Investigation on the photophysics of the narrow bandgap polymer for PDPPTT-T

• The simulated absorption spectra of oligomers are achieved.
• The lowest energy band of experimental absorption spectra is assigned.
• TA measurements give the relaxation process of polymer in CB solution and solid film.

We systemically study the electronic transition and photoexcitation process of PDPPTT-T in the solution and the solid film by theoretical and experimental methods. The chemical calculations show that in experimental absorption spectra the lowest energy band is assigned to the S0 → S1 transition featuring π–π∗ transition. The time-dependent TA spectra of the PDPPTT-T in the CB solution show that the relaxation process of excited polymer at the lower intensity is composed of the vibrational relaxation and the exciton decay, however, at the higher intensity, the exciton–exciton annihilation (EEA) would appear and participate in the relaxation process. In the solid film, the dynamic process of excited polymer gets the EEA process and the exciton relaxation involved, meanwhile the relaxation of polaron may be also involved. Through comparing the intensity-dependent of initial amplitude of PDPPTT-T in the CB solution and the solid film, it is found that the exciton generation and the EEA process could compete in the film at the high excitation intensity. Consequently, we provide a systematic analysis of dynamic processes of PDPPTT-T polymer in the solution and the solid film, which is beneficial for further understanding the photovoltaic process of polymers.