Temperature and layer thickness dependent in situ investigations on epindolidione organic thin-film transistors

Abstract

We report on performance evaluations as a function of layer thickness and substrate temperature for bottom-gate, bottom-gold contact epindolidione organic thin-film transistors on various gate dielectrics. Experiments were carried out under ultra-high vacuum conditions, enabling quasi-simultaneous electrical and surface analysis. Auger electron spectroscopy and thermal desorption spectroscopy (TDS) were applied to characterize the quality of the substrate surface and the thermal stability of the organic films. atomic force microscopy (AFM) was used to gain additional information on the layer formation and surface morphology of the hydrogen-bonded organic pigment. The examined gate dielectrics included SiO, in its untreated and sputtered forms, as well as the spin-coated organic capping layers poly(vinyl-cinnamate) (PVCi) and poly((±)-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE, from the class of polynorbornenes). TDS and AFM revealed Volmer-Weber island growth dominated film formation with no evidence of a subjacent wetting layer. This growth mode is responsible for the comparably high coverage required for transistor behavior at 90-95% of a monolayer composed of standing molecules. Surface sputtering and an increased sample temperature during epindolidione deposition augmented the surface diffusion of adsorbing molecules and therefore led to a lower number of better-ordered islands. Consequently, while the onset of charge transport was delayed, higher saturation mobility was obtained. The highest, bottom-contact configuration, mobilities of approximately 2.5 × 10cm/Vs were found for high coverages (50 nm) on sputtered samples. The coverage dependence of the mobility showed very different characteristics for the different gate dielectrics, while the change of the threshold voltage with coverage was approximately the same for all systems. An apparent decrease of the mobility with increasing coverage on the less polar PNDPE was attributed to a change in molecular orientation from upright standing in the thin-film phase to tilted in the bulk phase. From temperature-dependent mobility measurements we calculated activation barriers for the charge transport between 110 meV and 160 meV, depending on the dielectric configuration.

Solution of an elusive pigment crystal structure from a thin film: a combined X-ray diffraction and computational study

“…The presence of molecules with two distinct orientations in the same film is not uncommon 38 – 41 and the broad, weak nature of the diffraction peaks observed for films prepared at 200 K shows that the film is not well ordered. 42 The fact that films produced at 200 K are mostly disordered can potentially explain, and is consistent with, the observed poor device performance of EPI transistors prepared at substrate temperatures of 200 K, 5 as the degree of molecular ordering is strongly correlated with charge transport mobility in thin films. 43 …”

“…The first peak can be interpreted as standing molecules of EPI with a slight tilt of the long molecular axis away from the substrate normal (EPI has a molecular length of ∼12.6 Å). 5 The second peak gives a d -spacing similar to the intermolecular distance of π–π interactions, suggesting that molecules lying parallel to the substrate surface are also present. The presence of molecules with two distinct orientations in the same film is not uncommon 38 – 41 and the broad, weak nature of the diffraction peaks observed for films prepared at 200 K shows that the film is not well ordered.…”

“…Film thickness was monitored during deposition using a quartz microbalance where a conversion factor of Δ 1 Hz ≈ 0.083 nm was used. 5 During deposition, the substrate temperature could be set to temperatures in the range of ∼120–800 K (cooling provided by liquid N 2 ) and films were deposited at substrate temperatures of 200, 300, and 350 K. A detailed description of the experimental setup and sample mounting used is given elsewhere. 18 …”

“…In thin films of EPI, it was previously reported that the substrate temperature during film deposition can have a significant impact on the resultant transistor performance. 5 The strong influence of preparation conditions on film structure is well known, and it was recently shown for another hydrogen-bonded pigment, Tyrian purple, that a new polymorph can be induced at low substrate temperatures. 10 The performance variation in transistors produced from EPI deposited at different substrate temperatures points to structural differences in the thin films which are yet to be characterized.…”

Synthesis and Investigation ofN,N’-benzylated Epindolidione Derivatives as Organic Semiconductors

“…Unsubstituted Epi, in contrast, demonstrates quasi‐reversible reduction and oxidation and charge transport ambipolarity . The motivation for exploring epindolidione‐based materials is to find robust functional semiconductors for application in organic electronics that combine good performance with stability and low cost . Thus many successful molecular material concepts have been inspired by “old” dye and pigment chemistry .…”