Organic molecular crystals: from thin-films to devices : investigation of thin-film formation and electronic transport properties of polycrystalline perylene films

摘要

When Bardeen, Shockley and Brattain produced the first prototype transistor in 1947 no one could have imagined the invention's impact on today's life. It was the advances in thin-film technology which paved the way for the transistor to wide range applicability. Organic semiconductors have been subject to research since the early 1960s. In the mid 1980s, the research on organic semiconductors took another leap forward when Karl, Warta and Stehle published their research on ultrapure, high mobility organic photoconductors and on charge carrier transport in organic conductors. Organic electronics have entered everyday life as organic light-emitting diodes (OLEDs) can nowadays be found in a variety of applications. While OLEDs have accomplished the step from basic research to market readiness with sales exceeding $ 1B/yr in 2008, organic thin-film transistors (OTFTs) still lag behind. Nevertheless they are intensively studied as the material of choice in active matrix devices for flexible displays. One of the reasons for the absence of commercial OTFT applications is their slow switching speed due to low charge carrier mobility. The application of so-called dielectric surface modification (DSM) is one pathway towards high performance thin-film OTFTs. DSMs, which are based on organic molecules forming self-assembling monolayers or polymeric dielectrics, are studied due to their huge impact on OTFT device performance and on the film perfection of organic semiconductor thin-films. The appropriate choice of DSMs enables the combination of an arbitrary organic semiconductor with any substrate materials. Hence, the key factor to high performance and film perfection is the educated choice of thin and easily applicable DSM. However, the reason for the performance enhancement is not yet understood. This hinders the development of DSMs for high performance OTFTs. In this thesis two systematic approaches will be presented. DSMs were chosen by knowledge, which means that suitable DSMs were identified by a knowledge driven process. In addition, novel dielectrics were designed and synthesized specifically for this thesis. In case of DSMs chosen by knowledge, we were able to show that different DSMs can be classified into three types: low adhesion DSMs, high adhesion DSMs and intermediate DSMs. Perlyene films produced on low adhesion DSMs grow in highly textured films with large grains while perylene films produced on high adhesion DSMs show a perturbed growth mode which leads to an inferior film quality. The film quality of perylene films grown on intermediate DSMs lies between these two extremes. Furthermore, the measurements show that perylene OTFTs based on low adhesion DSMs outperform all other perylene based OTFTs. The application of novel dielectrics, based on derivates of tridecyltrichlorosilane (TTS) with different functional end-groups aimed at the specific tailoring of the adhesion energy, which is assumed to be the major factor in producing high performance OTFTs. The experiments confirm the importance of an educated choice of an appropriate DSM. We were able to tailor the adhesion energy of perylene molecules on a substrate by the application of deliberately designed DSMs. It was confirmed that perylene based OTFTs on carefully chosen DSMs outperform TFTs without DSM in terms of charge carrier mobility and film quality. This is indicative for an influence of the DSMs on the trap-state distribution and on the thin-film formation process. Both influences were investigated in detail. In case of the thin-film formation process, the influence of DSMs on film perfection was unraveled. We were able to show that the difference in adhesion energy for different DSMs influences the diffusivity. This is also influencing the island density and hence the film perfection. Furthermore, the influence of the balance between adhesion and cohesive energy on the growth of the first monolayers was demonstrated. The influence of DSMs on the trap-state distribution at the DSM/organic interface was investigated in detail. To explain all observed time- and temperature dependent effects in perylene TFTs, a model was developed that predicts the existence of states in the vicinity of the perylene HOMO level that need energy in order to be filled and in order to be emptied. By the application of a novel variation of temperature stimulated current (TSC) we were able to confirm the existence of this energy level. It was shown that the existence of trap states has a huge impact on charge carrier transport and in turn on device performance and that different DSMs lead to a different trap state density and distribution.