Designing molecular materials with very large exciton diffusion lengths would remove some of the intrinsic limitations of present-day organic optoelectronic devices. Yet, the nature of excitons in these materials is still not sufficiently well understood.
Here, Samuele Giannini and Wei-Tao Peng from Jochen Blumberger’s group introduce Frenkel exciton surface hopping, a highly efficient method to propagate excitons through truly nano-scale materials by solving the time-dependent Schrödinger equation coupled to nuclear motion. They find a clear correlation between diffusion constant and quantum delocalization of the exciton. In materials featuring some of the highest diffusion lengths to date, e.g. the non-fullerene acceptor Y6, the exciton propagates via a transient delocalization mechanism, reminiscent to what was recently proposed for charge transport. Yet, the extent of delocalization is rather modest, even in Y6, and found to be limited by the relatively large exciton reorganization energy. On this basis, a path is charted out for rationally improving exciton transport in organic optoelectronic materials.
Samuele Giannini, Wei-Tao Peng, Lorenzo Cupellini, Daniele Padula, Antoine Carof & Jochen Blumberger