Spectroscopic identification of quinacridone polymorphs for organic electronics

Quinacridone is a well known pigment, recently proposed as an environmentally friendly, air stable organic semiconductor. The polymorphism of quinacridone has been widely debated, since different polymorphs exhibit different colors, but the problem has not been addressed in relation to its use in organic electronics. Here, we give a detailed account of how the combination of different nondestructive spectroscopic techniques (Raman, infrared, and luminescence), also applicable to a working device, can be used to identify the quinacridone polymorphs. This is the necessary starting point to optimize the growth conditions towards the polymorph with the best performance for the sought application.