Orientation Control of Solution-Processed Organic Semiconductor Crystals To Improve Out-of-Plane Charge Mobility

Abstract

The crystallization of a series of triisopropylsilylethynyl (TIPS)-derivatized acene-based organic semiconductors drop cast from solution onto substrates was investigated as a function of the size of their conjugated cores. When drop cast onto a substrate, the molecules in TIPSpentacene crystals adopt a "horizontal" orientation, with the long axis of the pentacene core parallel to the substrate surface. For crystals comprising molecules with dibenzopyrene, anthanthrene, and pyranthrene cores, two-dimensional X-ray diffraction patterns revealed the existence of a second population of crystals adopting a "vertical" molecular orientation with the long axis of the acene core perpendicular to the substrate surface. The ratio of the population of TIPS-pyranthrene crystals with molecules adopting horizontal versus vertical orientations was controlled by varying the surface energy of the underlying substrate. These crystals displayed orientationdependent linear birefringence and linear dichroism, as observed by differential polarizing optical microscopy. Conductive atomic force microscopy (C-AFM) revealed a 42-fold improvement in out-of-plane hole mobility through crystals adopting the vertical molecular orientation compared to those adopting the horizontal molecular orientation.

Probing functional self-assembled molecular architectures with solution/solid scanning tunnelling microscopy

“…For example functionalization with triisopropylsilylethynyl (TIPS), which tends to interact strongly with the substrate, renders molecules with pentacene, anthanthrene and dibenzopyrene cores insensitive to the surface free energy of the underlying substrate. 63 Other approaches for controlling film morphology can make use of molecular layers: thin templating molecular layers can also be used to control the growth of device-ready films, either from the bottom of the deposited film (molecular template growth) 64 or from the top of the film, where an ordered layer (surface segregated monolayer) of low surface-energy molecules can promote ordering through the depth of the film. 65 Clearly, the presence of a substrate, as is required for STM, creates an opportunity for inducing order in molecular films.…”

A Review of Vertical Organic Transistors

“…There are basically two approaches: The processing conditions such as the evaporation rate, the substrate temperature, or the use of a a seed layer can be utilized to influence the growth of the semiconductor layer . The other approach is to chemically synthesize new materials that form crystals that have higher charge carrier mobility in vertical direction . At the same time, these efforts should also target a lower activation energy of transport in order to suppress destructive self‐heating effects at high current densities. •While vertical devices have already achieved excellent parameters, detailed understanding of device function and charge carrier transport is still limited.…”

Orienting and shaping organic semiconductor single crystals through selective nanoconfinement

“…14 For most solution-processable compounds, however, the presence of bulky side groups that impart solubility to the conjugated molecules typically prevent significant interactions between their p-planes and those of graphene. 15 To this end, nanoconfinement on the tens to hundreds of nanometer length scale is a promising strategy to control the orientation of molecular crystals. Because organic molecules are generally asymmetric, crystal growth occurs at different rates along different crystallographic directions.…”