Ultrafast formation of air-processable and high-quality polymer films on an aqueous substrate

Polymer solar cells are attracting attention as next-generation energy sources. Scalable deposition techniques of high-quality organic films should be guaranteed to realize highly efficient polymer solar cells in large areas for commercial viability. Herein, we introduce an ultrafast, scalable, and versatile process for forming high-quality organic films on an aqueous substrate by utilizing the spontaneous spreading phenomenon. This approach provides easy control over the thickness of the films by tuning the spreading conditions, and the films can be transferred to a variety of secondary substrates. Moreover, the controlled Marangoni flow and ultrafast removal of solvent during the process cause the films to have a uniform, high-quality nanomorphology with finely separated phase domains. Polymer solar cells were fabricated from a mixture of polymer and fullerene derivatives on an aqueous substrate by using the proposed technique, and the device exhibited an excellent power conversion efficiency of 8.44 %. Furthermore, a roll-to-roll production system was proposed as an air-processable and scalable commercial process for fabricating organic devices.


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Supplementary Figure 11. FT-IR spectra of the as-spun PTB7 film in N2 and the SS-BHJ film.

Supplementary Note 1| In-situ observation of spontaneous spreading and transfer
Supplementary Figure 1 shows that a polymer film was formed and dried within a few seconds on water after spontaneous spreading, and that the film could easily be transferred to a target substrate.

Supplementary Note 2| Variation of the thickness of the SS films
The area covered by the dripped solution before it dries is mainly determined by the S value when

Supplementary Note 4| Transfer of spontaneous spreading (SS) film to various substrates
The SS films can be formed conformally on nano-patterned or rough surfaces. Supplementary Figure   5a shows that an interference pattern was still apparent on the polyurethane (PU) substrate with sinusoidal gratings (period: 556 nm) after the SS films were transferred to the substrate. Scanning electron microscope (SEM) images also confirmed successful formation of the SS films ( Supplementary Fig. 5b).

Supplementary Note 5| Wide materials selection for SS film formation
Because the SS phenomenon occurs based on the interplay between solvents and substrates, the SS film formation process can be applied to various polymers. We demonstrated the SS film formation

Supplementary Note 7| Ultraviolet photoelectron spectroscopy (UPS) analysis
Supplementary Figure 8 shows the UPS spectra of the N2-processed SC-and SS-PTB7:PC71BM films on a Si wafer. The HOMO energy levels were determined by using the following equation: The correlation between the remaining solvent and the air stability of the polymer films was investigated by estimating the amount of oxygen in the films depending on the air-exposure time during film formation. For the experiment, the SC-BHJ films were prepared under N2; subsequently, the films were exposed to air for different times; finally, the films were completely dried in a N2-filled glove box, as depicted in Supplementary Figure 10a. The atomic ratio of oxygen to sulfur (O/S) in the SC-BHJ films as a function of the drying time in air is displayed in Supplementary Figure 10b The optical images show that as-spun PTB7 film with excess solvent is inhomogeneous with many aggregates after 1h of illumination in air, differently from two films without solvents, as shown in Supplementary Figure 12a. Furthermore, similar to the films exposed to air only (see Figure 3b), absorption of the as-spun PTB7 film with excess solvent were significantly decreased and hypochromic shift was observed in the absorption peaks at 625 nm and 680 nm, indicating the disruption of the backbone conjugation of BDT group in PTB7 molecules with air and light exposure, as compared with the rest, as shown in Figure Supplementary Figure 12b.
In Raman spectra of PTB7 films obtained under 514 nm excitation (Supplementary Figure 12c), peaks from BDT group (C=C) for the as-spun PTB7 film (1499 cm -1 ) were significantly lower and shifted than those of SC-PTB7 (1489 cm -1 ) and SS-PTB7 (1489 cm -1 ) films after 1h of illumination in air.
Furthermore, the relative intensity of peaks at ∼1535 and ∼1575 cm −1 due to fused thiophene and benzene group increased. These suggest that oxygen adsorbed through the path providing solvent molecules in the film during illumination in air contributes to accelerated cleavage of alkoxy groups and insertion of oxygen into C=C bonds in the backbone.

Supplementary Note 12| The performance of SS-PSCs with various DIO concentrations
Performances