Control of polymorphism in solution-processed organic thin film transistors by self-assembled monolayers

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications

Interfacial engineering has long been a vital means of improving thin-film device performance, especially for organic electronics, perovskites, and hybrid devices. It greatly facilitates the fabrication and performance of solution-processed thin-film devices, including organic field effect transistors (OFETs), organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light-emitting diodes (OLEDs). However, due to the limitation of traditional interfacial materials, further progress of these thin-film devices is hampered particularly in terms of stability, flexibility, and sensitivity. The deadlock has gradually been broken through the development of self-assembled monolayers (SAMs), which possess distinct benefits in transparency, diversity, stability, sensitivity, selectivity, and surface passivation ability. In this review, we first showed the evolution of SAMs, elucidating their working mechanisms and structure-property relationships by assessing a wide range of SAM materials reported to date. A comprehensive comparison of various SAM growth, fabrication, and characterization methods was presented to help readers interested in applying SAM to their works. Moreover, the recent progress of the SAM design and applications in mainstream thin-film electronic devices, including OFETs, OSCs, PVSCs and OLEDs, was summarized. Finally, an outlook and prospects section summarizes the major challenges for the further development of SAMs used in thin-film devices.

On the crystal forms of NDI-C6: annealing and deposition procedures to access elusive polymorphs

NDI-C6 has been extensively studied for its semiconducting properties and its processability. It is known to have several polymorphs and a high thermal expansion. Here we report the full thermal characterization of NDI-C6 by combining differential scanning calorimetry, variable temperature X-ray powder diffraction, and hot stage microscopy, which revealed two different thermal behaviours depending on the annealing process. The ranking of stability was determined by the temperature and energy involved in the transitions: Form α is stable from RT up to 175 °C, Form β is metastable at all temperatures, Form γ is stable in the range 175-178 °C, and Form δ in the range 178-207 °C followed by the melt at 207 °C. We determined the crystal structure of Form γ at 54 °C from powder. The analysis of the thermal expansion principal axis shows that Form α and Form γ possess negative thermal expansion (X1) and massive positive thermal expansion (X3) which are correlated to the thermal behaviour observed. We were able to isolate pure Form α, Form β, and Form γ in thin films and we found a new metastable form, called Form ε, by spin coating deposition of a toluene solution of NDI-C6 on Si/SiO₂ substrates.

Carbazole-Fused Polycyclic Aromatics Enabled by Regioselective Scholl Reactions

The synthesis of new carbazole-fused polycyclic aromatics with interesting geometry and useful properties was explored using Scholl reactions. As found from the Scholl reactions of substrates having two carbazole units linked at different positions through o-phenylene, oxidative coupling of carbazole units occurred in a regioselective manner with new carbon-carbon bonds preferably formed at C3 and C4 in N-alkyl carbazoles. A new N-containing aromatic bowl was characterized by single-crystal X-ray crystallography, and new p-type organic semiconductors exhibited field effect mobility of up to 0.070 cm²  V^-1  s^-1 in solution-processed thin-film transistors.

Manipulating Carrier Concentration by Self-Assembled Monolayers in Thermoelectric Polymer Thin Films

Conjugated polymers have attracted considerable attention for thermoelectric applications in recent years due to their plentiful resources, diverse structures, mechanical flexibility, and low thermal conductivity. Herein, we demonstrate a new strategy of modulating charge carrier concentration of chemical-doped polymer films by modifying the substrate with self-assembled monolayers (SAMs). The SAM with a trifluoromethyl terminal group is found to accumulate holes in the polymer thin films, while the SAM with an amino terminal group tends to donate electrons to the polymer films. Thermoelectric thin films of conjugated donor-acceptor copolymer exhibit high power factors of 55.6-61.0 μW m^-1 K^-2 on SAMs with polar terminal groups. These power factors are 49% higher than that on the SAM with the nonpolar terminal group and 3 times higher than that on pristine substrate. The high power factor is ascribed to the modulated charge carrier concentration and improved charge carrier mobility as induced by SAMs.