Wannier-Like Delocalized Exciton Generation In C-60 Fullerene Clusters: A Density Functional Theory Study

C-60 fullerene is widely used in organic devices; however, there are still unaccountable phenomena, such as the strong light absorption around 2.8 eV of C-60 films and favorable device performance of organic photovoltaics (OPVs) using C-60 rich bulk heterojunctions. We studied the excited states of C-60 fullerene clusters using large-scale time-dependent density functional theory taking thermal vibrations into account. A strong absorption peak around 2.8 eV appeared because of aggregation and we found that Wannier-like delocalized excitons spread over multiple molecules were generated in this energy region. This is contrary to the accepted theory that only Frenkel or charge transfer (CT) excitons are generated in organic materials. It is considered that the delocalized excitons with energies greater than electrical gap (E-g) become CT excitons after thermalization whereas those with energies lower than E-g become either CT excitons by obtaining thermal energy or Frenkel excitons after thermalization. In the exciton dissociation process, there is an essential channel in which the delocalized excitons are generated first and then subsequently become CT excitons. The delocalized exciton generation can explain both the strong absorption around 2.8 eV and enhanced OPV performance using C-60-rich bulk heterojunctions.