New semiconductors based on 2,2′-ethyne-1,2-diylbis[3-(alk-1-yn-1-yl) thiophene] for organic opto-electronics

We report the synthesis and properties of three novel acetylenic materials based on the new electron-rich building block, 2,2′-ethyne-1,2-diylbis[3- (alk-1-yn-1-yl)thiophene] (EBT). The synthesis of this new nonacene core is efficient and straightforward, and variation among n-hexyl, n-tetradecyl, and 2-ethylhexyl substituents substantially impacts the materials properties. Appending 2-ethylhexyl substituted diketopyrrolopyrrole (DPP) units to either terminus of the EBT core yields a series of low band gap molecules that are characterized in detail by a range of experimental microstructure and electronic structure probes and by density functional theory (DFT) computation. Detailed morphology/microstructure characterization of spin-cast films by X-ray diffraction and AFM reveals instructive microstructure and electronic/ photovoltaic response relationships in both organic field-effect transistors and bulk-heterojunction organic photovoltaic cells. Thus, the former devices exhibit hole mobilities (μ _h) as large as ∼0.2 cm ²/(V s) which fall as thermal annealing increases long-range order. The latter devices using PC ₆₁BM as the electron acceptor exhibit power conversion efficiencies as high as ∼2%, which appear to fall as the materials become less ordered. These results are in accord with a model where evolving grain boundaries and crystallinity impedes hole transport and excitonic charge generation.