We present two-dimensional white-light
spectroscopy (2DWL) measurements
of binary and ternary bulk heterojunctions of the polymer donor PM6
mixed with state-of-the-art nonfullerene acceptors Y6 or IT4F. The
ternary film has a shorter lifetime and faster spectral diffusion
than either of the binary films. 2D line shape analysis of the PM6
ground state bleach with a Kubo model determines that all three films
have similar amplitudes of fluctuations (Δ = 0.29 fs–1) in their transition frequencies, but different relaxation times
(ranging from 102 to 24 fs). The ternary film exhibits faster dynamics
than either of the binary films. The short lifetime of the ternary
blend is consistent with increased photoexcitation transfer and the
fast frequency fluctuations are consistent with structural dynamics
of aliphatic side chains. These results suggest that the femtosecond
fluctuations of PM6 are impacted by the choice of the acceptor molecules.
We hypothesize that those dynamics are either indicative, or perhaps
the initial source, of structural dynamics that ultimately contribute
to solar cell operation.