טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentDrori Tomer
SubjectRecombination Dynamics of Long-lived Photoexcitations in
Conjugated Polymers
DepartmentDepartment of Physics
Supervisors Professor Emeritus Eitan Ehrenfreund
Professor Yoav Eichen


Abstract

In this work we have investigated the recombination dynamics and recombination mechanisms of long-lived photoexcitations in a pi-conjugated polymer system, MEH-PPV. Using photo pumping we produced long-lived photoexcitations: neutral triplet excitons and charged polarons. The neutral excitations are the most common excitations in a neutral conjugated polymer, whereas the charged excitations, polarons, could be realized in a system composed of a mixture of a polymer with an excited-state electron acceptor. The nature of the photoexcitations was studied using spectroscopic methods such as photoluminescence and photo-induced absorption. The photoinduced absorption technique, in which we measure the change of absorption due to photoexcitations using the pump-probe method, was utilized in order to study the recombination dynamics of  the photoexcitations. Mixtures of these samples with C60 were used in order to study the polaron recombination dynamics. We showed that the recombination dynamics of the photoexcitations can be described in terms of a "bimolecular dispersive" recombination model, in which the basic mechanism is a bimolecular process with a very broad distribution of recombination rates. In the second part we report on the observation of a novel meta-stable polaron supporting phase in a pristine polymer film. Upon bias illumination of ultraviolet light for a long time, and in inert environment, we found that polarons can now be photoexcited at the expense of triplet excitons, which are the only photoexcitations in the pristine films. After relaxation in the dark, the film recovers back to the original pristine phase, and no photoexcited polarons could be detected anymore.