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

Introduction

Control of the nanomorphology in polymer solar cells (PSCs) is a key factor for maximizing the module power conversion efficiency (PCE) and enhancing the electrical properties related to exciton dissociation at the interfaces between donors and acceptors1,2,3, diffusion of charge carriers4,5, and charge collection at each electrode6,7,8. The nanomorphology of the blended film can be engineered via thermal and solvent annealing9,10 or the use of processing additives and so on, to achieve selective solubility11. Effective manipulations may favorably influence the crystallization and orientation of the polymers12.

Although spin-coating is the most appropriate laboratory-scale film formation process, scale-up of this process does not ensure uniformity, even within a device. This makes the spin-coating process incompatible with roll-to-roll (R2R) production under ambient conditions, where the latter is desirable for successful commercialization.

Furthermore, air processability of PSCs is a critical issue given that an inert environment is currently required for the entire process, unless the devices are stable under ambient conditions. Although poly(3-hexylthiophene) (P3HT)-based devices exhibit good air processability due to reversible degradation by oxygen, highly efficient polymers containing the benzodithiophene (BDT) group, such as poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th), tend to undergo significant degradation on exposure to oxygen and light13,14,15,16.

Here we propose a new approach to form high-quality organic films on an aqueous (water based) substrate under ambient conditions. A drop of organic material blended into a solvent, when dropped onto an aqueous surface, spreads spontaneously and rapidly by the Marangoni effect17, leading to the rapid and uniform formation of an organic layer that can be functionalized on various substrates once transferred. Interestingly, the nanomorphology of blended films can be efficiently managed in seconds during the process of spreading on the aqueous substrate. We verify that the ultrafast solvent removal by the spontaneous spreading process can impede the infiltration of oxygen into the film via the remaining solvent effectively.

The utility of the films in functional devices is demonstrated by applying the new film formation technique to the fabrication of highly efficient PSCs. PTB7-based PSCs are successfully fabricated by exploiting the high-quality nanomorphology within the bulk heterojunction (BHJ) films. Last, we demonstrate the potential of our method as a scalable process by transferring the spontaneously spreading (SS) film from water surface to a large plastic substrate using a custom-made R2R process.

Results

Spontaneously spreading process of polymer solutions

When chemical agents with low surface energy (for example, hydrocarbon solvents or detergents) are dropped into high-surface-energy media (for example, water), local surface tension gradients occur on the boundary between the surrounding materials, leading to surficial flow toward regions of higher surface tension. This spontaneous spreading is called Marangoni flow17,18. The spreading coefficient (S) dictates the speed of the spreading flow and is defined by the surface tensions at the three-phase contact line between a liquid droplet and liquid substrates19, where S=γ1–γ 2–γ 12 (γ 1 and γ 2 are the surface tensions of the base and polymer solutions, respectively, and γ 12 is the interfacial surface tension of the two solutions). Figure 1a illustrates the different behaviours of the polymer solution depending on S when a drop of polymer solution is dropped on the base solution. The flow of the solution is determined by the net force direction of the surface tensions. If S is positive, that is, the surface tension of the base solution,γ 1 (red arrow), is larger than the sum of γ 2 (green arrow) and γ 12 (blue arrow), the polymer solution will spread over the aqueous substrate surface and form a uniform polymer film. Otherwise, the polymer solution will ball on the aqueous substrate without dispersion.

Figure 1: Principle and formation process of spontaneous spreading.

(a) Spreading of polymer solution dropped onto the base solution is determined by the spreading coefficient, S. Positive S results in a uniform polymer film; otherwise, polymer drops aggregate; (b) Schematic illustrations of formation of an SS BHJ film on water and transfer to target substrates. Actual processing images are shown. (c–f) Transferred SS-PTB7:PC71BM films on various substrates: PDMS (c), PET (d), copy paper (e) and a curved surface of a vial (f).