Fig. 2: HSP-driven ink formulation and film formation mechanisms.
From: High-performing organic electronics using terpene green solvents from renewable feedstocks
a, 3D representation of the Hansen SSs of high-performing OPV materials, PM6 (green sphere) and BTP-eC9 (pink sphere). The position of the terpene solvents and co-solvents in Hansen space is denoted with green dots, while CF and DIO are presented in dark blue cubes. The binary solvent composition lines for the Eu:Tet, Lim:Ind, Pin:EPS and Men:Tet formulations are shown as dotted ‘guides to the eye’ in the diagram. b, PM6 solubility versus co-solvent content in each terpene-based formulation. The solubility profile aligns with Hansen binary composition lines. c, Diagram denoting the Hansen solubility binary solvent composition lines and the four stages of the bulk-heterojunction formation for organic photovoltaics. (1) First, the solution is deposited at the initial composition dictated by the closeness to the centre of the donor SS. (2) Second, as the low-BP terpene solvent evaporates, the overall concentration increases, shrinking the NFA SS to the point of saturation, where it begins crystallizing. (3) Then, as the higher-BP co-solvent starts evaporating, the formulation changes and moves across the solvent composition lines and reaches the edge of the NFA SS, where the donor domains start forming. (4) The remaining solvents keep evaporating until the final bulk heterojunction is formed. d, Solubility affinity of each solvent component versus boiling point. e, TGA evaporation curves denoting the remaining weight over time for a 10 µl droplet of each terpene-based formulation. f, In situ absorbance peak intensity versus absolute time tracking the formation of the PM6 (625 nm) and BTP-eC9 (830 nm) domains. The green and violet arrows represent the onset and end of solidification of PM6 and BTP-ec9, respectively. g, Normalized peak formation speed versus relative time of each formulation obtained by plotting the derivative of the normalized peak intensity over time (t). This derivative d(intensity)/dt explains how fast the absorbance peaks are forming and can provide a simplified view of the solidification process and its duration. These curves are superimposed next to one another to provide a better comparison at the film formation in relative time. The solid lines represent the formation of PM6, and the dotted lines, the formation of BTP-eC9. Panel c created with BioRender.com.
