This study is dedicated to prolonging the lifetime of OLEDs, and discussing the factors that affect the lifetime of OLEDs, including: the ITO surface roughness, the cleanliness of the ITO substrate, and the cleanliness inside the vacuum chamber. According to the AFM measurement, it is found that there are some spikes on the ITO surface on glass substrate, which will cause the burnt phenomenon of the encapsulated OLED, resulting in the failure of good reproducibility and lifetime. Therefore, a PEDOT:PSS layer is employed and first spin-coated on ITO, where the surface roughness Ra value is reduced from 3.31 to 1.58 nm. Then the OLED is fabricated on PEDOT:PSS layer to solve the problem of burning when lighting up. Thus its lifetime was prolonged. The ozone gas can effectively decompose the lipids harmful to organic materials released by the heat-resistant tape and vacuum glue inside the vacuum chamber for thermal evaporation. Therefore, this study uses ozone to clean the vacuum chamber, and then proceed to fabricate OLED devices to improve the efficiency and lifetime of OLEDs. Compared with the processes without using ozone to clean the chamber, it is found that the OLEDs fabricated in ozone-cleaned chamber, whether encapsulated with glass or not, had a relatively longer lifetime. The lifetime of OLED without glass encapsulation is extended from 9.9 to 34.3 min, and that of OLED with encapsulation is extended from 34.4 to 404.7 min. With ozone cleaned chamber, the OLED lifetime obtains much improvement.
In this paper, we adopt solution/evaporation hybrid processes to fabricate white organic light emitting diodes (WOLED) with a light-emitting area of 1.5 cm * 1.5 cm. The hole injection layer (PEDOT:PSS), p-type doped hole transport layer (NPB:F4-TCNQ), and emitting layer (UBH-215:UBD-07:DBP) are coated using the solution process while the electron transport layer (TPBI), LiF, and Al are coated through vacuum thermal evaporation. The light-emitting layer is a fluorescent material (UBH-215 as host) that combines the blue fluorescent material UBD-07 and the red fluorescent material DBP for a dual band light-emitting diodes. We found that if the red DBP doping concentration is too high, the overall emitting color tends toward reddish, and the luminance efficiency decreases. For WOLEDs whose red DBP doping concentration is 0.25%, the luminance reaches 192 cd/m2, and the current efficiency 0.94 cd/A at 9V voltage.
Introducing a hole injection layer between the ITO and emitting layer can enhance both the luminance and efficiency of a WOLED. In addition to the hole injection layer, it is found that inserting p-type doped HTL (NPB:F4-TCNQ) between HIL and EML using the spin- coating method can also improve luminance and efficiency. Furthermore, by optimizing and adjusting the dopant ratio and thickness of p-type doped HTL, the luminance reaches 393.1 cd/m2, the current efficiency reaches 1.32 cd/A under 9V voltage, and the OLED lifetime (half-value period) increases from the original 0.8 hours to 2 hours.
In this work, we prepared an organic phosphor and investigated its thermal as well as fluorescent properties. The
experimental results reveal that lab-made organic phosphor exhibits excellent thermal stability (Decomposition
temperature (Td) = 374 °C) and good fluorescent quantum yield (Φ = 0.88). The organic phosphor was then coated onto
the blue LED chip to form a white light-emitting diode. The size of blue LED die used throughout this work was 15 mil
square and the dominated wavelength was 460 nm. For the package process, the organic phosphor was firstly mixed with
the epoxy and dipped it into the LED bowl of lead frame. Secondly, the pure epoxy resin was full-filled within the lamp
model. For the measurement of spectrum and the C.I.E. value, it was found that the near white weight proportional of the
epoxy resin A, B and the organic phosphor were 1 : 1 : 0.1.
In this study, we dissolved hole transport layer (HTL) material NPB in THF (tetrahydrofuran) solvant, and spin-coated
the N,N'-Bis(naphthalene-l-yl)-N,N'-bis(phenyl)-benzidine (NPB) solution on the surface of Indium Tin Oxide (ITO)
anode to enhance the luminance efficiency and lifetime of flexible phosphorescent organic light emitting diodes
(POLEDs), where the 2,2',2''-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) was employed as hole
blocking layer (HBL) and its thickness was optimized. Such an improvement in the device performance was attributed to
the improved hole-electron balance. Finally, we employed 2,9-Dime-thyl-4,7-dphenyl-1,10-phenanhroline (BCP) or
TPBi as hole blocking layer. The maximum luminance efficiency reaching 24.4 cd/A can be obtained.
For improving the sensitivity of a D-type SPR fiber sensor, we simulated the optimum parameters, such as, the thickness of coatings, the length of sensor, and the angle of incidence for different ranges of refractive index. These simulations are based on SPR theory and the intensity and phase methods. It is clearly that, the sensitivity is improved by increasing the length of sensor and/or the thickness of the gold film. And the sensitivity of the phase method is higher than the intensity method by two orders. It is used to detect the
refractive index or concentration of gas or liquid in real-time, and it has some merits, such as, small, simple, cheaper, and in vivo test.
Organic electroluminescent devices using tris-(8-hydroxy) quinoline aluminum (Alq3) as the emissive layer and N,N'- diphenyl-N,N' bis (3-methylphenyl)-[1-1'-biphenyl]-4-4'- diamine (TPD) as the conventional hole transport layer have been fabricated. We tried to explore the charge transport mechanisms in the OLED device by the studying of temperature dependent luminescence over the temperature range from 10 K to 300 K. We found that first, at lower applied voltage, two peaks have been observed in the quantum efficiency with temperature, and they are due to deep trap levels (high temperature regime) and shallow trap levels (low temperature regime). With increasing voltage, the high-temperature peak shifts toward lower temperature but no significant shift of the low-temperature peak is observed, and when the voltage is over 10 V, superposition of the peaks causes the apparent saturation in the low temperature regime of the quantum efficiency. Second, up to a certain temperature luminescence intensity decreases with decreasing temperature and then saturated in the low temperature region. The quantum efficiency increases with decreasing temperature and finally reaches to almost a constant value. Meanwhile we tried to use Frenkel exciton model to explain the luminescence behavior of the device.
Temperature- and field-dependent electroluminescence and quantum efficiency have been investigated in tris-(8hydroxy) quinoline aluminum (Alq3)light emitting diode over the temperature range from 10K to 300K. It has been observed that up to a certain temperature luminescence intensity decreases with decreasing temperature and then saturated in the low temperature region. The quantum efficiency increases with decreasing temperature and finally reaches to almost a constant value. At lower applied voltage, two peaks have been observed in the quantum efficiency with temperature. The two peaks are attributed due to deep trap levels (high temperature regime) and shallow trap levels (low temperature regime) in Alq3.
In this work, electron-transporting chromophores were introduced to emission polymer to increase its electron affinity. Several emission chromophores also were synthesized to polycondensate with electron transporting chromophores. The influence of structure on optoelectric properties was investigated in detail. 2,5-Bis-(4- fluorophenyl)-1,3,4-oxadiazole and 9,10-dihydro-oxa-10- phosphaphenanthrene-10-oxide (DOPO) derivatives were used as electron transport and emission monomers, respectively. The DOPO derivatives that contain benzene, biphenylene or 1,4- naphthalene were synthesized. The emission units exhibits blue light as expected. Aromatic polyethers were obtained by nucleophilic displacement reaction of oxadiazole-activated bis(halide) monomers with bis(phenol) monomers. All the resulted polymers are thermally stable below 400 degrees C. The absorption peaks of these polymers vary between 325 nm - 350 nm, while the photoluminescence peaks vary between are 377 nm - 464 nm.
The reliable n+-ZnSSe metal-semiconductor-metal (MSM) blue-green light emitting diodes (LEDs) have been fabricated. The contact metal was CuGe/Pt/Au. The current transport mechanism agree very well with the back-to-back tunneling diodes. The kink phenomena were observed in the MSM current- voltage curves. In the metal-semiconductor interface, the element Zn in ZnSSe can be replaced by Cu results in some acceptor levels as radiative recombination centers in the MS interface. The peak wavelength in the LED electroluminescent (EL) spectra was strongly dependent on the injection currents from 5 to 40 mA. The peak wavelength and full width at half maximum are 510 and 10 nm, respectively, at 10 mA injection current. When the injection current increases to 15 mA, the peak wavelength shifted to 530 nm due to different recombination centers. Further increasing the injection currents, the peak wavelength shifted slightly to the long wavelength side.
We have found that the electrical and optical properties of GaN epilayers strongly depend on buffer TMGa flow rates and NH3 flow rates. At low flow rate of 4 sccm, the buffer layer quality was good so the concentrations of undoped GaN epilayers decreased and a stronger band-edge emission of 362 nm can be observed in photoluminescence spectra. The carrier density of the films can be reduced from 1018 to 1017 cm-3 by increasing the NH3 flow rate from 0.5 up to 1 SLM and comparatively increased the near-band-edge emission. So, with a NH3 flow rate as high as 1 SLM, the GaN epilayers with good optical quality can be obtained. The excitation power density of He-Cd laser influences the photoluminescence property of GaN epilayers. At high excitation power density of 637 W/cm2, the near-band-edge luminescence (362 nm) is dominant and the deep level luminescence (near 550 nm) appears very weak. But at low excitation power density, the luminescence from defect yellow band levels is stronger than the near-band-edge transitions.
This research attempted to use metals with lower work functions, such as Ti, Al, to form ohmic contacts to n-GaN. Then we used metals with higher work functions, such as Ni, Pd, Pt, and Au to form ohmic contacts to p-GaN. The work functions of these metals indeed influence the performance of ohmic contacts, indicating that the Fermi level of GaN is unpinned. The specific contact resistance measured and calculated by TCL model, was 2.35 X 10-3 (Omega) cm2 for as-deposited Ti/Al on GaN. After RTA processes at different temperatures in the range of 400 to approximately 900 degrees Celsius, the minimum (rho) c of 7.4 X 10-5 (Omega) cm2 can be obtained for RTA temperature of 600 degrees Celsius. The oxidized Ni/Au contact exhibited the lowest contact resistance of 1.02 X 10-2 (Omega) cm2, among Ni/Au, Pd/Au, Pt/Au contact schemes on p-GaN. It was also observed that the I-V curves of the triple-layer contact, Pt/Ni/Au, was near-linear while the others were rectifying even after annealing.
Experimental and theoretical results are presented for current-voltage and dynamic resistance-voltage characteristics of Hg1-xCdxTe ion-implanted p-n junction photodiodes with x approximately equals 0.22. By measuring the temperature dependence of the dc characteristics in the temperature range 25-140K, the dark current mechanism are studied. At high temperature and in low reverse bias region, the diffusion current dominates. On the other hand, at medium temperature and medium reverse bias, trap-assisted tunneling plays an important role. At low temperature and in the medium reverse bias region, band-to-band tunneling is the key leakage current source.However, when the temperature is further lowed down to 25K and the applied reverse bias is very small, the band-to-band tunneling current will be ruled out and the trap-assisted tunneling mechanism dominates again. We have measured 1/f noise in HgCdTe photodiode as a function of temperature, diode bias, dark current. The dependence of 1/f noise on dark current was measured over a wide temperature range on devices. The temperature dependence of the 1/f noise was found to be the same as the temperature dependence of the surface generation and leakage currents. We obtained the maximum specific detectivity value and the maximum signal-to-noise ratio are about 3.51 X 1010 cm Hz1/2/W and 5096 respectively.
Bulks layers of both GaSb and InGaSb have been successfully deposited by metal-organic chemical vapor deposition (MOCVD). The transition energies in the GaSb/InxGa1-xSb single quantum well have been calculated using a model which takes into account the elastic strain and quantum well effects. The shifts of transitions energies as a function of In solid compositions and well widths for 300 K and 12 K, respectively, were calculated. The transition energies from conduction bands to light holes at 12 K were found to be higher than the GaSb energy-gap with In composition below 0.3. Thus, the light hole band cannot be confined in the quantum wells with In composition between 0 and 0.3. The photoluminescence (PL) spectra of quantum well structures with different well widths were presented. The transition energies obtained from the PL spectra were compared with theoretical predictions.
The ohmic contact properties of Ag/AuGeNi/n-GaSb and AuGeNi/n-GaSb systems were investigated in this paper by measuring the barrier height and specific contact resistance with various sintering temperatures. The lowest specific contact resistance was about 8 X 10-3 - 8 X 10-4 cm2 for the Ag/AuGeNi/n-GaSb contact system when the sintering temperature was 400 degree(s)C for 2 min. Rutherford backscattering spectroscopy (RBS) was also used to study the interface between Ag/AuGeNi and GaSb during heat treatment. Pd/n-GaSb Schottky contacts have been studied experimentally before and after temperature annealing. I-V characteristics, Auger electron spectroscopy (AES), RBS, and x- ray diffraction patterns were used to determine and identify electrical performance of Pd/n- GaSb Schottky diodes. When the annealing temperature is increased to 450 degree(s)C, the rectifying property becomes bad. In this case, Pd interdiffusion and Ga, Sb out-diffusion to form Ga5Pd compound is very serious.
Undoped GaSb epitaxial layers have been grown on (100) GaSb and GaAs substrates by low pressure MOCVD. It was found that the layer morphologies were strongly dependent on TMSb/TEGa (V/Ill) ratios. The mirrorlike surface can be easily obtained under V/Ill ratio in the range of 68 at growth temperature 600 C and growth pressure 100 torr. Beyond this range the surface deteriorated seriously. The epilayers were characterized by electron diffraction patterns and photoluminescent measurements. The boundexciton (BE) peaks and strong acceptor band peak in PL spectra were observed from the sample grown under V/Ill ratio of 6. 84 on GaSb substrates. PL peak intensity was found to be a function of the V/Ill ratios. When V/Ill ratios increased beyond the range of 68 the BE peaks disappeared and PL spectra became roughened. The full width at half maximum (FWHM) of acceptor-band peaks in PL spectra was dependent on V/Ill ratios ensuring that obtained from the analysis of surface morphology. IV characteristics of the pn diodes fabricated on the sample of undoped-GaSb/GaSb:Te was measured. The electrical properties of undoped GaSb were studied from the epilayers grown on GaAs semiinsulating substrates. The hole concentration increased and mobility decreased with growth temperature between 520 and 635C under V/1116. 84. For 550 C grown epilayers: as V/Ill ratio increased above 6. 64 or decreased below 6. 64 the hole concentration increased and hole mobility decreased. .
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