Origin of size effect on efficiency of organic photovoltaics

It is widely accepted that efficiency of organic solar cells could be limited by their size. However, the published data on this effect are very limited and none of them includes analysis of light intensity dependence of the key cell parameters. We report such analysis for bulk heterojunction solar cells of various sizes and suggest that the origin of both the size and the light intensity effects should include underlying physical mechanisms other than conventional series resistance dissipation. In particular, we conclude that the distributed nature of the ITO resistance and its influence on the voltage dependence of photocurrent and dark current is the key to understanding size limitation of the organic photovoltaics (OPV) efficiency. Practical methods to overcome this limitation as well as the possibility of producing concentrator OPV cells operating under sunlight concentrations higher than 10 suns are discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3567930]