Theoretical study of thiophene oligomers for photovoltaics: thiophene-furan for films and cyanoacrylated thiophene as sensitizer for ZnO

• Autor(a) principal: Bastos, Vinicius Alves
• Data de Publicação: 2023
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/43/43134/tde-30102023-110406/
• topic: BSE/GW; BSE/GW; corante sensitizando ZnO; DFT; DFT; dye sensitizing ZnO; Furan; Furano; Thiophene; Tiofeno

Thiophene oligomers (oligothiophenes) are important organic materials for the photogeneration of electrical energy. Still, there are some issues concerning their application in films, such as the non-planarity of the linear chains, affecting molecular packing. An interesting suggestion reported in the literature is the co-polymerization of thiophene and furan since furan oligomers (oligofurans) are highly soluble and show a planar backbone. Thus, for films, it would be interesting to use this thiophene-furan composition. In another direction, an important point is the photoinduced charge transfer from the organic region of a photovoltaic device to the semiconductor contact. Another recent suggestion was adopting short cyanoacrylated oligothiophenes (nTCs) as sensitizers in Dye-Sensitized Solar Cells (DSSCs) based on ZnO. Here, we perform a theoretical investigation of thiophene (T) and furan (F) oligomers/co-oligomers with 1 to 4 units, and nTCs containing 1 and 3 thiophene units. We adopt MP2-corrected HF, DFT (PBE), and hybrid HF/DFT (PBE0) methodologies to study the structural and ground-state electronic structure properties and the GW method and the Bethe-Salpeter equation (BSE) for quasiparticle and optical excitations. We show that avoiding TT sequencing in the thiophene-furan chains leads to planarity. Furthermore, thiophene-furan co-oligomers incorporate good electronic and optical properties of both thiophene and furan precursors: IPs similar to oligofurans and HOMO-LUMO and optical gaps similar to oligothiophenes. Regarding the nTCs, they present a donor-acceptor character with the HOMO electronic density mainly on the thiophene part and the LUMO on the cyanoacrylate termination. Consequently, the IPs are very similar to those for the thiophene-only oligomers, and the HOMO-LUMO gaps are smaller than for the oligothiophenes. The first optical excitation is mainly HOMO->LUMO, and for 3TC is around 2.5eV, in a good range for sunlight harvesting. We further performed a PBE/PBE0 investigation of a single 3TC attached to the ZnO (10-10) surface (3TC-ZnO). We see that the 3TC-HOMO is in the ZnO gap and does not communicate with the surface states, while the 3TC-LUMO is in the conduction band (CB) region, and hybridizes with the crystal states (charge-transfer states). The 3TC-ZnO setup thus really provides properties suitable for application in DSSCs.