Charge-transfer and other excitonic state in conjugated polymer : fullerene blends-implication in photovoltaics

"May 2014."Dissertation Supervisor: Dr. Suchi Guha.Includes vita.Over the last few decades there has been tremendous progress in organic photovoltaics (OPVs), with efficiencies reaching over 10%. Still, many factors including the origin and the dynamics of charge carrier involved are debatable. New and sensitive techniques are constantly being devised to identify the origin of free charges. At the same time, a lot of research has also been devoted to synthesize low bandgap material such that its absorption spectra overlap with that of the solar spectrum. The most important hindrance in organic semiconductors is the formation of bound electron-hole (exciton) charge pair upon photoexcitation. Additional energy is required to dissociate the bound pair to generate free charges for photovoltaic application. The most popular and efficient way to dissociate excitons is to fabricate a bulk heterojunction solar cell, which comprises of a blend of at least two polymers: one donor and the other acceptor. It is very well established that the presence of fullerene (acceptor) helps in transfer of the negative charges from the donor polymer to fullerene, making the exciton slightly less bound. The nanometer scale islands further help in migration of charges. A crucial aspect of our studies has been evaluating the role of various excitonic states such as charge-transfer and triplet excitonic states in device efficiencies. The focus of this work was on diketopyrrolopyrrole (DPP)- based donor-acceptor (D-A) type conjugated copolymers which have low bandgap energies and have been known to show high efficiency in organic photovoltaics. These copolymers have D-A unit present in the same chain, which lowers the optical bandgap of the material. Variation of either the donor or the acceptor fraction offers an option to tune the optical bandgap by using the same D-A chromophores. The D-A configuration also results in the separation of positive and negative charges within the same polymeric chain, which is the intramolecular charge-transfer excitonic state. We analyze the intramolecular charge-transfeIncludes bibliographical references (pages 125-136)