Search

The luminescence spectra of InGaN/GaN multiple quantum wells light-emitting diodes under low level injection current (<4 mA) during aging process was investigated for the first time. Comparing the electroluminescence (EL) spectra of LEDs before and after aging time it was found that the peak wavelength and the full width at half maximum (FWHM) decreased with stress time and the changes of EL spectrum had two different stages-drastic decrease at the early stress stage and slow decrease later showing the same trend with the output optical power of LEDs, which indicates that the effective polarization electric field of LEDs becomes weak during the aging process and the change has a clear correlation with the increase of the defects in the multiple quantum wells of LEDs. Electrical measurement revealed that junction capacitance (C(j)) under the same junction voltage (V(j) = 1.8 V) and the junction voltage (V(j)) with the same injection current 1 mA calculated by ac small-signal IV method increased along with aging time, which explicates that the carrier density under the same low injection increases as the aging time increases. Analyses indicate that the polarization field in the quantum well is more seriously screened by the increased carriers captured by defects activated during stress time, the weaker effective polarization electric field makes the tilt of the energy band smaller, the energy radiated through the band edge and the density of energy states of the band edge increase which leads to the behaviors of peak wavelength and the FWHM of InGaN/GaN multiple quantum wells LEDs under low level injection current.

The growth mechanism and crystallization phase state were investigated by the methods of atomic force microscopy (AFM) and X-ray diffraction (XRD). The pentacene films were deposited with a self-assembling monolayer by thermal evaporation on p(+)-Si wafer substrates at room temperature and annealed at a constant temperature (80 degrees C) for 120 min. The experimental results show that pentacene films were grown with terraces island structure with the diameter of island of about 100 nm and constituted a layer consisting of faceted grains with a average step height between terraces of 1.54 nm x s(-1), which were accord with the long axis length of pentacene molecule, and the film were vertically grown on the substrate surface. The crystallization of pentacene thin films is shown in XRD pattern. The increase in the thin film thickness introduced a second set of diffraction peaks, which were attributed to the pentacene triclinic bulk phase. The critical thickness of both phases is 150 and 80 nm, respectively. At a film thickness of 150 nm, the triclinic phase diffraction peaks become the dominant phase. This is contrast to the XRD spectrum of very thin film of 80 thickness, where the thin film phase is the only contribution.

The changes of exciton generation region are influenced by varying electric field, which affect the color and efficiency performance of devices. Firstly, The authors fabricated two types of phosphorescent light emitting devices, device 1:ITO/PEDOT : PSS/PVK : Ir(ppy)s : DCJTB (100:2:1 wt)/BCP(10 nm)/Alq3 (15 nm)/Al, and device 2: ITO/PEDOT : PSS/ PVK : Ir(ppy)3 (100:2 wt)/BCP (10 nm)/Alq3(15 nm)/Al. The authors investigated the influences of electric field on exciton generation region in single-layer and multi-doped structure devices. Analysis of the electroluminescence spectrum under different voltages indicates that the emitting of Ir(ppy)3, PVK and DCJTB was enhanced with the increase in applied voltages. Compared to Ir(ppy)3, the emitting of PVK and DCJTB was prominently enhanced. This is because under high electric field it is easier high energy carrier to generate high energy exciton, and the emitting of wide-band-gap material PVK is stronger; on the other hand, the authors investigated the results from the aspect of energy band gap. DCJTB is narrow-band-gap material, which can capture carrier comparatively easily and emit stronger light. At the same time, we obtained a new emission peak located at 460 nm, which becomes comparatively weak with increasing voltage. In order to explore the reason, we fabricated the device: ITO/ PEDOT: PSS/PVK : BCP : Ir(ppy)3 (x:y:2 wt)/Alq3 (15 nm)/Al. The 460 nm emission peak doesn't disappear by changing the mass ratio of x and y. The authors speculate that the emission peak relates to PVK and BCP.

The authors fabricated OTFT devices with different insulator materials. The OTFTs with PMMA as the dielectric layer exhibit better properties, including a mobility of 0.207 cm2 x V1 x s(-1), an on/off current ratio of 4.93 x 10(3), and a threshold voltage of 4.3 V. However, the OTFTs based on oxidized silicon dielectric layer perform not so well, with a mobility of 0.039 cm2 x V(-1) x s(-1), an on/off current ratio of 5.98 x 10(2), and a threshold voltage of 5.4 V. In order to explain the difference in performances, we compared the surface roughness of the oxidized silicon film with that of the PMMA film according to the results of atomic force microscopy, and found that the former had a roughness mean square (RMS) of 1.579 nm and the latter was more smooth with an RMS of 0.216 nm. The quality of the pentacene films deposited onto oxidized silicon and PMMA was also studied by atomic force microscopy and X-ray diffraction. From the results of AFM, the authors found that the pentacene film deposited on PMMA had high thin film quality with larger grain size and less crystal grain boundaries. From the results of XRD, the authors found that the pentacene film deposited on PMMA had clear diffraction peak, showing that the pentacene film deposited on PMMA had greater crystallite quality once again. Therefore, OTFTs with PMMA as insulator layers have advantages over those OTFTs with oxidized silicon dielectric layer.

Improving on the sealing and high pressure conditions of traditional hydrothermal method, vertical ZnO nanorod arrays were synthesized on indium tin oxide substrate by employing Zn(NO3)2 x 6H2O, (CH2)4N6 as the starting materials in the presence of polyethylenimine(PEI) at ambient pressure and low temperature (92 degrees). Between the substrate and the nanorods, a layer of ZnO flim was prepared as buffer layer and seed layer. The ZnO film was gained by spin-coating zinc acetate solution on indium tin oxide substrate, then annealed at 350 degrees C for 20 min, which can make zinc acetate decompose into zinc oxide. The zinc acetate spin-coating and decomposition procedure was carried out twice to ensure a complete and uniform coverage of ZnO seeds. The second layer was annealed at 500 degrees C for 30 mini Different spin-coating speeds were adopted, one was 2500 r x min(-1), and the other was 5000 r x min(-1). XRD result indicated that the seed layer with 5000 r x min(-1) has better alignment than the layer with 2500 r x min(-1). The aligned seeds with 5000 r x min(-1) show only a (002) reflection, indicating their complete c-axis texturing, whereas the spin-coated seeds give a powder pattern because they rest at all angles on the substrate. SEM result shows that the layer is made up of grains with an the average size of about 30 nm. Well-aligned ZnO nanorod arrays were synthesized by putting the substrate with ZnO seeds into the precursor solutions vertically for one hour. The nanorod arrays were taken out and rinsed with clean ethanol and pure-water for several times, blown dry with a stream of nitrogen, then annealed at 400 degrees C for 30 min in order to wipe off the organic solvent. At room-temperature, the SEM and XRD were measured. SEM results indicate that the crystal structure of most of ZnO nanorods is hexagonal wurtzite crystallographic phase structure, mainly vertical to the substrate. ZnO nanorods have good crystallization, the diameter of the rods is around 40 nm, and the length is above micrometer. The XRD results showed the nanorod arrays have (002) and (004) angles, and the (002) is quite strong. Absorption spectra of the nanorod arrays shows ZnO essence absorption and strong ultraviolet absorption, indicating that ZnO has good quality. Optical properties were studied, and the excitation spectra of the nonorod arrays showed a strong and narrow peak at 387 nm with FWHM smaller than 30 nm and a weak blue peak.

Many organic matters including heavy metal ions can validly utilize the singlet and triplet for luminescence owiog to the spin-orbit coupling. As a result, the internal quantum efficiency can easily achieve a value higher than traditional organic light emitting diodes in theory. There is a strong luminescence of PVK in PVK : PBD : Rubrene system. PL spectra excited by 345 nm of PVK : PBD : Rubrene thin film has a 410 nm PVK luminescent peak and a 560 nm Rubrene peak. EL still has a PVK luminescent peak, which should be kept from happening. Excitons can not adequately transferred from the matrix solution to Rubrene. The doping with Ir(ppy)3 improves the PVK : PBD : Rubrene system performance. PL spectra excited by 345 nm of PVK : PBD : Ir(ppy)3 : Rubrene with low concentration of Rubrene has a 510 nm Ir(ppy)3 peak and a new 548 nm one. However, the Ir(ppy)3 peak is smaller and the Rubrene one is bigger in EL spectra. Notably a strong and single luminescence of Rubrene is obtained in EL and PL spectra excited by 345 nm of PVK : PBD : Ir(ppy)3 : Rubrene with high concentration of Rubrene. Meanwhile, the Ir(ppy)3 luminescent peak disappears. The mechanism originates from the phosphorescent effect of Ir (ppy)3. The singlet excitons can basically be transferred from PVK : PBD or Ir(ppy)3 to Rubrene. But most excitons from Ir (ppy)3 can directly tunnel to the fluorescent material and come into being singlet states that can return to ground states and cause luminescence. Rubrene can accept proportional excitons with low concentration. While the concentration of Rubrene is higher, excitons can be entirely accepted by Rubrene. The effect also restricts the luminescent intensity of Ir(ppy)3 and boosts up that of Rubrene. Furthermore, the energy transfer in PVK : PBD : Ir(ppy)3 : Rubrene system is primary the Forester energy transfer. Excitation spectra of Rubrene and emission spectra of Ir(ppy)3 have a large overlap revealing that there is a strong energy transfer and further confirmed the phosphorescent effect of Ir(ppy)3. The doping system with phosphorescence material and small molecules can enhance the brightness and internal quantum efficiency.

ZnO films were deposited on different structural substrate by rf-reactive Magnetron sputtering. The optical characteristics of ZnO films were studied by X-ray diffraction and optical transmission spectrum. The ZnO films deposited on the Al2 O3 / AlN compound substrate had better crystallized and had a higher transmittance compared to the ones on AlN substrate. The optical characteristics of ZnO films were studied after all samples with a series of annealing temperature from 200 degrees C to 500 degrees C. When the annealing temperature was 400 degrees C, crystallization and c-axis (002) oriented of the ZnO film got best, and average optical transmittance reached 88% in the range visible light. While annealing temperature went beyond 450 degrees C, the crystallized structure of ZnO films was broken; the distance between O and Zn atoms became bigger. The authors found that the higher annealing temperature make against crystallization of ZnO thin film and increased density of defect states and dispersion mechanisms and reduced optical characteristics of ZnO film, and average optical transmittance of ZnO films reached 80% in the range of visible light at 500 degrees C.

The new complex Tb0.5 Eu0.5 (TTA)3Dipy was synthesized. By doping polymer PVK with Tb0.5 Eu0.5 (TTA)3Dipy, the EL device was fabricated with the structure of ITO/PVK: Tb0.5 Eu0.5 (TTA)3Dipy/PBD/Al using PVK: Tb0.5 Eu0.5 (TTA)3Dipy as the emitting layer. Compared with the blend PVK: Eu(TTA)3, the Tb3+ acts as an energy transfer bridge, which enhances the energy transfer efficiency between PVK and Tb0.5 Eu0.5 (TTA)3Dipy. As a result, the emission of Eu3+ is enhanced with the quenching of the emission of PVK. The process of energy transfer was studied.