Inhomogeneity of the Ultrafast Excited State Dynamics in Organic Photovoltaic Materials Measured at Nanoscale

Solid-state organic light conversion devices are known to intrinsically possess structural and electronic inhomogeneity, which limits their performance. To better understand the local transport dynamics in these systems, there is a great need to probe transport processes in the condensed phase and to follow the optical excitation dynamics with fast time resolution and high spatial resolution. Here, femtosecond pulse-sequence near-field optical microscopy, which allows monitoring of fast coherent processes in the excited state at the nanoscale has been demonstrated. Utilizing phase-locked femtosecond light pulses in a two-photon regime, we have investigated the fast excited state dynamics in bulk heterojunction film consisting of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] as a donor and dimeric perylene diimide (DPDI) as a nonfullerene acceptor as well as in the neat film of DPDI. Specifically, we have observed and analyzed the inhomogeneity of the ultrafast fluorescence oscillations as a function of the interpulse delay at the nanoscale. The oscillating fluorescence amplitude profile indicated dynamics with a characteristic time of ∼450 fs that can be associated with the intermolecular energy transport in DPDI system.