Design, Synthesis and Characterization of Novel Low Band Gap Conjugated Polymers for Use in Bulk Heterojunction Photovoltaic Solar Cells
Design, Synthesis and Characterization of Novel Low Band Gap Conjugated Polymers for Use in Bulk Heterojunction Photovoltaic Solar Cells

Author: Athanasius Justin Manji

Publisher:

Published: 2012

Total Pages: 318

ISBN-13:

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This work details the synthesis of a functionalised all-donor polymer, poly(3- hexylthioacetate thiophene) (P3HTT) [42] along with four new donor-acceptor (D-A) copolymers. Two of the four copolymers are based on fluorene (F) and carbazole (C) as electron donors copolymerised with DEBT to give, PFDEBT [62] and PCDEBT [63] respectively, while the other two are based on the same electron donors, but copolymerised with DES to give PFDES [64] and PCDES [65] respectively. All the polymers exhibited good solubility in common organic solvents and good thermal stability. The spectrophotometry analysis of polymer [42] in chloroform showed that it emits in the green region with 26% photoluminescence quantum yield (PL I". The optical response of copolymers [62] and [63] in chloroform solution and thin films are similar; both have two broad absorption bands with peaks at short and long wavelengths which are within comparable range. In addition, both emit in the deep red region of the emission spectrum and have similar PL values of 55 % and 51 % respectively. The electrochemical responses of the polymers were measured using cyclic voltammetry. The results obtained for polymer [42] were different from what was expected; its band-gap was similar to that of P3HT and its energy levels were elevated. In contrast, the band-gaps of the copolymers were tuned in the range of 1.47 to 1.65 eV The results showed that the acceptor units have exhibited the same effect of band-gap lowering in the two sets of the copolymers, for instance, the LUMO energy level of copolymers [62] and [63] is - 3.32 eV and those of copolymers [64] and [65] is - 3.57 eV Conversely, different effect was observed in the values of the HOMO energy levels, for example, the values for [62] and [64] which contain the same donor (fluorene) are - 4.86 and - 5.07 eV respectively; while those of copolymers [63] and [65] composed of carbazole are - 4.97 and - 5.04 eV respectively. This may suggest that the acceptor unit reduced the band-gap by lowering the LUMO energy level but at the same time elevated the HOMO energy level probably due to the presence of EDOT.

Synthesis and Characterization of Novel Low Band Gap Semiconducting Polymers for Organic Photovoltaic and Organic Field Effect Transister Applications
Synthesis and Characterization of Novel Low Band Gap Semiconducting Polymers for Organic Photovoltaic and Organic Field Effect Transister Applications

Author: Dickson Aleroh

Publisher:

Published: 2012

Total Pages:

ISBN-13:

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This thesis describes the synthesis, characterization and device properties of a range of conjugated polymers incorporating 3,6-dilakylthieno[3,2-b ] thiophene. We report a new and facile synthesis for the preparation of 3,6-dialkylthieno[3,2-b ]thiophene, which is readily scaled up to the multi-gram scale. With this synthesis in hand, we initially investigated the properties of poly(thienothiophene-alt-vinylene) polymers incorporating both straight and branched side-chains. Two different polymerization methods were investigated to synthesise the conjugated polymers, namely Stille and Gilch polymerization. The Gilch route was found to lead to high molecular-weight polymers with less eis-defects in the backbone. The polymers were found to be largely amorphous by X-ray diffraction measurements, although there were clear signs of aggregation by optical investigations. Field-effect transistors fabricated with these polymers exhibited charge carrier mobilities up to 0.02 cm2 V-I S-I for the straight chain analogue, with the branched polymer displaying lower mobilities. Blends with PC71BM were found to exhibit solar cell device efficiencies up to 2.5 %, with significant differences observed for polymers containing two different side-chains. In the third chapter we investigated the properties of ethynylene-linked 3,6-dialkylthieno[3,2- b ] thiophene polymers. The simple homo-polymers were found to exhibit much worse device performance than the analogous vinylene-containing polymers in transistor devices. Co- polymers with a range of electron accepting monomers were also synthesized. These displayed low optical energy gaps and signs of aggregation in the solid state. Transistors were fabricated and their performance examined. In the final part of this thesis, co-polymers bearing 3,6-dialkylthieno[3,2-b ]thiophene donor and squaraine acceptor units were synthesized. These zwitterionic conjugated polymers displayed band gaps as low as 1.0 eV. The influence of the nature of the side-chains and co- monomer was investigated with regard to their optoelectronic properties.