Ultra-Violet Electroluminescence of ZnO Nanorods/MEH-PPV Heterojunctions by Optimizing Their Thickness and Using AZO as a Transparent Conductive Electrode.

In this paper, a series of ITO/ZnO/ZnO nanorods/MEH-PPV/Al were prepared with different thicknesses of MEH-PPV that were changed from 15, 10 to 7 nm. The electric field in the devices was analyzed. An increase in the electric field on ZnO made hole injection easy and the electrons tunnel fast through thinner MEH-PPV to ZnO. This made the carriers prefer to recombine inside the ZnO layer, and the emission of ZnO was predominant under direct current (DC) bias. Furthermore, another device was fabricated with the structure of AZO (Al-doped ZnO)/ZnO/ZnO nanorods/MEH-PPV/Al. Ultra-violet (UV) electroluminescence (EL) at 387 nm from ZnO band edge emission was realized under DC bias. The turn-on voltage of the devices having AZO as the electrode is lower than that of ITO, and the EL power is enhanced. This work also studies the effect of inserting LiF underneath the Al electrode and above the layer of MEH-PPV. The LiF film inserted caused an obvious decrease in turn-on voltage of the devices and a pronounced increase in the EL power. The mechanism of electroluminescence enhancement is also discussed.

Study on the Overshoot Effect of Doped PhOLED with Transient Electroluminescence.

The accumulation carriers and the trapped carriers are found in many organic light-emitting diodes (OLEDs) more or less, which can lead to a great loss of carriers and weaken the performance of devices. We have investigated a host-guest-system containing the green phosphorescent emitter tris[2-phenylpyridinato-C2,N]iridium(Ⅲ) [Ir(ppy)3] and one host material with transient electroluminescence (EL). The charge recombination, accumulation and light emission mechanisms of the phosphorescent organic light-emitting diodes (PhOLEDs) with different host materials were analyzed. The structure was fabricated as ITO/NPB(30 nm)/host: Ir(ppy)3/BCP(10 nm)/Alq3(20 nm)/LiF(0.7 nm)/Al(100 nm),the hosts were CBP, PVK and TAZ respectively. These results showed the transient EL was strongly dependent on host materials. Compared to devices of host material CBP and PVK, only those with the host material TAZ as the emitting layer exhibited strong electroluminescence overshoots between 1 and 3 µs after turning off the voltage pulse at room temperatures. To further elucidate the generality of the overshoots, we monitored their dependence on the dopant concentration. The transient EL results in host-guest-system devices demonstrated a direct link between the strong overshoot effect and charge trapping in the emitting guest molecules. The excessive electrons in the guest sites could be a major factor inducing significant strong overshoot phenomenon in the TAZ: Ir(ppy)3 layer. We attributed these overshoot effect to the electrons accumulated on Ir(ppy)3 sites and accumulated holes in the vicinity of the HBL/EML interface. As a result, we obtained a better understanding of carriers' dynamics and recombination process of PhOLEDs after turning off the voltage pules. The new understanding of the charge carriers and exciton dynamics of PhOLEDs is instrumental in directing the efforts of developing stable and high-efficiency PhOLEDs.

[Understanding the Effected Efficiencies of Polymer Solar Cells Employing Different Fullerene Multiadducts as Acceptors].

The application of fullerenes with two or more adducts as acceptors has greatly enhanced the performance of bulk-heterojunction solar cells with poly (3-hexylthiophene) (P3HT) as the donor. The enhancement is caused by a substantial increase in the open-circuit voltage due to a rise in the fullerene lowest unoccupied molecular orbital (LUMO) level when going from monoadducts to multiadducts. While the increase in the open-circuit voltage is obtained with many different polymers, most polymers other than P3HT show a substantially reduced photocurrent when blended with fullerene multi-adducts such as bis adduct of Phenyl-C61 -butyric acid methyl ester (bis-PCBM) or the indene-C60 bis-adduct (ICBA). Here we investigate the reasons for this change in performance of polymer solar cells(PSCs) based on [6,6]-phenyl C70-butyric acid methyl ester (PC70BM), ICBA and bis-PC70BM as the acceptors and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7)as the donor. The cell configuration is ITO/PEDOT：PSS/active layer /LiF/Al .The PSCs with PC70BM, ICBA and bis-PC70BM show PCEs of 7.29%, 4.92% and 3.33%, respectively. The change of performance of the PSCs could be mainly attributed to the different excition generation and charge collection of PSCs employing different fullerene multi-adducts as acceptors.

The Inside Charge Behavior of Organic Light-Emitting Diodes Investigated with Transient Electroluminescent Measurements.

Transient electroluminescent measurement system is used to study the inside charge behavior of prepared organic light-emitting diodes (OLEDs) in this article. Two rectangular pulses with a fixed time interval are supplied on the device. We can analyze the inside charge storage and the emitting process by measuring the transient EL and transient current of the device. OLEDs based on the m-MTDATA∶3TPYMB (1∶1) system was prepared. We found that the stable EL intensity increase when the second pulse is supplied. And the increment reduced with the increased current. We also find the electroluminescent under the second pulse decayed faster than that of the first pulse. This is because of the quenching due to the polaron-exciton effect (TPQ) is more serious in the emitting layer under the second pulse.

[Effect of annealing pressure on P3HT : PCBM nanoscale morphology and photovoltaic properties].

The considerable performance enhancement of P3HT-based solar cell after thermal annealing can be attributed to the thermodynamically driven partial crystallization and phase segregation of each component. In the present work, thermal annealing was executed by delivering P3HT : PCBM blend films onto the preheated susceptor in a PECVD chamber filled with high purity nitrogen gas. The pressure of inner chamber could be set steadily and precisely in the range of 1 to 1 850 mTorr at 150 °C. It was found that the phase segregation scale of two components was tuned to a certain extent by varying the annealing pressure, whereas the polymer crystallinity was slightly affected. According to the pressure settings, polymer solar cells (PSCs) were fabricated in the following structure: ITO/PEDOT : PSS/P3HT : PCBM/Al. All of the device parameters exhibited the similar trend--an initial increase followed by a decrease, and reached a peak at 1500 mTorr with successive increase in annealing setting pressure. PSC annealed under 1 500 mTorr shows overall high performances with the power conversion efficiency up to 3. 56%. The UV-Vis absorption spectra of annealed blend films shows that the vibronic absorption peaks (shoulders) at 510, 550 and 600 nm became more pronounced under higher setting pressure, which is attributed to better crystalline P3HT with increased π-π stacking of polymer molecules. The AFM results further suggest that high annealing pressure (>1000 mTorr) promoted the domain formations of P3HT or PCBM; moreover, a moderate phase segregation, as a result of an appropriate annealing pressure (1500 mTorr), facilitates polymer crystallization which ensures the high charge (hole) mobility and consequently increased short-circuit current and fill factor.

[Improved color purity of green OLED device based on Au thin film].

Au was used as anode in some kind of organic electroluminescent devices. Sometimes transparent Au electrodes are required, which means that the thickness of Au electrode should be as thin as possible. Therefore, two metals together forming an electrode become a choice. In the present paper, translucent Au/Al layer was inserted to anode side, and OLED device with the structure of ITO/Al (16 nm)/Au (10 nm)/TPD (30 nm)/AlQ (30 nm)/LiF (0.5 nm)/Al was prepared. There is a spectral narrowing phenomenon on the device ITO/TPD (30 nm)/AlQ (30 nm)/LiF (0. 5 nm)/Al, and through analysis and experiment it was found that this phenomenon comes from selective permeability to light of Au thin film rather than the microcavity effect. The device maintains wide viewing angle, without the angular dependence. And the color purity of device with Au thin film is improved.

[A new method to improve the luminous efficiency of organic light-emitting device: metal thin layer].

Inserting metal thin layer into organic electroluminescent device often brings some unexpected effect. By inserting 5 nm Au thin layer between MoO3 and TPD, we prepared the OLED device with the structure of ITO/MoO3(5 nm)/Au(5 nm)/ TPD/AlQ/LiF/Al. The luminous efficiency is improved as compared to the device without the Au. By analysis, we believe that the Au between MoO3 and TPD formed a small trap for hole to reduce the current density in device. At the same time, since the light transmittance of green light (AlQ) of 5 nm Au is greater than 80%, the brightness of the device was not significantly affected. That is the reasons for the improved luminous efficiency. This work provides some new ideas and experimental evidence for how to improve the luminous efficiency of OLED devices.

[Study on the safety of blue light leak of LED].

In this paper, the blue light properties of LED illumination devices have been investigated. Against the status quo of China's LED lighting, we measured the spectrum component of LED lamps and analyzed the photobiological safety under the current domestic and international standards GB/T 20145-2006/CIE S009/E: 2002 and IEC62471: 2006 standards as well as CTL-0744_2009-laser resolution, which provides the reference to the manufacture of LED lighting lamps as well as related safety standards and laws. If the radiance intensity of blue light in LED is lower than 100 W x m(-2) x Sr(-1), there is no harm to human eyes. LEDs will not cause harm to human eyes under normal use, but we should pay attention to the protection of special populations (children), and make sure that they avoid looking at a light source for a long time. The research has found that the blue-rich lamps can affect the human rule of work and rest, and therefore, the LED lamps with color temperature below 4 000 K and color rendering index of 80 are suitable for indoor use. At the same time, the lamps with different parameters should be selected according to the different distances.

Heterojunction of ZnO nanoparticle/PMMA and its ultraviolet electroluminescence.

A ZnO nanoparticle layer was synthesized by the sol-gel method on ITO glass. Devices with a ZnO nanoparticle layer/polymethylmethacrylate (PMMA) heterojunction structure were then fabricated and the room temperature electroluminescence (EL) spectra were studied. Under DC bias, ultraviolet (UV) EL at 390 nm from the ZnO band edge emission was observed, and defect-related emission of ZnO at 600 and 760 nm were also detected. The EL mechanisms are discussed in terms of the carrier tunneling process.

[White organic light-emitting diodes applied for lighting technology].

Lighting accounts for approximately 22 percent of the electricity consumed in buildings in the United States, with 40 percent of that amount consumed by inefficient incandescent lamps. This has generated increased interest in the use of white electroluminescent organic light-emitting devices (WOLEDS) as the next generation solid-state lighting source, owing to their potential for significantly improved efficiency over incandescent sources, combined with low-cost, high-throughput manufacturability. The research and application of the devices have witnessed great progress. WOLEDS have incomparable advantages for its special characteristics. This progress report sketched the principle of WOLEDS and provided some common structures, and further investigation of the mechanism of different structures was made. Meanwhile, the key technologies of WOLEDS were summarized. Finally, the latest research progress of WOLEDS was reviewed.

[Tunneling electroluminescence of the ZnO nanorods/MEH-PPV heterojunction devices].

In the present paper, zinc oxide nanorods (ZnO-NRs) were synthesized by the hydrothermal method at lower temperature and the polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) were used to fabricate an electroluminescent device based on the structure of "ITO/ZnO-seed/ZnO-NRs/MEH-PPV/Al". The measurement of current-voltage curve shows that it has a well rectifying characteristic. When a reverse bias exceeding 17 volts was applied to the ZnO-NRs/ MEH-PPV heterojunction, both emissions of ZnO and MEH-PPV were detected. And the intensity of near ultraviolet emission from ZnO is stronger than that of the orange-red emission belonging to MEH-PPV. It was also shown that the light power is gradually promoted with the increase in the reverse bias. Unfortunately, although carriers were injected into the heterojunction under the positive bias, we could not get any emissions. Its mechanism is attributed to the specificity of the organic inorganic hybrid heterojunction and the nanometer effects of ZnO-NRs. It is concluded that tunneling electroluminescence happens in this prepared device under the reverse bias.

[Bulk heterojunction solar cell based on porphyrin compounds].

Three kinds of porphyrins which can abbreviate as TPP, TPPCu and TMPPFeCl were synthesized by one-step method with mixed solvents. Then these porphyrin materials were used as donors to fabricate organic solar cells with PCBM as accepter by the solution processing of spin-coating method. The structure is ITO/porphyrin : PCBM/Al. The photovoltaic characterizations of these devices were investigated. The device based on TPP : PCBM shows the best performance with an open circuit voltage (V(OC)) of 0.52 V, a short circuit current (J(SC)) of 0.98 mA x cm(-2), and fill factor (FF) of 30.1%. Then the influence of different weight ratio of TPP : PCBM was researched. The best weight ratio of TPP : PCBM is 1 : 1. Increasing or decreasing the quatity of TPP would make J(SC) and V(OC) of the device deterioration and have little effect on the FF.

[Luminescence characteristics of PVK doped with Ir(Fppy)3].

In the present work, the photoluminescence (PL) and electroluminescence (EL) characteristics of Tris[2-(2,4-difluorophenyl)pyridine]iridium(III) (Ir(Fppy)3) doped poly(n-vinylcarbazole) (PVK) with different doping concentrations were investigated. And a blue phosphorescent organic light-emitting diode (OLED) with the structures of ITO/PEDOT : PSS/PVK : Ir(Fppy)3/BCP/Alq3/LiF/Al was fabricated. The experimental results show that the luminescence performances of devices are different as the doping concentration of Ir(Fppy)3 is different. When the doping concentration of Ir(Fppy)3 is lower, the luminescence of PVK can be found in EL spectra. When the doping concentration is too high, concentration quenching may occur. As the doping concentration is suitable, the luminescence of PVK can not be found, only the luminescence of Ir(Fppy)3 can be found in EL spectra. It is concluded that the device with doping concentration of 4% has the best photoelectric performance according to its current density-voltage-luminance curve.

[Study on the movement of the carrier recombination region in organic light-emitting diodes (OLEDs) based on DPVBi/Alq3].

Series of organic light emitting devices with basic structure of ITO/PCBM: PVK(x Wt%, approximately 40 nm)/DPVBi(30 nm)/Alq3 (30 nm)/Al were fabricated in order to investigate the carrier recombination region movement in these devices. The carrier injection-dependent, the carrier transport-dependent and the voltage-dependent carrier recombination region movements were investigated respectively by modifying cathode with lithium fluoride, by changing the doping concentration of PCBM and by changing the voltage on the devices. The physical mechanism behind the voltage-dependent carrier recombination region movement was discussed.

[Value of contrast-enhanced ultrasound imaging in monitoring malignant tumor during argon-helium cryosurgery].

OBJECTIVE: To assess the value of contrast-enhanced ultrasound imaging in monitoring the therapeutic effect of argon-helium cryosurgical treatment of malignant tumors. METHODS: Before and after argon-helium cryosurgical treatment, 42 patients underwent contrast-enhanced ultrasound imaging, conventional ultrasound imaging and enhanced CT or magnetic resonance imaging (MRI) for examining the number of tumor foci and the size of necrotic areas. RESULTS: A total of 80 tumor lesions were detected by contrast-enhanced ultrasound imaging. Compared with conventional ultrasound imaging, contrast-enhanced ultrasound imaging detected a significantly greater number of tumors and the intratumoral necrotic areas (96 vs 19) as well as a significantly increased mean size of necrotic areas (5.7∓3.6 cm vs 2.8∓1.7 cm), showing no significant differences from the results by enhanced CT and MRI (94 and 5.5∓3.3 cm, P=0.872 and 0.978, respectively). The short-term therapeutic effect of argon-helium cryosurgery evaluated by contrast-enhanced ultrasound imaging were also similar to that assessed by enhanced CT or MRI (P=0.906). CONCLUSION: Contrast-enhanced ultrasound imaging has important values in monitoring malignant tumors during argon-helium cryosurgical treatment and in evaluating the short-term therapeutic effect of the treatment.

[Effect of mixed interface on the performance of solution-processed phosphorescent OLEDs].

In the present work, in order to improve electron injection and transport at the interface of the hole blocking layer (HBL) and the electron transport layer (ETL) in the hole-domain solution processed phosphorescent organic light emitting devices (PhOLEDs), the mixed interface layer (MIL) was fabricated by partially co-doping hole blocking material 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) and electron transport material tris(8-quinolinolato) aluminum (Alq3) between HBL/ ETL. The MIL thickness was kept at 10nm, while the doping ratio of these two materials varied. Under a given electric field, the devices with the MIL at any mixed ratios all show much higher luminance and current density than those with a typical interface. For example, the luminance power and current density at 10 V for a typical device are 1.03 microW and 5.13 mA x cm(-2), while in case of mixed interface are 3.64 microW and 18.1 mA x cm(-2), respectively. From data results and theoretical analysis, the possible derivation of these improvements is considered to be the reduced electron accumulation at the interface resulting from the reduced electron injection energy barrier and lowered transport mobility by BCP material, which leads to an increase in electron amount in the emission layer and therefore the higher luminance and current density.

[Effects of hole-injection layers on the performance of blue organic light-emitting diodes].

The present work investigates the effects of different buffer layers on the performance of blue organic light-emitting diodes (OLEDs), and compares them with the device with no buffer layer. Two kinds of blue OLEDs with 4,4'-bis(2,2'-diphenyl vinyl)-1,1'-biphenyl (DPVBi) as the emitting layer, N, N'-bis-(1-naphthyl)-N, N'-1-diphenyl-1,1 '-biphenyl-4, 4'-diamine (NPB) as the hole transporting layer, and copper phthalocyanine (CuPc) and poly(3,4-ethylenedioxythiophene) : poly (styrenesulphonate) PEDOT : PSS as the hole injection layer respectively were fabricated with the structures of ITO/CuPc/NPB/DPVBi/BCP/Alq3 /Al and ITO/PEDOT : PSS/NPB/DPVBi/BCP/Alq3/Al. Moreover, the effects of different preparation technology of CuPc on the performance of OLEDs were also investigated. It was found that the performance of the devices with a hole injection layer is better than that of the device without any hole-injection layer. Although the luminance and efficiency of the water-soluble CuPc based device are worse than that of the device with thermally evaporated CuPc, but better than that of the device with water-soluble PEDOT : PSS. So the water-soluble CuPc is a good hole injection material because it is easier to fabricate the film than traditional CuPc.

[Luminescence characteristics of PVK doped with Eu-complex Eu(UVA)3Phen].

The present work investigates the photoluminescence (PL) and electroluminescence (EL) characteristics of Eu-complex Eu (UVA)3Phen doped PVK with different doping concentrations. The results indicate that there exists Forster energy transfer from PVK to Eu(UVA)3 phen in the mixed system. It can get good color purity by optimizing the doping concentration of host and guest materials. And the authors can obtain the best doping concentration to be 4% in EL device.

[Influence of the active layer thickness on the performance of bulk heterojunction solar cell].

Bulk heterojunction polymer solar cells based on the blend of MEH-PPV (poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene]) and PCBM (1-(3-mehyloxycarbonyl)propyl-phenyl[6,6]C61) were fabricated. The thickness of the active layer was controlled by changing the concentration of MEH-PPV : PCBM (1 : 4 in weight ratio) solution and spin speeds. Investigation of the effects of active layer thicknesses on the performance of the photovoltaic devices indicates that, when the spin-coated speeds are lower than 4,000 r x min(-1) (round per minute), the open-circuit voltage (V(oc)) remains almost unchanged at approximately 0.8 V, whereas the short-circuit density (J(sc)) monotonically increases and the fill factor (FF) decreases slightly. The spin speeds that are higher than 5,000 r x min(-1) rpm result in the V(oc) and J(sc) both reduced. The V(oc) decreases from 0. 78 V at the spin-speed of 5,000 r x min(-1) to 0.67 V at 8,000 r x min(-1), and the J(sc) even decreases from 3.96 mA x cm(-2) at 5 000 r min(-1) to 1.76 mA x cm(-2) at 8,000 r x min(-1). J(sc) depends on the mutual impact of light absorption and carrier transport, while a contradicting effect from the two aspects is caused by varying the thickness of the active layer. The thicker the active layer, the more the excitons induced by light absorption. However, the build-in electric field becomes weaker and the pathway becomes longer for transporting the opposite charge carriers derived from exiciton separation to their corresponding electrodes at the same time, which makes the probability of charges collection by respective electrodes lower. With respect to the reduced V(oc), it may be attributed to the increased proportion of exciton dissociation at the interfaces of MEH-PPV and PCBM with the relevant electrodes.

[Study on luminescence characteristics of polymer doped with two different phosphorescent materials].

With the development of organic light-emitting diodes, interests in the mechanisms of charge carrier photo generation, separation, transport and recombination continue to grow. Phosphorescent organic light-emitting diodes have gained considerable interest in the last 10 years because of high luminance efficiency. In the present paper, the authors investigated the optical and electrical characteristics of the devices based on Ir(ppy)3 and Ir(piq)3 doped PVK matrix emission layer at room temperature. The PL spectra show that the energy transfer from PVK to Ir(piq)3 is harder than that of PVK to Ir(ppy)3. The Luminescence characteristics of devices with different doping ratio show that it is not the energy transfer from matrix, but the recombination of injected carriers on phosphorescent molecules that is the main origin of EL emissions. From the viewpoint of energy level, the characteristics of carrier-trap and transport in Ir(piq)3 are better than in Ir(ppy)3 due to the high HOMO and low LUMO in Ir(piq)3.