Nucleation and growth of organic thin films on polymeric dielectrics

  • Nukleation und Wachstum organischer Dünnfilme auf polymeren Dielektrika

Mokros, Daniel; Wuttig, Matthias (Thesis advisor); Taubner, Thomas Günter (Thesis advisor)

Aachen (2016)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

During the last years, scientific attention on organic semiconductors has significantly increased. Major advantages of this material class are their usual mechanical flexibility, low cost production and the opportunity to develop transparent thin film devices. First devices using organic light emitting diodes (OLEDs) have reached market readiness and are applied in smart phone devices and TVs. The research on organic solar cell (OSC) concepts led to a significant increase in efficiency. The development of organic thin film transistors (OTFTs) is also a goal in the field of organic electronics. Further progress in this field demands an in-depth understanding of the respective physical material properties. One important aspect is given by the growth mechanisms of the semiconductor thin films. Due to the complex structures and structural anisotropies of organic molecules, an optimization of the crystalline order of the thin film is crucial for electronic and optical properties.One promising approach to control thin film quality at very early stages of growth, bases on the variation of the surface free energy (SFE) of the substrate via polymeric dielectric surface modifications (DSMs). The interaction between substrate and active organic semiconductor material impacts the adhesion and the diffusion of the molecules on the substrate surface. For further development of this approach of SFE control, it is crucial to get a microscopic pictureof the participating effects. Thus, a detailed study was performed in this thesis in order to reveal the fundamental growth mechanisms of the p-type semiconductor perylene on polymericdielectric surfaces and to develop a consistent growth model . The selected polymers are poly(methyl methacrylate) (PMMA), polystyrene (PS) and polydimethylsiloxane (PDMS). These three polymers cover a critical range in the adhesion of perylene. The adhesion of peryleneon PMMA lies above the cohesion energy of perylene itself, which is again comparable to the adhesion energy on PS. The adhesion energy on the PDMS substrate lies significantly below the cohesion of perylene. For this study, a silicon dioxide substrate served as reference system. In this work, it was revealed that the modification of the adhesion energy does not only determine the diffusion length of the molecules on the substrates, but also the critical nucleus size of perylene. The critical nucleus size is a central quantity for the description of the island formation during thin film growth. Two models were applied for the evaluation of the critical nucleus. The first model bases on classical rate equation theory. The second model extracts the critical nucleus size by applying capture zone analysis of the island distribution. The application and comparison of these two different theoretical models delivered consistent results regarding the nucleus size and it was clearly shown that it increases with decreasing adhesion energy of the substrate.The low adhesion regime on PDMS additionally exhibits a transition towards a very distinctive growth mode of perylene with characteristic morphologies and high crystalline order. In this work, a growth model was developed, which is able to explain the island formation on PDMS.The observed features of the perylene thin films on PDMS were mainly addressed to dewetting. Beside the large extend of the islands and a very high crystalline order, the PDMS-modified system exhibited indications for the formation of a polymorphic perylene phase and showed hopper growth and cloverleaf structures. In order to investigate if the dewetting driven growth effects on PDMS are more generic for organic thin films, a further active material was applied as comparator systems to perylene. The n-type semiconductor PTCDI-C13 was characterized in its growth properties. In a preliminary investigation, the distinctive layer-by-layer-growth on silicon dioxide was characterized and afterwards compared to the growth on the low adhesion DSM PDMS. At room temperature,PTCDI-C13 exhibited three different growth regimes on silicon dioxide. Starting with a highly ordered layer-by-layer growth, the system showed indications for a thin film phase. Later on, an increase of disorder was observed and exhibited grainy island structures between the layers due to imperfect layer growth. This polycrystalline grains dominate the whole surface for thick films.The growth transition can be suppressed by a sufficient amount of thermal energy during or after deposition. However, in the case of PTCDI-C13 the application of the PDMS modification causes significant dewetting effects, which result in the growth of needle structures with lengths of a few hundred nanometers. It appeared that these needle-like structures correspond to a local equilibrium form,which rearranges to compact 3D-islands via post-annealing to minimize contact to the PDMS substrate. The results of this thesis and the comparison between the growth of perylene andPTCDIC-13 on PDMS revealed the impact of DSMs and the potential of a systematic variation of the surface free energy. It has been shown, that the central growth parameters of diffusion and critical nucleus size can be tuned systematically. Below the critical limit, where the cohesion of the thin film is stronger than its adhesion on the substrate, significant dewetting mechanisms induce distinctive, kinetically driven morphologies, which lead to new opportunities for the application in electronic devices.