4.4 Using additives
Additive treatment is an effective means to optimize the morphology. This method is to add a small amount of additives which are quite different from the properties of the main solvent into the solution, and to control the morphology of the active layer by changing the state of the solution and film-forming kinetics.[24, 114] The introduction of additives can generally reduce solvent volatilization rate, prolong film-forming time and enhance crystallinity. At the same time, additives generally have selective solubility, which can adjust the precipitation sequence of molecules and effectively adjust the phase separation structure.[115] In addition, adding solid additives can inhibit the further phase separation and improve the stability.[116]