Solution-processable oligomeric and small molecule semiconductors for organic solar cells
Le Borgne, Mylène
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Organic solar cells appear as a promising technology within photovoltaic field owing to their low-cost fabrication and their great flexibility enabling a widespread distribution. For now, they are still at the prototype stage due to their limited efficiency and lifetimes. Many efforts were realized in designing new materials as they are involved in every steps of the photovoltaic process and thus they dictate the cell efficiency. Along this thesis, two series of electron-donating semi-conductors were designed and synthesized. The first series consist in three oligomers containing three diketopyrrolopyrrole units, a well-studied chromophore. Those oligomers absorb up to the near infra-red region, a very interesting feature for light harvesting. Through the engineering of electron-rich spacers, various twists were generated in the oligomers backbone. The oligomer showing a coplanar conformation appears to be the most crystalline and thus exhibits the best charge transport properties with a hole mobility of 10-3 cm².V-1.s-1. However, bulk heterojunction organic solar cells, this high crystallinity results in an unfavorable morphology and a PCE inferior to 1%. As for the second series, the four small molecules combined 3,3’-(ethane-1,2-diylidene)bis(indolin-2-one) (EBI), an electron deficient unit, and various electron-rich units such as thiophene (EBI-T), benzofuran (EBI-BF) and bithiophene (EBI-2T). Among EBI derivatives, EBI-BF demonstrated the highest hole mobility of 0.021 cm².V-1.s-1 in field effect transistors due to its coplanar conformation. Meanwhile, in bulk heterojunction solar cells, the highest PCE of 1.92% was obtained with EBI-2T:PC61BM blend owing to a more appropriate morphology and the broadest absorption spectrum of EBI-2T.