Debkumar Rana

• Laser spectroscopy is a powerful tool for the investigation of structural and dynamical changes in diverse material systems. The work presented in this thesis is focused on the investigation of organic semiconductors. Work has also been done on ionic liquids, which also present a highly attractive substance class. In the first part of the Ph.D. thesis, time-resolved studies of the effect of an external static electric field on dynamical properties of an organic semiconductor-based system are presented. The important processes in organic semiconductor devices strongly depend on the presence of internal and external electric fields. This is important when trying to understand the generation and decay mechanism of charge pairs generated in organic semiconductor-based devices. Here, we have used femtosecond transient absorption spectroscopy to investigate the polaron pair dynamics in neat Poly (3-hexylthiophene-2,5-diyl) (P3HT) thin films and P3HT films sandwiched between different electrical contacts in presence of external static electric field. Field-induced dissociation of polaron pairs, generated after photoexcitation mainly from hot excitons, to charge carriers is observed in the P3HT device increasing the reverse bias. We have also studied the polaron-pair dynamics of a typical donor/acceptor heterojunction device made from phenyl-C61-butyric acid methyl ester (PCBM) as acceptor and P3HT as donor with different donor/acceptor concentration. In the second part of the thesis, we have used density functional theory (DFT) to model different systems. We have investigated the effect of external fields on structural parameters in P3HT and the change in reorganization energy and hence the electron-phonon coupling ‘Huang-Rhys factor’, due to change in external electric field in P3HT, have been calculated. In addition, here, we provide a summary of our work on ionic liquid systems on water-ionic liquid interactions in frequency domain using Raman spectroscopy and DFT.