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

Abstract

It is well known that benzothieno­[3,2-b]­[1]­benzothiophene (BTBT) derivatives qualify as some of the best-performing organic semiconductors (OSCs), whereas their mobility anisotropies (e.g., the realizations of in-plane isotropic charge-transport characteristics) have not been thoroughly understood. Herein, we systematically study the mobility anisotropies of three types of BTBT derivatives, including mono-, asymmetrically, and symmetrically substituted BTBTs, to understand their possibility to achieve low anisotropy (μmax/μmin < 1.5) in mobility. Among them, all of the mono- and asymmetrically substituted BTBTs show anisotropic charge-transport characteristics in angular resolution mobilities (ARMs) due to their imbalanced transfer integrals of six neighboring contacts within the respective layers of a layered herringbone (LHB) packing. It is noteworthy that long alkyl chains (n ≥ 10) and phenyl substitutions are found to promote the mobility orientation toward low anisotropy in BTBTs. Moreover, we discuss the effects of the substitution position on mobility anisotropy through 1,6-C12-BTBT, 2,7-C12-BTBT, 3,8-C12-BTBT, and 4,9-C12-BTBT, which suggest that 2,7- and 3,8-substitutions are more suitable for developing low anisotropy BTBT molecules. Finally, we report the synthesis of 2,7-bis­(4-ethylphenyl) benzothieno­[3,2-b]­[1] benzothiophene (DBEP-BTBT), which shows hole mobilities in the range of 0.284 to 0.366 cm2/V–1 s–1, with an in-plane low anisotropy of μmax/μmin = 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 cm2/V–1 s–1, with μmax/μmin = 1.05 in the calculated ARM.

Theoretical Insights into the Hole Mobility Anisotropy of Heteroarenes and Their Derivatives

“…Unfortunately, the angles θ T of 2,9-C 8 -DNTT and 2,9-C 10 -DNTT were measured at 50.9° ( V P / V T = 1.04) and 45.8° ( V P / V T = 0.91), respectively, mismatching the required angle θ T of 62°. Indeed, it has been proven that symmetric substitution with the alkylated-phenyl group significantly contributes to realizing low anisotropy of angular mobility in anthracene and BTBT. , Based on these precedents, we hypothesize that symmetric-substituted DNTTs hold substantial promise for realizing isotropic charge mobility. This could potentially be accomplished through minor alkyl modification on 2,9-DPh-DNTT or 3,10-DPh-DNTT.…”

“…Based on the systematic analysis of the collected heteroarenes and their derivatives, we propose that DNTT, NDT, BNDT, ADT, BADT, and DNBDT are feasible precursors for designing low anisotropic or even isotropic molecules. In this case, we speculate two possible approaches to achieve low anisotropy or even isotropy hole mobility in heteroarenes: (1) symmetrically introducing alkylated-phenyl groups to gradually adjust the molecular packing and intermolecular charge-transfer characteristics to promote the isotropic mobility by altering the alkyl chain length, as the key parameters of θ T and R may be fine-tuned by varying the alkyl chain length in anthracene and BTBT, ,, and (2) symmetrically introducing naphthyl groups to enhance the intermolecular interaction (C–H···π) at the transverse contacts and to balance the transfer integrals of six neighbor contacts, as heteroarenes often have strong chalcogen–chalcogen interactions between the molecules, preventing C–H···π interactions (transverse contact) and favoring π–π interactions (parallel contact). ,, However, the two speculations above require further empirical validation in future experiments.…”

“…Previously, the anisotropic ratios of OSCs in angular mobility were categorized into seven levels: low anisotropy (μ max /μ min < 1.5), mid-anisotropy (1.5 < μ max /μ min < 2.0), high anisotropy (2.0 < μ max /μ min < 5.0), higher anisotropy (5.0 < μ max /μ min < 10.0), huge anisotropy (10.0 < μ max /μ min < 50.0), huger anisotropy (50.0 < μ max /μ min < 100.0), and complete anisotropy (μ max /μ min > 100.0). , In this case, we apply these classifications to distinguish the mobility anisotropy of heteroarenes and their derivatives. In principle, the OSCs that achieve low anisotropy in angular mobility must meet two key conditions: (1) the LHB packing motif with the inversion symmetry (simplified as LHB-IS, V P 1 = V P 2 and V T 1 ≈ V T 2 ≈ V T 3 ≈ V T 4 ) and (2) the transfer integrals that fulfill either (a) V P ≈ V T or (b) V P ≪ V T .…”

“…The unsubstituted BTBT with its LHB packing arrangement has theoretically shown complete anisotropy of μ max /μ min = 113.4, while its derivative, DPh-BTBT, maintains the LHB packing motif and exhibits midanisotropy of μ max /μ min = 1.71, indicating that the molecular extension via symmetric substitution has potential to reduce mobility anisotropy in BTBTs. To further develop isotropic OSCs along this structural motif, we focus on diacene-fused thieno­[3,2- b ]-thiophenes (e.g., DNTT, DATT, and DBTTT), which contribute to study the effect of π-conjugation system extensions on mobility anisotropy.…”

“…In 2015, Schweicher et al demonstrated that symmetric tertbutyl-substituted benzothieno­[3,2- b ]­[1]­benzothiophene (ditBu-BTBT) not only showed a hole mobility of 17 cm 2 V –1 s –1 in SC-OFET but also exhibited nearly isotropic mobility in theoretical calculation. More recently, a p-type 2,7-bis­(4-ethylphenyl) BTBT (DBEP-BTBT) showed remarkably blue fluorescence and theoretically exhibited in-plane low-anisotropy of 1.29 in angular hole mobility, while an n-type D­(PhFCO)-BTBT showed nearly isotropic mobility with a μ max /μ min ratio of 1.05 . These findings suggest that heteroarenes and their derivatives have the potential to achieve low mobility anisotropy in single crystals.…”