Abstract
We studied the molecular packing and crystallization of a highly regio-regular semiconducting polymer poly(3-hexylthiophene) (P3HT) on both single layer graphene and silicon as a function of temperature, during cooling from the melt. The onset of crystallization, crystallites' size, orientation, and kinetics of formation were measured in situ by synchrotron grazing incidence X-ray diffraction (GIXD) during cooling and revealed a very different crystallization process on each surface. A favored crystalline orientation with out of plane π-π stacking formed at a temperature of 200 °C on graphene, whereas the first crystallites formed with an edge-on orientation at 185 °C on silicon. The crystallization of face-on lamellae revealed two surprising effects during cooling: (a) a constant low value of the π-π spacing below 60 °C; and (b) a reduction by half in the coherence length of face-on lamellae from 100 to 30 °C, which corresponded with the weakening of the 2nd or 3rd order of the in-plane (k00) diffraction peak. The final ratio of face-on to edge-on orientations was 40% on graphene, and 2% on silicon, revealing the very different crystallization mechanisms. These results provide a better understanding of how surfaces with different chemistries and intermolecular interactions with the polythiophene polymer chains lead to different crystallization processes and crystallites orientations for specific electronic applications.
Highlights
The crystallinity of the polymer film strongly influences charge transport in the active layer of an electronic device, which will impact its overall efficiency
In order to predict and control the electronic properties of a single sheet graphene/semiconducting polymer film layer, it is important to understand how the polymer crystallizes on a mono-layer of graphene, and how different crystallization on graphene is from a typical substrate, e.g. silicon or silicon oxide
Our findings reveal several differences with this previous study, and clearly show that the way crystallites form on graphene is fundamentally different than on the less interacting silicon surface, which results in different crystallite orientations, kinetics of crystallization, and a very different final ratio of face-on to edge-on lamellae, which imply largely different opto-electronic properties
Summary
The crystallinity of the polymer film strongly influences charge transport in the active layer of an electronic device, which will impact its overall efficiency. We show a detailed in situ study of the kinetics and process of crystallization of a 85 nm thick P3HT film spun from orthodichlorobenzene (o-DCB) on both graphene and silicon (with native oxide) as a function of temperature during slow cooling from the melt.
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