Prolongation of the singlet exciton lifetime of nonfullerene acceptor films by the replacement of the central benzene core with naphthalene

A comprehensive study on the relationship between the structure and the photophysical and photovoltaic properties of acceptor-donor-acceptor (A-D-A) type nonfullerene acceptors (NFAs) is of pivotal importance to obtain design guidelines for high-performance organic photovoltaics (OPVs). In this study, we synthesized a D unit, NTT, in which the central benzene core, indacenodithieno[3,2-b]thiophene (IT), is replaced by naphthalene. Notably, NTTIC, an A-D-A type NFA with NTT as the D unit, showed a longer singlet exciton lifetime in the film than the IT-based benchmark NFA, ITIC. When paired with a conjugated polymer donor, PBDB-T, the NTTIC-based OPV device exhibited a higher power conversion efficiency (PCE = 9.95%) than the ITIC-based device (9.71%). Furthermore, due to the larger domain size of NTTIC in PBDB-T:NTTIC, the exciton diffusion (ED) at the donor/acceptor interface and the charge transfer (CT) were slower (14 ps in total) than those of PBDB-T:ITIC (7 ps in total). Nevertheless, the total efficiency of the ED and CT in PBDB-T:NTTIC was similar to that in PBDB-T:ITIC (95%) owing to the longer singlet exciton lifetime of the NTTIC film. These results demonstrate the high potential of naphthalene-cored D units of A-D-A type NFAs to achieve a long singlet exciton lifetime and a resultant high PCE in NFA-based OPVs.

Automated summary

The comprehensive study on the relationship between the structure and the photophysical and photovoltaic properties of acceptor-donor-acceptor (A-D-A) type nonfullerene acceptors (NFAs) is crucial for design guidelines in high-performance organic photovoltaics (OPVs). The inclusion of naphthalene-cored D units in A-D-A type NFAs shows potential for achieving a long singlet exciton lifetime and resulting in high power conversion efficiency in NFA-based OPVs.