Role of the Charge Transfer State in Organic Donor–Acceptor Solar Cells

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Abstract
Charge transfer complexes are interfacial charge pairs residing at the donor–acceptor heterointerface in organic solar cell. Experimental evidence shows that it is crucial for the photovoltaic performance, as both photocurrent and open circuit voltage directly depend on it. For charge photogeneration, charge transfer complexes represent the intermediate but essential step between exciton dissotiation and charge extraction. Recombination of free charges to the ground state is via the bound charge transfer state before being lost to the ground state. In terms of the open circuit voltage, its maximum achievable value is determined by the energy of the charge transfer state. An important question is whether or not maximum photocurrent and maximum open circuit voltage can be achieved simultaneously. The impact of increasing the CT energy—in order to raise the open circuit voltage, but lowering the kinetic excess energy of the CT complexes at the same time—on the charge photogeneration will accordingly be discussed. Clearly, the fundamental understanding of the processes involving the charge transfer state is essential for an optimisation of the performance of organic solar cells.