By generating two free charge carriers from a single high-energy photon, singlet fission (SF) promises to significantly improve the efficiency of a class of organic photovoltaics (OPVs). However, SF is generally a very inefficient process with only a small number of absorbed photons successfully converting into triplet states. In this Letter, we map the relation between stacking geometry and SF yield in crystals based on perylenediimide (PDI) derivatives. This structure-function analysis provides a potential explanation for the SF yield discrepancies observed among similar molecular crystals and may help to identify favorable geometries that lead to an optimal SF yield. Exploring the subtle relationship between stacking geometry and SF yield, this Letter suggests using crystal structure engineering to improve the design of SF-based OPVs.
Keywords: excited-state dynamics; organic solar cell; solar energy; triplet formation.