Sharp Corp announced Nov 4, 2011, that the cell conversion efficiency of its compound semiconductor triple-junction photovoltaic (PV) cell reached 36.9%.
The company announced the world's highest efficiency of 35.8% in 2009 and, this time, it broke the world record by 1.1%. The conversion efficiency was measured at Japan's National Institute of Advanced Industrial Science and Technology (AIST). The area of the cell is about 1cm2.
Compound semiconductor triple-junction PV cells absorb lights with different wavelengths by using three types of cells (top, middle and bottom cells) to improve conversion efficiency. As in the case of the cell that Sharp announced in 2009, the company combined InGaP (indium gallium phosphide) top cells, GaAs (gallium arsenide) middle cells and InGaAs (indium gallium arsenide) bottom cells.
Also, the new PV cell is manufactured in the same process as the former cell. First, the top, middle and bottom cells are formed on a GaAs substrate in this order. Then, the GaAs substrate is removed, and the cells are mounted on a silicon (Si) supporting substrate with the bottom cells in contact with the substrate. This is called "backward buildup method."
The reason why Sharp succeeded in improving conversion efficiency while using the same cell structure and manufacturing method is that the company reduced series resistance components at the tunnel junction parts that connect the top and middle cells and the middle and bottom cells. Though the company did not disclose a specific method, it improved maximum output power by reducing the resistance components.
As for fill factor, which is an indicator for photovoltaic cells, Sharp increased it from 85.3% in 2009 to 87.5%. The other properties of the cell have hardly changed.
Sharp plans to commercialize the latest achievement both in space and above ground. It aims to have the cell approved as a part suited for use in space in 2013 in cooperation with Japan Aerospace Exploration Agency (JAXA).
The company intends to commercialize the cell in 2014 or 2015 after testing it in space. At that time, it plans to use the backward buildup method for transcription on a film substrate so that the weight of the cell can be reduced.
For use above ground, Sharp expects that the cell will be used for light-harvesting systems capable of condensing sunlight by several hundred times with lenses, etc. It will spend about a year conducting a field test of the cell from 2012. Because the amount of the current flowing in the cell increases at the time of light condensing, the reduction in series resistance, which contributed to improving conversion efficiency this time, will probably be useful.
When sunlight is condensed by several hundred times, the conversion efficiencies of Sharp's PV cells tend to increase by 10%, compared with a case where sunlight is not condensed. Therefore, in theory, it is possible to achieve a cell conversion efficiency of 45% or higher when sunlight is condensed by several hundred times.