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  • 1.
    Iqbal, Javed
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Department of Chemistry, University of Agriculture, Faisalabad 38040, Pakistan.
    Enevold, Jenny
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wang, Jia
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Revoju, Srikanth
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Eliasson, Bertil
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    An arylene-vinylene based donor-acceptor-donor small molecule for the donor compound in high-voltage organic solar cells2016In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 155, p. 348-355Article in journal (Refereed)
    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. 

  • 2.
    Revoju, Srikanth
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Molecular design, synthesis and performance evaluation of phenothiazine-based small molecules for efficient organic solar cells2018Doctoral thesis, comprehensive summary (Other academic)
    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.

  • 3.
    Revoju, Srikanth
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Biswas, Subhayan
    Eliasson, Bertil
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Sharma, Ganesh D.
    Asymmetric triphenylamine–phenothiazine based small molecules with varying terminal acceptors for solution processed bulk-heterojunction organic solar cells2018In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 9, p. 6390-6400Article in journal (Refereed)
    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.

  • 4.
    Revoju, Srikanth
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Biswas, Subhayan
    Eliasson, Bertil
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Sharma, Ganesh D.
    Effect of acceptor strength on optical, electrochemical and photovoltaic properties of phenothiazine-based small molecule for bulk heterojunction organic solar cells2018In: Dyes and pigments, ISSN 0143-7208, E-ISSN 1873-3743, Vol. 149, p. 830-842Article in journal (Refereed)
    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.

  • 5.
    Revoju, Srikanth
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Biswas, Subhayan
    Eliasson, Bertil
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Sharma, Ganesh D.
    Phenothiazine-based small molecules for bulk heterojunction organic solar cells: Variation of side-chain polarity and length of conjugated system2019In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, p. 232-242Article in journal (Other academic)
    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.

  • 6. Yadagiri, B.
    et al.
    Narayanaswamy, K.
    Revoju, Srikanth
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Eliasson, Bertil
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Sharma, Ganesh D.
    Singh, Surya Prakash
    An all-small-molecule organic solar cell derived from naphthalimide for solution-processed high-efficiency nonfullerene acceptors2019In: Journal of Materials Chemistry C, ISSN 2050-7526, E-ISSN 2050-7534, Vol. 7, no 3, p. 709-717Article in journal (Refereed)
    Abstract [en]

    Two small molecules BYG-1 and BYG-2 with fluorene donor and benzothiadiazole acceptor units connected to the terminal naphthamide group via ethyne linker were designed and synthesized. In this work we have discussed the effect of fluorine atoms connected with electron withdrawing benzothiadiazole unit to the fluorene core (BYG-1). In this study, we have fabricated solar cells with small-molecular donor and acceptor materials in the device architecture of bulk-heterojunction, using highly conjugated BYG-1 and BYG-2 as electron acceptors along with an appropriate small molecule donor (SMD). After improving the device architecture of the active layer using a suitable donor-to-acceptor weight ratio with solvent vapour annealing, we achieved power conversion efficiencies of 8.67% and 7.12% for BYG-1 and BYG-2, respectively. The superior photovoltaic performance of the fluorine-substituted BYG-1 can be attributed to its higher crystallinity, more balanced charge transport mobilities and efficient exciton dissociation.

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