Dewetting of an Organic Semiconductor Thin Film Observed in Real‐time

Abstract

applicationsofthinsolidfilms,dewetting isoftenconsidereda problem affecting growth of smooth films as well as a limitation for devicestability. However,itcanalsobeused asa nanopatterning technique to produce arrays of nanostructures with controlled lateral distances and size. It has recently been shown that dewetting and thermal stability are of particular importance for organic semiconductors, [2‐5] because crystalline thin film of these materials are bound by weak van der Waals forces and therefore molecular diffusion sets in already at moderate temperatures. Dewetting studies therefore are important for applications of organic semiconductors in devices such as thin film transistors, [6] organic photovoltaics and organic light emitting diodes. In this paper we study the planar aromatic molecule diindenoperylene (DIP) as a model system, which has been shown to exhibit high structural order, [7] good charge carrier mobility, [8] and interesting opto-electronic properties such as a long exciton diffusion length. [9] Further,arecentstudyshowsthatunderappropriate growth conditions DIP forms nano-dots on top of a wetting layer indicating its potential for self-assembled nanostructures. [10] Besides its relevance for the long-time stability of devices, dewetting also offers a challenge for the fundamental understanding of these molecular materials. Dewetting is a complex process that can be started by an external stimulus such as a thermal or mechanical perturbation and is closely related to the growth of the potentially metastable thin film. Adding to the complexity molecular semiconductor films often exhibit polymorphism and therefore one or more metastable structures are common during growth. Also, organics show strong surface anisotropy because of the generally anisotropic molecular building blocks, which has to be taken into account for dewetting theories, as simple models for isotropic liquids do not apply. In particular for very thin films such as a molecular monolayer (ML) the discreteness of theunderlyingcrystallatticebecomesimportant andtherefore continuum models have to be altered as shown in reference [11]. Experimental data for dewetting of such ultra-thin crystalline organic layersis scarce though and in particular the kinetics and mechanisms of dewetting require further elucidation. In this paper, we report clear evidence of DIP ML dewetting via the formation of bilayer islands. We use real time X-ray scattering to determine the time constant of interlayer molecular diffusion during dewetting and use AFM to determine the dewetted morphology and in-plane length scales.