Grazing-incidence wide-angle X-ray scattering (GIWAXS) is suited to molecular packing, crystal orientation, and crystallisation in thin films. This note keeps the composition ratios, spectral parameters, and times reported for D18/BTP-eC9-4F organic films, and lines them up with the other measurements in the same source.

What GIWAXS measures

Synchrotron X-rays hit the film at a grazing angle of about 0.1°–1°. The source states that the evanescent wave from total reflection at the interface can keep the sampling depth within 200 nm, which raises the scattering signal from an ultrathin surface layer.

A two-dimensional detector records the scattering vector $q = k_f - k_i$. The $q_x$, $q_y$, and $q_z$ components carry in-plane and out-of-plane reciprocal-space information. After geometric, polarisation, and Lorentz–Ewald corrections, detector pixels are mapped into $q$-space.

Quantity What it reads Typical question
Diffraction rings or arcs orientation distribution random, or textured?
Azimuthal intensity texture and orientation fractions how do face-on and edge-on fractions change?
Peak position and width lattice spacing, coherence length, microstrain did packing or crystallite size change?
In-situ time series crystallisation and structural evolution which stages does spin-coating pass through?

Two-dimensional GIWAXS patterns and line cuts at different donor:acceptor ratios

Synchrotron layout, marking the electron gun, storage ring, beamlines, and experimental stations

Incident beam, film, scattering vector, and a two-dimensional detector

Packing and orientation in D18/BTP-eC9-4F

The source compares films with donor:acceptor (D:A) ratios from 1:1.2 to 0.2:1.2. As the donor D18 fraction falls, the features associated with π–π stacking weaken.

Quantity D:A = 1:1.2 D:A = 0.2:1.2 Change reported in the source
Out-of-plane crystallite coherence length (CCL) 22.0 Å 16.0 Å shorter
In-plane lamellar spacing 20.7 Å 20.0 Å smaller
In-plane CCL 59.9 Å 28.8 Å shorter
Face-on fraction 45.9% 33.5% lower
Edge-on fraction 26.7% 35.2% higher

Two-dimensional GIWAXS patterns and line cuts at different donor/acceptor ratios

The source places the lower donor fraction together with a drop in packing order, coherence length, and face-on fraction. The DOI is 10.1038/s41467-025-64032-7.

Lamellar spacing, crystallite coherence length, and stage duration versus D:A ratio

Watching spin-coating crystallise

In-situ GIWAXS splits film formation into four stages:

  1. Solution. The solvent has not yet left, and no clear crystalline peak is seen.
  2. Nucleation. Solvent evaporates, concentration rises, the (010) scattering peak appears, and the π–π stacking distance starts to shorten.
  3. Crystal growth. The coherence length increases and the stacking distance settles.
  4. Solid film. After the solvent has gone, stacking distance, coherence length, and orientation level off.

The source reports that stage III lasts 5.5 s at D:A = 1:1.2 and 1.5 s at D:A = 0.2:1.2. UV–vis absorption shows the D18 peak stabilising earlier, while the BTP-eC9-4F peak red-shifts sooner and more clearly as the donor is reduced. GIWAXS shows the corresponding shortening of the crystal-growth stage.

Two-dimensional scattering and crystallisation metrics during spin coating

Other measurements kept beside GIWAXS

Method What the source used it for
Atomic force microscopy (AFM) surface roughness and morphology
Transmission electron microscopy (TEM) internal structure and donor/acceptor phase separation
Femtosecond transient absorption (fs-TA) exciton dissociation, diffusion, and hole-transfer kinetics

The reported root-mean-square roughness falls from 0.721 nm at D:A = 1:1.2 to 0.561 nm at 0.2:1.2. The fibrillar texture nearly disappears in the low-donor films.

The two fs-TA time constants lengthen as the donor fraction drops. The interfacial exciton-dissociation time $\tau_1$ goes from 0.28 ps to 0.84 ps, and the domain exciton-diffusion time $\tau_2$ goes from 0.86 ps to 1.62 ps. The source discusses those changes next to the crystallisation and orientation differences seen by GIWAXS.

Organic solar-cell film and its functional layers

What the geometry does not show

Near $q_r \approx 0$, grazing incidence leaves a missing wedge in reciprocal space. The source suggests filling that region with multi-angle incidence or a complementary reflectivity measurement. GIWAXS read together with UV–vis, AFM, TEM, and fs-TA is how the source compares structure with spectra, morphology, and charge dynamics.