The Preparation And Characteristics Of Rubrene Single Crystal Materials And Devices

In view of the advantages in emission efficiency, exciton diffusion length and carrier mobility, rubrene has been widely used in organic light-emitting diodes, organic field-effect transistors, organic solar cells and other areas. The mobility of rubrene single crystal in organic field-effect transistors was measured to be about24.5cm2/Vs, which is the record in organic semiconductor devices.In recent years, researches on organic spintronics have made rapid progresses. However, organic spin injection field-effect transistors and organic spin valves with large magnetoresistance at room temperature remain challenging. Due to their weak spin-orbital interaction and weak hyperfine interaction, organic semiconductors are expected to have long spin diffusion length in theory. However, the spin diffusion lengthes are found to be just tens of nanometers in organic semiconductors. The short spin diffusion length might due to the low carrier mobility and large amounts of grain boundaries, defects and impurities in amorphous or microcrystalline thin films, which can significantly reduce the diffusion length. Rubrene has best performance in carrier mobility. By preparing high quality rubrene single crystals which have less grain boundaries, defects and impurities, long spin diffusion length can be expected. Thus, in this thesis, spintronics devices based on rubrene single crystals were prepared and the properties of the devices and rubrene materials were investigated.The detailed contents and main results are given below:First, rubrene single crystals were prepared using the method of physical vapor transport (PVT). The structural properties of the prepared rubrene crystals were studies by XRD and AFM. The AFM images showed clear layered surface structures of the rubrene crystals, which confirms the layered growth mechanism of rubrene. Subsequently, we studied the PL spectrum of rubrene single crystal. We observed strong optical anisotropy by analysis of the spectrum. The photoluminescence peak located in yellow and green zone at low temperature, while when the temperature rises, the peak changes to orange zone due to the self-absorption.Secondly, we prepared asymmetric ferromagnetic electrodes devices with2-3u m using photolithography and e-beam evaporation methods. We chose conventional ferromagnetic material iron, cobalt and nickle as the electrodes. After that, we studied the oxidation of electrodes under the condition of crystal growth using AES method. In order to solve the oxidation problem,5nm copper protective layer was deposited on the top of ferromagnetic electrodes to prevent the oxidation. Then we studied the hysteresis loops of different asymmetric electrodes using AGM method. The result showed that the magnetization reversed separately.Finally, we studied the device property under different temperature and magnetic field using a semiconductor characterization system. We observed that the devices showed typical diode characteristics. The IV curve changes obviously as the temperature increases, and the device resistance is inversely proportional with temperature. When applied a large external magnetic field under low temperature, the Ni/Rubrene/Co devices shows a magnetoresistance as large as44%. But the magnetoresistance disappeared at room temperature.