Molecular Disorder in Crystalline Thin Films of an Asymmetric BTBT Derivative

Abstract

The molecule 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) is an organic semiconductor with outstanding performance in thin-film transistors. The asymmetric shape of the molecule causes an unusual phase behavior, which is a result of a distinct difference in the molecular arrangement between the head-to-head stacking of the molecules versus head-to-tail stacking. Thin films are prepared at elevated temperatures by crystallization from melt under controlled cooling rates, thermal-gradient crystallization, and bar coating at elevated temperatures. The films are investigated using X-ray diffraction techniques. Unusual peak-broadening effects are found, which cannot be explained using standard models. The modeling of the diffraction patterns with a statistic variation of the molecules reveal that a specific type of molecular disorder is responsible for the observed peak-broadening phenomena: the known head-to-head stacking within the crystalline phase is disturbed by the statistic integration of reversed (or flipped) molecules. It is found that 7–15% of the molecules are integrated in a reversed way, and these fractions are correlated with cooling rates during the sample preparation procedure. Temperature-dependent in situ experiments reveal that the defects can be healed by approaching the transition from the crystalline state to the smectic E state at a temperature of 145 °C. This work identifies and quantifies a specific crystalline defect type within thin films of an asymmetric rodlike conjugated molecule, which is caused by the crystallization kinetics.

Mobility anisotropy in the herringbone structure of asymmetric Ph-BTBT-10 in solution sheared thin film transistors

“…The alternating broadening of the 00L diffraction peaks can be explained by defects within the crystal structure. 50 The coating speed has a strong influence on the width of the diffraction peaks. The peak widths (Δ q z ) of the 00 ± 2 Bragg peaks were determined and analysed.…”

“…The Ph-BTBT-10 films were then deposited by Bar-Assisted Meniscus Shearing (BAMS) as previously reported. 50 The solution shearing deposition was carried out at a substrate temperature of 105 °C and with coating speed of 1 and 10 mm s −1 . Note that all the fabrication process was carried out under ambient conditions and no post-thermal treatments were required.…”

Mixing‐Induced Orientational Ordering in Liquid‐Crystalline Organic Semiconductors

“…The thermal analyses demonstrated that substituted alkyl chains acquire conformational degrees of freedom in the thermotropic SmE phase of PE-BTBT-C n . Most likely, the intralayer flip-flop motion of the molecules (i.e., rotation of molecules about their short axis) [48][49][50][51] is coupled with the melting of alkyl chains, which eventually allows the emergence of the SmE phase with disordered long-axis orientations. By contrast, the mobility of flip-flop and conformational molecular motion should be quite low under ambient conditions, after the molecules have fully crystallized in both the b-LHB and d-LHB phases.…”

Comparative Study of Charge-Transport Behavior of Edge-on- and Face-on-Oriented Diketopyrrolopyrrole-Based Conjugated Copolymers Bearing Chalcogenophene Units

“…Consequently, we hypothesize that the disordered (amorphous) domains of these D–A-type conjugated copolymers play significant roles in determining the PFET charge-transport behavior. The spin-cast polymer films generally contain both disordered and ordered crystalline domains, with the disordered phases in the polymeric films associated with the rate-determining step for charge transport . Hence, the electrical characteristics of the PFETs were analyzed using the Gaussian disorder model (GDM), which focuses on the charge-transport behavior of the amorphous domains of the polymer film .…”