Browsing by Author "Tegegne, Newayemedhin"

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    Photophysics and photovoltaic application of direct heteroarylation derived isoindigo based copolymers
    (Stellenbosch : Stellenbosch University, 2018-03) Tegegne, Newayemedhin; Schwoerer, Heinrich; Steenkamp, Christine M.; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH SUMMARY: Environmental friendliness, ease of fabrication, low cost and mechanical flexibility make organic solar cells a potential future of renewable energy sources. The structure of organic materials such as conjugated polymers play an important role in their optoelectronic properties that are relevant to power conversion efficiency of organic solar cells. Copolymers have an internal donor-acceptor coupling that will reduce the band gap from the respective donor or acceptor units. Fundamental photophysical properties of such copolymers is crucial to understand efficiency limiting factors. The dynamics associated with charge transfer in the molecules, in the solid films and bulk hetrojunction composites were studied using fs-transient absorption spectroscopy in three copolymers. The result on a bithiophene-isoindigo copolymer in dilute solution showed an intramolecular charge transfer state generation rate of 2 ps. The bulk heterojunction film of a blend of P2TI:PCBM71, showed a fast charge generation (<250 fs). But only 40 % of the charge carriers could stay longer than 2 ns. This is due to a poor charge percolation pathways in the active layer morphology. The low power conversion efficiency in P2TI:PCBM71 based solar cells is due to poor percolation pathway to charge carriers. Moreover, we studied effect of side chains on photophysics of two terthiophene-isoindigo copolymers. The result showed when the length of the alkyl side chains at position 3 and 4 of the first and the last terthiophene unit increases from (C8H17) by four methyl units to (C12H25), the intramolecular charge transfer rate slows down from 4.5 ps to 13 ps. The longer side chains also lowers exciton life time by creating a barrier for interchain interaction. Exciton diffusion is less efficient when the side chains are longer.