Ultrafast Energy Transfer and Structural Dynamics of the PTB7 Polymer on a MoS₂ Monolayer

Energy transfer across a heterogeneous interface is an important topic to understand detailed functioning mechanisms of solar cells and photocatalysts. Here, we used mega-electronvolt ultrafast electron diffraction (MeV UED) as a sensitive time-resolved ”thermometer” to simultaneously measure structural dynamics and energy transfer between a polymer (PTB7) and an atomic thin MoS₂ monolayer. Optical excitation of the polymer to the excited state relaxes quickly through the heterojunction interface to the monolayer MoS2. The thermal energy transfers from the polymer to the atomic layer can be described by a thermal transport model. The time-resolved structural dynamics of polymer suggests a bond dissociation located specifically at the C-O sidechain during the flattening motion of the two aromatic conjugated rings in the excited state, providing the fundamental mechanism of the photo-instability of a polymer in the applications of solar cell materials.