Fractal contact patterns for high efficiency silicon concentrator solar cells

One of the main obstacles to the diffusion of photovoltaic energy is the low ratio between the produced energy value and the system cost; to overcome this limitation it is possible to resort to photovoltaic systems based on concentrated sunlight. A concentrator system is based on a large number of mirror or lenses concentrating the sunlight over a small area where an array of high efficiency photovoltaic cell is placed. The main advantage of this approach is that it is possible to employ only a small cells surface, being the most expansive and complex system parts, to produce large amounts of energy. The photovoltaic received must therefore be designed to handle the high current density produced under concentration, which are responsible for not negligible collection losses. These losses are caused by an early direct polarization of the PN junction induced by the presence of voltage drop across the contact resistance. It appears that the resistance of the front contact grid and surface layers is responsible, being exposed to high current densities, for large part of such losses. The methods currently used to cope with this problem shifts to the contact of the cell back but requires complicated solar cells design and fabrication processes, increasing significantly the cost of the cells. The method we propose to overcome this difficulty is to lay the electric contacts on the cell front according to a fractal geometry. In particular this geometry scales in such a way that contacts carrying high current are thick, while low current contacts are thinner. Different contact geometry , both fractal and not, are compared through the simulation of IV curves and maximum power for each device under several light concentration level. To create the simulation of the device responses under illlumination, an original numerical method has been developed exploiting an iterative algorithm based on non-linear maps, allowing to solve general contact patterns on conducting material. The results evidence fill factor gains of 8 to 15 percent under concentration under 300 suns for fractal contacts cells with respect to traditional patterns.