Charge Dynamics at Heterojunction between Face-on/Edge-on PCPDTBT and PCBM Bilayer: Interplay of Donor/Acceptor Distance and Local Charge Carrier Mobility

The bulls heterojunction organic solar cell has shown much promise as a cost-effective energy harvesting device, while despite recent progress in boosted power conversion efficiency, critical photophysical process at the interface of electron donor and electron acceptor is subject of ongoing debate. Here we investigate the impact of polymer orientation of cychlopentadithiophene benzothiadiazole copolymer (PCPDTBT) on the charge separation (CS) and recombination (CR) at the bilayer heterojunction of polymer and methanofullerene (PCBM). The charge carrier dynamics at contrasting face-on-rich or edge-on interface controlled via side-alkyl chain modification are monitored by flash photolysis time-resolved microwave conductivity (TRMC). The data are analyzed using Singlet exciton diffusion to donor-acceptor interface with quenching term at high excitation density. We show that CS is more efficient for the face-on-rich interface than edge-on, while CR is in turn retarded for the latter. Along with computational validation based on density functional theory, molecular dynamics, and the Marcus-Levich-Jortner model, our work provides a useful guide regarding interplay of polymer/fullerene interface, exciton/charge dynamics, and local charge carrier mobility.