Study On Optical Gain And Charge Transport Propetries Of Organic Semiconductor Materials

The easily processed organic semiconductors have been first used in the field of organic light-emitting diodes (OLEDs). The recent development of organic semiconductor laser builds on the rapid development of OLEDs. Organic semiconductors have a number of properties making them good candidates for organic laser application, for example, they are intrinsical "four-level" systems, they have broad spectra across visible spectrum, they can be easily processed into optical quality films by solution processing and they are capable of charge transport, open up the possibility of electrically pump organic laser. In1996Tessler et al. demonstrated one of the first organic semiconductor laser using conjugated polymer film, poly(p-phenylenevinylene)(PPV), as active medium. After this demonstration, remarkable progress has been made in the field of organic semiconductor lasers, which are based on several types of organic semiconductors and resonator structures. However, the realization of electrically pumped organic semiconductor lasers is still a great challenge due to its high threshold current density (up to～kA/cm2). Therefore, the combination of low photopumped laser threshold and high charge-carrier mobility are expected for organic semiconductor laser materials.There have been two simple approaches, amplified spontaneous emission (ASE) and time-of-flight (TOF) measurements to study the optical gain and charge-carrier mobility of organic semiconductor materials, respectively. This thesis is mainly discussed the optical gain and charge-transport properties of novel organic semiconductor materials.We first reported a detailed study of optical gain and charge-transport properties of a new fluorescent conjugated polycarbazole (OCZ). OCZ has a bulky side chain of4-[tris-(4-octyloxyphenyl)methyl]phenyl which contributes to its high fluorescence quantum yield (17%) and good solubility. ASE with a high net gain coefficiency up to47cm-1at pump intensity of48kW/cm2was observed from the OCZ based waveguides. The threshold value of8kW cm-2are comparable to that of PPV and PFO, indicate that the OCZ is a novel polymer laser material. Further, the hole mobility values of the OCZ films was found in the range of2～6×10-5cm2/Vs for different film thicknesses. These results show that the introduction of the big side chain is beneficial for the fluorescence efficiency of the polycarbazoles polymer without sacrificing the charge mobility. However, the hole mobiltiy of the OCZ film is too low for the electrically pumped organic laser.The charge-carrier mobilities of organic single crystals generally three or four order of magnitude higher than that of organic amorphous films. The highly luminescent crystals of oligo(p-phenylenevi-nylene)s (OPVs) exhibit ASE with slightly higher thresholds. Here, we first measured the ASE threshold of slice like orange-emitting CN-OPV crystals with different sizes and thicknesses. The ASE threshold are thickness dependent, because the thinner crystal experienced lower optical loss caused by crystal defect and interface scattering. In addition, we observed that the ASE threshold the OPVs crystals were closely correlated to the radiative decay rate (Kr), and the large Kr correspond to the low ASE threshold.Then, a symmetric waveguide structure of air/crystal/air comprising a slab orange-emitting crystal of CN-OPV has been developed for organic crystal lasers. The crystals exhibit high photoluminescence efficiency of30%and have smooth surfaces and edges, which ensures strong self-cavity optical confinement and provides optical feedback for the laser oscillation. A single-mode laser oscillation is observed around569nm with a minimum Full width at half height (FWHM) of1.5nm and a threshold of10.2kW/cm2which is one of the lowest values for organic crystal lasers. The leaky loss caused by the substrate is prevented by the air/crystal/air waveguide structure.Then the LEDs based on green-emitting CN-OPV crystals were fabricated to investigated the electroluminescent (EL) properties OPV crystals. The crystals are grown by the physical vapor transport (PVT) method, and exhibit a very high ΦPL of75%. Meanwhile, they have large sizes, regular shape, and flat surfaces. In addition, they are flexible and bendable, thus can easily adhere to the substrate/electrode to provide high-quality electronic contacts. The ambipolar behavior of the CN-OPV crystals is confirmed using the TOF technique. All these properties indicate that they are the promising candidates as the light-emitting layer of LEDs. The device structure is Glass/Au/crystal (700nm)/LiF/Al. Intense green EL from a diode has been successfully demonstrated at an applied voltage below20V. However, The crystal based LEDs possessed a high operating voltage and a very low EL efficiency. This may be caused by misaligned energy level between the electrodes and crystals. In summary, the OPVs crystals combining both high carrier mobility, high ΦPL, and low photopumped laser threshold. These results indicate that OPVs crystals are promising for organic laser diodes application.Extensive work has been carried out in the field of organic semiconductor lasers in order to reduce the ASE threshold. We investigate the emission spectra from the edge of optically pumped waveguide based on thin films of small molecular,2,5,2’,5-tetrakis(2,2-diphenylvinyl)biphenyl (TDPVBi). Two different propagation modes ASE are observed which are defined as guided mode and cutoff mode, respectively. The guided mode ASE exhibits a threshold of24kW/cm2and a net gain of29cm-1at pump intensity of50kW/cm2. On the other hand, the cutoff mode ASE has a exceptional low threshold of below0.9W/cm2,which is attributed both to the cavity enhancement effects and the low optical loss. Therefore, Our results suggest that the cutoff mode is a promising route for the reduction of ASE threshold. In addition, the ambipolar charge transport properties of the TDPVBi film were measured using TOF method. Both the hole and electron mobilities are4×10-5cm2/Vs. This result also suggests that the TDPVBi film is a attractive light-emitting material for OLEDs.