Determining the Correlation between Excited State Dynamics and Donor and Acceptor Structure in Nonfullerene Acceptors

Abstract

The design of nonfullerene acceptors (NFAs), which comprise donor and acceptor moieties, has led to significant improvement in organic photovoltaic (OPV) device performance over the past few years. Although there have been many NFAs synthesized and device-optimized, there have been fewer studies on the excited-state photophysics of donor-acceptor-containing NFAs. Using a combination of femtosecond transient absorption spectroscopy and time-resolved photoluminescence (PL), we have investigated the excited-state photophysical processes in three donor-acceptor-containing NFAs, where the chemical structure of the NFAs are systematically modified to allow direct comparison (D-A1-A1-D, D-A1-D, and D-A1-A2, where D represents 9,9-di-n-propylfluorene, A1 represents benzothiadiazole, and A2 represents dicyanovinyl). A large triplet population was observed for the dimer NFA (D-A1-A1-D), whereas for D-A1-D or D-A1-A2, the nonradiative decay rate was significantly decreased, and higher PL quantum yields observed. Furthermore, a singlet exciton relaxation process between S1∗ and S1 on the picosecond timescale was observed for each of the NFAs, although the mechanism was not always the same. Torsional relaxation was found to dominate the picosecond singlet relaxation for D-A1-D and D-A1-A2 in solution and film, and for D-A1-A1-D in solution. In the film, the singlet excited state of D-A1-A1-D predominantly relaxed via resonant excitonic energy transfer. The observation of singlet relaxation on the tens of picosecond timescale also provides a plausible explanation to the reduced VOC loss commonly associated with NFA-based OPV devices.