M.Sc Thesis

M.Sc StudentShusterman Olga
SubjectPhotoinduced Charge Transfer in (Tetrathiophene/Acceptor)
DepartmentDepartment of Physics
Supervisors PROFESSOR EMERITUS Eitan Ehrenfreund


Photoinduced charge transfer in (tetrathiophene)/(acceptor) systems

This work deals with the excited electronic states of some conjugated oligomers.  Their well-defined molecular structure and unique chain length allow structure-property relations to be studied and compared to those of longer polymers having a distribution of chain length. The opto-physical properties of the materials have been studied by several experimental methods. The experimental techniques used in this study are optical absorption, photo-luminescence and photoinduced absorption. We focused our research on one type of conjugated oligomers: oligo-thiophenes (Tn). In this report, we show our results on tetra-thiophene (T4) derivatives and the effect of having in their close vicinity acceptor groups, such as C60 and Viologen. We have observed photoinduced electron transfer from the oligomers to the electron acceptor groups, similar to what has been observed in long conjugated polymers. Such charge transfer results in long-lived charged species, which may be useful in opto-electronic devices such as light emitting diodes and photovoltaic or solar cells.

The main result of the photoinduced electron transfer is the quenching of the photoluminescence by the addition of the acceptors. Quenching by a factor of up to 100 was observed. This is taken as an evidence for charge separation, which prevents radiative recombination. The positive radical tetra-thiophene ion, which is left after photoexcitation, is studied using photoinduced absorption spectroscopy, showing long lived (~mili-seconds) photoexcited species, with nearly mono-molecular recombination mechanism, which is thermally activated (activation energy around 0.07 eV).