Example:P3HT orientation analysis

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This tutorial describes how to quantify the orientation distribution of the semiconducting polymer P3HT. This orientation analysis is meant to determine the relative amounts of the material oriented in different ways. The same kind of analysis can be applied to other semiconducting polymers or small-molecules. In fact, the same conceptual steps can be applied more broadly to determining any kind of orientation distribution (though one must be careful in interpreting the relationship between reciprocal-space peaks, which will be different for each material's specific unit cell).

P3HT orientation

P3HT crystallizes into the unit cell shown below. P3HT crystals are anisotropic, often appearing as long needle-like structures. Note that this unit cell is highly idealized: the alkyl side-chains have considerable freedom and thus disorder (they may even be liquid-like). Moreover, real samples of P3HT organize in a more disordered state than shown here. Although the lamellar stacking is well-defined, and the aromatic (π-π) stacking is also well-defined, one typically does not observe any scattering peak along the backbone direction: i.e. there is usually no well-defined persistence of order in this direction. It is also likely that subsequent chains do not organize with the well-defined registry shown in the cartoon. Overall, P3HT is thus better thought of as being semi-crystalline or liquid-crystalline: it exhibits considerable molecular order (giving rise to scattering peaks), but is not an extended well-ordered crystal.

Cartoon of an idealized P3HT unit cell.

When P3HT is cast as a thin-film, this unit cell can adopt a variety of orientations. Although most materials form 3D powders when cast from solution, P3HT adopts a relatively well-defined orienation with respect to the substrate. It seems that the alkyl side-chains preferentially segregate to the film-air and film-substrate interfaces (presumably in order to lower the interfacial energy), which drives the material overall into the edge-on orienation shown below:

Absent any other driving force, the above configurations are assumed to be in-plane isotropic (2D powders). In such a case, one would expect to see reciprocal-spaces corresponding to these states of:

Analysis: In-plane powder

TBD

Caveats

  • Standing-up: The preceeding analysis has assumed that the 100 scattering along qr arises from face-on material. However, both face-on and end-on material would give rise to scattering at that position in reciprocal-space. To differentiate between those two possibilities, one must invoke additional data. Specifically, one can look at the 010 (π-π) peak: for end-on, the π-π peak will also be in-plane (along qr), whereas for face-on, one should observe substantial intensity of the π-π peak in the out-of-plane direction (near qz axis).

Analysis: In-plane aligned

TBD

See also grating alignment for caveats related to the in-plane angle (ϕ).

Literature

Development/description of analysis method

Application of method

Related papers

Angular correction (curvature of Ewald sphere)

sin(angle) correction

Other orientation analyses