Interplay between Long-Range Crystal Order and Short-Range Molecular Interactions Tunes Carrier Mobility in Liquid Crystal Dyes

Abstract

We investigated the influence of molecular packing on the optical and electrical properties of the liquid crystalline dye 4,7-bis[5-(2-fluoro-4-pentyl-phenyl)-2-thienyl]-2,1,3-benzothiadiazole (FPPTB). FPPTB is crystalline at room temperature, exhibits a nematic phase at temperatures above 149 °C and is in an isotropic melt at temperatures above 230 °C. Solution processed FPPTB films were subject to thermal annealing through these phase transition temperatures and characterized with X-ray diffraction and polarized optical microscopy. Cooling FPPTB films from the nematic and isotropic phases increased crystal domain size, but also induced local structural variations in the molecular packing of crystalline FPPTB. The decrease in long-range order was correlated with an increase in short-range π–π interactions, leading to changes in molecular aggregation which persisted even when the FPPTB films were cooled to room temperature. Annealing-induced changes in molecular aggregation were confirmed with optical spectroscopy. The carrier mobility in FPPTB films increased over 2 orders of magnitude from (2.2 ± 0.4) × 10–5 cm2 V–1 s–1 in as-spun films to μ = (5.0 ± 0.8) × 10–3 cm2 V–1 s–1 in films cooled from the isotropic melt. We discuss the relationship between thermal stability and high carrier mobility values in terms of the interplay between long-range molecular order and increased π–π interactions between molecular pairs in the FPPTB film.

Vapor-Deposited Glasses with Long-Range Columnar Liquid Crystalline Order

“…For many applications involving organic electronics and optoelectronics, the delocalization of electronic states within the material is highly relevant. Charge carrier mobility in organic materials is controlled by several factors, ,− and large π-conjugation between molecules is of predominant importance in nonpolymeric organic electronic materials . To gain some insight into this, we compared UV–vis absorption (UV–vis) spectra for vapor-deposited glasses of the phenanthroperylene-ester deposited at two temperatures with a glass prepared by cooling the equilibrium liquid crystal state of this system.…”

Impedance Spectroscopy for Emerging Photovoltaics

“…If n = 0.5, the CPE reduces to a Warburg element, and Q = 1/ A w . While the description of the CPE may appear arbitrary, this element is often used to describe relaxation processes with a distribution of time scales (i.e., time-dependent or dispersive processes), such as charge transfer at a nonhomogeneous electrode surface, or carrier transport in amorphous semiconductors. , In this case, the ideal capacitor is substituted by a CPE element with Q = C and n > 0.5 in the circuit. The distribution in relaxation times (0.5 < n < 1) results in a “depressed” or distorted semicircle in the Nyquist plot.…”

“…Signatures of dispersive transport can be identified by fitting the impedance spectrum with an equivalent circuit model composed of an R-CPE circuit. If n = 1, the CPE reduces to an ideal capacitor, and the transport is not dispersive …”

“…Their model consists of a series resistance R s that accounts for contacting resistances, R rec C μ to account for the generation and recombination of photogenerated charge, and R tr C tr to account for carrier transport. The carrier lifetime is given by τ = R rec C μ , and the carrier transit time τ tr is defined as τ = R C tr tr tr (18) and represents the time it takes for a carrier to be injected and transported across the device. C tr accounts for the low carrier mobilities observed in organic semiconductors and is determined by both C geo and C μ .…”

“…If n = 1, the CPE reduces to an ideal capacitor, and the transport is not dispersive. 18 Extracting the carrier transit time from the admittance spectra is relatively straightforward due to the clear signature of slow carriers in the complex capacitance C ̃spectrum and the susceptance B(ω) spectra, where Y = G − iB.…”