The correlations of the electronic structure and film growth of 2,7-diocty[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) on SiO₂

Abstract

Combining ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), atomic force microscopy (AFM) and small angle X-ray diffraction (SAXD) measurements, we perform a systematic investigation on the correlations of the electronic structure, film growth and molecular orientation of 2,7-diocty[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) on silicon oxide (SiO). AFM analysis reveals a phase transition of disorderedly oriented molecules in clusters in thinner films to highly ordered standing-up molecules in islands in thicker films. SAXD peaks consistently support the standing-up configuration in islands. The increasing ordering of the molecular orientation with film thickness contributes to the changing of the shape and lowering of the leading edge of the highest occupied molecular orbital (HOMO). The end methyl of the highly ordered standing molecules forms an outward pointing dipole layer which makes the work function (WF) decrease with increasing thickness. The downward shift of the HOMO and a decrease of WF result in unconventional downward band bending and decreased ionization potential (IP). The correlations of the orientation ordering of molecules, film growth and interface electronic structures provide a useful design strategy to improve the performance of C8-BTBT thin film based field effect transistors.

Role of Oxide/Metal Bilayer Electrodes in Solution Processed Organic Field Effect Transistors

“…In the valence band region, Ag and Au show clear Fermi levels at zero binding energy. The valence band edge of C 8 -BTBT is 2.13 eV below the E F. With the work function which is estimated from the onset of the secondary electron cutoff, the obtained HOMO value is 5.40 eV, which is in a very good agreement with the cyclic voltammetry and optical absorption data 9 as well as that measured by UPS 29 . Such a deep HOMO level is one of the reasons that causes large contact resistance with bare Au or Ag for C 8 -BTBT based OFETs.

Figure 5UPS spectra of Au, Ag, C 8 -BTBT, C 8 -BTBT/MoO 3 and C 8 -BTBT/WO 3 in secondary electron cutoff (measured with 9 V of bias) and valence region.

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Experimental and theoretical evidence of a supercritical-like transition in an organic semiconductor presenting colossal uniaxial negative thermal expansion

“…The negative expansion in BHH-BTBT can be labeled as ‘colossal’ according to the classification proposed and used by Goodwin et al , 13 van Heerden et al , 20 and Henke et al 10 where ‘giant’ means | α | ≥ 25 MK –1 , ‘colossal’ | α | ≥ 100 MK –1 and ‘massive’ | α | ≥ 400 MK –1 . We also show that this exceptional thermal expansion is completely absent in a closely related compound, the well-known, much studied, 21 – 23 and commercially available bis(octyl)[1]benzothieno[3,2- b ][1]benzothiophene ( C8-BTBT , Scheme 1 ), despite a very similar crystal packing.…”

Effects of Ambient Gases on the Electrical Performance of Solution-Processed C8-BTBT Thin-Film Transistors

“…Typically, C8-BTBT molecules orient themselves with the long-axis ( c -axis) direction along the SiO 2 /Si substrate. A herringbone arrangement of BTBT core parts appears in the in-plane direction [14]. Thiophene peaks are located at 1314 cm −1 and 1465 cm −1 , while the C–H in-plane peak appears at 1547 cm −1 [6, 21].…”

“…C8-BTBT is a representative small molecule organic semiconductor material [5]. Extensive research has been performed to study its charge transport mechanisms [9], low-cost fabrication methods [10, 11], growth and microstructure formation on various substrates [12–14], metal/semiconductor contact characteristics [15, 16], and strategies to increase its carrier mobility [11, 17–19]. Thus far, there is no systematic study on the impact of ambient gases on the electrical performance of C8-BTBT-based devices.…”