Spatial Extent of Wave Functions of Gate-Induced Hole Carriers in Pentacene Field-Effect Devices as Investigated by Electron Spin Resonance

Abstract

An electron spin resonance (ESR) method is applied to a pentacene field-effect device to investigate gate-induced hole carriers in such devices. Clear field-induced ESR signals are observed, demonstrating that all of the field-injected carriers have S = 1/2 spins. Anisotropic ESR signals due to unpaired pi electrons show the molecular orientation at the interface in the devices. The spatial extent of the spin density distribution (wave function) of the carriers is evaluated from the ESR linewidth, accounting for the hyperfine structure, to be of the order of 10 molecules.

Electron spin resonance observation of charge carrier concentration in organic field-effect transistors during device operation

“…The g value exhibits a minimum at θ = 0 • and a maximum at θ = 90 • , as in the cases of previous FI-ESR studies of pentacene FETs fabricated on Al 2 O 3 or parylene insulators. 31,32 The observed angular dependence is described well by the anisotropy of the g tensor of π electrons of pentacene, whose principal axes are shown in Fig. 3(b).…”

“…This result is independent of the insulator material. 31,32 As the spin concentration increases, peak-to-peak ESR line width ( H pp ) exhibits clear narrowing, as shown in Fig. 2(c).…”

“…Similar behavior has been reported by Matsui et al in the pentacene FET with parylene insulator. 31 They ascribed this behavior to the motional narrowing of the ESR spectrum reflecting the trap-and-release process in the charge transport. The deeply trapped carriers with long trapping time, which is dominant when the charge concentration is small, give large line widths.…”

“…26 So far, this method has been applied to various OFETs of poly(3-alkylthiophene), [26][27][28][29][30] pentacene, [31][32][33] thienothiophene-based high-mobility materials, 34,35 and rubrene single crystals, 36 etc., giving microscopic information such as spin-charge relation, carrier wave function, motional effect of charge carriers, and local molecular orientation at the device interface. In particular, direct determination of spin concentration within the channel during device operation played a crucial role in clarifying the operation mechanism of OFETs, as was demonstrated for P3HT transistors using top-contact geometry, 37 and subsequently, for rubrene single-crystal transistors with bottom-contact geometry; 38 the spin concentration exhibited a clear decrease by applying V ds together with V gs in the linear region, whereas no V ds dependence was observed for the saturation region.…”

“…31,32 In order to obtain the accurate spin concentration within the channel, we fabricated the top-contact FETs with the length of the top electrodes to be sufficiently small compared to the channel length. Furthermore, the spin concentration underneath the electrodes was subtracted from the total concentration.…”