Comprehensive Study on the Mobility Anisotropy of Benzothieno[3,2-b][1]benzothiophenes: Toward Isotropic Charge-Transport Properties

It is well known that benzothieno[3,2-b][1]benzothiophene(BTBT) derivatives qualify as some of the best-performing organicsemiconductors (OSCs), whereas their mobility anisotropies (e.g.,the realizations of in-plane isotropic charge-transport characteristics)have not been thoroughly understood. Herein, we systematically studythe mobility anisotropies of three types of BTBT derivatives, includingmono-, asymmetrically, and symmetrically substituted BTBTs, to understandtheir possibility to achieve low anisotropy (& mu;(max)/& mu;(min) < 1.5) in mobility. Among them, all ofthe mono- and asymmetrically substituted BTBTs show anisotropic charge-transportcharacteristics in angular resolution mobilities (ARMs) due to theirimbalanced transfer integrals of six neighboring contacts within therespective layers of a layered herringbone (LHB) packing. It is noteworthythat long alkyl chains (n & GE; 10) and phenylsubstitutions are found to promote the mobility orientation towardlow anisotropy in BTBTs. Moreover, we discuss the effects of the substitutionposition on mobility anisotropy through 1,6-C-12-BTBT, 2,7-C-12-BTBT, 3,8-C-12-BTBT, and 4,9-C-12-BTBT,which suggest that 2,7- and 3,8-substitutions are more suitable fordeveloping low anisotropy BTBT molecules. Finally, we report the synthesisof 2,7-bis(4-ethylphenyl) benzothieno[3,2-b][1] benzothiophene(DBEP-BTBT), which shows hole mobilities in the range of 0.284 to0.366 cm(2)/V-1 s(-1),with an in-plane low anisotropy of & mu;(max)/& mu;(min) = 1.29. Notably, a reported n-type BTBT derivative, namely,D(PhFCO)-BTBT, was first found to show isotropic electron mobilities,which ranged from 0.228 to 0.238 cm(2)/V-1 s(-1), with & mu;(max)/& mu;(min) = 1.05 in the calculated ARM.