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Title: Understanding the structure-property relationship in diketopyrrolopyrroles for organic electronics
Author: Sharma, Lisa
ISNI:       0000 0004 9359 9779
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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The effects of chain length, encapsulation and asymmetry of conjugated DPP-based polymers and small molecules were investigated. The first part of this thesis explored how chain length can affect the optical properties of conjugated DPP-based materials. Previously in our group, a series of thienyl-DPP oligomers (n = 1-5) and their polymeric counterpart were synthesised via a series of Suzuki-Miyaura cross-couplings. However, the tetramer was found to be impure. Thus, in this work the tetramer was re-synthesised via alternative reaction conditions. In addition, the DPP monomer, dimer and polymer were also re-synthesised for this study. The optical properties of the DPP oligomers were studied and it was observed that as the chain length increased from monomer to dimer, the extinction coefficient did also, at which it reached a maximum. Then, as the chain length increased from the dimer to the pentamer, the value largely decreased, suggesting that a saturation limit had been reached. On the other hand, this was challenged by the polymer’s largely red-shifted absorption compared to any other compound in the study. However, as the expected size of the red shift should decrease with extending conjugation length, this result was unexpected. It is possible that this may have been due to aggregation of the polymer chains, thus suggesting that the absorption of the DPP compounds does not rely solely on the ECL alone. The second part of this thesis explores the effect of encapsulation of the DPP core, on the performance of OPV devices. Two series of DPP-based polymers doped with an increasing amount of an encapsulated DPP monomer were synthesised, and the corresponding OPV devices were fabricated. In general, the encapsulated devices achieved higher Voc values than the reference, however the optimum amount of encapsulation was found to vary in different polymer systems. It is possible that through the introduction of the encapsulated dopant, this alters the donor-acceptor interface by either increasing the difference in the static dipole moment, reducing the ratio of the CT-state density or reducing the reorganization energy, thus resulting in an increase of the Voc. However, these results may have been affected by differences in polymer molecular weight. Three additional ring sizes of the encapsulated monomer were also investigated, followed by the synthesis of a series of encapsulated DPP-BDT based polymers. It was observed that the ring size of the encapsulated monomer had an influence over the optical properties and performance of the corresponding polymers, thus highlighting the importance of choosing the optimum ring size. The synthesis of an encapsulated and linear chained DPP-bi-thiophene co-polymer consisting of ethylene glycol side chains was also attempted, for application in bioelectronics. The final part of this thesis explores the different synthetic attempts towards a novel DPP polymer consisting of a higher DPP to thiophene ratio than previously reported. Based upon previous electronic calculations, this polymer is theorised to show increased tolerance towards energetic/torsional disorder, due to secondary overlap contribution between the thiophene/DPP units. Several synthetic building blocks were synthesised, among them alkylated thienyl pyrrolinone esters. These were used to develop a novel synthetic methodology towards fully asymmetric DPP derivatives. Four fully asymmetric DPP derivatives were synthesised, including a pair of structural isomers. It was observed that their optical and theoretical properties were found to lie in-between that of their symmetrical counterparts. Thus, it is possible to fine tune properties such as absorption and optical band gap via such asymmetric design strategies.
Supervisor: Bronstein, Hugo Sponsor: ERC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Organic Electronics ; Diketopyrrolopyrroles ; DPP ; DPP polymers ; Conjugated polymers ; Organic solar cells ; Organic photovoltaics ; Conjugated materials ; Encapsulated polymers ; Conjugated oligomers ; Conjugated small molecules ; Organic conjugated polymers ; DPP small molecules