|M.Sc Student||Ella Olejnik|
|Subject||Photoinduced Absorption Studies of Alkyl Sensing|
|Department||Department of Physics||Supervisors||Professor Emeritus Ehrenfreund Eitan|
|Full Professor Lifshitz Efrat|
|Full Thesis text|
In this work we investigate the recombination dynamics and recombination mechanism of long-lived photoexcitations in π- conjugated polymeric systems.
Due to the semiconducting and optical properties together with the attributes of organic materials such as processability, flexibility and strength the π-conjugated polymers form a class of electronic materials.
The experimental methods which been used in this study are optical absorption, photoluminescence and photoinduced absorption.
We report on the study of an alkyl-sensing co-polymer. The unique property of the new polymer is the sensitivity to the presence of different alkyls. We studied the variations in the optical properties upon exposure to alkyls.
We have observed photoinduced charged polarons both in the unexposed and alkyl exposed co-polymers. In both we have determined that the recombination process is bimolecular, but with a broad distribution of recombination rates ("bimolecular dispersive"). The polarons have a similar spectrum in both cases but the recombination time is enhanced by a factor of
~40 upon exposure. In both unexposed and exposed copolymers the photoinduced infrared active vibration (IRAV) discrete lines overlap the lower polaron absorption band, resulting in "anti-resonance" due to the Fano effect.
In addition to photoinduced polarons we have identified additional species which absorb at around 1.5 eV (the HE band). The recombination time of these species is much shorter than the polarons recombination time and it does not change upon exposure to alkyls.
Just below the HE (at 0.7-1.4 eV) we have observed peculiar behavior and photobleaching even before exposure to methyl iodide. The intensity dependence of the PIA in the range 0.7-
1.4 eV shows a very peculiar behavior. The positive photoinduced absorption at 1.4 eV, initially increase with the laser intensity and then, with a further increase in intensity it decreases; in parallel the negative photoinduced absorption ("photobleaching") is nonexistent at low intensities, then increases with increasing intensity and then with further increase in intensity it decreases. The modulation frequency dependence also shows a peculiar behavior: with increasing frequency the signal grows unlike any other photoinduced excitation that usually decreases with increasing frequency. We account for this behavior by assuming that in this energy region, at least two species contribute simultaneously: one is absorbing (like polaron or triplet exciton) and is less absorbing under illumination (like traps). The two species have different recombination times and different laser intensity dependence giving rise to the observed behavior.