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), psexiphenyl (6P), and alkyl chain substituted a,u-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.

Interface Properties of Organic para-Hexaphenyl/α-Sexithiophene Heterostructures Deposited on Highly Oriented Pyrolytic Graphite

“…53 Importantly, no indication of the presence of new electronic states is found in UPS. Vacuum-level alignment and therefore the absence of interface dipoles are observed.…”

Toward a comprehensive understanding of molecular doping organic semiconductors (review)

“…This can be seen in the isosurface plots of the C 10 BTBT HOMO and the respective dopant LUMOs, as shown in Figure 9a (bottom). Because material pairs with similarly sized conjugated cores tend to form ordered structures with intimate mixing on the molecular level [174], mixed growth of dopant and COM can be expected for C 10 BTBT and all the TCNQ derivatives. The specular XRD data for increasing F 4 TCNQ content in the films (fabricated via spin-coating onto SiO 2 substrates) is shown in Figure 9b (top) and evidences that, indeed, mixed crystalline growth occurred: In addition to the peak series of pure C 10 BTBT indicating upright-standing growth in its known single-crystal structure (marked by yellow circles), a second series was observed already at a dopant ratio as low as 2% (red squares).…”

Structure formation in perfluoropentacene:diindenoperylene blends and its impact on transient effects in the optical properties studied in real-time during growth

“…The structural properties of these systems are influenced by different effects, such as intermolecular interactions and steric properties of the compounds, which affect the mixing (i.e., phase separation vs. intermixing) 10 as well as the crystalline ordering behavior. 11 The optical and electronic properties of the system can depend strongly on the film structure [12][13][14][15][16][17] and investigations of the underlying mechanisms of structure formation and ordering behavior are necessary for a detailed understanding of these phenomena.…”