Charge Transport in Twisted Organic Semiconductor Crystals of Modulated Pitch

Abstract

Many molecular crystals (approximately one third) grow as twisted helicoidal ribbons from the melt, and this preponderance is even higher in restricted classes of materials, for instance charge transfer complexes. Previously, twisted crystallites of such complexes present an increase in carrier mobilities. Here, the effect of twisting on charge mobility is better analyzed for a mono-component organic semiconductor 2 2,5-bis(3-dodecyl-2-thienyl)-thiazolo [5,4-d]thiazole (BDT) that forms twisted crystals with varied helicoidal pitches and makes a possible correlation with carrier mobility. These films were analyzed by Xray scattering and Mueller matrix polarimetry to characterize the microscale organization of the polycrystalline ensembles. Carrier mobilities of organic field effect transistors were five times higher when the crystals were grown with the smallest pitches (most twisted) compared to those with the largest pitches along the fiber elongation direction. A ten-fold increase was observed along the perpendicular direction.Simulation of electrical potential based on scanning electron micrographs and density functional theory suggests that the twisting-enhanced mobility is mainly controlled by the fiber organization in the film. A greater number of tightly packed twisted fibers separated by numerous smaller gaps permits better charge transport over the film surface compared to fewer big crystallites separated by larger gaps.

Twisted Crystalline Organic Semiconductor Photodetectors

“…[ 27 ] The same trend has been observed for many polymers, such as poly(3‐hydroxybutyrate), [ 28 ] poly(vinylidene fluoride), [ 29 ] and poly(ethyleneterephthalate), [ 30 ] and numerous small molecules, including mannitol, [ 31 ] coumarin, [ 32 ] resorcinol, [ 33 ] tetraphenyl lead, [ 10 ] and BDT. [ 13 ]…”

“…This heterochiral twisting, coupled with splaying of helicoidal lamellae to fill the space of the spherulite, can create heterogeneities along the light path that mimic circular birefringence (CB) or circular dichroism (CD) arising from the stacking anisotropic lamellae with a systematic sense of offset. Such optical activity associated with the film morphology, as opposed to the molecular or crystal structure, is a common feature of banded spherulites [ 44 ] and has been observed in melt‐processed films of phthalic acid, [ 45 ] aspirin, [ 46 ] CTCs, [ 11 ] TTF, [ 14 ] and BDT, [ 13 ] among others. One possibility for the signal in Figure 5D–F could be the coupling of linear birefringence (LB) and LD, in particular a term of the form 1/2 (LD′·LB−LD·LB′) where the primes indicate birefringence and dichroism measured at a second of orthogonal axes at 45° with respect to the principal axes of the experiment.…”

“…[ 11 ] A monocomponent semiconductor, 2,5‐bis(3‐dodecyl‐2‐thienyl)‐thiazolo[5,4‐d]thiazole (BDT), shows improved charge mobilities for TFTs comprising twisted crystal active layers compared to straight crystal active layers, and mobilities vary inversely with pitch. [ 13 ] While large gaps were present between straight crystals, twisting promoted the formation of more frequent but smaller gaps, resulting in better film connectivity for charge transport. Modest improvements in mobility were also observed for TFTs comprising twisted tetrathiafulvalene (TTF) crystals compared to those comprising straight TTF crystals.…”

Lamellae Grating Assembly in Ring-Banded Spherulites

“…Lotz et al claimed in 2017 that some of the issues have resisted analyses for decades, and despite great studies, adequate information regarding RBS self-assembly is still lacking . Extensive research has been conducted on the morphology of ring bands on their top surfaces to grasp the fundamental assembly behavior of lamellae, − rather than studying the 3D-interior structure of a banded spherulite. , In 1952, Keller proposed that a ring-band pattern was developed in the polyethylene (PE) spherulite when several helical lamellae continuously radiated from the nuclease. , In 1953, Watson and Crick proposed the DNA double-helix model, which was established with X-ray diffraction patterns on DNA molecules, and they proved the base pairs of hydrogen-bonds hold the DNA double-helical structures through the ribose and phosphate backbone chains. Unlike DNA molecules, polymeric lamellae in spherulites do not have molecular–structural strong H-bonding for maintaining such a stable helical-twist conformation.…”

Structural origins of two-dimensional elastic bending in a nonaromatic organic molecular crystal

“…8 Recently, several studies have been successfully utilized the bend to strengthen the fluorescence 9 and carrier mobilities. 10,11 Mechanical flexible crystals are usually classed as plastic crystals and elastic crystals according to the deformation manner that responds to the mechanical force stimulus. For plastic crystals, a general structure characteristic of the known examples is that the molecule assembles via π⋯π stacking interactions to bound molecular columns and the adjacent columns usually interact by weak interactions, typically light halogen bonding 12 and weak hydrogen interactions, 13,14 to form the slip plane.…”