Arrays of Pentacene Single Crystals by Stencil Evaporation

Abstract

In order to realize high-performance organic thin-film transistors (TFT), two parameters of the organic semiconducting layer are desired: single crystallinity for high mobility, and patterning for low off currents. High-quality single crystals can be fabricated using vapor techniques such as physical vapor transport (PVT) but they require high temperatures close to thermodynamic 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 equilibrium, for example, 240°C for pentacene. Such high temperatures are not ideal for TFT fabrication on plastic substrates and limit the use of PVT in flexible electronics applications.In this work arrays of pentacene single crystals were directly deposited at low temperature of 40°C by vacuum thermal evaporation through micro-fabricated stencil masks (stencil lithography). By decreasing the stencil aperture size down to 1 µm x 1 µm, we were able to limit the nucleation area until only one grain per aperture is nucleated and grown. We studied systematically scaling effects for large singe crystal growth and discuss details of the growth morphology. We found for instance that the formed pentacene crystals are one monolayer thick and the crystal area is much larger than the aperture size. This can be explained by the diffusion of adsorbed molecules on the surface laterally under the shadow mask, where they are protected from other impinging molecules. The diffusion away from the impinging area under the aperture affects the nucleation density inside this area and was used to calculate the diffusion length to nucleation λ N = 0.66 ± 0.11 µm of pentacene on SiO 2 at 40°C. Our diffusion-driven growth of organic single crystals by stencil lithography is a direct method to grow patterned arrays of single crystalline organic thin-film semiconductor layers.

Growth Of Organic Semiconductor Thin Films with Multi-Micron Domain Size and Fabrication of Organic Transistors Using a Stencil Nanosieve

“…17 We have previously described how below a certain aperture size this surface diffusion becomes a dominant factor in the growth of pentacene on SiO 2 . 15 We observe a similar effect when evaporating C 10 -DNTT on ODTS-treated SiO 2 through a stencil mask. Differential interference contrast (DIC) microscopy and atomic force microscopy (AFM) images show the evolution of the organic semiconductor film morphology as a function of the aperture size a as well as the aperture-to-aperture distance d (Figure 1).…”

“…In this case impinging molecules are rapidly added to the growing crystallites and only molecules adsorbed along the perimeter of the aperture may diffuse underneath the mask. 15 The Stranski−Krastanov film growth observed in the exposed regions is typical for C 10 -DNTT. 18 It consists of two-dimensional layered growth up to 5 monolayers (∼20 nm-thick) covered by several μm-long needles and ∼100 nm-thick.…”

“…When neighboring apertures are situated farther than the surface diffusion length, the surface concentration of C 10 -DNTT molecules remains too low to trigger nucleation and no film growth is observed. 15 The few grains observed in Figure 1a may be the result of heterogeneous nucleation on surface defects. On the other hand, when considering large sparse apertures (a = 5 μm, d = 5 μm, Figure 1b), sufficient supersaturation is reached within the aperture, giving rise to multiple nucleation events.…”

“…To this end, we have demonstrated the use of stencil masks with microfabricated apertures to reduce and localize nucleation sites on the substrate during organic film growth. 15 This approach requires that the aperture size is close to the surface diffusion length of adsorbed molecules, that is, in the micrometer range. This poses a severe constraint on the lateral dimension of the film formed through a single aperture.…”

“…Therefore, strategies that limit nucleation density during vacuum film growth are desirable to obtain large crystalline domains. To this end, we have demonstrated the use of stencil masks with microfabricated apertures to reduce and localize nucleation sites on the substrate during organic film growth . This approach requires that the aperture size is close to the surface diffusion length of adsorbed molecules, that is, in the micrometer range.…”