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Polymer Side-Chain Elucidating Thermochromism of Benzodithiophene-Dithiophenylacrylonitrile Copolymers –Polymer Solubility Correlation of Thermochromism and Photovoltaic Performance

Abstract

Nine pBCNx polymers based on donor--acceptor benzo[1,2-b:4,5-b’]dithiophene-(E)-2,3-di(thiophen-2-yl)acrylonitrile main-chain structure and n-octyl/2-ethylhexyl as the side-chain substituents have been designed and synthesized. The thermochromism of pBCNx polymers have been carefully examined in both solution and thin film. With additional evidences from Raman spectroscopy and AFM microscopy, we have deciphered the varied colour of nine pBCNx polymers in solution and the pronounced thin-film thermochromism only observed for certain pBCNx polymers, i.e., pBCN4, pBCN5, pBCN6, and pBCN8. It is the alkyl side-chain and hence the solubility or aggregation of pBCNx polymers as a main factor for the varied colour in solution and the varied thermochromism in thin film. We have identified that a stronger main-chain and main-chain interaction takes place on (E)-2,3-di(thiophen-2-yl)acrylonitrile moiety. Whereas poor to moderate solubility pBCN3, pBCN5’, and pBCN7 exhibit pronounced thermochromism in solution, pBCN4, pBCN5, pBCN6, and pBCN8 are four polymers having good solubility in solution and showing thermochromism in thin film state. We took navy blue pBCN3 (poor solubility), magenta pBCN4 (good solubility), and reddish purple pBCN5’ (moderate solubility) in the fabrication of bulk heterojunction organic photovoltaics (OPVs). With a solvent annealing treatment, power conversion efficiency of OPVs has been improved significantly from 4.7% to 6.2% for the good solubility pBCN4, but there is little and no improvement for the moderate solubility pBCN5’ and poor solubility pBCN3, respectively. Our study verifies that the good OPV performance of pBCNx can be correlated to the pronounced thin-film thermochromism. A good solubility of the polymer is the common characteristics of significant thin-film thermochromism and good OPV performance.

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Publication details

The article was received on 28 Feb 2017, accepted on 22 May 2017 and first published on 23 May 2017


Article type: Paper
DOI: 10.1039/C7PY00338B
Citation: Polym. Chem., 2017, Accepted Manuscript
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    Polymer Side-Chain Elucidating Thermochromism of Benzodithiophene-Dithiophenylacrylonitrile Copolymers –Polymer Solubility Correlation of Thermochromism and Photovoltaic Performance

    C. Chen, H. Chen, C. Lin, C. Chen, J. Golder, Y. Lan and J. Wang, Polym. Chem., 2017, Accepted Manuscript , DOI: 10.1039/C7PY00338B

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