Influence of the substrate platform on the opto-electronic properties of multi-layer organic light-emitting field-effect transistors

Abstract

In this paper, we present a study of the effects of the influence of the substrate platform on the properties of a three layer vertical hetero-junction made of thin films of α,ω-diperfluorohexyl-4T (DHF4T), a blend of tris(8-hydroxyquinolinato)aluminium (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), and α,ω-dihexyl-quaterthiophene (DH4T).The hetero-junction represents the active component of an Organic Light-Emitting Transistor (OLET). The substrate platforms investigated in this study are: glass/Indium-Tin-Oxide (ITO)/Poly(methyl-methacrylate) (PMMA) and Si ++ /Silicon oxide (SiO 2 )/PMMA. The first platform is almost completely transparent to light and therefore is very promising for use in OLET applications. The second one has been chosen for comparison as it employs standard microelectronic materials, i.e. Si ++ /SiO 2 . We show how different gate materials and structure can affect the relevant field-effect electrical characteristics, such as the charge mobility and threshold voltage. By means of an Atomic Force Microscopy analysis, a systematic study has been made in order to correlate the morphology of the active layers with the electrical properties of the devices.

Correlation between gate-dielectric morphology at the nanoscale and charge transport properties in organic field-effect transistors

“…44 Notably, the computed R rms of the annealed systems is in excellent agreement with the typical values (about 0.5 nm) measured in the PMMA samples deposited on ITO and thermally annealed to temperatures above the T g of PMMA. 34 As anticipated, the surface roughness of the gate dielectric is certainly related, to some extent, to the charge transport properties of the overlying organic semiconductor. Consequently, parameters measuring the morphological features of the surfaces in terms of the mean values, such as the R rms , are usually invoked to assess the quality of dielectric surfaces for the subsequent deposition of ordered layers of organic materials.…”

“…24,45,46 However, recent experiments provide evidence of a limited correlation between the averaged surface properties of the dielectric and the overall device performance. 34 Indeed, parameters measuring the surface morphology as averages over large areas, such as the R rms , do not constitute reliable indicators to dene the potential local ordering of organic molecules over the dielectric layer and the formation of efficient, interconnected charge percolation paths. Inversely, the subnanometer scale of the single events involved in the transport of charge in the organic semiconductor layer points to a potential role of the local organization of the dielectric surface in affecting the device performance.…”

“…3) was also included, with a magnitude (1.5 Â 10 6 V m À1 ) corresponding to the typical operation conditions of OFETs. 34,55 From the hopping rates between site pairs, charge percolation paths were simulated by assuming the injection of a single positively charged carrier (hole) from a randomly selected open site on the le side of the grid and propagating the events in time until the carrier reaches the right side of the grid. The mobilities and charge densities were then evaluated as averages of the values obtained from 5000 independent simulations for each of the samples considered.…”

“…To assess the dependence of the computed properties with the nature of the polymeric substrate, a comparison with poly-4-vinylphenol (PVP) was also performed. The use of PMMA and PVP as a dielectric layer generally leads to better charge transport properties with respect to inorganic materials, as for example SiO 2 , [33][34][35][36] as they are able to combine good compatibility with organic materials, ease of processing, and a relatively high dielectric constant in thin lms. 4,37 The morphology of the polymer dielectric layer depends on several factors, including molecular weight and tacticity.…”

Study on copper phthalocyanine and perylene-based ambipolar organic light-emitting field-effect transistors produced using neutral beam deposition method

“…To realize a balanced carrier conduction, the application of single ambipolar materials or the combination of two unipolar materials through co-evaporated (blended) or bilayered structures has been utilized thus far. [11][12][13] In most cases of single-component-and blend-based OLEFETs, however, efficient light emission is yet to be reported owing to the difficulties in achieving a balanced injection and carrier transport. Instead, although there is a physical separation between the hole and electron transports, bilayered heterojunction-based OLEFETs have demonstrated well-balanced ambipolarity with high carrier mobilities, and have a consequently improved luminescence through the tuning of the gate voltage.…”