Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
How Charges Separate Very Efficiently in Bulk Heterojunctions
Michael McGehee
Department of Materials Science and Engineering
Stanford University
It is remarkable that electrons and holes separate efficiently in organic solar cells given that their coulomb attraction is substantially larger than the thermal energy. We have developed a model based on extensive structural characterization and kinetic Monte Carlo simulations that explains how the charge carrier separate. We find that it is critically important to use a hopping rate that is based on charge carrier mobilities measured by terahertz absorption measurements instead of those obtained by diode or transistor measurements because the hopping rate away from the donor-accepter interface is not limited by charges being trapped or held up at grain boundaries. Charge separation is possible because hops away from the interface are substantially faster than recombination. Consequently the charge carriers can attempt to get away from each other thousands of times before they separate or recombine.
We also find that in most highly efficient solar cells there is a three-phase morphology in which there are pure regions of polymer and fullerenes along with a mixed region. There are energetic offsets that push electrons and holes out of the mixed region and into the pure region. One can control the size and composition of the phases by adjusting factors such as the regioregularity and molecular weight of the polymer, the polymers sidechains, the donor:acceptor ratio, the choice of fullerenes, the use of processing additives and annealing conditions. We will show how the sidechains of polymers can be designed to optimize the interactions fullerenes have with them.
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International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.
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