Suppressing Energy Loss due to Triplet Exciton Formation in Organic Solar Cells: The Role of Chemical Structures and Molecular Packing

XK Chen and TH Wang and JL Bredas, ADVANCED ENERGY MATERIALS, 7, 1602713 (2017).

DOI: 10.1002/aenm.201602713

In the most efficient solar cells based on blends of a conjugated polymer (electron donor) and a fullerene derivative (electron acceptor), ultrafast formation of charge-transfer (CT) electronic states at the donor-acceptor interfaces and efficient separation of these CT states into free charges, lead to internal quantum efficiencies near 100%. However, there occur substantial energy losses due to the non-radiative recombinations of the charges, mediated by the loweset-energy (singlet and triplet) CT states; for example, such recombinations can lead to the formation of triplet excited electronic states on the polymer chains, which do not generate free charges. This issue remains a major factor limiting the power conversion efficiencies (PCE) of these devices. The recombination rates are, however, difficult to quantify experimentally. To shed light on these issues, here, an integrated multi-scale theoretical approach that combines molecular dynamics simulations with quantum chemistry calculations is employed in order to establish the relationships among chemical structures, molecular packing, and non- radiative recombination losses mediated by the lowest-energy charge- transfer states.

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