A study on the effect of film crystallinity and morphology on charge carrier concentration-dependent hole mobility in pentacene thin-film transistors: advantages of high deposition rate

Abstract

The combined effect of deposition rate and substrate temperature on the film crystallinity, morphology, and electronic properties of pentacene is studied. It is shown that the channel mobility in polycrystalline pentacene thin-film transistors is relatively immune to substrate temperature, and the films offer good hole mobility when grown at a high rate. This is advantageous when high throughput with low deviation in electrical parameters over devices are required. The surface morphology is characterized by atomic force microscopy measurements and the crystallinity is studied using x-ray diffraction. The effect of growth parameters on the crystalline phases of pentacene is correlated to the charge carrier transport. It is found that the field-effect mobility is primarily affected by the crystalline phases of the film rather than the grain size. The charge carrier dependence of the hole mobility is used to parameterize the dispersion (width) in the density of states (DOS) of the highest occupied molecular orbital of the films in the transistor channel region. It is found that the presence of multiple phases in the path of the charge carrier flow increases the dispersion of the DOS.

A New Approach to Polymorphism in Molecular Crystals: Substrate‐Mediated Structures Revealed by Lattice Phonon Dynamics

“…[47] In fact, the architecture of the film strongly influences the transport properties of the semiconducting layer, as revealed by the effect of film crystallinity and morphology on charge carrier mobility in thin film transistors of pentacene, where it was shown that a good hole mobility could be obtained once the film was deposited at high rate. [50] This observation is fully consistent with the idea that at high rate we have a 2D growth regime, [47] with a corresponding poor interlayer correlation perpendicular to the dielectric interface. Besides, in our conditions of pentacene growth, [47] high rates yield 2D nucleation, leading to a quasi-layer-by-layer growth, forming continuous films, more suitable for charge transport.…”

“…Besides, in our conditions of pentacene growth, [47] high rates yield 2D nucleation, leading to a quasi-layer-by-layer growth, forming continuous films, more suitable for charge transport. [14,50,51] On the contrary, low deposition rates yield a 3D growth, leading to ill-connected grains. [50] These findings have their counterpart in the two upper spectra of Figure 1a, where the TF spectrum at low rate exactly matches that of the HT bulk phase, and is therefore labelled 3D.…”

“…[14,50,51] On the contrary, low deposition rates yield a 3D growth, leading to ill-connected grains. [50] These findings have their counterpart in the two upper spectra of Figure 1a, where the TF spectrum at low rate exactly matches that of the HT bulk phase, and is therefore labelled 3D. This is also confirmed by the intra-molecular spectra of Figure 1b.…”

“…[26,47] As a consequence, the 2D structure growing at the interface produces strong effects on the mobility of the active layers, increasing the efficiency of the organic semiconductor. [50] These considerations definitely open the route to a renewed interest on the polymorphism beyond the bulk crystals. This new approach of the organic structures at the interfaces (SIP) obviously requires more investigations if we want to further improve the knowledge on the transport and structural properties of organic semiconductors.…”

Terahertz Raman scattering as a probe for electron–phonon coupling, disorder and correlation length in molecular materials

“…Indeed, a very recent paper that appeared in the literature has thoroughly studied the effect of film crystallinity and morphology on the charge carrier mobility in thin film transistors of pentacene. 84 A good hole mobility is indeed obtained once the film is deposited at a high rate, an observation that fully agrees with our view of the actual formation of a 2D film with a corresponding low inter-layer correlation close to the dielectric interface, where charge transport is more efficient. 64,65 It is quite sensible that the disorder introduced in the layer-by-layer growth at the higher rate produces a slight loss of correlation among the various MLs, sufficient to disrupt the phase matching in the direction perpendicular to the substrate.…”