Phase separation spin-coating process

The film formation in fhe spin-coating process for the polymer/fuller-ene blend system in the mixture solvent is a complex process because it is a nonequilibrium state that both thermodynamic and kinetic parameters can influence phase separation, and the system contains four components with dissimilar physical/chemical properties. We found the donor/acceptor components in the active layer can phase separate into an optimum morphology during the spin-coating process with the additive. Supported by AFM, TEM, and X-ray photoelectron spectroscopy (XPS) results, a model as well as a selection rule for the additive solvent, and identified relevant parameters for the additive are proposed. The model is further validated by discovering other two additives to show the ability to improve polymer solar cell performance as well. 

Phase separated devices show improved photovoltaic performance. The phase separation is controlled as the blends separate into an interpenetrating network. During the spin-coating process, the extent of phase separation from the well mixed state, which exists in the solution, is limited by the evaporation time of the volatile solvent. When the solvent has fully evaporated, a further rearrangement is impossible and the morphology of the blend is locked in place. 

Figure 7.4 A schematic model describing the owing to solvent evaporation only. The interface film formation during the spin coating process, between the polymers destabilizes (iv) and the After the initial spin-off stage where both film phase separates laterally (v, vi).

Schematic drawings showing the effect of the molecular weight, on the formation of the surface-phase separated structure of the PS/PMMA blend film during spin-coating process. Morphologies observed for PS with molecular weights of (c) 4kD, (d) 435kD.

Several photochromes of different structures (diarylethene and spirobenzopyran) were encapsulated in PS matrices to form composite nanoparticles with diameters between 70 and 150 nm [31], TEM images did not show phase-separated dye crystals in the nanoparticles. Hybrid films were prepared by spin-coating and investigated for their photochromic properties. After UV irradiation, the films changed their color according to the embedded photochrome. The reversibility of this process was shown by irradiation with light of 500-650 nm, which reinstalled the original state. 

In addition to the microphase separation phenomenon, in the presence of an interface, the affinity of one of the blocks by the interface influences the flnal rearrangement of the block copolymer at the outmost surface as has been already reported, for instance by Coulon et al. [96] for the case of polystyrene-6-poly (methyl methacrylate) block copolymers (Fig. 5.12). Initially, upon spin coating the block copolymers are rather disordered due to the fast evaporation process. However, upon annealing reorganization occurs and nanometer scale phases rich in each of the components are observed. Finally, the difference in the surface energies of the components forces the orientation of these domains parallel to the surface, with the lower-surface-energy block located at the surface [96-98]. 

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