Abstract
The use of processing additive is a powerful approach for the optimization of active layer performance in organic photovoltaic devices. The researches always focus on the system of P3HT:PCBM system and study the morphology in film state. In this study, we focus on the P3HT system with different amount of OT to investigate the film formation during spin coating by in-situ GIWAXS/GISAXS and normal incident optical interferometry. Analysis (assuming a single liquid layer) of OI results indicates that there are generally 3 stages in the film-forming process: (1) the spin-off or flow-dominated stage, where the film-thinning rate decreases very quickly according to the Meyerhofer equation, (2) the evaporation-dominated stage of plateaued film-thinning rate, and (3) the freezing-in or vitrification stage where the film-thinning rate approaches to zero. In previous case (pure P3HT), we show the formation of a thin layer with high-refractive index at the air-liquid interface in the transition between flow and evaporation dominated stage and it will determine final film’s morphology. Therefore, the air-liquid interface is the significant role during the spin-coating process. In this case, the high-boiling point additive will enhance the air-liquid interface during spin-coating. From the OI observation, the film forms a low refractive index layer at the air/liquid interface (ca. 10 nm in thickness), which indicates that the high boiling OT accumulates on the surface due to CB evaporated soon. Especially, the OT diffuse back to the P3HT-rich layer in the vitrification stage. From the GIWAXS results, it shows the phenomenon that the orientation was randomized and the highest intensity became lower. It may represent the original P3HT crystallites may be disturbed by OT diffusion then randomize the original orientation. In the GISAXS observation, P3HT agglomerates became smaller continuously and invariant keeps increasing even in vitrification stage, may represent that it still grows the new agglomerate makes the average size decrease. Combined all the results, adding a high boiling point additive will prolong the period of film formation. The additive will accumulate on the surface then diffuse back to randomize the crystallites and maybe also make the agglomerates smaller.