Photophysics and photovoltaic application of direct heteroarylation derived isoindigo based copolymers

Abstract

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.

AFRIKAANSE OPSOMMING: Organiese sonselle is moontlik die gesig van toekomstige hernubare energiebronne weens die omgewingsvriendelikheid, lae vervaardigingskoste en meganiese buigsaamheid daarvan. Die struktuur van organiese materiale, soos gekonjugeerde polimere, speel n belanrike rol in hul opto-elektriese eienskappe wat tersaaklik is tot die drywing omskakelingsdoeltreffendheid van organiese sonselle. Ko-polimere het interne skenker-ontvanger koppelings wat die bandgaping van die onderskeie donor of ontvanger eenhede verklein. Om die faktore wat die doeltreffendheid beperk te verstaan is dit belangrik om die fundamentele fotofisiese eienskappe van sulke ko-polimere te verstaan. Die dinamika geassosier met ladingsoordrag in die molekule (in die soliede films en in heterogene massa-aansluiting materiaal) is bestudeer deur middel van fsoorgangsabsorbsie spektroskopie in drie ko-polimere. Die resultaat vann bitiofeenisoindigo (bithiophene-isoindigo) ko-polimeer in verdunde oplossing, het n intramolekulłre tempo van ladingsoordrag van 2 ps getoon. Die heterogene massa-aansluiting film met n mengsel van P2TI:PCBM71, het n vinnige ladingsopwekking van <250 fs getoon. Slegs 40% van die lading draers kon egter langer as 2 ns bestaan, weens swak deursype-llingskanale vir ladings in die aktiewe lae se morfologie. Die lae drywing omskakelingsdoeltreffendheid in P2TI:PCBM71 gebaseerde sonselle is ook weens die swak lading deursypell-ingskanale. Die effek wat sykettings op die fotofisika van twee tritiofeen-isoindigo (terthiophene-isoindigo) ko-polimere is verder ondersoek. Die resultate wys dat wanner die lengte van die alkiel sykettings by posisie 3 en 4 van die eerste en laaste tritiofeen eenheid vergroot van (C8H17) met vier metiel eenhede tot (C12H25), raak die intramolekulłre ladingsoordrag tempo stadiger van 4.5 ps na 13 ps. Die langer syketting verlaag ook die leeftyd van die elektron-holte paar (exciton) deur interketting wisselwerking te verhinder. Diffusie van elektron-holte pare is minder effektief wanneer die sykettings langer is.