Solution-processed double-layered hole transport layers for highly-efficient cadmium-free quantum-dot light-emitting diodes.

The search for heavy-metal-free quantum-dot light-emitting diodes (QD-LEDs) has greatly intensified in the past few years because device performance still falls behind that of CdSe-based QD-LEDs. Apart from the effects of nanostructures of the emitting materials, the unbalanced charge injection and transport severely affects the performance of heavy-metal-free QD-LEDs. In this work, we presented solution-processed double hole transport layers (HTLs) for improving the device performance of heavy-metal-free Cu-In-Zn-S(CIZS)/ZnS-based QD-LEDs, in which N,N'-Bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine (TPD) as an interlayer was incorporated between the emitting layer and the HTL. Through optimizing the thickness of poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenyl-amine (TFB) and TPD layers, a maximum external quantum efficiency (ηEQE) of 3.87% and a current efficiency of 9.20 cd A-1 were achieved in the solution-processed QD-LEDs with double-layered TFB/TPD as the HTLs, which were higher than those of the devices with pristine TFB, TPD and TFB:TPD blended layers. The performance enhancement could be attributed to the synergistic effects of the reduction of the hole injection barrier, the increase of the hole mobility and suppressed charge transfer between the HTL and the emitting layer. Furthermore, the best ηEQE of 5.61% with a mean ηEQE of 4.44 ± 0.73% was realized in the Cu-In-Zn-S-based QD-LEDs by varying the annealing temperature of TPD layer due to the more balanced charge injection and transport as well as smooth surface of TPD layer.

[Research on spectral response of CdSe quantum dots dopted polymer solar cell].

In the present paper, bulk heterojunction polymer solar cells based on P3HT:PCBM and P3HT:PCBM:QDs active layer were fabricated and measured respectively. The experimental result showed that the addition of QDs can broaden the spectral response and enhance photoinduced electron transfer. The conversion efficiency of device with QDs is about 25% higher than that without QDs.

[Effect of rubrene position on the EL performance of the device and analysis of the exciton recombination zone].

By using an ultrathin 5, 6, 11, 12-tetraphenylnaphthacene (rubrene) layer deposited on the top of host materials, the influence of rubrene layer position on the electroluminescence (EL) spectra of organic light-emitting devices (OLEDs) was stud ied. When the rubrene layer is located at the interface between N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,18-biphenyl)-4,4'-di-amine (NPB) and tris(8-hydroxyquinoline) aluminum (AlQ) layer, EL luminescence of the device is nearly coming from rubrene emission. From the analysis of the EL spectra of the devices with different rubrene layer position, the solely contribution of rubrene and AlQ, respectively, to the luminescence of the devices is determined by spectra unmixing. Based on the analysis, the exciton length in AlQ layer is determined to be about 15-20 nm. The exciton transport and recombination characteristics are also discussed in this work.

[Emitting characteristics and carrier composite regions variety of the blend of terbium complex and poly-n-vinylcarbazole].

A new rare earth complex TbGd(BA)6(bipy)2 was synthesized, which was used as an emitting material in electroluminescence. By doping with poly-N-vinylcarbazole (PVK), the stability and conductivity of terbium complex were improved. The photoluminescence of PVK, terbium complex and their blend indicated that energy transfer from PVK to terbium complex occurred. In the present paper, single-, double- and multi-layer devices were fabricated. In the double-layer devices, holes and electrons were recombined at the interface of emitting layer (EML) and electron transporting layer (ETL). With increasing the thickness of Alq3, especially at highly electric field, carrier composite region transferred closely to ETL. However, in multi-layer devices, carrier composite region was restricted at the interface of emitting layer and holes blocking layer (HBL) due to high HOMO level of BCP. Enhancing electric field, emission from terbium complex was getting saturation gradually and weak emission from PVK was observed. From the optimized multi-layer device, bright and green emission from terbium complex was obtained with the highest EL brightness of the device of 213 cd x m(-2) at 13.5 V.

[The emission mechanisms of two kinds of Tb complexes doped in PVK system].

Two kinds of new rare earth complexes Tb(p-MBA)3phen (sample I) and Tb(p-ClBA)3 phen (sample II) were synthesized, and photoluminescence (PL) and electroluminescence (EL) characteristics of the terbium complex and PVK blended system were investigated. As to the sample I , the emission of PVK was completely restrained in the EL spectra of the blended film; while in the PL spectra, obvious emission of PVK appeared besides the emission of Tb3+. But as to the sample II, only the green emission from Tb3+ can be observed in both PL and EL spectra of the doping system. So there are two processes that occur in sample I, one is the incomplete Förster energy transfer process from PVK to the Tb complex, and the other is the car rier charge trapping process. The main mechanism of sample II is the energy transfer process.

[Effects of PBD on the electroluminescence (EL) of Tb complex doped PVK system].

Electroluminescent (EL) properties of the terbium complex [Tb(m-MBA)3 phen]2 x 2H2O doped PVK system were investigated. Two kinds of devices with the structures of ITO/PVK: Tb complex/PBD/LiF/Al and ITO/PVK: Tb complex: PBD/PBD/LiF/Al were fabricated. PBD emission appears in the EL spectra of ITO/PVK: Tb complex/PBD/LiF/Al in comparison with ITO/PVK: Tb complex: PBD/PBD/LiF/Al. The reason is that PBD acts as a hole blocking material in the emitting layer (PVK: Tb complex: PBD) and thus the combination of excitons mostly occur within this layer. The energy of terbium complex emission comes from two different ways, namely energy transfer and direct combination of carriers. By altering the doping weight ratio of PBD, a set of devices were fabricated with the configuration of ITO/PVK: Tb complex: PBD/PBD/LiF/Al. Both the efficiency and brightness of these devices decrease with the rising of the PBD doping weight ratios. The efficiency of the carriers trapping may decrease with the rising of PBD doping ratios, for PBD may block the hoping of hole or electron between the PVK chains, causing a decrease in the brightness and efficiency. The PBD doping ratios have little effect on the efficiency of Förester energy transfer based on our synthesis.

[Electroluminescent device based on terbium complex].

Rare earth complex Tb(BA)3phen has been synthesized, which is used as an emitting material in electroluminescence. The properties of monolayer device with the rate of 1000 r x min(-1) and the impure concentration of 1:5 are the best. And the highest brightness of this device reached 26.8 cd x cm(-2) at a fixed bias of 21 V. Bright green emission can be obtained from the optimized double-layer device and the highest EL brightness of the device reaches 322cd x m(-2) at the voltage of 22 V.

[Photovoltaic character of organic EL devices MEH-PPV/Alq3].

An organic photovoltaic(PV) cell, ITO/MEH-PPV/Alq3/LiF/Al, was fabricated. The MEH-PPV and Alq3 are the electron-acceptor and donor in the cell, respectively. The respond region matchs the adsorption of Alq3 film. Under UV light with 0.5 mW x cm(-2), the cell shows a short-circuit current of 2.4 microA x cm(-2), open-circuit voltage of 2.6 V, a fill factor of 0.71, and a power conversion efficiency of 0.9%. It was found that the PV cell indicates electroluminescence (EL) performance and could emit orange light at DC voltage. The maximum luminance is about 1 000 cd x cm(-2) at 15 V.

[Luminescence properties of rare earth complexes Tb(BSA)4].

A new rare earth complex Tb(BSA)4 was synthesized and studied. Pure green and narrow band emission was generated from the device with structure ITO/PVK:Tb(BSA)4 /Alq3 /LiF/Al, where PVK was used to improve the film-formation and hole-transport property of the Tb(BSA)4. The absorption mechanism, and the photoluminescence and electroluminescence mechanisms are discussed. It has been proved that there exists energy transfer from PVK to Tb(BSA)4 and the mechanisms of photoluminescence and electroluminescence are different. The effect of different ratios of PVK on the device characteristics is also studied.

[Electroluminescent device based on rare earth terbium complex].

Rare earth complex TbY(m-MBA)6 (phen)2.2H20 have been synthesized, which were used as emitting materials in electroluminescence. Single-layer devices and bilayer devices with Alq as electron transmission layer have been fabricated. The electroluminescent properties of the devices were studied. The electroluminescent mechanism of the devices was proposed by measuring and analyzing the emission and the excitation spectra of the emissive layer. Y3+ may play the role to promote the energy transfer from ligand to Tb3+ and the possible energy transfer process of the device was preliminarily discussed.

[Emission mechanism in the Tb complex doped PVK system].

Excitation and photoluminescence (PL) spectra of the mixture terbium complex and PVK were investigated. The excitation spectrum of Tb complex dispersed PVK film is very similar to that of the pure PVK film, indicting that energy transfer occurs from PVK to Tb complex. For the overlap between the excitation spectrum of Tb complex dispersed PVK film and the PL spectrum of PVK film is very little, the ratio of occurrence for Förester energy transfer is very little. The emission of Tb3+ mainly comes from the excited ligand, which comes from the ligand capture of electron-hole pairs. In the electroluminescence (EL) spectra, the emission of PVK is completely restrained and only emission of Tb3+ is occured, which origins from the different mechanism in comparison with photoluminescence.

[The luminescence characteristic of a new novel coprecipitate rare earths complexes].

A new novel coprecipitate rare earths complexes La0.6Eu0.4(BSA)3phen has been synthesized and was chosen as the emitter material in the organic electroluminescent devices: ITO/PVK: La0.6Eu0.4(BSA)3phen/Alq/Al where PVK was used to improve the film-forming and hole-transporting property of the La0.6Eu0.4(BSA)3phen. It has been proved that it exists Förster energy transfer process from La3+ to Eu3+. This device has also been compared with the devices: ITO/PVK: Eu(BSA)3phen/Alq/Al and ITO/PVK: Tb0.6Eu0.4(BSA)3phen/Alq/Al. From the data, it indicated the device has the good purity of red color and good commutation property. The maximum EL brightness of 102 cd x m(-2) had been got.