August 23, 2013
10:00am (334 JFB)
Title: PHOTOEXCITATIONS OF NON LUMINESCENT CONJUGATED POLYMERS
We describe the use of the picosecond pump-probe correlation technique to measure the transient photomodulation (PM) spectrum and decay kinetics in various pi – conjugated polymers (PCPs) films and blends. Using two ultrafast laser systems, we covered a broad spectral range from 0.25 – 2.5 eV in the time domain from 200 fs to 1 ns with 150 fs time resolution. We also used steady state PM spectroscopy, photoluminescence (PL), electro-absorption and doping-induced absorption for studying the excitations and linear and nonlinear optical properties of PCPs and guest/ host blends.
In particular we studied two different types of PTV polymer derivatives. One polymer is the ordered regio-regular (RR) and regio-random (RRa) – PTV in which the dark exciton, 2Ag is the lowest excited state. In these polymers the photoexcited exciton has very fast decay kinetics due to internal conversion to the dark exciton, which results in weak PL emission; thus these two polymers are non-luminescent. The other PTV derivative is the imide – PTV which is more luminescent, but still not as luminescent as other polymers. The excitation dynamics in this polymer is dominated by singlet fission into triplet excitons that is also a non-radiative decay channel. In addition we also demonstrate transient strain spectroscopy in RR – PTV thin films, where the ultrafast energy release associated with the exciton fast decay kinetics gives rise to substantial static and dynamic strains in the film that dramatically influences the film’s transient PM response.