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Molecular design, synthesis and performance evaluation of phenothiazine-based small molecules for efficient organic solar cells
Umeå University, Faculty of Science and Technology, Department of Chemistry. (Bertil Eliasson)ORCID iD: 0000-0001-5746-017X
2018 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Design, syntes och utvärdering av fenotiazin-innehållande små molekyler till effektiva organiska solceller (Swedish)
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 [en]
Organic solar cell, small molecule donors, phenothiazine, power conversion efficiency
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
URN: urn:nbn:se:umu:diva-144465ISBN: 978-91-7601-835-4 (print)OAI: oai:DiVA.org:umu-144465DiVA, id: diva2:1180097
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
List of papers
1. An arylene-vinylene based donor-acceptor-donor small molecule for the donor compound in high-voltage organic solar cells
Open this publication in new window or tab >>An arylene-vinylene based donor-acceptor-donor small molecule for the donor compound in high-voltage organic solar cells
Show others...
2016 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 155, p. 348-355Article in journal (Refereed) Published
Abstract [en]

A donor-acceptor-donor (D-A-D) molecule has been designed and synthesized for use as the electron donating material in solution-processed small-molecule organic solar cells (OSCs). The D-A-D molecule comprises a central electron-accepting (2Z,2'Z)-2,2'-(2,5-bis(octyloxy)-1,4-phenylene)bis(3-(thiophen-2-yl)acry lonitrile) (ZOPTAN) core, which is chemically connected to two peripheral and electron-donating triphenylamine (TPA) units. The ZOPTAN-TPA molecule features a low HOMO level of -5.2 eV and an optical energy gap of 2.1 eV. Champion OSCs based on a solution-processed and non-annealed active material blend of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) and ZOPTAN-TPA in a mass ratio of 2:1 exhibits a power conversion efficiency of 1.9% and a high open-circuit voltage of 1.0 V. 

Keywords
Organic solar cell, Small-molecule donor, Fullerene acceptor, Solution processing, High open-circuit ltage, Thermal stability
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-126290 (URN)10.1016/j.solmat.2016.06.018 (DOI)000381529100040 ()
Available from: 2016-11-08 Created: 2016-10-03 Last updated: 2018-06-09Bibliographically approved
2. Effect of acceptor strength on optical, electrochemical and photovoltaic properties of phenothiazine-based small molecule for bulk heterojunction organic solar cells
Open this publication in new window or tab >>Effect of acceptor strength on optical, electrochemical and photovoltaic properties of phenothiazine-based small molecule for bulk heterojunction organic solar cells
2018 (English)In: Dyes and pigments, ISSN 0143-7208, E-ISSN 1873-3743, Vol. 149, p. 830-842Article in journal (Refereed) Published
Abstract [en]

Two new acceptor–π(donor)–donor–π(donor)-acceptor small molecules SM1 and SM2 with benzodithiophene (BDT) donor core unit linked via phenothiazine (PTZ) donor units with 1,3–indanedione and malononitrile end capping units, respectively, have been designed and synthesized and used as electron donor along with PC71BM as acceptor for solution-processed bulk heterojunction organic solar cells. The influence of these end-capping groups on their thermal, optical, electrochemical properties and photovoltaic performance was investigated. After the optimization of the donor to acceptor weight ratio and solvent vapor annealing, the organic solar cells based on SM1:PC71BM and SM2:PC71BM active layers showed the power conversion efficiency (PCE) of 6.20% and 7.45%, respectively. Hence, the end-capping acceptor units for the two small molecules revealed a significant influence on the PCE of the organic solar cells, which resulted from differences in film absorption, molecular packing and charge transport properties.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Small molecule donors, Bulk heterojunction solar cells, Solvent vapor annealing, Power conversion efficiency
National Category
Physical Chemistry
Identifiers
urn:nbn:se:umu:diva-143018 (URN)10.1016/j.dyepig.2017.11.048 (DOI)000423246900101 ()
Available from: 2017-12-14 Created: 2017-12-14 Last updated: 2018-06-09Bibliographically approved
3. Asymmetric triphenylamine–phenothiazine based small molecules with varying terminal acceptors for solution processed bulk-heterojunction organic solar cells
Open this publication in new window or tab >>Asymmetric triphenylamine–phenothiazine based small molecules with varying terminal acceptors for solution processed bulk-heterojunction organic solar cells
2018 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 9, p. 6390-6400Article in journal (Refereed) Published
Abstract [en]

Three compounds consisting of the electron-donating triphenylamine–phenothiazine conjugate backbone and each of the electron-withdrawing groups 3-ethylrhodanine, malononitrile and 1,3-indandione have been synthesized and used as donors in blends with [6,6]-phenyl-C70-butyric acid methyl ester (PC71BM) for organic solar cell devices. After improvements of the active layer structure by a selected donor-to-acceptor weight ratio and a two-step solvent and thermal annealing, the organic solar cells showed power conversion efficiency (PCE) values in the range of 4.79–7.25%. The highest PCE was obtained for the bulk heterojunction device with the indandione compound, which can be attributed to its better absorption profile, higher crystallinity, more balanced electron and hole transport, higher charge collection efficiency and reduced recombination, in comparison with the photovoltaic cells from the other two compounds. DFT-calculated characteristics, absorption spectra and cyclic voltammetry of the compounds, along with X-ray diffraction patterns of the blend films, are used to validate the photovoltaic results.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2018
Keywords
Organic solar cell, asymmetric small molecule donor, charge-transfer compound, bulk heterojunction, solvent annealing, thermal annealing, electron acceptor, power conversion efficiency
National Category
Organic Chemistry Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-144464 (URN)10.1039/C7CP08653A (DOI)000429280100040 ()29441385 (PubMedID)
Available from: 2018-02-03 Created: 2018-02-03 Last updated: 2018-06-09Bibliographically approved
4. Phenothiazine-based small molecules for bulk heterojunction organic solar cells: Variation of side-chain polarity and length of conjugated system
Open this publication in new window or tab >>Phenothiazine-based small molecules for bulk heterojunction organic solar cells: Variation of side-chain polarity and length of conjugated system
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-144478 (URN)
Available from: 2018-02-03 Created: 2018-02-03 Last updated: 2018-06-09

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