Lowering the driving voltage and improving the luminance of blue fluorescent organic light-emitting devices by thermal annealing a hole injection layer of pentacene

We chose pentacene as a hole injection layer（HIL） to fabricate the high performance blue fluorescent organic lightemitting devices（OLEDs）. We found that the carrier mobility of the pentacene thin films could be efficiently improved after a critical annealing at temperature 120℃. Then we performed the tests of scanning electron microscopy, atomic force microscopy, and Kelvin probe to explore the effect of annealing on the pentacene films. The pentacene film exhibited a more crystalline form with better continuities and smoothness after annealing. The optimal device with 120℃ annealed pentacene film and n-doped electron transport layer（ETL） presents a low turn-on voltage of 2.6 V and a highest luminance of 134800 cd/m^2 at 12 V, which are reduced by 26% and improved by 50% compared with those of the control device.

Effect of NaCl doped into Bphen layer on the performance of tandem organic light-emitting diodes

To improve the performance of tandem organic light-emitting diodes （OLEDs）, we study the novel NaCl as n-type dopant in Bphen：NaCl layer. By analyzing their relevant energy levels and cartier transporting characteristics, we discuss the mechanisms of the effective charge generation layer （CGL） of Bphen：NaCl （6 wt%）/MoO3. In addition, we use the Bphen：NaC1 （20 wt%） layer as the electron injection layer （ELL） combining the CGL to further improve the performance of tandem device. For this tandem device, the maximal current efficiency of 9.32 cd/A and the maximal power efficiency of 1.93 lm/W are obtained, which are enhanced approximately by 2.1 and 1.1 times compared with those of the single- emissive-unit device respectively. We attribute this improvement to the increase of electron injection ability by introducing of Bphen：NaCl layer. Moreover, the CGL is almost completely transparent in the visible light region, which is also important to achieve an efficient tandem OLEDs.

Improvement of electron injection of organic light-emitting devices by inserting a thin aluminum layer into cesium carbonate injection layer

We investigate the electron injection effect of inserting a thin aluminum（Al） layer into cesium carbonate（Cs2CO3）injection layer. Two groups of organic light-emitting devices（OLEDs） are fabricated. For the first group of devices based on Alq3, we insert a thin Al layer of different thickness into Cs2CO3 injection layer, and the device＇s maximum current efficiency of 6.5 cd/A is obtained when the thickness of the thin Al layer is 0.4 nm. However, when the thickness of Al layer is 0.8 nm, the capacity of electron injection is the strongest. To validate the universality of this approach, then we fabricate another group of devices based on another blue emitting material. The maximum current efficiency of the device without and with a thin Al layer is 4.51 cd/A and 4.84 cd/A, respectively. Inserting a thin Al layer of an appropriate thickness into Cs2CO3 layer can result in the reduction of electron injection barrier, enhancement of the electron injection, and improvement of the performance of OLEDs. This can be attributed to the mechanism that thermally evaporated Cs2CO3 decomposes into cesium oxides, the thin Al layer reacts with cesium oxides to form Al–O–Cs complex, and the amount of the Al–O–Cs complex can be controlled by adjusting the thickness of the thin Al layer.

Improvement of the Injection and Transport Characteristics of Electrons in Organic Light-Emitting Diodes by Utilizing a NaC1 N-Doped Layer

The injection and transport characteristics of electrons are enhanced by using sodium chloride （NaC1） as an n-type dopant doped into a 4, 7-diphnenyl-1, lO-phe-nanthroline （Bphen） electron-transporting layer, which improves the performance of organic light-emitting diodes （OLEDs）. Meanwhile, a NaCl-doped Bphen layer can effectively influence electrical characteristics of the devices, and significantly improve the current and power efficiency. The turn-on voltage and the operation voltage of the optimal device are decreased drastically from 6.5 V and 10.8 V to 3.3 V and 5 V, respectively, compared with those of the reference device. The maximum current efficiency and power efficiency of the optimal device are 7.0 cd/A and 4.4 Im/W at the current density of 16.70 mA/cm~, which are about I. 7 and 4 times higher than those of the reference device, respectively. Moreover, the enhancement of the injection and transport ability for electrons is attributed not only to the reduced energy barrier between A1 cathode and Bphen, but also to the increased mobility of electrons by the doping effect of NaCl. Therefore, both the electron injection and transport ability are enhanced, which improve the carrier balance in OLEDs and lead to the better device efficiency.

Efficiency of a blue organic light-emitting diode enhanced by inserting charge control layers into the emission region

We demonstrate high current efficiency of a blue fluorescent organic light-emitting diode （OLED） by using the charge control layers （CCLs） based on Alq3 . The CCLs that are inserted into the emitting layers （EMLs） could impede the hole injection and facilitate the electron transport, which can improve the carrier balance and further expand the exciton generation region. The maximal current efficiency of the optimal device is 5.89 cd/A at 1.81 mA/cm2 , which is about 2.19 times higher than that of the control device （CD） without the CCL, and the maximal luminance is 19.660 cd/m2 at 12V. The device shows a good color stability though the green light emitting material Alq3 is introduced as the CCL in the EML, but it has a poor lifetime due to the formation of cationic Alq3 species.

Efficiency of Blue Organic Light-emitting Diodes Enhanced by Employing an Exciton Feedback Layer

We report that a novel exciton feedback effect is observed by introducing the bis（2-methyl-8-quinolinolato）（4- phenylphenolato）Muminum （BAlq） inserted between the emitting layer （EML） and the electron transporting layer in blue organic light emitting diodes. As an exciton feedback layer （EFL）, the BAlq does not act as a traditional hole blocking effect. The design of this kind of device structure can greatly reduce excitons＇ quenching due to accumulated space charge at the exciton formation interface. Meanwhile, the non-radiative energy transfer from EFL to the EML can also be utilized to enhance the excitons＇ formation, which is confirmed by the test of photolumimescent transient lifetime decay and electroluminescence enhancement of these devices. Accordingly, the optimal device presents the improved performances with the maximum current efficiency of 4.2 cd/A and the luminance of 24600cd/m2, which are about 1.45 times and 1.75 times higher than those of device A （control device） without the EFL, respectively. Simultaneously, the device shows an excellent color stability with a tiny offset of the CIE coordinates （△x = ±0.003, △y = ±0.004） and a relatively lower efficiency roll-off of 26.2% under the driving voltage varying from 3 V to 10 V.

Improved Performance of Pentacene Organic Field-Effect Transistors by Inserting a V_(2)O_(5) Metal Oxide Layer

We fabricate pentacene-based organic field effect transistors(OFETs),inserting a transition metal oxide(V_(2)O_(5))layer between the pentacene and Al source−drain(S/D)electrodes.The performance of the devices with V_(2)O_(5)/Al S/D electrodes is considerably improved compared to the pentacene−based OFET with only Al S/D electrodes.After the 10-nm V2O5 layer modification,the effective field-effect mobility of the devices increases from 2.7×10^(−3) cm^(2)/V⋅s to 8.93×10−1 cm^(2)/V⋅s.Owing to the change of the injection property,the effective threshold voltage(Vth)is changed from−7.5 V to−5 V and the on/off ratio shifts from 102 to 104.Moreover,the dispersion of sub−threshold current in the devices disappears.These performance improvements are ascribed to the low carrier injection barrier and the reduction of contact resistance.It is indicated that V2O5 layer modification is an effective approach to improve pentacene-based OFET performance.

Performance Improvement of Ambipolar Organic Field Effect Transistors by Inserting a MoO_(3) Ultrathin Layer

We fabricate N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide and pentacene heterostructure organic field effect transistors with a MoO_(3) ultrathin layer between Al source-drain electrode and active layer.By inserting the MoO_(3) layer,the injection barrier of hole carriers is lowered and the contact resistance is reduced.Thus,the performance of the device is improved.The device shows typical ambipolar transport characteristics with effective hole mobility of 4.838×10^(-3) cm^(2)/V·s and effective electron mobility of 1.909×10^(-3) cm^(2)/V·s,respectively.This result indicates that using a MoO_(3) ultrathin 1ayer is an effective way to improve the performance of ambipolar organic field effect transistors.

Low driving voltage in an organic light-emitting diode using MoO_3/NPB multiple quantum well structure in a hole transport layer

The driving voltage of an organic light-emitting diode（OLED） is lowered by employing molybdenum trioxide（MoO3）/N,N＇-bis（naphthalene-1-yl）-N,N＇-bis（phe-nyl）-benzidine（NPB） multiple quantum well（MQW） structure in the hole transport layer.For the device with double quantum well（DQW） structure of ITO/[MoO3（2.5 nm）/NPB（20 nm）]2/Alq3（50 nm）/LiF（0.8 nm）/Al（120 nm）],the turn-on voltage is reduced to 2.8 V,which is lowered by 0.4 V compared with that of the control device（without MQW structures）,and the driving voltage is 5.6 V,which is reduced by 1 V compared with that of the control device at the 1000 cd/m2.In this work,the enhancement of the injection and transport ability for holes could reduce the driving voltage for the device with MQW structure,which is attributed not only to the reduced energy barrier between ITO and NPB,but also to the forming charge transfer complex between MoO3 and NPB induced by the interfacial doping effect of MoO3.

Highly Efficient Simplified Organic Light-Emitting Diodes Utilizing F4-TCNQ as an Anode Buffer Layer

We demonstrate that the electroluminescent performances of organic light-emitting diodes （OLEDs） are significantly improved by evaporating a thin F4-TCNQ film as an anode buffer layer on the ITO anode. The optimum Alq3-based OLEDs with F4-TCNQ buffer layer exhibit a lower turn-on voltage of 2.6 V, a higher brightness of 39820cd/m^2 at 13 V, and a higher current efficiency of 5.96cd/A at 6 V, which are obviously superior to those of the conventional device （turn-on voltage of 4.1 V, brightness of 18230cd/m^2 at 13 V, and maximum current efficiency of 2.74calla at 10 V）. Furthermore, the buffered devices with F4-TCNQ as the buffer layer could not only increase the efficiency but also simplify the fabrication process compared with the p-doped devices in which F4-TCNQ is doped into β-NPB as p-HTL （3.11 cd/A at 7 V）. The reason why the current efficiency of the p-doped devices is lower than that of the buffered devices is analyzed based on the concept of doping, the measurement of absorption and photoluminescence spectra of the organic materials, and the current density-voltage characteristics of the corresponding hole-only devices.

Color tuning of Eu-Tb co-doped borophosphate glasses for white light through valence state adjustment

The dependence of color points of white light on the composition of borophosphate glasses co-doped with europium (Eu) and terbium (Tb) has been investigated in terms of valence change of rare earth ions. Under ultraviolet (UV) excitation, the white light is observed to be from a combination of 4f65d→4f7 band transition emission at 425 nm for Eu2+, 5D0→7FJ (J=1, 2) line-emissions at 593 nm and 611 nm for Eu3+, and 5D4→7F5 band transition emission at 545 nm for Tb3+. By varying the glass composition, the resultant emission color can be tuned efficiently. Eventually, the optimized white light with commission international de I'Eclairage (CIE) coordinate of (0.3382, 0.2763) and the correlate color temperature (CCT) at 5010 K are achieved.

Enhanced performance of C_(60) N-type organic field-effect transistors using a pentacene passivation layer

We investigated the properties of C_（60）-based organic field-enect transistors（OFETs）（？） a pentacene passivation layer inserted between the C_（60） active layer and the gate dielectric.After modification of the pentacene passivation layer,the performance of the devices was considerably improved compared to C_（60）-based OFETs with only a PMMA dielectric.The peak field-effect mobility was up to 1.01 cm^2/（V·s） and the on/off ratio shifted to 10~4.This result indicates that using a pentacene passivation layer is an effective way to improve the performance of N-type OFETs.