The impact of tetrahedral capping groups and device processing conditions on the crystal packing, thin film features and OFET hole mobility of 7,14-bis(ethynyl)dibenzo[b,def]chrysenes

Abstract

A series of tetrahedrally capped acetylene dibenzochrysene derivatives were modelled and then synthesised resulting in compounds with very similar electronic properties, but differing physical properties depending on the capping group. By varying the size of the tetrahedral capping group we have undertaken a study on the relationship between the molecular structure and charge mobility in organic transistors. Thin film characterisation techniques (near-edge X-ray absorption fine structure spectroscopy, scanning electron microscopy, atomic force microscopy and X-ray diffraction) were coupled with organic field effect transistor (OFET) hole mobility data to explore the interplay between molecular structure, molecular packing, substrate dielectrics and transistor hole mobility. Using this strategy we identified a lead compound, TMS-DBC 2b, that has a one-dimensional slipped stack packing motif with a high degree of π–π overlap that produces hole mobilities as high as 0.012 cm² V⁻¹ s⁻¹ when deposited on SiO₂-ODTS substrates. Further refinement of the substrate temperature and deposition rate results in TMS-DBC 2b achieving hole mobilities as high as 1.17 cm² V⁻¹ s⁻¹. These results show that subtle modification of the tetrahedral capping groups can alter the crystal packing and thin film microstructure of these angular dibenzochrysene materials, which dramatically influences the hole mobility in transistors. Further refinement of the device processing conditions on the lead compound results in a significant increase in transistor mobility.