Synthetic principles directing charge transport in low-band-gap dithienosilole-benzothiadiazole copolymers
Hansen, Michael Ryan
Amb, Chad M.
Choudhury, Kaushik Roy
Mavrinskiy, Alexey V.
Brédas, Jean Luc
Reynolds, John R.
KAUST DepartmentPhysical Sciences and Engineering (PSE) Division
KAUST Solar Center (KSC)
Chemical Science Program
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AbstractGiven 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.
SponsorsWe acknowledge the funding of this work as follows. J.RR: the Air Office of Scientific Research (FA9550-09-1-0320) for new materials development. J.RR. and F.S.: the Office of Naval Research (N00014-11-1-0245) for transport measurements and device construction/testing. J.-L.B.: the Office of Naval Research (N00014-11-1-0211) for computational studies. KM.: the German Science Foundation (Korean-German IR TG), the European Community's Seventh Framework Programme ONE-P (grant agreement no. 212311), DFG Priority Program SPP 1355, DFG MU 334/32-1, DFG Priority Program SPP 1459, and ESF Project GOSPEL (ref no.: 09-EuroGRAPHENE-FP-001) for characterization studies. M.R.H. acknowledges Dr. Robert Graf for helpful discussions and Prof. Hans Wolfgang Spiess for his continued support. P.M.B. acknowledges Dr. Uwe Rietzler and Dr. Rudiger Berger for their support at the AFM facilities of MPIP-Mainz.
PublisherAmerican Chemical Society (ACS)