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Self-assembly Of Organic Semiconductor Materials And Theoretical Studies

Posted on:2014-02-20Degree:DoctorType:Dissertation
Country:ChinaCandidate:C Y ZhangFull Text:PDF
GTID:1228330398459929Subject:Physical chemistry
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Over the past decades, organic thin film field effect transistors (OFETs) have been the focus of intensive research due to their potential applications in low cost and flexible electronics using a wide variety of different organic semiconductor materials. The exciting process has been witnessed in solar cells, sensors, memories, flexible flat-panel displays, e-paper’s operation and so on. The performance of these devices is closely correlated with the crystalline structure and the molecular packing state of organic semiconductors thin films. Among the semiconductor materials, rubrene (5,6,11,12-tetraphenylnaphthacene) is a fluorescent and small molecular weight organic molecule with highest field effect mobility in his single crystalline form (40cm2/V.s), which makes it one of the most promising organic semiconductor materials. Many studies on rubrene single crystal have been carried out and rubrene single crystal devices with high performances have been demonstrated. However, the poor crystallinity of rubrene thin films in conventional thermal evaporation procedures results in the low charge carrier mobility, which limits the practical applications in organic electronics. In order to fabricate rubrene thin films with high crystalline quality, several special methods such as low pressure hot wall deposition, deposition in situ vacuum annealing, and ramping substrate temperature slowly have been designed. Some researcher explored6,13-pentacenequinone (PQ) as an crystallized insulating molecule to induce the crystallization of rubrene in vacuum-deposited thin film tansistors. Besides the techniques mentioned above, self-assembled monolayers (SAMs), with their well-ordered structures, have been widely applied to controll the site-nucleation of organic seminconductor crystalline films in recent years. Generally, the growth behavior and microstructure of crystalline films are highly dependent on functionalized SAM decoration on the substates. The reported charge mobilities were in the range of10-2~10-1cm2/V.s, which is still lower than the critical value of1 cm2/V.s for technical applications.It is very challenging to increase the crystallinity of rubrene and charge carrier mobility in thin film transistors. Due to the great development of computational power over the past few years, computer simulations have been proven to be valuable tools to study self-assembly of semiconductor molecules. Regrettably, an explicit and complete understanding toward the crystallization of rubrene films and site-selective mechanism of organic semiconductor molecules appeals to the assistance of theoretical research.In this dissertation, we achieved the transformation of rubrene thin films from amorphous to crystalline state after in-situ thermal post annealing. The SiO2gate dielectric was modified with self-assembled monolayers consisting of phospholipids. The OFETs devices based on rubrene crystalline films exhibited high performance. Based on the experimental results and several kinds of functional small molecular weight organic molecules newly reported, Molecular dynamics and Quantum mechanics study have been performed to study the deposition and growth behavior of rubrene. Additionally, we also investigate the site-selective growth and crystallization of another functional small molecular weight organic semiconductor molecule: N,N-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8), which is considered as the most promising and important materials to fabricate n-channel OFETs. The theoretical simulation provides a detailed atomistic level insight into the growth process and the molecular packing state. More importantly, it gives a hint for the further investigation to obtain a wide variety of high quality crystalline thin films.The important and valuable results in this dissertation can be summarized as follows:(1) Site-selective Deposition Behavior onto SAMs with Different Packing Density and Step-Edge Induced Area-Selective Growth of Rubrene:A Combined Molecular Dynamics Study and Quantum Chemistry Calculation.The site-selective deposition of rubrene on different densely packed self-assembled monolayers consisting of phospholipids was investigated by a series of molecular dynamics (MD) simulation. Our simulated results reveal that the packing density of SAMs can directly control the site-selective depositon of rubrene molecules. Additionally, a mixed system composed of bare silica substrate and SAMs covered substrate was constructed to investigate the edge-induced area-selective growth. On the mixed system, the rubrene molecules preferentially deposit along the step edge of alkyl chain SAMs induced by the binding energy difference. In combination of MD simulation and Density functional theory (DFT-D) results, we found that the typical conjugated structures of rubrene molecules were formed during the deposition process. Based on the simulation results, we have provided a microscopic perspective on the step-edge induced area selective growth of rubrene molecules, which could offer theoretical assistance to the crystallinity and controllable patterning of organic semiconductors.(2) Growth of Rubrene Crystalline Thin Films Using Thermal Annealing on SAMs and the Influence of Self-Assembled Monolayers on Growth and Crystallization of Rubrene Films by Molecular Dynamics Study.We used DPPC and dioleoylphosphatidylcholine (DOPC) SAMs pattern as the inducing layer to obtain crystalline quality rubrene thin films after in-situ thermal post annealing. Crystalline rubrene thin films can be obtain on DPPC liquid expanded phase (LE), liquid condensed phase (LC) and DOPC LE phase. The OTFTs devices based on rubrene crystalline films, which were obtaind on DPPC LC phase, exhibited high performance. The charge mobility can be achieved as high as0.98cm2/V.s. Furthermore, molecular dynamics (MD) simulation has been performed to study the deposition behavior and crystallization of rubrene films on different SAMs. Four simulated systems with different self-assembled monolayers (SAMs) were constructed to investigate the microscopic configuration of rubrene deposition, interfacial reaction, and further discuss how the template structures affect the crystallization of rubrene molecules. The simulated results reveal that surface binding energy and structural order of SAMs, exhibits a significant effect on rubrene growth and crystallinity. SAMs with ordered arrangement are in favor of inducing crystallization of rubrene films. The simulated results provide a microscopic perspective on the growth and crystallization of rubrene films. It could be helpful to develop modes to design appropriate template structures for achieving semiconductor with high performance in realistic systems.(3) Control Over Patterning of Thin Organic Films of PTCDI-C8via Molecular Dynamics and Quantum Mechanics Study.Based on the experimental study newly reported, we explored MD and Density functional theory to investigate the step-edge induced area selective growth of N,N-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8). The simulated results confirmed that the organic molecules preferably deposited along the pattern edge and diffused until the gold step edge was covered completely by PTCDI-C8. Then further growth was driven by strong intermolecular π-π stacking, which lead to the lateral growth. The quantum mechanics study demonstrated the stacking mode of organic molecules at a microscopic level.
Keywords/Search Tags:self-assembled monolayers, organic semiconductor, rubrene, organicfield effect transistors (OFETs), PTCDI-C8, step-edge-induced area-selective growth, molecular dynamics(MD), quantum mechnics (QM)
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