Phase-separation and mixing in thin films of co-deposited rod-like conjugated molecules

Abstract

We report on the formation of organic-organic thin film heterostructures by co-sublimation, which exhibit either mixing of compounds on the molecular scale or pronounced phase separation into ordered domains on the micrometre scale. The mixed structures may be used as active layers in organic field effect transistors, while phase separation is useful in organic bulk hetero-junction solar cells. Pairs of five rod-like conjugated molecules were co-deposited onto silicon oxide substrates in vacuum. With the five materials: pentacene (PEN), alpha-quaterthiophene (4T), alpha-sexithiophene (6T), p-sexiphenyl (6P), and alkyl chain substituted α,ω-dihexylsexithiophene (DH6T), we investigated molecule pairs, which differ in the length of the molecular conjugated core (CC) and the overall molecular length. Material pairs with similarly sized CC, such as 4T/PEN and 6T/6P, showed the formation of ordered layered structures with intimated mixing on a molecular level. On the other hand, pronounced phase separation was observed for material pairs of dissimilar CC lengths, e.g. 4T/6T and PEN/DH6T. We propose that this can be generalized as design rule for the formation of either mixed or phase separated co-deposited molecular films.