Mobility of Small Molecules in Solid Polymer Film for π-Stacked Crystallization

Yue Liu,Xinping Zhang

doi:10.3390/cryst11091022

Abstract

Crystallization or π-stacked aggregation of small molecules is an extensively observed phenomenon which favors charge transport along the crystal axis and is important for the design of organic optoelectronic devices. Such a process has been reported for N,N′-Bis(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (EPPTC). However, the π-stacking mechanism requires solution–air or solution–solid interfaces. The crystallization or aggregation of molecules doped in solid films is generally thought to be impossible, since the solid environment surrounding the small molecules does not allow them to aggregate together into π-stacked crystals. In this work, we demonstrate that the movement of the EPPTC molecules becomes possible in a solid polymer film when it is heated to above the glass transition temperature of the polymer. Thus, crystal particles can be produced as a doped matrix in a thin solid film. The crystallization process is found to be strongly dependent on the annealing temperature and the annealing time. Both the microscopic and spectroscopic evaluations verify such discoveries and characterize the related properties of these crystals.

Highlights

Crystals 2021, 11, 1022. https://Crystallization of organic molecules through π-stacking is an important approach to improve the charge transport performance in semiconductors and achieve high-efficiency optoelectronic devices [1,2,3]
We report the crystallization of perylene molecules in the solid thin film of polymers, where the polymer needs to be annealed to its glass transition temperature so that the solid film becomes sufficiently softened and flexibilized to allow the motion of the doped small molecules
Blending small molecules with polymers is an important approach to construct efficient optoelectronic devices based on heterojunctions

Summary

Introduction

Crystallization of organic molecules through π-stacking is an important approach to improve the charge transport performance in semiconductors and achieve high-efficiency optoelectronic devices [1,2,3]. Due to the planar structures of the perylene molecules, they can aggregate into one-dimensional crystals at the solution–solid, vapor–solid, and solution–air interfaces, as well as at those between different solvents [1,3,17,19] Such a crystallization process has not been observed within solids. Since the glass transition has a threshold temperature, the formation of the crystal particles in the solid polymer film has a threshold temperature, which is found to be about 120 °C This temperature, the crystallization process the crystal particles in the solid polymer film has a threshold temperature, which is illustrated in Figure 1b is found to not be possible even with a long time of annealing.

Annealing

Annealing Time Dependence

Conclusions