Tuning charge carrier transport and optical birefringence in liquid-crystalline thin films: A new design space for organic light-emitting diodes

Chang-Min Keum,Antal Jakli,Lu Han,Kristiaan Neyts,Vikash Kaphle,Akram Al-Shadeedi,Robert J Twieg,Malte C Gather,Hongping Zhao,Björn Lüssem,Scott D Bunge,Michiel Koen Callens,Shiyi Liu

doi:10.1038/s41598-018-19157-9

Abstract

Liquid-crystalline organic semiconductors exhibit unique properties that make them highly interesting for organic optoelectronic applications. Their optical and electrical anisotropies and the possibility to control the alignment of the liquid-crystalline semiconductor allow not only to optimize charge carrier transport, but to tune the optical property of organic thin-film devices as well. In this study, the molecular orientation in a liquid-crystalline semiconductor film is tuned by a novel blading process as well as by different annealing protocols. The altered alignment is verified by cross-polarized optical microscopy and spectroscopic ellipsometry. It is shown that a change in alignment of the liquid-crystalline semiconductor improves charge transport in single charge carrier devices profoundly. Comparing the current-voltage characteristics of single charge carrier devices with simulations shows an excellent agreement and from this an in-depth understanding of single charge carrier transport in two-terminal devices is obtained. Finally, p-i-n type organic light-emitting diodes (OLEDs) compatible with vacuum processing techniques used in state-of-the-art OLEDs are demonstrated employing liquid-crystalline host matrix in the emission layer.

Highlights

Liquid-crystalline organic semiconductors exhibit unique properties that make them highly interesting for organic optoelectronic applications
The I–smectic A (SmA) and SmA–crystalline solid (Cr) transitions occur at 127 °C and 100 °C, respectively, on cooling[21], but the phase transition temperature in thin-films can differ from the bulk values[35]
Since the non-polarized incident light is kept to be normal to the substrate plane, the variation in the spectrum after heating could be explained by a change in the molecular orientation from the as-prepared initial state

Summary

Introduction

Liquid-crystalline organic semiconductors exhibit unique properties that make them highly interesting for organic optoelectronic applications. It was demonstrated that a simple off-centre spin-coating method enables the formation of highly aligned molecular packing structure of the organic semiconductor[5]; a solution shearing method alters the π–π stacking distance between co-facially stacked molecules and introduces lattice strain[6]; and chemically tailoring of the organic/metal contact interface was shown to influence the microstructure of solution-cast organic thin films[7]. These processes resulted in molecular structures in organic field-effect transistors with high charge carrier mobility. Liquid-crystalline organic semiconductors show pronounced electrical and optical anisotropies, which provide a wide-range of opportunities to optimize charge carrier transport in organic semiconductors and to tune the light propagation in optoelectronic devices, e.g. OLEDs25

Methods

Results

Conclusion