Manipulation of Charge Delocalization in a Bulk Heterojunction Material Using a Mid-Infrared Push Pulse

In organic bulk heterojunctionmaterials,charge delocalization has been proposed to play a vital role in thegeneration of free carriers by effectively reducing the Coulomb attractionvia an interfacial charge transfer exciton (CTX). Pump-push-probe(PPP) experiments produced evidence that the excess energy given bya push pulse enhances delocalization, thereby increasing photocurrent.However, previous studies have employed near-infrared push pulsesin the range & SIM;0.4-0.6 eV, which is larger than the bindingenergy of a typical CTX. This raises the doubt that the push pulsemay directly promote dissociation without involving delocalized states.Here, we perform PPP experiments with mid-infrared push pulses atenergies that are well below the binding energy of a CTX state (0.12-0.25eV). We identify three types of CTXs: delocalized, localized, andtrapped. The excitation resides over multiple polymer chains in delocalizedCTXs, while it is restricted to a single chain (albeit maintaininga degree of intrachain delocalization) in localized CTXs. TrappedCTXs are instead completely localized. The pump pulse generates a"hot" delocalized CTX, which promptly relaxes to a localizedCTX and eventually to trapped states. We find that photo-excitinglocalized CTXs with push pulses resonant to the mid-infrared chargetransfer absorption can promote delocalization and, in turn, contributeto the formation of long-lived charge separated states. On the otherhand, we found that trapped CTXs are non-responsive to the push pulses.We hypothesize that delocalized states identified in prior studiesare only accessible in systems where there is significant interchainelectronic coupling or regioregularity that supports either inter-or intrachain polaron delocalization. This, in turn, emphasizes theimportance of engineering the micromorphology and energetics of thedonor-acceptor interface to exploit the full potential of amaterial for photovoltaic applications.