The Influence Mechanism Of Molecular Orientation And Charge Transport Properties Of Organic Thin Film

Due to the advantages of facile manufacturing, low casts, compatibility with flexible substrates, and potential for large-scale roll-to-roll production, organic thin film transistors (OTFTs) have shown potential applications in wide fields such as flexible displays, flexible storage, radio frequency identification devices (RFIDs) and sensors. However, the charge carrier mobilities of organic semiconductors are still at a low level, and unable to meet the requirements of microelectronic devices. Researches have shown that the charge carrier mobility is strongly affected by the molecular orientation, which can be controlled by the interplay between molecule-molecule interaction versus molecule-substrate interaction, and different applications require different molecular orientations on the substrate. Therefore, to understand deeply the influence mechanism of molecular orientation and charge transport of organic thin film from the microscopic view is the essential issue to improve the performance of organic semiconductor devices.Based on the comprehensive methodology of combining analogue simulation, theoretical calculation and experimental verification, the growth processes of pentacene thin film are studied in this paper. The transformation mechanism of molecular orientation has been elucidated and the influences of substrate temperature and external electric field on the molecular orientation have been analyzed. Besides, we have theoretically investigated the charge carrier mobilities under different process parameters and the optimal ranges are determined, which are verified by the experiments finally. The main contents and results are summarized as follows:The theoretical analysis and molecular dynamics (MD) simulations are performed to investigate the initial growth of pentacene on amorphous SiO2 (a-SiO2) substrate, where the initially grown lateral-oriented cluster will transfer to the normal-oriented one at a critical size. The reorientation behavior is considered to be governed by the competition between the molecule-molecule interactions and the molecule-substrate interactions. Comparing the bare a-SiO2 substrate, the existence of the sylanol groups on the a-SiO2 substrate will increase the molecule-substrate interactions.The influences of substrate temperature and external electric field on the molecular orientation have been analyzed using MD simulations. It's found that during the optimal temperature range, the pentacene molecules tend to form the normal-oriented, well-ordered cluster driven by the dominant molecule-molecule interactions. A higher substrate temperature results in the disordered, lateral orientation with the weakening of the molecule-molecule interactions. Besides, under the electric field parallel to the pentacene long axes or short axes with proper strength, the pentacene cluster undergoes a conformational transition from a lying-down conformation to a standing-up one. Contrary to that, when an electric field perpendicular to the molecular plane is applied to the pentacene cluster, the pentacene molecules would keep the lying-down conformation regardless of changes in the electric field strength.Based on the Marcus theory, the charge transport properties of pentacene crystal and cluster have been calculated by employing a multi-scale theoretical approach combining molecular dynamics simulations, density function theory (DFT), and kinetic Monte Carlo simulations (KMC), and the effect of substrate temperature on the charge transport have been provided. It is found that the mobility of pentacene cluster would increase first and then decrease with the increase of substrate temperature. A reasonable substrate temperature range has been determined.Finally, a series of pentacene OTFTs have been fabricated by physical thermal deposition under different substrate temperature. The experimental results agree well with the theoretical calculations.