Solution-processed quantum dot light-emitting diodes with PANI:PSS hole-transport interlayers

For solution-processed quantum dot light-emitting devices (QD-LEDs), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/poly(N-vinylcarbozole) (PEDOT:PSS/PVK) bilayers have been widely used as the hole injection/transport layer. The high work function of the hole transport layer is crucial for high electroluminescence efficiency with balanced electron/hole charge injection. Herein, we report improvement of the performance of QD-LEDs by inserting a polyaniline (PANI)-poly (p-styrenesulfonic acid) (PSS) (PANI:PSS) hole-transport layer between the PVK and PEDOT:PSS layers. The insertion of the PANI:PSS layer significantly shifted the electronic energy levels of the PVK layers to lower values, which reduced the energy barrier of holes traveling to the QD layer by 0.22 eV. The QD-LEDs with PANI:PSS interlayer exhibited superior electric and electroluminescent characteristics. The hole-only devices with PANI:PSS interlayer also presented high hole injection and transport capability. Ultraviolet photoelectron spectroscopy (UPS) was used to investigate the electronic energy level alignment of the QD-LEDs with/without the PANI:PSS interlayer. The device performance results of QD-LEDs and hole-only devices indicated enhanced electric and electroluminescent characteristics for the PANI:PSS-inserted QD-LEDs with high hole conduction capability, in agreement with UPS findings.     

Optimization of carrier transport layer: A simple but effective approach toward achieving high efficiency all-solution processed InP quantum dot light emitting diodes

High performance quantum dot light emitting diodes (QD-LED) are being considered as a next-generation technology for energy efficient solid-state lighting and displays. In recent years, cadmium (Cd)-based QLEDs have made great progress in performance, which is close to commercial applications. However, the performance of environmentally friendly Cd-free QD-LED still needs to be improved. In this letter, using InP/ZnS quantum dots (QDs), an environmentally friendly red QDs material, as the light emitting layer, low-cost all-solution processed red InP/ZnS QD-LED are fabricated. The optimized device with a hybrid multilayered structure employing an organic double hole transport layer (HTL) with doping small molecules (TFB/PVK:TAPC) and an inorganic ZnMgO nanoparticles (NPs) electron transport layer (ETL), here TFB, PVK and TAPC represent poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4’-(N-(p-butylphenyl))-diphenylamine)], poly (9-vinlycarbazole) and 1,1-bis [4-[N,N′-di (p-tolyl)amino]phenyl]-cyclohexane, respectively. The best device exhibits a peak current efficiency (CE) of 7.58 cd A⁻¹, which is 2.4 times higher than the control device using PVK (HTL) and ZnO (ETL). At the same time, turn-on voltage dropped from 2.8 V (control devices) to 2.4 V. These superb QD-LED performances originate not only from the improved hole injection by the introduction of a double hole layer and the reduced the quenching of excitons by using ZnMgO NPs ETL but also from increasing the hole mobility with doping of small molecule materials in PVK to balance the carrier transportation. This work provides a simple and feasible idea with optimization the carrier transport for realizing high-efficiency QD-LED devices.     

Highly efficient all-solution processed blue quantum dot light-emitting diodes based on balanced charge injection achieved by double hole transport layers

Solution-processed blue quantum dot light-emitting diodes (QLEDs) suffer from low device efficiency, whereas the balance of electron and hole injection is critical for obtaining high efficiency. Herein, synergistical double hole transport layers (D-HTLs) are employed, which use poly(9-vinylcarbazole) (PVK) stacked on poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB). The fabrication of D-HTLs is achieved by using dimethyl formamide (DMF) as the solvent for PVK, with which the underlying TFB layer almost remains unwashed and undamaged during the spin-coating process of PVK layer. TFB/PVK D-HTLs form the stepwise energy level for hole injection, which reduces the hole injection barrier and favors the carrier balance in the emission layer (EML). The optimized blue QLED with TFB/PVK D-HTLs shows a maximum external quantum efficiency (EQE) of 13.7%, which is 3-fold enhancement compared to that of the control device with single TFB HTL. The enhancement of the QLED performance can be attributed to the improvement of surface morphology and charge injection balance for the stepwise D-HTLs based QLEDs. This work manifests the positive effect on performance boost by selecting appropriate solvents towards stepwise D-HTLs formation and paves the way to fabricate highly efficient all-solution processed light emitting diodes.     

Parasitic emission in inkjet-printed InP-based quantum dot light-emitting diodes

Indium phosphide-based colloidal quantum dot (QD) light-emitting diodes represent a promising technology for various lighting applications. To promote this innovative technology closer to an industrialized production environment, the fabrication methods should be adapted. Hence it is necessary to replace the common spin-coating process under an inert atmosphere, by a more cost-efficient inkjet-printing process at ambient conditions. However, in our case, this transfer results in devices with limited performance and parasitic emission channels besides the desired QD emission. In this paper, we identify the physical origin of these parasitic emission channels for three different device layouts depending on the QD material as well as the number of inkjet-printed layers. For the first type of devices, a recombination process on the dopant of the electron transporting layer (ETL) as well as an exciplex formation at the interface between QDs and ETL was identified. For the next device layout, the introduction of a hole-conducting matrix embedding the QDs leads to a shift of the parasitic emission with contributions from the matrix material. Finally, the integration of a hole injection layer leads to a reduction of the undesired emission processes. For all three kinds of devices, the spacial separation of the dopant in the ETL from the QDs is a critical factor, since it directly influences the parasitic emission channels.     

Advantage of InGaN-based light-emitting diodes with trapezoidal electron blocking layer

In this study, a trapezoidal-shaped electron blocking layer is proposed to improve efficiency droop of InGaN/GaN multiple quantum well light-emitting diodes. The energy band diagram, carrier distribution profile, electrostatic field, and electron current leakage are systematically investigated between two light-emitting diodes with different electron blocking layer structures. The simulation results show that, when traditional AlGaN electron blocking layer is replaced by trapezoidal-shaped electron blocking layer, the electron current leakage is dramatically reduced and the hole injection efficiency in markedly enhanced due to the better polarization match, the quantum-confined Stark effect is mitigated and the radiative recombination rate is increased in the active region subsequently, which are responsible for the alleviation of efficiency droop. The optical performance of light-emitting diodes with trapezoidal-shaped electron blocking layer is significantly improved when compared with its counterpart with traditional AlGaN electron blocking layer.     

Controlling charge balance using non-conjugated polymer interlayer in quantum dot light-emitting diodes

The balance of electron–hole charge carriers in quantum dot (QD) light-emitting diodes (QLEDs) is an important factor to achieve high efficiency. However, poor interfacial properties between QDs and their adjacent layers are likely to deteriorate the electron–hole charge balance, resulting in the poor performance of a QLED. In this paper, we report an enhanced efficiency in red-emitting inverted QLEDs by modifying the interface properties between QDs and ZnO electron transport layer (ETL) using a thin layer of non-conjugated polymer, poly(4-vinylpyridine) (PVPy). Based on the precise control of the electrical properties with PVPy, the maximum efficiency of the QLED is enhanced by 30% compared to the device without a PVPy layer. In particular, the efficiency at low current density region is significantly increased. We investigate the effect of the PVPy interlayer on the performance of QLEDs and find that this thin layer not only shifts the energy levels of the underlying ZnO ETL, but also effectively blocks the leakage current at the ETL/QD interface.     

A new I-V model for light-emitting devices with a quantum well

This letter reports a new current versus voltage model for light-emitting devices with a quantum well where electrons and holes are injected and recombine. The current is entirely caused by the recombination of electrons and holes. Historically, the equation used for light-emitting diodes (LEDs) and laser diodes (LDs) has been the renowned Sah-Noyce-Shockley (SNS) diode equation. In this equation at typical forward bias condition, most of the current is caused by the diffusion of carriers over the depletion region. It is clear that this condition is different from what actually happen in LEDs and LDs. We thus looked into the fundamental of carrier transport and developed a new model for devices with a quantum well. Based on the new model, calculated I-V curves agree well with measurement results of GaN/sapphire LEDs with GaInN quantum wells. In calculation, junction temperature T_j rather than case temperature T_c is used to achieve better agreement.     

基于化学溶液法制备的氧化锌量子点发光二极管研究

氧化锌（ZnO）量子点是一种宽直接带隙半导体纳米颗粒,具有激子束缚能大、绿色环保、量子效应等优点,引起广泛关注。近期,将通过化学溶液法制备的ZnO量子点应用到发光二极管的研究成为热点。文章综述了近几年ZnO量子点发光二极管研究进展,重点介绍了各种结构的ZnO量子点发光二极管最新研究成果,并对ZnO量子点发光二极管的发展趋势进行了展望。     

Unveiling photophysical properties of phenanthro[9,10-d]imidazole derivatives for organic light-emitting diodes

Recently, two phenanthro[9,10-d]imidazole derivatives exhibited excellent advantages in organic light-emitting devices (i.e. high luminous efficiency, high carrier mobility, and low turn-on voltage). However, the relationship between their photophysical properties and the structural characters or intermolecular interactions remain elusive, which is considerable importance to further performance improvement. Currently, density functional theory (DFT) and time-dependent DFT (TD-DFT) have become powerful tools to rationalize photophysical properties and to design new materials with improvement performance. The simulated electron absorption and emission wavelengths of compounds 1 and 2 are in good agreement with the experimental ones. For the studied compounds, the involvement of tert-butyl moiety has negligible effect on energy level and distribution of frontier molecular orbitals (FMOs), whereas greatly affects electron transition of deep energy level and charge transport property. Synergy of π-π and CH···π intermolecular interactions is responsible for the bipolar carrier transport, while CH···π for hole transport. The incorporation of NH₂ on phenanthro[9,10-d]imidazole and NO₂ on diphenylamino part is an effective way to tune FMOs energy level and intramolecular charge transfer, leading to the substantial enhancement of the second-order nonlinear optical (NLO) response. Our work is also important for understanding photophysical properties and designing photoelectric materials of phenanthro[9,10-d]imidazole derivatives.     

High-performance azure blue quantum dot light-emitting diodes via doping PVK in emitting layer

Highly bright and efficient azure blue quantum dot-based light-emitting diodes (QD-LEDs) have been demonstrated by employing ZnCdSe core/multishell QDs as emitters and the crucial development we report here is the ability to dramatically enhance the efficiency and brightness through doping poly vinyl(N-carbazole) (PVK) in the emissive layer to balance the charge injection. The best device displays remarkable features like maximum luminance of 13,800 cd/m², luminous efficiency of 6.41 cd/A, and external quantum efficiency (EQE) of 8.76%, without detectable red-shift and broadening in electroluminescence (EL) spectra with increasing voltage as well as good spectral matching between photoluminescence (PL) and EL. Such azure blue quantum-dot LEDs show a 140% increase in external quantum efficiency compared with QD-LEDs without PVK. More important, the peak efficiency of the QD-LEDs with PVK dopant is achieved at luminance of about 1000 cd/m², and high efficiency (EQE > 8%) can be maintained with brightness ranging from 200 to 2400 cd/m². There are two main aspects of the role of PVK in the proposed system. Firstly, the lower HOMO of PVK than (poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) can reduce the potential barrier for 0.4 eV at the interface of QDs and hole transport layer which could result in higher hole injection efficiency along with good EQE as compared to TFB-only HTLs. Secondly, with PVK acting as buffer layer of TFB and QDs, the exciton energy transfer from the organic host to the QDs can be effectively improved.     

Bright,efficient, deep blue-emissive polymer light-emitting diodes of suitable hole-transport layer and cathode design

This study reports the fabrication of efficient deep blue-emissive polymer light-emitting diodes (PLEDs), incorporating a polyfluorene derivative of nonsymmetric and bulky aromatic groups at C-9 position as the light-emissive layer. Another poly(fluorene-co-triphenylamine) (PFO-TPA) derivative of the highest occupied molecular orbital level, −5.3 eV, is used as the hole-injection and -transport layer in the anode part. The thermally crosslinking of styryl groups in PFO-TPA inhibits the solvation of an interlayer in constructing the multilayer device architecture of PLEDs. While applying a cesium carbonate (Cs₂CO₃)/Aluminum (Al) cathode rather than Calcium (Ca)/Al, the device has the superior performance (i.e. one order of magnitude higher). Experimental results indicate that the interfacial reactions at the polymer/Ca junction, as characterized in this study, significantly degrade the luminescence properties and the device performance. Moreover, Cs₂CO₃/Al is a highly favorable cathode in fabricating polyflourene-based PLEDs. The device of the optimal configuration has a decent deep blue emission centered at 430–450 nm of the Commission Internationale de l’Eclairage chromaticity coordinates, (0.15, 0.14), with a maximum brightness of 35054.2 cd/m² and luminous efficiency of 14.0 cd/A (at 2975.0 cd/m²).     

Colloidal quantum-dot LEDs with a solution-processed copper oxide (CuO) hole injection layer

Solution-processed copper oxide (CuO) thin films are introduced as a hole injection layer (HIL) for quantum dot-based light-emitting diodes (QD-LEDs). AFM, XPS and UPS measurements are utilized for the characterization of the thermally-annealed CuO films. The optimized CuO-based QD-LEDs exhibited an external quantum efficiency (EQE) of 5.37% with a maximum brightness over 70,000 cd/m². The key parameters including the current efficiency and power efficiency of CuO-based QD-LEDs are comparable to the commonly-used PEDOT:PSS-based QD-LEDs using the same structure, further demonstrating that CuO is an effective hole injection layer for QD-LED applications.     

具有微米/纳米复合粗化ITO的GaN基LED

Three types of textured indium-tin-oxide （ITO） surface, including nano-texturing and hybrid micro/nano-texturing with micro-holes （concave-hybrid-pattem） or micro-pillars （convex-hybrid-pattern）, were applied to GaN-based light-emitting diodes （LEDs）. The nano-texturing was realized by maskless wet-etching, and the micro-texturing was achieved by standard photolithography and wet-etching. Compared to LED chips with flat ITO surface, those with nano-pattern, concave-hybrid-pattern, and convex-hybrid-pattern exhibit enhancement of 11.3%, 15.8%, and 17.9%, respectively, for the light-output powers at 20 mA. The electrical performance has no degradation. Moreover, the convex-hybrid-pattern show higher light-output efficiency under small injection current, while the concave-hybrid-pattern exhibit better light-output efficiency at large injection current. The light- extraction efficiency is simulated by use of two-dimensional finite difference time domain method, and the numer- ical results are consistent with the experiments.     

AlGaN/GaN层叠结构插入层改善氮化镓基发光二极管抗静电性能研究

在GaN基发光二极管的uGaN与nGaN之间插入AlGaN/GaN层叠结构,增大了外延层的张应力,降低了外延层中的穿透位错密度,改善了外延材料的质量。对比了AlGaN/GaN层叠结构中不同Al组分对LED的抗静电能力的影响,含6.8%铝组分AlGaN/GaN层叠结构的LED人体模式抗静电能力提高到了6000V,合格率超过了95%。     

Improved III-nitrides based light-emitting diodes anti-electrostatic discharge capacity with an AlGaN/GaN stack insert layer

Through insertion of an AlGaN/GaN stack between the u-GaN and n-GaN of GaN-based light-emitting diodes(LEDs),the strain in the epilayer was increased,the dislocation density was reduced.GaN-based LEDs with different Al compositions were compared.6.8%Al composition in the stacks showed the highest electrostatic discharge(ESD) endurance ability at the human body mode up to 6000 V and the pass yield exceeded 95%.     

Solution-processed quantum dot light-emitting diodes based on NiO nanocrystals hole injection layer

Nickel oxide (NiO), as a kind of p-type transition metal oxide (TMO) has shown promising applications in photoelectric devices. In our work, the NiO nanocrystals (NCs) are fabricated by a simple solvothermal method using tert-butyl alcohol and nickel acetylacetonate as precursors at 200 °C for different reaction times. The diameters and valence band edge of the prepared NiO NCs are increased with the increase reaction time from 12 h, 24 h–36 h. The band gaps of the NiO NCs were decreased with the increase time. Selected area electron diffraction (SAED) shows that the NiO NCs is polycrystalline structure. X-ray diffraction (XRD) indicates that the NiO NCs is cubic crystal form. X-ray photoelectron spectroscopy (XPS) shows that the as-prepared NiO NCs have a core of NiO and some form of Ni₂O₃ and NiOOH states on its surface. Further, the obtained NiO NCs is applied on quantum dot light-emitting diode (QLED) as hole injection layer (HILs), showing excellent hole injection properties. Particularly, the NiO NCs for 24 h obtains the best results due to its high band gap and pure cubic crystal phase. Highly bright orange-red QLED with peak luminance up to ∼25580 cd m⁻², and current efficiency (CE) of 5.38 cd A⁻¹ are achieved successfully based on the high performance NiO HIL, further, the device obtained relative long operational lifetime of 11491 h, which has been improved by more than 6- fold as compared to 1839 h for the device based on PEDOT.     

球形量子点量子阱中的电子拉曼散射研究

本文对HgS/CdS球形量子点量子阱中的电子拉曼散射进行了研究;推导了拉曼散射截面的表达式,给出了有关的选择定则,获得了丰富的拉曼散射谱线,给出了相邻能级差对于不同阱宽的色散图谱并对谱线特点进行了分析和讨论,讨论结果直接给出了电子的重要微观信息。     

Investigation of light output performance for gallium nitride-based light-emitting diodes grown on different shapes of patterned sapphire substrate

GaN-based blue light-emitting diodes (LEDs) on various patterned sapphire substrates (PSSs) are investigated in detail. Hemispherical and triangular pyramidal PSSs have been applied to improve the performance of LEDs compared with conventional LEDs grown on planar sapphire substrate. The structural, electrical, and optical properties of these LEDs are investigated. The leakage current is related to the crystalline quality of epitaxial GaN films, and it is improved by using the PSS technique. The light output power and emission efficiency of the LED grown on triangular pyramidal PSS with optimized fill factor show the best performance in all the samples, which indicates that the pattern structure and fill factor of the PSS are related to the capability of light extraction.     

Improved electron injection from silver electrode for all solution-processed polymer light-emitting diodes with Cs2CO3:conjugated polyelectrolyte blended interfacial layer

This study demonstrates the incorporation of a Cs₂CO₃:conjugated polyelectrolyte blended interfacial layer between the emissive layer and a silver (Ag) cathode, for realizing all-solution processed polymer light-emitting diodes. For a device with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as the emissive layer, this approach improves the maximum luminance of approximately 80,000 cd/m² and maximum current efficiency of 10.6 cd/A. It is clarified that the interfacial layer prevents Ag nanoparticles from penetrating into the emissive layer, resulting in yellow–green emission from F8BT. We also demonstrate the possibility of all-solution processed polymer light-emitting diodes utilizing solution-processed Cs₂CO₃:conjugated polyelectrolyte interfacial layer and Ag nano-ink.     

Optimizing carrier balance of a red quantum-dot light-emitting electrochemical cell with a carrier injection layer of cationic Ir(III) complex

Solid-state light-emitting electrochemical cells (LECs) with promising features of solution processability, low-voltage operation and compatibility with inert cathode metals have shown great potential in display and lighting applications in recent years. Among the reported emissive materials for LECs, ionic transition metal complexes (iTMCs) have relatively higher electroluminescence (EL) efficiencies due to their phosphorescent property. However, the red iTMCs generally exhibit moderate color saturation and low emission efficiency, limiting their display applications. To improve color saturation and device efficiency of red LECs, efficient quantum dots (QDs) with narrow emission bandwidth are good alternative emissive materials. In this work, efficient and saturated red QD LECs employing iTMC carrier injection layers to provide in situ electrochemical doping are demonstrated. The thicknesses of iTMC and red-QD layers are systematically adjusted to achieve the best carrier balance. In the optimized device, the iTMC carrier injection layer facilitates hole injection into the red-QD layer while electrons are injected from the cathode into the red-QD layer directly since the electron injection barrier is low. The Commission Internationale de I'Eclairage (CIE) coordinates of the EL spectra approach the red standard point of National Television System Committee (NTSC). High external quantum efficiency and current efficiency reaching 9.7% and 16.1 cd A⁻¹, respectively. These results confirm superior carrier balance in such a simple iTMC/QD bilayer device structure. Furthermore, compared with iTMC LECs, less degree of device efficiency roll-off upon increasing device current is observed in QD LECs since a shorter excited-state lifetime of fluorescent QDs reduces the probability of collision exciton quenching. Saturated and efficient red EL with mitigated efficiency roll-off from red-QD LECs employing iTMC carrier injection layers confirms that they are good candidates of saturated light sources for displays.