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Phenothiazine-based small molecules for bulk heterojunction organic solar cells: Variation of side-chain polarity and length of conjugated system
Umeå University, Faculty of Science and Technology, Department of Chemistry. (Bertil Eliasson)
Umeå University, Faculty of Science and Technology, Department of Chemistry.
2019 (English)In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, p. 232-242Article in journal (Other academic) Published
Abstract [en]

Three small molecules denoted SM1, SM2 and SM3, with the phenothiazine donor moiety connected to benzothiadiazole and 3-ethylrhodanine acceptor units through thiophene π-linkers have been synthesized for use in organic solar cells with PC71BM as electron acceptor. SM1 has a 2-ethylhexyl group at the phenothiazine nitrogen, while SM2 and SM3have a 2-(2-methoxyethoxy)ethyl group at that N. Opto-electronic and dielectric properties, charge carrier mobilities, morphology of active layers, and photovoltaic properties were investigated in detail. The three molecules have wide absorption bands with high molar absorption coefficients and relatively low HOMO levels (−5.21 to −5.27 eV). Compared with the N-alkylated SM1, both SM2 and SM3 exhibit red-shifts of the long-wavelength absorption band in thin films, and show enhanced crystallinity in thin films with smaller stacking distances, higher hole mobility, and higher dielectric constant. After solvent vapourannealing, the power conversion efficiencies (PCEs) were significantly improved for the solar cell devices, from 1.69 to 3.95% for SM1, 2.78–6.62% for SM2 and 3.22–7.16% for SM3. This increase in PCEs was due to the enhancement in Jsc and FF attributed to the formation of nanoscale domains of donor and acceptor resulting in efficient charge separation, balanced charge transport and suppressed charge recombination. These results demonstrate that use of an oxygen-containing side chain, in conjunction with alkyl side chains for solubility during solar cell processing, is an alternative and effective strategy for achieving high-performance small molecule donor materials.

Place, publisher, year, edition, pages
Elsevier, 2019. p. 232-242
Keywords [en]
Organic solar cell, Small molecule donor, Bulk heterojunction, Solvent annealing, Power conversion efficiency, Dielectric constant
National Category
Chemical Sciences Organic Chemistry Condensed Matter Physics
Identifiers
URN: urn:nbn:se:umu:diva-144478DOI: 10.1016/j.orgel.2018.11.024ISI: 000453572400034OAI: oai:DiVA.org:umu-144478DiVA, id: diva2:1180002
Funder
Knut and Alice Wallenberg Foundation, KAW 2011.0055The Kempe Foundations
Note

Originally included in thesis in manuscript form 

Available from: 2018-02-03 Created: 2018-02-03 Last updated: 2019-01-10Bibliographically approved
In thesis
1. Molecular design, synthesis and performance evaluation of phenothiazine-based small molecules for efficient organic solar cells
Open this publication in new window or tab >>Molecular design, synthesis and performance evaluation of phenothiazine-based small molecules for efficient organic solar cells
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Design, syntes och utvärdering av fenotiazin-innehållande små molekyler till effektiva organiska solceller
Abstract [en]

Photovoltaics offers one of the most promising routes to generate electricity in a clean way. As an emerging technology in photovoltaics, organic solar cells (OSC) have attracted a great deal of attention owing to their potential low-cost, lightweight, flexibility and solution processability. Although power conversion efficiencies above 12% have been achieved at this date, there is a great interest for new ideal materials to further improve the PCEs and address device durability, which are major concerns for the commercialization of this technology. The main objective of this thesis is to design and synthesize phenothiazine-based conjugate small molecules and explore their use as electron donor components in OSCs. Phenothiazine is a non-planar moiety with unusual “butterfly” type of geometry, which is known to reduce molecular aggregation and intermolecular excimer formation.

In the first study of this thesis, a small molecule based on a cyano-arylenevinylene building block with deep HOMO level was prepared. Although a high open-circuit voltage of 1.0 V was achieved, the tendency of the small molecule to crystallize in the active layer at a higher temperature and with time hindered the attainment of an optimal phase morphology required for the achievement of a higher efficiency. In the second and third studies, phenothiazine was used as a π-system bridge and as a core unit to construct small molecules based on symmetric and asymmetric frameworks with varying terminal electron-withdrawing groups. The electron-withdrawing property of the terminal units was found to have a significant influence on the optical absorption properties, electronic energy levels, molecular ordering, charge carrier mobility and morphology of the resulting active layers. In the fourth study, side-chain modification of the phenothiazine unit of symmetrically configured small molecules with an oxygen-containing (methoxyethoxy ethyl) side chain resulted in the enhancement of the dielectric constant. Although absorption properties were unchanged in solution, a dense π-π stacking was observed in the solid state.

In summary, it is demonstrated that phenothiazine is a promising candidate and worth exploring donor material for OSCs. Its versatility as a π-linker and as a central core unit in symmetric and asymmetric configurations has been explored. The use of nonplanar building blocks such as phenothiazine for the construction of donor materials is an interesting strategy for controlling molecular aggregation and difficult solution processability of small molecules if it is combined with a judiciously designed conjugate backbone.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2018. p. 90
Keywords
Organic solar cell, small molecule donors, phenothiazine, power conversion efficiency
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:umu:diva-144465 (URN)978-91-7601-835-4 (ISBN)
Public defence
2018-02-28, KBE301 - Lilla Hörsalen, KBC-huset, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2018-02-07 Created: 2018-02-04 Last updated: 2018-06-09Bibliographically approved

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Revoju, SrikanthEliasson, Bertil

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