Preparation, Growth Mechanism Exploration And Optoelectronic Properties Of High-Quality Organic Crystals

Organic crystal with highly ordered molecular arrangement and low impurity content has the high carrier mobility and high thermal stability, and with clear crystal structure also has provided a model to investigate the intrinsic properties of material, such as the basic intermolecular interactions, the relationship between the molecular stacking modes and the optoelectronic properties (luminescent efficiency and carrier mobility).Organic crystals with high luminescent efficiency, high carrier mobility and low amplified spontaneous emission (ASE) threshold could be as promising materials for the electrically pumped laser which has not yet been achieved. The molecular stacking mode could be adjusted by introducing the special intermolecular interactions, to improve the luminescent efficiency and carrier mobility of the crystal, but in the view of supramolecular chemistry and crystal engineering, designing the intermolecular interaction by the chemical synthesis to control the molecular stacking mode in the crystal is more complicated. As we know, physical doping is a very effective approach to realize the high solid-state luminescent efficiency. However, in terms of the organic crystal, the lattice mismatch and the weak intermolecular interaction which are the reasons for the difficulties of large-size doping crystal growth should be considered.The three molecules of trans-1,4-Distyrylbenzene (trans-DSB), tetracene and pentacene have similar linear configurations and'herringbone'intermolecular arrangements in the crystals. The emission spectrum of trans-DSB overlaps well with the absorption spectra of tetracene and pentacene, and thus the three materials are ideal for preparing the large-size doping crystals by choosing the appropriate crystal growth conditions.Physical vapor transport (PVT) is the common method to grow the large-size and high-quality organic crystal which usually has the characteristics of thin thickness and smooth surface and is suitable for the fabrication of the device. Because the crystal growth process is carried out under the high temperature condition, the molecules in vapor have the high kinetic energy and are conducive to a more stable stacking mode during the formation of the crystal. Based on the characteristics of PVT method to crystal growth, trans-DSB as the host, tetracene and pentacene as the guest, respectively, have been chosen, and the guest molecules with high kinetic energy benefit to embed into the host lattice which has been intensely disturbed due to the high-temperature growth condition. Meanwhile the host and guest molecules are structural compatibility to ensure the smaller lattice mismatch that the high doping concentration could be achieved in the crystal. Atomic force microscopy (AFM) and X-ray diffraction (XRD) results show that the ordered structures of doped crystals are retained but a slight increase of layer spacing compared with the undoped trans-DSB crystal, indicating that the guest molecules embedded into the host lattice occupy the original location of host molecules in the crystal growth process. As tetracene and pentacene molecules in the doped crystals avoid the self-aggregation induced luminescence quenching, and the luminescent efficiencies of tetracene-doped trans-DSB and pentacene-doped trans-DSB crystals reach to 74% and 28% by the energy transfer, respectively. The ASEs are also observed due to the decrease of the self-absorption in the doped crystals. The mutually perpendicular host-guest molecular dipole arrangements result in that the orientation factorκ2 (0.004-0.008) and the Forster distance R0 (1.5-1.8 nm) are small, and thus higher doping concentration in doped crystals compared with amorphous doped films are necessary to ensure the energy transfer efficiently. Further, the white-emission doped crystal could be obtained by controlling the molar ratio of host and guest molecules (tetracene:pentacene:trans-DSB = 1:1.35:23.1). There are the energy transfers not only from trans-DSB to tetracene and pentacene, but also from tetracene to pentacene by the analysis of time-resolved fluorescence spectra of the white-emission doped crystal.Then the field-effect transistors and diodes based on doped crystals with high luminescent efficiency are fabricated to investigate the influence of guest doping concentration and crystal thickness on the carrier mobility. In the FETs devices, the hole mobility for the undoped trans-DSB crystal is about 6.67×10-3 cm2/Vs and those for the doped crystals with almost entire energy transfer are about 1.41×10-3 cm2/Vs (molar ratio, tetracene:trans-DSB= 1:15) and 1.17×10-3 cm2/Vs (molar ratio, pentacene:trans-DSB= 1:13), respectively. Although the mobility of doped crystal decreases slightly, it is still in the same order of magnitude with that of the undoped crystal, because the guest molecules embedded in doped crystals don't destroy the whole crystal ordered structure. Doped crystals by energy transfer achieve high luminescent efficiency while maintain the characteristics of high mobility, which are promising materials for the electrical injection luminescence and laser research. In the diodes devices, the mobility increases with the crystal thickness at the same electrical field because the relatively thicker crystal can reduce the influence of interface contact between the ITO/PEDOT electrode and the crystal on the charge injection. Further, taken cyano substituents oligo (para-phenylene vinylene) (CN-DPDSB) crystal with low ASE threshold (23 KW/cm2), high luminescent efficiency (-95% ) and bipolar carrier transport balance (μelectron≈0.9-1.3×10-2 cm2/Vs;μhol≈2.5-5.5×10-2 cm2/Vs) for example, the diode device structure can endure the current density of～5 KA/cm2, which still has a great gap with the threshold current required to the organic electrically pumped laser (～102-103 KA/cm2). However, it is analyzed that the requirements to organic crystals applied in electrically pumped laser include:i) developing crystal materials with high luminescence, high mobility and low ASE threshold; ii) improving the interface contact of crystal device to increase the effective charge injection.