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Universal diffusion-limited injection and the hook effect in organic thin-film transistors.

ABSTRACT: The general form of interfacial contact resistance was derived for organic thin-film transistors (OTFTs) covering various injection mechanisms. Devices with a broad range of materials for contacts, semiconductors, and dielectrics were investigated and the charge injections in staggered OTFTs was found to universally follow the proposed form in the diffusion-limited case, which is signified by the mobility-dependent injection at the metal-semiconductor interfaces. Hence, real ohmic contact can hardly ever be achieved in OTFTs with low carrier concentrations and mobility, and the injection mechanisms include thermionic emission, diffusion, and surface recombination. The non-ohmic injection in OTFTs is manifested by the generally observed hook shape of the output conductance as a function of the drain field. The combined theoretical and experimental results show that interfacial contact resistance generally decreases with carrier mobility, and the injection current is probably determined by the surface recombination rate, which can be promoted by bulk-doping, contact modifications with charge injection layers and dopant layers, and dielectric engineering with high-k dielectric materials.

SUBMITTER: Liu C

PROVIDER: S-EPMC4954995 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The key impact and significance of a multilayer polymer-based dielectric system on the remarkable photoresponse properties of zinc phthalocyanine (ZnPc)-based photosensitive organic field-effect transistors (PS-OFETs) have been systematically analyzed at various incident optical powers. A combination of inorganic aluminum oxide (Al2O3) and organic nonpolar poly(methyl methacrylate) (PMMA) is used as the bilayer dielectric configuration, whereas in the trilayer dielectric system, a bilayer polymer dielectric, consisting of PMMA, as the low-k dielectric polymer, on top of poly(vinyl alcohol) (PVA), the high-k polar dielectric, has been fabricated along with Al2O3 as the third layer. Before fabricating the OFETs, a systematic optimization of the nature of growth of the ZnPc molecules, deposited on PMMA-coated glass substrates at different substrate temperatures (T s) was performed and examined by atomic-force microscopy, field-emission scanning electron microscopy, X-ray diffraction, and Raman analysis. At 90 °C, the fabricated PS-OFETs with the Al2O3/PVA/PMMA trilayer dielectric configuration showed the best p-channel behavior, with an enhanced and remarkable photoresponsivity of R ∼ 9689.39 A W-1 compared to that of the Al2O3/PMMA bilayer dielectric system (R ∼ 2679.40 A W-1) due to the polarization of the dipoles inside the polar PVA dielectric, which increases the charge transport through the channel. The charge carrier mobility of the device also improved by one order (μh ∼ 1.3 × 10-2 cm2 V-1 s-1) compared to that of the bilayer dielectric configuration (μh ∼ 3.5 × 10-3 cm2 V-1 s-1). The observed specific detectivity (D*) and NEP values of the bilayer dielectric system were 6.01 × 1013 Jones and 2.655 × 10-17 W Hz-1/2, whereas for the trilayer dielectric system, the observed D* and NEP values were 5.13 × 1014 Jones and 1.043 × 10-17 W Hz-1/2, respectively. Additionally, the operating voltage of each of the fabricated devices was also very low (-10 V) due to the influence of the inorganic high-k Al2O3 dielectric layer. The electrical stability of all of the fabricated devices was also investigated by bias stress analysis under both light and dark conditions in vacuum. To the best of our knowledge, the photoresponsivity (R) reported here with an Al2O3/PVA/PMMA trilayer dielectric configuration is the highest reported value for thin film-based PS-OFETs, at a remarkably low operating voltage of -10 V, on low-cost glass substrates without indium tin oxide or/and Si/SiO2.

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Universal diffusion-limited injection and the hook effect in organic thin-film transistors.

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