Directional Crystallization Using a Thermal Gradient

Electroluminescent displays, lighting (organic light-emitting diodes (OLED), circuits on flexible substrates, and photovoltaic cells are all applications of organic electronics that rely on industrial organic semiconductors (OSC).

The Linkam GS350 Stage in the Université Libre de Bruxelles laboratoryDue to a fundamental lack of the understanding of how molecular structure and supramolecular organization affects optoelectronic properties Prof. Yves Henri Geerts and his colleagues at Université Libre de Bruxelles have undertaken a study of single crystal thin films of Terthiophene by directional crystallization by means of a thermal gradient using the Linkam GS350 stage.

Optoelectronic properties can also be affected by the method of fabrication, therefore determining a method to control deposition and crystallisation is important.

As part of his research Prof. Yves Henri Geerts used polarized optical microscopy (POM) and X-ray diffraction to characterise the shape, size, and orientation (in and out of the plane of the substrate) of the crystals produced by the thermal gradient technique.

A sample of Terthiophene was placed on a cover slip on the hot side of the stage and is slowly translated to the cold side at a constant speed until all the sample is on the cold side.

The cover slip and sample are slowly translated from the hot side to the cold side at a constant speed.One side is above the melting temperature (hot side) and the other at a temperature below the crystallization temperature (cold side).

The conclusion of the experiment was that temperature gradients could potentially be used to control crystal growth and these conditions induce a preferential fast growth direction perpendicular to the gradient direction. In addition it is found that nucleation and growth can be decoupled for OSC crystallizing from the melt in a temperature gradient and that these conditions lead to the generation of highly textured thin films with uniaxial in-plane orientation of crystallites.

By Caroline Feltham