Layer-by-layer morphology of spray-deposited polymer blends

Organic solar cells fabricated via spray deposition offer a route to low-cost, low-energy and large-scale devices. However, while spray coating enables scalable fabrication, the relationship between deposition parameters and nanoscale morphology remains poorly understood, particularly for layer by-layer processing.
Grazing incidence scattering techniques are ideal to probe thin-film morphology and interfaces in solar cells. In particular, time-of-flight grazing-incidence neutron scattering (ToF-GISANS) of the polymeric bulk heterojunction enables simultaneous characterisation of crystalline and amorphous domains at both the surface and in the bulk, providing insights into exciton transport pathways. Here, we employ ToF-GISANS to study spray-deposited layers of varying thicknesses using the model active layer polymer system poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM). Structural length scales stemming from phase separation and agglomeration along the film depth are investigated for layer-by-layer deposition, mimicking film formation and thickness build up during spray deposition.
The ToF-GISANS results are complemented by grazing-incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS), and correlated with UV-VIS spectroscopy, surface profilometry, and AFM. Surface profilometry and AFM are used to quantify the film thickness and the surface roughness. Changes in crystallinity are assessed by the position of absorption peaks in UV-Vis measurements and π-π stacking distances obtained from GIWAXS.
This multiscale characterisation, enables a deeper understanding of the spray-deposited film formation and nanostructure control, which is essential for optimizing solar cells.