Electron Transfer, Vibrational Coherence and Intrinsic Dopants in Conjugated Polymer/fullerene Blends

In this thesis, we utilize two-dimensional electronic spectroscopy (2D ES) to study the photoexcited dynamics in conjugated polymer/fullerene blends. A model system, poly(3-hexylthiophene) (P3HT) /PCBM blends was chosen in these studies. Three topics—ultrafast electron transfer, vibrational coherence and dynamics of intrinsic doped electrons are investigated. In conjugated polymer/fullerene blends, the mechanism for ultrafast electron transfer, preceding or in the meantime of vibrational relaxation remains a puzzle. By utilizing 2D ES, we report dynamics maps showing the pathways of charge transfer that clearly expose the significance of hot electron transfer. We observe vibrational coherence is directly transferred from the P3HT exciton to the P3HT hole polaron in the crystalline domain. This result suggests that the wavefunction of the P3HT cation exhibits a similar spatial extent as the photogenerated exciton. To further explore vibrational coherence, we propose a protocol that utilizes 2D coherence amplitude maps to separate the vibrational coherence from ground- and excited- electronic states. By applying this method to the P3HT film, we find that the vibrational coherence from excited electronic states has a dephasing time of ~ 244 fs. The long dephasing time of the vibrational coherence suggests that it might be involved in the electron transfer process in P3HT/PCBM blends. By using the vibrational coherence, we identify a photoinduced absorption band from 640 to 690 nm that could at least partially be attributed to the absorption of the charge-transfer product. We find that a fraction of the charge-transfer states are generated by either direct photoexcitation or rapid electron transfer within 30 fs. Furthermore, we also observe the spectral signatures of intrinsic doped electrons, i.e. anions, in P3HT domains in both steady-state absorption and 2D ES. We find that the anion does not quench excitons or charges in the first 400 fs.Ph.D