Dynamics in Electronically Excited States of Diketopyrrolopyrrole–Thiophene Conjugated Polymer Thin Films

Conjugated polymers based on diketopyrrolopyrrole (DPP) are a versatile basis for a variety of high-performance organic semiconductors. Two conjugated polymers with DPP as an electron acceptor were prepared. The DPP-based polymers were completed with quaterthiophene (PDPP4T) as a donor in the first case; for the second donor–acceptor conjugated polymer, two thiophenes have been replaced by thieno-thiophene (PDPPTTT). Time-resolved two-photon photoemission spectroscopy is employed to study the lifetimes of electronically excited states. On the basis of the 2PPE spectra, we propose four unoccupied states between the determined valence band maximum and LUMO of the thin films. Electron kinetic energy resolved lifetimes range from a few 100 fs up to 1.4 ps and show also a slower component around 10 ps. After annealing, the ultrafast decay of the lowest-energy electrons in the upper polaron state terminated after about 20 ps. This happens in PDPPTTT for the high-energy level as well. We suggest that it is connected with improved charge transport properties of the film.