies on the amount of light that approaches the p-n junction. A shallow, lightly doped emitter (n-layer) increases light gain and improves efficiency of power generation.
With modified screens and front metal electrodes, they reduced shading loss of front grid electrodes by 25 percent compared to previous cells.
Mitsubishi Electric will begin introducing this multi-crystal silicon cell technology into its mass-produced photovoltaic modules by fiscal 2011. They also hope to increase output of solar power generation systems by combining the technology with its PV inverters, which have the industry's highest energy conversion efficiency rate. Printed electronics has the potential to match and even exceed all this but it has yet to do so.