Simple considerations for the design of solar cells based on polycrystalline semiconductor materials

Solar cells based on poly-crystalline semiconductor thin films have great potential for decreasing the cost of photovoltaic energy. However, this kind of solar cells have characteristics very different from those fabricated on crystalline silicon for which the carrier transport and behavior is clearly known. Instead, for hetero-junction solar cells made on less known polycrystalline materials the design is almost empirical. In this work, several physical aspects related to the behavior of polycrystalline thin film solar cells will be discussed, and some considerations for an adequate design of this kind of solar cells will be made. For example, the recombination at the grain boundaries and its influence on the short circuit current as a function of the crystallite sizes on the active material is considered. Based on this, the appropriate thickness of each Layer and their resistivity will be discussed. As an example, these considerations will be applied to CdS / CdTe hetero-junction solar cells, taking into account typical properties of CdTe thin films used for solar cells.