Ultrafast Excited State Dynamics and Functional Interfaces Probed by Second Harmonic Generation

Abstract

Understanding the ultrafast excited state dynamics in organic semiconductors after optical excitation is a key requisite on the road towards efficient organic solar cells. Additionally, the creation of functional interfaces built from organic molecular switches and the read-out of the photochromic state are essential for molecular electronics. In this thesis, static second harmonic generation (SHG) measurements were utilized to investigate the photochromism of different indolylfulgimide derivatives immobilized on silicon. During this, the influence of chemical modifications on the switching efficiencies (cross-sections) and the non-linear optical contrast between the switching states were investigated. In the second part of this thesis, femtosecond time-resolved second harmonic generation measurements were used to investigate the ultrafast decay mechanism of optically induced electronically excited states in organic semiconductors and donor/acceptor systems. These led to observations of relaxation into dimer induced states, charge trapping at native silicon oxide and ultrafast vibronic relaxation. For the donor/acceptor configurations, depending on the molecular orientation at the interface and the excitation energy, the creation of charge transfer states were investigated.