Hot dissociation, hole shrinkage, and hole re-delocalization in P3HT/PCBM organic photovoltaics

Despite recent progress in increasing the power conversion efficiencies of organic photovoltaics, the mechanism behind charge separation against Coulombic attractions remains unclear. To date, studies have emphasized both ultrafast charge transfer and/or separation, otherwise known as ``Hot" dissociation, in addition to the historical ``Cool" dissociation. Entropic diffusion has also been emphasized without being clearly distinguished from the above dissociations. Thus, we herein reveal theoretically a threshold relating separation distance to charge separation followed by entropic diffusion after the threshold has been exceeded. We also theoretically disclose explicit separation pathways to reach the threshold, i.e., hot dissociation accompanied by shrinkage and re-delocalization of holes rather than electrons, which causes ultrafast charge separation, omitting the contact charge transfer state. The majority of free carriers were experimentally found to be generated by hot dissociation in ordered interfaces. Finally, the separation pathways were unable to reach the threshold in amorphous interfaces, which accounts for the experimentally low efficiencies of free carrier generation in amorphous blends.