| Organic light-emitting diodes (OLEDs) are interesting candidates for prospective electronic devices in the field of flat-panel display technology, because they have advantages such as self-luminescence, low operating voltage below DC 5.0 V, high luminance and efficiency, broad viewing angle, short response time, etc. over the liquid crystal display. However, there are still some problems, for example, on the luminescence efficiency and operational stability, which limit their further application on large-scale displays. The systemic studies of the behaviors of organic film growth, photoluminescence and electrical transport of organic semiconductors, can help us to understand intensively the physics involved in OLEDs and to improve the devices performance. In this thesis, based on model syetems, the growth behavior of organic semiconductors, such as tetracene, and photoluminescence(PL) and electrical transport of organic films, were studied by means of Scanning tunneling Microscopy(STM), in situ PL spectroscopy and impedance spectroscopy(IS).First, we investigated growth behavior and adsorption geometries of tetracene on the Ru(1010) surface by means of STM and theoretical Density Functional Theory(DFT) calculations. The STM images reveal that tetracene molecules are randomly distributed on the surface, and in the flat-lying adsorption geometry with their long axes aligned mainly along the [1210] and [0001] directions. For the tetracene overlayer near to a monolayer, formation of a parquet-like pattern was observed. The DFT calculations with DMol3 showed an adsorption energy of 4.72 eV for tetracene adsorbed between the top and the second Ru atomic rows with its long molecular axis along the [1210] direction, and a slightly smaller adsorption energy of 4.42 eV for tetracene adsorbed with its long molecular axis perpendicular to [1210], consisting qualitatively with the orientational distributions observed by STM. The growth behavior of tetracene on Ru(1010) is predominately determined by the adsorbate-substrate interaction, and the lateral intermolecular interactions have little influence on the growth.Secondly, thickness dependence and thermal effect on the PL properties of Alq3 thin film deposited on glass substrate were performed in situ by means of photoluminescence spectroscopy. Upon deposition of Alq3 on the glass substrate, the PL intensity of Alq3 emission increased continuously with the film thickness, and finally tended to a saturated value, while PL peak position showed a red-shift of about 12 nm for the Alq3 film thickness range from 2 to 500 nm. The thickness dependent PL intensity of Alq3 emission can be understood with the radiative decay of excitons in Alq3 film and the non-radiative decay of excitons caused by the interaction of excitons with the substrate. The thickness dependent red-shift of Alq3 emission, especially the sharp red-shift at the beginning of Alq3 deposition, was attributed to the change from the 2D to 3D excition state.Temperature dependent PL spectra of Alq3 films showed an increasein PL intensity for the film annealed below 130℃, and a decrease in PL intensity and a blue-shift of Alq3 emission about 9 nm for the film annealed up to 150℃. Both changes in PL intensity, and especially in the PL peak position, were attributed to crystallization effect of Alq3 film upon annealing.Finally, we investigated the transport behavior of NPB by using impedance spectroscopy and J-V measurements based on a single-layer structure of ITO/NPB/Al. The J-V characteristic of the device obeys Ohmic law below 0.6 V, and is in the SCLC regime above 0.6 V. The density of a thermally generated charge carrier is determined to be about 3.1×1016 cm-3. Impedance spectroscopy measurements showed that the device can be modeled by an equivalent parallel RC network with a contact resistance Rs in series, in which the bulk capacitance C of NPB remains unchanged while the resistance R decreases along with bias voltage. By the fit value of bulk capacitance, the relative dielectric constant NPB is determined to be aboutεr=3.6. The variation of the whole admittance of the device with frequency and bias can be understood by carrier dispersion in NPB film.
|
PDF Full Text Request