PffBT4T

Conformational and energetic disorder in organic semiconductors reduces
charge and exciton transport because of the structural defects, thus
reducing the efficiency in devices such as organic photovoltaics and
organic light-emitting diodes. The main structural heterogeneity is
because of the twisting of the polymer backbone that occurs even in
polymers that are mostly crystalline. Here, we explore the relationship
between polymer backbone twisting and exciton delocalization by means of
transient absorption spectroscopy and density functional theory
calculations. We study the PffBT4T-2DT polymer which has exhibited even
higher device efficiency with nonfullerene acceptors than the current
record breaking PCE11 polymer. We determine the driving force for
planarization of a polymer chain caused by excitation. The methodology is
generally applicable and demonstrates a higher penalty for nonplanar
structures in the excited state than in the ground state. This study
highlights the morphological and electronic changes in conjugated polymers
that are brought about by excitation.