Influence of the Annealing Temperature on the Photovoltaic Performance and Film Morphology Applying Novel Thermocleavable Materials

Martin Helgesen Petersen, Morten Bjerring, Niels Chr. Nielsen, Frederik C Krebs

    Research output: Contribution to journalJournal articleResearchpeer-review


    Di-2-thienyl-2,1,3-benzothiadiazole (DTBT) bearing thermally cleavable ester groups in different positions were prepared and copolymerized with alkylsubstituted cyclopentadithiophene (CPDT). The polymers were found to have band gaps in the range of 1.66−2.03 eV and were explored in polymer photovoltaic devices as mixtures with soluble methanofullerenes. The positioning of the ester groups proved to be very significant despite the identical conjugated backbone of 2-methyl-2-hexyl 5-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4-b′]dithiophen-2-yl)-2-(7-(3-(((2-methylhexan-2-yl)oxy)-carbonyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-3-carboxylate (T1) and 2-methyl-2-hexyl 2-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4-b′]dithiophen-2-yl)-5-(7-(4-(((2-methylhexan-2-yl)oxy)carbonyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-3-carboxylate (T2). Power conversion efficiencies of up to 1.92% were observed for polymers bearing ester groups on the 4-positions of the thienyl groups (T2), but shifting them to the 3-positions (T1) reduced the efficiency significantly to 0.18%. The thermal behavior of the polymers was studied with thermogravimetric analysis (TGA) that showed a weight loss around 200 °C corresponding to elimination of the ester side chains followed by a second weight loss around 300 °C corresponding to loss of CO2 via decarboxylation. The temperature of thermocleavage of the active layer films was optimized to 265 °C whereby the T2:PCBM solar cells maintained a significant performance giving efficiencies up to 1.49%.
    Original languageEnglish
    JournalChemistry of Materials
    Issue number19
    Pages (from-to)5617-5624
    Publication statusPublished - 2010

    Bibliographical note

    This work was supported by the Danish
    Strategic Research Council (DSF 2104-05-0052 and 2104-
    07-0022), the Danish National Research Foundation
    (through support of inSPIN), and the Danish Biotechnological
    Instrument Center (DABIC).


    • Polymer solar cells
    • Solar energy

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