Charge separation by high electronic excited state transition in organic photovoltaics*

Organic photovoltaics (OPVs) are promising alternative energy devices with low cost, flexibility, light weight. Though the power convergence efficiencies (PCEs) of OPVs have reached more than 11%, the PCEs are lower than Silicon solar cells. Therefore, it is indispensable to achieve high PCEs, but one of the reasons for difficulty in improving PCEs is that the charge separation process in OPVs is not clear. In this research, we investigated why hot process and cool process which are currently discussed in the charge separation process differ depending on the molecules.
We investigated the energy profile from exciton states to charge separated states in two kinds of atomistic interface, P3HT/PCBM and PCPDTBT/PCBM. Namely, we prepared bilayer interfaces of P3HT/PCBM and PCPDTBT/PCBM, which were made by conducting molecular dynamics simulations with 48 donor molecules and 144 acceptor molecules. Then, we calculated electronically excited states in those interfaces by applying semi-empirical quantum calculations and then revealed charge separation pathway reaching 4 nm of electron-hole distance. Finally, we discussed critical effects of charge delocalization on the charge separation pathway and charge recombination rate.