Modeling electrical transport in blend heterojunction organic solar cells

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Abstract
In order to understand how to enhance the performance of blend heterojunction organic solar cells, we have developed a theoretical model that enables us to investigate the impact of a number of materials and device parameters on device performance. A full description of the charge transport of photogenerated holes and electrons in a continuum model of the blend is given. The injection of charges at anode and cathode is treated according to previous models. The input of our model is the optical absorption in the active layer of the cell, as calculated within previous optical models, and measured as well as estimated transport parameters. By applying the model to devices with the active layer consisting of an alternating copolymer of fluorene blended with a fullerene derivative, we conclude that the most limiting parameter in these devices is the low hole mobility in the blend. Electron-hole recombination is strongly suppressed compared to Langevin recombination, due to the donor-acceptor separation.