Study On The Photoelectric Properties Of The Ambipolar Organic Single-crystal Field-effect Transistors

The highly ordered arrangement of organic molecules and low impurity make it possible for the organic single crystals to obtain high thermodynamic stability and high carrier mobility. In addition, the clear stacking model of the organic single crystal also provide us a convenient model to deeply research on the relationship of the intermolecular interaction, molecular arrangement and the optoelectronic properties of the material(for example, charge injection efficiency and luminescent crystals and crystal carrier mobility).Organic field-effect transistors(OFETs), especially ambipolar OFETs, are efficient physical prototype to study on the carrier mobility. The special geometry of the OFETs also provide the precious opportunity to in-situ observe the carrier recombination, electroluminescence and other physical process. At the same time, the OFETs based on high quality organic single crystals are capable of providing ultra-high current density and withstand the injection and migration. In the organic lighting emitting field-effect transistors(OLETs), the carrier recombination zone can be control far away from the injection electrodes, which protects the excited states from quenching by the electrodes effectively. Further, in the ambipolar OFETs, the concentration of the charge in the recombination is very low and the recombination degree of the electron and hole is very high, which can effectively avoid the interactions of the charges and excited excitons which are very common in theorganic lighting emitting diodes(OLEDs). As the result of that, the roll off problems of the current efficiency will not exist in the OLETs even when worked in very high current density. In summary, the ambipolar OFETs based on organic single crystals are very promising for electric pumped organic lasers.However, the development of the materials science hasn’t been able to provide any suitable materials for the organic laser. In order to meet the demandes of the OLETs, improving the quality and developing new structure of the organic single crystals is urgently needed. For now, only a few organic single crystals can meet the demands of the ambipolar OFETs. Most of the organic materials can only work in P-type in the OFETs. Even if the calcium was taken use as the electron injection electrode, the signal of the electron’s injection and transition can still be hardly observed. Therefore, the development of the novel organic crystalline materials to achieve balanced carrier injection and mobility in the OFETs is a very meaningful and important work. In addition, the preparation of the OFETs also requires the organic materials can be easily grown as high quality slice like single crystals. The physical vapor transport method(PVT) is an efficient method commonly used to grow the large size high quality organic single crystals. In addition, the organic crystals grown by the PVT method can be usually very thin and obtain smooth crystal surfaces, which is very suitable for preparation and research of the high performance single crystal OFETs. In the process of the PVT, the crystals grow under high temperature, so that the molecules with relatively high kinetic energy in the gas phase, can be stacked in more stable manner to form the crystals.In this paper, we take use of the β-CNDSB, which has successfully been grown as flaky single crystals. By carefully adjusting the parameters during the PVT, the growth condition were optimized. We have carefully studied the basic structural properites of the obtained slice like single crystals, such as the crystal structure and crystal face orientation, etc. After that, the single crystals were used to fabricate the OFETs with various structures. In the transistors, the typical ambipolar injection and transition characteristics were observed. The highest mobility of electrons and holes are up to 2.50 cm2V-1s-1 and 2.10 cm2V-1s-1, respectively. The light emission of the transistors from edge of the single crystals inthe channel which shows typical waveguided characteristic have also been achieved when the calcium/gold electrodes was chosen as the electron and hole’s injector, respectively.Subsequently, based on the structure of the β-CNDSB, the β-PBTA and α-PBTA were designed and synthesized. Their slice like single crystals were obtained by the PVT method, respectively. With the help of the Institute of the Crystal Materials, Shandong Univeristy, the crystal structure of the crystals was successfully obtained by the XRD measurements. The slice-like single crystals of the β-PBTA and α-PBTA were taken used to fabricate the OFETs. The maximum electron and hole mobility respectively achieaved 4.43 cm2V-1s-1 and 24.16 cm2V-1s-1 in the OFETs based on the β-PBTA single crystals. The light emission of the transistors from edge of the single crystals in the channel which shows typical waveguided characteristic have also been achieved in the OFETs with calcium/gold electrodes. However, the single crystals of the β-PBTA and α-PBTA exhibits no amplified spontaneous emission(ASE) in our test. The result indicated that the materials’ structure should be continously optimized.In the last chapter, we carefully investgated the relationship between the crystal face orientation, function groups in the interface, energy band structure of the crystal and the electrical properties(injection and transition of the carriers). The result shows that the functional groups and hydrogen bonds matrix probablely are very conducive for the injection of the carriers, especially elecrons. The calculation of the energy bands shows the mobility are very similar when the carrier transport along the π-π stacking and hydrogen bonds matrix.In conclusion, our research indicateds that: With carefully design of the molecular structure, the materials can be very beneficial to form the slice like single crystals by the PVT method; the presence of the cayno group is benefit to the injection, especially for the electrons; the strong hydrogen bonds network structure in the single crystal play the very important role in promoting the injection and transport process of the carriers.