Micro-Raman imaging of isomeric segregation in small-molecule organic semiconductors

Chiung-Wei Huang,Joanna M Atkin,John E Anthony,Peter J Diemer,Oana D Jurchescu,Xiao You,Anthony J Petty

doi:10.1038/s42004-019-0122-7

Abstract

Charge transport in organic semiconductors is highly sensitive to film heterogeneity and intermolecular interactions, but probing these properties on the length scales of disorder is often difficult. Here we use micro-Raman spectroscopy to assign vibrational modes of isomerically pure syn and anti 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES ADT) by comparing to density functional theory calculations. With polarization-dependent measurements, we determine the orientation of crystallites in pure isomers. In mixed-isomer samples, we observe narrow linewidths and superposition spectra, indicating coexistence of isomerically pure sub-domains on length scales smaller than the probe area. Using the ring breathing modes close to 1300 cm−1 as indicators of the pure isomer crystalline sub-domains, we image their spatial distribution with 200-nm resolution. These results demonstrate the power of micro-Raman spectroscopy for investigating spatial heterogeneities and clarifying the origin of the reduced charge carrier mobility displayed in mixed-isomer diF-TES ADT.

Highlights

Charge transport in organic semiconductors is highly sensitive to film heterogeneity and intermolecular interactions, but probing these properties on the length scales of disorder is often difficult
We report on thin films prepared by solvent-assisted crystallization (SAC) on PFBT-treated Au electrodes, yielding improved film quality with highly ordered microstructure in the diF-TES ADT films[20,28,29]
The experimental Raman spectra of syn and anti diF-TES ADT films on PFBT-treated Au electrodes are shown in Fig. 2, with the associated density functional theory (DFT) calculations

Summary

Introduction

Charge transport in organic semiconductors is highly sensitive to film heterogeneity and intermolecular interactions, but probing these properties on the length scales of disorder is often difficult. Using the ring breathing modes close to 1300 cm−1 as indicators of the pure isomer crystalline sub-domains, we image their spatial distribution with 200-nm resolution These results demonstrate the power of micro-Raman spectroscopy for investigating spatial heterogeneities and clarifying the origin of the reduced charge carrier mobility displayed in mixed-isomer diF-TES ADT. The charge carrier mobility, and thereby device performance in OTFTs, is very sensitive to the molecular structure of the organic semiconductor, as well as its solid state packing and microstructure, which are highly dependent on processing conditions[5,6,7,8,9,10,11,12]. The fluorinated ADT 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES ADT) is among the most promising molecules for incorporation in OTFTs (Fig. 1a) with a charge carrier mobility as high as 6 cm[2] V−1 s−1 reported in vapor grown single crystals of this material[18]. The vertical orientation was induced by selective F–H and F–S interactions between the molecule and the fluorine in the SAM surface[24], an orientation that is favorable for in-plane π-orbital overlap and that can improve charge mobility by a factor of ten[19]

Methods

Results

Conclusion