Dynamics Of Exciton Formation And Charge Carrier Transport In Conjugated Polymer Under Photoexcitation

Conjugated polymer has many applications in the semiconductor industry: Polymer Light-Emitting Diodes, Polymer Field-Effect Transistor and Polymer So-lar Cell. There are tremendous attention paid to the research on polymer solar cell as people are getting more aware of environment protection as well as the com-prehension of developing new kind of energy. This thesis focues on two highly discussed aspects based on the research of Alan Heeger’s group:(1) Charge gen-eration, or excition formation process; (2) Charge transport, or dynamics of charge carrier motion in conjuatged polymer system.Molecule dynamics is empolyed to uncover the time profile of exciton forma-tion, which can be divided into two stages:the localization of electron-hole pairs and the relaxation process. Under photoexcitation, the electron transition rapidly forms the electron-hole pair which also leads to its localization, which triggers the total energy to shift violently and oscillate. The oscillation during the first 40 fs induces the excitation to step into the second stage, i.e., relaxation. After the relax-ation process of about 850 fs, the total energy, lattice energy and electron energy reach certain values while the lattice configuration and electron eremain localized, and indicating the formation of a singlet exciton. Because electron-hole pair is lo-calized before exciton is formed, it is highly likely that the electron-hole pair will be separated before the formation of exciton in the process of conducting polymeric solar cell. The corresponding result has been published on Journal of Physical Chemistry B.In a real bulk heterojunction polymer solar cell, after exciton separation in the heterojunction,the resulting negatively-charged carrier, a polaron, moves along the chain of the acceptor, which is believed to be of significance for the charged carrier transport properties in a polymer solar cell. During the negative polaron transport, due to the external light field, the polaron is re-excited and induces deep localiza-tion, also forming a new local distortion of the alternating bonds. It is revealed that the excited polaron moves more slowly than the ground-state polaron. Fur-thermore, the velocity of the polaron moving along the polymer chain is crucially dependent on the photoexcitation. With an increase in the instensity of the opti-cal filed, the localization of the excited polaron has been deepning, with a slowing down of the polaron’s velocity. Mostly, the deep trapping effect of charged carrier in composite conjugated polymer solar cell presents an opportunity for the future application in nanoscale memory and imaging devices. The corresponding result has been published on Nanoscale.