Geometry Control of Source/Drain Electrodes in Organic Field-Effect Transistors by Electrohydrodynamic Inkjet Printing

Abstract

In this work we study the influence of dielectric surface and process parameters on the geometry and electrical properties of silver electrodes obtained by electrohydrodynamic inkjet printing. The cross-section and thickness of printed silver tracks are optimized to achieve a high conductivity. Silver overprints with cross-section larger than 4 μm² and thickness larger than 90 nm exhibit the lowest resistivity. To fabricate electrodes in the desired geometry, a sufficient volume of ink is distributed on the surface by applying appropriate voltage amplitude. Single and multilayer overprints are incorporated as bottom contacts in bottom gate organic field-effect transistors (OFETs) with a semiconducting polymer as active layer. The multilayer electrodes result in significantly higher electrical parameters than single layer contacts, confirming the importance of a careful design of the printed tracks for reliable device performance. The results provide important design guidelines for precise fabrication of electrodes in electronic devices by electrohydrodynamic inkjet printing.

Keywords: electrohydrodynamic inkjet printing; organic field-effect transistors; printed electronics.

Figure 1

(a) Schematic top view representation of DGP overprints with various interline spacing (s) and different number of printing passes (n). (b) Exemplary cross-section profile underlining the geometrical parameters W, L and h_max.

Figure 2

Optical microscope images of DGP overprints on SiO₂ surface with different types of treatment. Scale bar equal 100 μm.

Figure 3

(a) Optical microscope images of DGP printed lines on plasma-treated SiO₂ surface with different amplitudes (A). (b) Cross-section profiles of the respective overprints. B = 200 V, f = 500 Hz, s = 100 μm. Scale bar equal 200 μm.

Figure 4

Influence of the amplitude of the applied voltage (A) on the geometrical features of the overprints on (a) width (W) and edge-to-edge distance, defining the channel length (L), and (b) maximum height of the overprints (h_max). B = 200 V, f = 500 Hz, s = 100 μm.

Figure 5

(a) Influence of the frequency of the voltage waveform (f) on the linewidth (W) of the overprints and on the resulting channel length (L), for s = 100 μm. (b) Optical microscope images for f = 100 Hz, f = 500 Hz and f = 1000 Hz, respectively. A = 300 V, B = 200 V, s = 100 μm. Scale bar equal 50 μm.

Figure 6

Summary of the geometrical features of the overprints fabricated with different A and f. (a) Width, (b) maximum height and (c) cross-section of the overprints increases for the higher voltage amplitude.

Figure 7

Summary of the resistivity measurements for DGP printed single layer overprints as a function of their cross-section (a) and their maximum height (b) for different A. The blue dashed lines represent the resistivity reported by the ink supplier.

Figure 8

(a) Optical microscope images of DGP overprints on plasma-treated SiO₂ surface with different number of printed layers (n). (b) Cross-section profiles of the respective overprints. A = 250 V, B = 200 V, f = 100 Hz, s = 100 µm. Scale bar equal 200 µm.

Figure 9

Summary of the resistivity measurements of single and multilayer overprints. (a) Two-point probe resistivity as a function of the overprint cross-section for different A. (b) Resistivity presented as a function of number of printed layers (n). The blue dashed lines represent the resistivity reported by the ink supplier. B = 200 V, f = 100 Hz.

Figure 10

(a) Transfer and (b) output characteristics of bottom contact/bottom gate organic field-effect transistors (OFETs) with DPP-DTT (poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole -alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)]) as active layer, comprising single (n = 1), double (n = 2) and triple (n = 3) overprints as electrodes. V_SD = −40 V for transfer characteristics.