Synthetic principles directing charge transport in low-band-gap dithienosilole-benzothiadiazole copolymers

Pierre Beaujuge, Hoinok Tsao, Michael Ryan Hansen, Chad M. Amb, Chad Risko, Jegadesan Subbiah, Kaushik Roy Choudhury, Alexey V. Mavrinskiy, Wojciech Pisula, Jean Luc Brédas, Franky So, Kläus Müllen, John R. Reynolds

Research output: Contribution to journalArticlepeer-review

109 Scopus citations

Abstract

Given the fundamental differences in carrier generation and device operation in organic thin-film transistors (OTFTs) and organic photovoltaic (OPV) devices, the material design principles to apply may be expected to differ. In this respect, designing organic semiconductors that perform effectively in multiple device configurations remains a challenge. Following "donor-acceptor" principles, we designed and synthesized an analogous series of solution-processable π-conjugated polymers that combine the electron-rich dithienosilole (DTS) moiety, unsubstituted thiophene spacers, and the electron-deficient core 2,1,3-benzothiadiazole (BTD). Insights into backbone geometry and wave function delocalization as a function of molecular structure are provided by density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level. Using a combination of X-ray techniques (2D-WAXS and XRD) supported by solid-state NMR (SS-NMR) and atomic force microscopy (AFM), we demonstrate fundamental correlations between the polymer repeat-unit structure, molecular weight distribution, nature of the solubilizing side-chains appended to the backbones, and extent of structural order attainable in p-channel OTFTs. In particular, it is shown that the degree of microstructural order achievable in the self-assembled organic semiconductors increases largely with (i) increasing molecular weight and (ii) appropriate solubilizing-group substitution. The corresponding field-effect hole mobilities are enhanced by several orders of magnitude, reaching up to 0.1 cm 2 V -1 s -1 with the highest molecular weight fraction of the branched alkyl-substituted polymer derivative in this series. This trend is reflected in conventional bulk-heterojunction OPV devices using PC 71BM, whereby the active layers exhibit space-charge-limited (SCL) hole mobilities approaching 10 -3 cm 2 V -1 s -1, and yield improved power conversion efficiencies on the order of 4.6% under AM1.5G solar illumination. Beyond structure-performance correlations, we observe a large dependence of the ionization potentials of the polymers estimated by electrochemical methods on polymer packing, and expect that these empirical results may have important consequences on future material study and device applications. © 2012 American Chemical Society.
Original languageEnglish (US)
Pages (from-to)8944-8957
Number of pages14
JournalJournal of the American Chemical Society
Volume134
Issue number21
DOIs
StatePublished - May 18 2012

ASJC Scopus subject areas

  • Biochemistry
  • Colloid and Surface Chemistry
  • Chemistry(all)
  • Catalysis

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