Low Temperature Phase Identification in Organic Semiconductor Thin Films

In-situ structural studies of organic semiconductor thin films at low temperature are used to monitor the presence and appearance of different structural phases which have an impact on the emission properties of the materials studied.

1 Introduction

Organic semiconductors have the potential to be used in novel, high performance, and flexible optoelectronic devices. For these materials, the physical properties and device performance are strongly correlated with how the molecules are arranged in the thin film device (i.e. the crystal structure). Non-ambient X-ray diffraction experiments enable structural changes to be characterised over a range of temperatures, providing insights into how the organic semiconductor devices perform in different environments.

TES and F2-TES are anthradithiophene derivative based organic semiconductors which behave as singlet fission materials. These materials have potential applications in quantum computing and their emission properties are known to be temperature dependent. As the physical and chemical properties of these materials are strongly dependent on their crystal structure, it is important to carry out structural analysis of the crystalline phases present over a wide range of temperatures for a full understanding of their emission behavior and to determine whether different structures (phases) exist.

Crystal structures with the precise atomic positions of both TES and F2-TES are known from single crystal diffraction data. TES has one known phase recorded at low temperature (90 K), while F2-TES has two known phases; a high temperature phase recorded at 240 K, and a low temperature phase recorded at 155 K. It is not known whether the phase behaviour differs in thin films, which is of relevance for devices.

Therefore, the aim of the present study is to identify the crystalline phases present in thin films of TES and F2-TES over a range of temperatures from 85 to 300 K and to compare this with the previously determined single crystal structures and phases. This information is then used to better understand the emission properties of the thin films over a wide temperature range.

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