Molecular Reorganization in Organic Field-Effect Transistors and Its Effect on Two-Dimensional Charge Transport Pathways

Abstract

Charge transport in organic thin film transistors takes place in the first few molecular layers in contact with the gate dielectric. Here we demonstrate that the charge transport pathways in these devices are extremely sensitive to the orientational defects of the first monolayers, which arise from specific growth conditions. Although these defects partially heal during the growth, they cause depletion of charge carriers in the first monolayer, and drive the current to flow in the monolayers above the first one. Moreover, the residual defects induce lower crystalline order and charge mobility. These results, which are not intuitively explained by electrostatics arguments, have been obtained by combining in situ real time structural and electrical characterization together with ex situ AFM measurements, on thin films of a relevant n-type organic semiconductor, N,N'-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis dicarboximide grown by sublimation in a quasi-layer-by-layer mode at different substrate temperatures.

Changes of the Molecular Structure in Organic Thin Film Transistors during Operation

“…There, molecules can lie planar as demonstrated by AFM and STM measurements, 20 or adopt a configuration different from the crystal phase. 2 Because of the looser packing, their energy barriers for re-orientation are smaller with respect to molecules inside the domains.…”

MINERVA: A facility to study Microstructure and INterface Evolution in Realtime under VAcuum

“…Although the system in its current configuration does not have optical viewports set at 74 • for in situ spectroscopic ellipsometry, a technique which has been successfully combined with GIXD measurements, 24 the available ports could accommodate fibre-optic feedthroughs for complementary optical techniques such as in situ differential reflectance. 25,26 In addition, connections for enabling electrical measurements of films in organic field-effect transistors (OFETs) during or after deposition 27,28 can be easily integrated using the current flange ports.…”

Real-time x-ray scattering study of the initial growth of organic crystals on polymer brushes

“…X-ray scattering measurements enable real-time structural monitoring during the earlystage growth of a small molecule organic layer. X-ray scattering in the anti-Bragg configuration 12,[16][17][18][19][20][21][22] or energydispersive x-ray reflectivity [23][24][25] methods are useful tools for characterizing the coverage of individual small molecule organic thin film layers in real-time. Real-time x-ray scattering methods have been applied to organic films deposited onto bare SiO 2 or SAM-treated SiO 2 surfaces.…”

“…We also used the Parratt model, 27 a dynamical calculation, to explain the x-ray scattering intensities in place of the closed-form kinematical calculation used in previous real-time x-ray scattering studies. 12,[16][17][18][19][20][21][22] Since Parratts formalism can explain more accurately the dynamical effects, such as multiple reflections and refraction effect, this dynamical calculation allowed us to explain consistently both in situ full-range x-ray reflectivity curves and time-dependent scattering intensities measured at specific angles.…”