Quantum Dots for Light Emitting Diodes

Copyright © 2013 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 13, 3173–3185, 2013 Quantum Dots for Light Emitting Diodes Khan Qasim∗ , Wei Lei∗ , and Qing Li Display Research Centre, School of Electronic Science and Engineering, Southeast University, Nanjing 210096, P. R. China REVIEW In this article we discuss the development and key advantages of quantum dot based light emitting diode (QD-LED) and other applications based on their color purity, stability, and solution processibility. Analysis of quantum dot based LEDs and the main challenges faced in this field, such as the QD luminescence quenching, QD charging in thin films, and external quantum efficiency are discussed in detail. The description about how different optical down-conversion and structures enabled researchers to overcome these challenges and to commercialize the products. The recent developments about how to overcome these difficulties have also been discussed in this article. Keywords: Quantum Dots, Light Emitting Diodes, QD-LED Composite Structures, Crosslinking, Transport Layers, Quantum Efficiency. CONTENTS 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Synthesis Method and Growth Mechanisms of Cd-Based Nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Homogeneous Nucleation Method . . . . . . . . . . . . . . . . . . . 2.2. Nucleation and Growth Kinetics Method . . . . . . . . . . . . . . 2.3. CdSe/ZnS Core–Shell Nanocomposit Structures . . . . . . . . 3. Characteristics of Cd Based Nanocrystals . . . . . . . . . . . . . . . . . 3.1. Temperature Probes of (CdSe)ZnS Quantum Dots . . . . . . . 3.2. Types of Core–Shell Structures i.e., Type-I and Type-II Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Enhanced Photoluminescence of (c/s) Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Quantum Dots Application in Light Emitting Diode . . . . . . . . . 4.1. Organic and Inorganic Composite Structures Based LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Energy Band Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. All-Inorganic QD-LED . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Fabrication Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Large-Area LED via Phase Separation . . . . . . . . . . . . . . . . 5.2. Contact Printing and Inkjet Printing and Transfer Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. The Recent Developments and Goles . . . . . . . . . . . . . . . . . . . . 6.1. Innovative QDs, Hybridization, and Cross-Linking Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Updated QD-LED Structure Possibilities . . . . . . . . . . . . . . 7. Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3173 3174 3174 3175 3175 3176 3176 3176 3176 3176 3176 3177 3179 3180 3180 3181 3182 3182 3183 3183 3183 3183 1. INTRODUCTION Atomic-like electronic structures, where the nanoparticles exhibit size-quantization effects are often called quantum ∗ Authors to whom correspondence should be addressed. J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 5 dots. The adjustable and sharp emission characteristics of these materials make them interesting candidates to be used in biodetction, biosensors, biomedicine, solar cell and light emitting diodes.1–14 Specific advantages of these QDs over the organic phosphors include their range of emission frequencies, improved stability and high density of absorbing states; ensembles of QDs with nearly continuous absorption spectra from their bandgap. The last feature permits the simultaneous excitation of various size QDs with a single light source and consequently facilitates the mixed color emission by controlling the mixing ratio of QDs. Moreover, their narrow emission spectra (full width half maximum (FWHM)) of ∼ 30 nm compared to those of inorganic phosphors with FWHM of 50∼100 nm make QDs outstanding sources of nearly pure color emission.1 15 16 However, the inherent instability and narrow absorption profiles of the organic phosphors and the broad emission spectra of inorganic often makes it difficult to precisely access the desired mixed colors.17 QDs are inorganic semiconductor nanocrystals with typical diameters from 2 to 8 nm and their electronic properties that are closely related to the size and shape of the crystals. They are generally composed of group II and VI elements (e.g., CdSe and CdTe) or group III and V elements (e.g., InP and InAs) in the periodic table.18 Their physical dimensions are usually smaller than the exciton Bohr radius i.e., R ≪ a0 (the bulk exciton Bohr radius such as R = 10 Å in II–VI crystallites) that leads to the three dimensional quantum confinement effects. These are responsible for their unique optical and electronic characteristics.7 19–25 Their absorption spectrum has discrete features which move to the higher energy 1533-4880/2013/13/3173/013 doi:10.1166/jnn.2013.7146 3173 REVIEW Quantum Dots for Light Emitting Diodes with decreasing dot diameter and a “band edge” luminescence spectrum, which is tunable with dot size. Electronic characterization of these dots has principally relied on optical methods19 and to a lesser extent on electrochemical methods.16 Therefore, quantum dots have a great deal with electrical and excitation/electroluminescence connection, which made them attractive for the optoelectronic based device applications. Bawendi and his colleagues demonstrated electroluminescence from a solid-state assembly of nearly monodispersed ( < 4%) nanocrystallites of CdSe embedded in a matrix of polyvinylcarbazole ∼ PVK and an oxadiazole derivative ∼ t-Bu-PBD which was experimented by Colvin et al.19 that will be discussed later in this article. The crystallites have reasonable photoluminescence quantum yields, strong absorption cross-sections when dispersed in organic solvents (∼10%), and their derivatized surfaces allow incorporation and chemical manipulation into the organic matrices. Therefore, because of Qasim et al. these similarities they can be used to design hybrid inorganic/organic devices. The crystallites are unlike more traditional chromophores, however, because the wavelength of the emission can be exactly chosen by selecting an appropriate dot size, without the need for tedious synthetic procedures often required to change the emission spectrum of organic chromophores.26 The quantum dots of CdSe surface was passivated with trioctylphosphine oxide (TOPO) as it protects the nanocrystals from chemical degradation and electronically passivates the surface. 2. SYNTHESIS METHOD AND GROWTH MECHANISMS OF Cd-BASED NANOCRYSTALS 2.1. Homogeneous Nucleation Method Initially, the nearly monodispersed CdSe QDs were prepared by relatively simpler synthetic route to produce of K. Qasim was born in 1983 in Swabi, KPK, Pakistan. He completed his masters in Electronics Engineering at Quaid-i-Azam University, Islamabad, Pakistan in 2008. He joined COMSATS Institute of Information Technology, Islamabad, Pakistan, as research associate/ lecturer in 2008. In September 2010, he started his Ph.D. under the supervision of Professor Lei Wei in Display Research Center, School of Electronics Science and Engineering, Southeast University, P. R. China. His current research is on Quantum Dots nanocrystals synthesis and its applications in Light Emitting Diodes and photovoltaics applications. Lei Wei received his Ph.D. degree in Electronic Engineering, Southeast University. After completing his Ph.D. in 1993 he joined School of Electronics Science and Engineering, Southeast University, as an Assistant Professor. He became Associate professor in 1996, full Professor in 2000 and the voice Dean in 2008. In 2010 he became the executive vice Dean of the school of Electronic Science and Engineering, Southeast University. He also serves as the General Secretary of Chinese Association of Display Technology, General Secretary of Chinese Vacuum Society of Jiangsu Province, Vice Director of Chinese Association of Vacuum Nanoelectronics, member IEEE Society (Electron Device), and member of SID. His research interests include nano-materials for displays, field effect transistor based on nanowires, 3D display technologies, and micro-displays. He has published more than 150 papers in the scientific journals and holds 28 patents. Since 1995, he has finished 22 collaborative projects between university and display industries. Qing Li received her B.S. and M.S. from University of Science and Technology, Nanjing (China), and Ph.D. from Southeast University (China). Since 1995, she has joined School of Electronic Science and Engineering, Southeast University. She worked in the Electronic Devices Institute, Nanjing, China from 1990∼1995. During 2010∼2011, she worked at University of Central Florida, USA as a senior visiting scholar. In 2012, she worked at University of Rennes 1 in France. She is a senior member of the Society for Information Display. Her study field covers flat display technology and optic electronic devices research. The main research work focuses on PDP, Xe-Ne flat lamp with the correlative material in plasma devices, such as protective layer, and LCD as well as optical devices using liquid crystal. Currently, she has more than 40 publications and 28 patents. 3174 J. Nanosci. Nanotechnol. 13, 3173–3185, 2013 Qasim et al. J. Nanosci. Nanotechnol. 13, 3173–3185, 2013 peak at 349 nm was found as the common nuclei for the growth of CdSe quantum rices, rods, tadpoles, and branched-nanocrystals. 2.2. Nucleation and Growth Kinetics Method Bullen et al.36 synthesized CdSe nanocrystals by the nucleation and growth kinetics in Octadecene. The oleic acid concentration and effects of temperature on the kinetics of both nucleation and growth were investigated. It was found that increasing the oleic acid concentrations led to a smaller numbers of nuclei, smaller nuclei size, and larger final particle sizes. The FWHM for CdSe nanocrystals was affected by the after injection time at 275  C and the data depicted that with the increasing time the FWHM was decreased and the mean crystallite radius was increased as time increases. Finally, they have shown unambiguously that the capping agents not only determine the rate of growth but also play a major role in determining the size and number of the nuclei formed during the injection. The presence of a vast excess of capping agent may lead to larger particle sizes. For studying the growth of CdSe QDs octadecene provides a useful medium. The nucleation process consumes about 2–8% free of cadmium. 2.3. CdSe/ZnS Core–Shell Nanocomposit Structures Core–shell type structured quantum dots exhibit novel properties making them attractive from both practical and experimental point of view.27 37–43 Overcoating nanocrystallites with higher band gap inorganic materials have been shown to improve the photoluminescence quantum yields by passivating surface nonradiative recombination sites. To improve the stability and quantum yield different groups are working on the synthesis engineering of quantum dots. Dabbousi and his colleagues synthesized highly luminescent (CdSe)ZnS composite quantum dots with CdSe cores ranging from 23 to 55 Å in diameter. Due to ZnS passivation the effects on the electronic structure and optical properties were explored. It was also reported that these QDs have narrow photoluminescence (FWHM ≤ 40 nm) which mostly lies in the visible spectrum from blue through red with quantum yields of 30–50% at room temperature. The absorption spectra for (CdSe)ZnS dots are slightly red-shifted and broader from their respective bare dot spectra. The PL spectra for the coated dots are also much more intense owing to their higher quantum yields. The PL quantum yield increased by 5–15% for bare dots to the values ranging 30–50% for dots passivated with ZnS. Overcoating with ZnS suppresses deep trap emission by passivating most of the vacancies and trap sites on the crystallite surface, resulting in PL which is dominated by band-edge recombination. 3175 REVIEW high-quality monodispersed (< 5% rms in diameter) samples of CdE (E = S, Se, Te) nanometer size crystallites, with the emphasis on CdSe.27–33 The synthesis begins with the rapid injection of organ, metallic reagents into a hot coordinating solvent to produce a temporally discrete homogeneous nucleation. Slow growth and annealing in the coordinating solvent results in uniform surface derivatization and regularity in core structure. Size selective precipitation provides powders of nearly monodispersed nanocrystallites which can be dispersed in a variety of solvents. The addition of methanol increases the average polarity of the solvent and reduces the energetic barrier to flocculation with and without size selective precipitation on the absorption spectrum. This shows the broad absorption features that correspond to a sample with an average size of 35 Å 10%. Slow addition of methanol results in the flocculation of larger particles after dispersing in 1-butanol. Titration of methanol again produces flocculation of larger particles and dispersion in 1-butanol. A final size-selective precipitation from 1-butanol yields with optical absorption and an average size of ∼37 Å 5%. The dramatically sharpness in the absorption was observed relative to that of initial growth solution and reveals transitions at 530 and ∼ 400 nm which were previously cloaked by polydispersity.34 For the fractionation process to work effectively it is crucially important that the shape and surface derivatization of the initial crystallites be uniform and that the initial polydispersity in size be relatively small. Without any size sorting, nearly monodispersed CdSe quantum structures with different shapes were reproducibly synthesized by using the alternative cadmium precursors, cadmium–phosphonic acid complexes.35 A reasonably large excess of the cadmium precursor, which is less reactive than the Se precursor was found beneficial for the system to reach the desired balance between the nucleation and growth. The shape evolution and growth kinetics of these elongated nanocrystals were consistent with the diffusion-controlled model proposed previously. Experimental results indicate that coordinating solvents and two ligands with distinguishable coordinating abilities are both not intrinsic requirements for the growth of elongated CdSe nanocrystals. Initial Cd to Se precursor ratio (Cd:Se ratio) was found as an important fact for determining the shape of the resulting nanocrystals. The general trend was that the highest average aspect ratio increased as the initial Cd:Se ratio was increased. Moreover, the more selenium precursor exists at the initial stage, the faster nucleation occurred and the more nuclei were generated in the nucleation stage. Consequently, the remaining monomer concentration after the nucleation stage in the solution decreased significantly as the initial Cd:Se monomer ratio decreased. This results in nanocrystals with a lower aspect ratio. A unique magic sized nanocluster with the first absorption Quantum Dots for Light Emitting Diodes Quantum Dots for Light Emitting Diodes 3. CHARACTERISTICS OF Cd BASED NANOCRYSTALS REVIEW 3.1. Temperature Probes of (CdSe)ZnS Quantum Dots The photoluminescence efficiency of (CdSe)ZnS also strongly dependents on temperature in the range from 100 to 315 K. The QDs dispersed in poly(lauryl methacrylate) i.e., PLMA matrix for optical measurements as previously described.44 The PL intensity from 50 to 55 Å (CdSe)ZnS QDs in PLMA matrices increases by a factor of ∼5 when the temperature is decreased from 315 to 100 K, and the peak emission band is blue-shifted by 20 nm over the same range.35 The temperature change is linear, appreciable, and reversible (−13% per K) for the range close to the ambient conditions. Experimental data shows the phenomenon of decreasing PL efficiency with the increase in temperature, in the range 100 to 315 K. As the temperature increases the PL spectra are red-shifted, PL intensity decreased, and FWHM is increased. There is a 20 nm red shift observed in the range 100 to 200 K prominently while as the spectrum FWHM is increased from 22 nm to 26 nm for the full range. The dramatic change in the PL intensity was observed, which is a 5-fold decrease with increasing temperature over the full range. Over the range from 250 to 315 K, the PL intensity decreased linearly (@− 1% per K) with increasing temperature. 3.2. Types of Core–Shell Structures i.e., Type-I and Type-II Quantum Dots There are two type core–shell QDs; the type-I is the type in which the band gap of the shell (the higher band gap material) is higher than the core (the lower band gap material) i.e., the band offsets are such that the valance band of the shell is of lower energy than that of the core and the conduction band of the shell is of higher energy than the core of QDs. The common Type-I QDs are CdSe/ZnS (c/s), CdSe/CdS (core/shell (c/s)), and InAs/CdSe (c/s).27 35 45–47 As a result both holes and electrons are confined in the core of QDs. In type-II QDs, in contrast, they have both the conduction and valence bands in the core lower (or higher) than in the shell. As a result, one carrier is mostly confined to the core, while the other is mostly confined to the shell. Type-II QDs have many novel properties that are fundamentally different from the type-I QDs because of their different band structures. Moreover, the separation of charges in the lowest excited states of type-II nanocrystals make these materials more suitable for photovoltaic and light applications, where the QDs are the chromophores and one of the photocarriers is injected from the QD into a matrix to produce the electron-hole pair.48 49 For example from for CdTe/CdSe QDs, the hole is mostly confined to the CdTe core, while the electron is 3176 Qasim et al. mostly in the CdSe shell. In CdSe/ZnTe QDs, the electron mostly confined in the CdSe core, while the hole is mostly in the ZnTe shell, as the band offsets are reversed.50 3.3. Enhanced Photoluminescence of (c/s) Quantum Dots QDs have proven themselves as efficient exciton donors in energy transfer. It is possible to transfer energy in the form of virtual photon (so called “dark exciton” or the exciton having intermediate energy) from CdSe/ZnS core/shell QDs donor to organic/inorganic acceptor.6 51–54 In quantum dots, because of the presence of transition metal atoms, such as cadmium (Cd), leads to spin–orbit coupling and hole–electron exchange interaction that mix the hole and electron spin states. In CdSe QDs electron spin-mixing results in a non-emissive exciton, so called ‘dark exciton’, which is about 0.13 meV (as in CdSe bulk) and 12.5 meV (for the smallest QD with few nm diameter) below the emissive exciton state.55 Thermal mixing of these dark and emissive exciton results in the efficient excitation with lifetime in the range 3–30 ns (depending on the QD core/shell size distribution, structure, and the fidelity of the organic capping layer), and luminescence could be exceeded 80% in solution.27 56 However, there still needs to debate in literature to show the reverse phenomenon such as, the organic donor to transfer energy into the QDs acceptor.57 Anikeeva’s group, fabricated a hybrid structure of organic/QDs and demonstrated the triplet exciton energy transfer (ET) phenomenon from phosphorescent molecules (organic) to the monolayer of CdSe/ZnS core/shell QDs.58 Triplet exciton harvesting and energy transfer to an efficient lumophores has been previously used in organic light emitting device (OLED), and has shown potency to be used in quantumdot based LEDs (QD-LEDs), the new emerging display technology.59 60 They reported an increase in PL lifetime as well as PL intensity for a CdSe/ZnS QDs layer due to exciton energy-transfer from phosphorescent organic thin film. The demonstrated energy transfer of triplet excitons to luminescent QDs could also benefit the development of QD optoelectronic devices such as QD-LEDs which we will discuss bellow in this article. 4. QUANTUM DOTS APPLICATION IN LIGHT EMITTING DIODE 4.1. Organic and Inorganic Composite Structures Based LEDs The journey towards the QD-LEDs was started in 1994 by Colvin et al. and they reported the first light emitting diode based on the QDs nanocrystals and polymer.1 They used P-paraphenylene vinylene (PPV) as the organic material while cadmium selenide (CdSe) nanocrystals as the inorganic material. In such hybrid structures the holes J. Nanosci. Nanotechnol. 13, 3173–3185, 2013 Qasim et al. J. Nanosci. Nanotechnol. 13, 3173–3185, 2013 layer of nanocrystals favors the electron transport, and also embeds the exciton recombination zone. It was found that ZnS overcoating does not increase the device performance while increases photoluminescence. The energy band diagram structure of the device, the presence of ZnS overcoating is associated with an additional energy mismatch (energy barrier) at the interface between the inorganic ETL and organic HTL, because of the higher band gap of ZnS. This barrier mostly reduces the flow of holes from the HTL into the nanocrystal layer and so in the case of electrons into the PPV structure. So the barrier becomes more effective in preventing carriers’ injection into the nanocrystal core with thicker ZnS overcoating. The performance was also greatly influenced by the dot size and inorganic layer thickness also reported as a highly stable operation for more than 50 hrs with the external quantum efficiency of 0.1%. 4.2. Energy Band Alignment Different struggles were carried out to get the full colored emission from the QDs and so a group of researchers at MIT figured out some facts about the II–VI nanocrystals by studying its photoluminescence characteristics. Judicious choice of the exact II–VI semiconductor nanocrystals can lead us to access the full and even different regions of the visible spectrum.44 They achieved nearly full color emission using semiconductor nanocrystalspolymer (QD-polymers) composites. By exciting these composites using blue light or ultraviolet sources over the entire visible range with narrow emission and high photoluminescence (PL) quantum yields was observed. It was also stated that mixed colors can be easily produced by controlling the mixing ratio of different sized QDs and showing the ability to tune nearly all the colors within the CIE chromaticity diagram and demonstrating the potential application in biomedicine sensing and full color display.7 64 65 The ZnS-overcoated QDs (CdS)ZnS were used to generate violet to blue colors and (CdSe)ZnS QDs were used generate bluish-green to red colors. It is necessary to stabilize these QDs in an appropriate matrix to retain their initial PL efficiency in order to apply to light-emitting devices. It is shown that ZnS overcoating around CdSe core considerably improves the luminescence efficiency and the particle stability.66 However, a wide application of the QDs to display has not been achieved due to subsequent luminescence quenching and aggregation of the QDs in a matrix. Charge transport and its effects on luminescence is one of the valued phenomena of the light emitting diode. There is a lot of debate in the literature on this and Hikmet et al. has explored some of these phenomena by fabricating luminescent colloidally synthesized core–shell CdSe/ZnS nanocrystal QDs and poly(3,4-ethylenedioxythiophene): 3177 REVIEW and electrons recombine in the organic and inorganic layers and emit light. Two structures were proposed, the so called structure (ITO/CdSe/PPV/Mg) as A and structure (ITO/PPV/CdSe/Mg) as B. Since the work function of inorganic CdSe nanocrystals is lower than the organic PPV and its turn-on voltage is ∼ 4 V, smaller than 7 V for PPV, so it started emitting light earlier than PPV. In forward biased, the indium tin oxide (ITO) was treated as the anode and Mg as the cathode. The samples were emitting light with luminance of about ∼100 cd/m2 in the visible region under the normal light. For structure A since PPV is closer to Mg electrode, so green light was prominently observed than the QDs emission. This phenomenon was valued to the PPV emission only because this layer is considered as weak electron transport medium and so the electron–hole recombination zone was shifted into the PPV layer closed to Mg electrode in such systems. For structure B, under the forward biased, the light emission was observed at a voltage of ∼ 4 V and the emission was considered only due to the QD nanocrystals. As the operating voltage for structure B is lower than the structure A therefore it is supposed that the QD nanoparticles electron affinity in case of structure B is higher than that of PPV. This high electron affinity in these crystals not only enhancing the emission of light but it also shifts the recombination region into the QDs-inorganic layers. It is also reported that the hetero-junctions in the polymer structures can bring the recombination region away from the electrodes and produce large electric potential across the junctions of the layers that can increase the tunneling of the holes into the CdSe and electrons into the PPV.61 62 The excitons formed can then diffuse easily into the low band gap materials emitting visible light with the combination of QDs emitted colors. It is observed that structure B has 3–10 times higher quantum efficiency than the structure A. External quantum efficiency of these devices is quite low (0.001–0.01%). However, these diodes are not optimized and need to be explored in more details and engineered with new structures. The generation of QDs based organic/inorganic composite structures opens a new window towards the display technology and is an active area of research and development in the recent years. Mattoussi and his companions use PPV as HTL, CdS as emissive layer, ITO as anode, and Aluminum (Al) as cathode.63 The demonstrated that PPV serves primarily as holes transporting medium and wider band gap semiconductor e.g., ZnS as passivating materials for nanocrystals. They found that the electroluminescence signal was almost exclusively generated within the nanocrystals layer, with a very weak emission from the PPV layer at higher applied voltage. The organic PPV favors the holes transport injected from anode and restricts the flow of electrons. This results in moving the excitonic recombination zone away from the anode and prevents quenching of the emission. The Quantum Dots for Light Emitting Diodes REVIEW Quantum Dots for Light Emitting Diodes polystyrenesulfonate as conducting polymer followed by the deposition of metal electrode.66 The main purpose of using this conducting polymer was to planarize the ITO surface because it contains small particles causing electrical shorts. Light emission was observed at about 5 V when PEDOT was treated as anode and at 7 V when it was treated as cathode. In order to accelerate electrons to 2∼3 eV and generate excitation a drift velocity of 106 m/s must be reached. In order to achieve such high velocity it is necessary to apply high electric field (∼108 V/m) with a high mobility (∼10−2 m2 /V s). Therefore, the threshold voltage and efficiency of luminescence depend on how the cell is biased indicates that the mechanism observed here dependent on injection of electrons and holes into the quantum dots. To observe this biasing dependency, different cathode materials were used having obviously different energy levels. For Ba cathode when biased positively, the efficiency was much lower than the Al and Au because of its higher work function and difference between the PEDOT energy level. The turn-on voltages for the devices having Au, Al and Ba electrodes were 3 V, 4 V and 5 V, respectively. Besides the energy levels, which are important for the injected charge balance, their mobility is also a very important parameter. Mobility of the carriers is important in order to get the recombination process of these carriers to take place away from the interfaces and electrodes for efficient light emission. Various reports have been shown in the area of charge injection, charge separation, and charge transport, in systems containing quantum dots.34 63 67 68 Another considerable fact is the space charge limited (SCL) currents which appear as soon as the concentration of injected carriers exceeds the concentration of carriers produced by thermal excitation. That’s why it is important to have a good ohmic contact between the semiconductors with the electrode so that it can continuously supply electrons and must satisfy the following conditions: Hole injection:  = Vv = VHOMO and Electron injection:  = Vc = VLUMO . Where  is the work function of the electrode, Vv valence band of the nanocrystal, Vc conduction band of the nanocrystal, VHOMO and VLUMO are the energy levels of higher occupied molecular orbital (HOMO) and lowest un-occupied molecular orbital (LUMO), respectively. However, all of these devices showed very low efficiencies. It was believed that difficulty in holes injection into QDs due to the low lying valence band energy level of CdSe QDs was considered to be the cause of the observed low efficiencies. By dispersion of QDs into organic materials, the electrophotoluminescence can be tuned. In this regards, a struggle is reported here, where they incorporated QDs in an organic hole transporting material, N ,N ′ -Diphenyl-N ,N ′ bis(3-methylphenyl)-1,1′ -biphenyl-4,4′ -diamine (TPD).69 3178 Qasim et al. For this purpose different layered structures were investigated, such as, QDs with and without TPD, and only TPD based. Three different devices were structured from the dispersed QDs into TPD with varied QDs average diameter size of 3.2, 4.1, and 5.4 nm. Nearly ohmic behaviors observed at low voltages while the current was dramatically increased at higher voltages. There was no prominent change in the turn-on voltages. So it is clear that electroluminescence from these LEDs with a smaller dot size was observed at a higher operating voltage because the small dots have wider band gaps, and hence low-lying hole levels or higher hole injection barriers for the QDs, which is being discussed already in detail.15 TPD influences the hole conduction and hence influence on the electro-optical properties of the devices. However, it was believed that the current–voltage characteristics were mainly due to TPD alone and the electroluminescence of the QD-TPD devices most likely results from the isolated QDs. The TPD plays an important role in the charge injection because of the intimate contact between the hole conducting material and the embedded QDs. In order to summarize this talk, different material dispersion in QDs with enhanced conductivity and less background luminescence could help to achieve this goal, which would let to understand the charge transport and injection mechanisms in the QD-LEDs. QD-LEDs can emit a full white color with the combination of polymers. A polymer based QDs emitting diode was fabricated and mainly two kinds of CdSe nanoparticles with different particle size were used.70 Blue light emitting polymer, green emitting 3 nm CdSe and red emitting 7 nm CdSe QDs, where the excitation was caused by energy transfer from the polymer, jointly resulted to white emission. By carefully controlling the blend ratio, pure white color from the hybrid device can be produced. It was also stated that the blue-emitting matrix polymer makes the device preparation process simpler due to its easy processibility. The energy transfer from the polymer to the quantum dots enables the quantum dots to emit green and orange–red light and blue light emitted from the polymer, the device generates pure white emission. In the year 2004, many groups of researchers were working on the QD-LED and in this regards, tri-layers structures were being tried. So in the line of this research, polymers based QD-LEDs was investigated by sandwiching QDs core–shell nanocrystals in polymers HTL and ETL. Where PVK was used as HTL and an oxadiazole derivative (butyl-PBD) as ETL while CdSe/ZnS is the emissive layer (EM) were spin coated using layer-by-layer technique.71 It was stated that water solution of QDs nanocrystals haven’t affected the underneath PVK film thickness as compared to the toluene solution of QDs.72 These devices with water-soluble nanocrystals tend to aggregate and form organized, closely packed structures. The close packing can be further improved by annealing. Such clusterized behavior was also been previously demonstrated at J. Nanosci. Nanotechnol. 13, 3173–3185, 2013 Qasim et al. 4.3. All-Inorganic QD-LED Due to the life time limitations of the polymers used for transport layers in QD-LEDs, scientists and researchers are looking forward to the inorganic materials to replace the organic ones. With these efforts a group of researchers tried to use NiO an inorganic material as the hole transporting layer material.37 In this work they explored the pros and cons of the inorganic holes transporting layer (HTL). It was believed that replacing the TPD (organic) film with a high band gap (3 eV) inorganic hole transporting layer that is electrically and chemically more stable. Moreover, early studies have shown that transparent p-type NiO thin films can be obtained by sputtering of Ni or NiO targets at near room temperature and can be used as hole transporting/injecting layers in light emitting devices.76–78 Quenching of QD electroluminescence by the high density of free holes in NiO and the imbalance of holes and electrons are two crucial challenges that have to be overcome in order to build efficient NiO QD-LEDs.79–81 By careful optimization, the NiO thin film conductivity has enabled the authors to improve the charge balance inside the device, reduced the quenching of quantum luminescence, and achieved QD-LED with maximum EL of 0.18% and brightness up to 3000 cd/m2 for the best device. However, the present QD-LEDs with NiO (inorganic) are less efficient than the best reported QD-LEDs that utilize TPD (organic) hole transport layer. J. Nanosci. Nanotechnol. 13, 3173–3185, 2013 From a recent article published by Mulvaney’s group, they also researched on the all-inorganic QD-LED where each layer is inorganic. P -type NiO (sol–gel), colloidal QDs, and n-type ZnO were used as HTL, EML and ETL respectively, and the device showed QDs only electroluminescence with the peak EL brightness of 249 cd/m2 .37 In the previous report, the desire to used inorganic transporting layer is because they are expected to operate at higher current and less susceptibility problems.82 83 It is highly desirable to avoid vacuum chamber deposition in order to achieve the ultimate goal of ‘printable electronics’ that’s why the p-type NiO sole–gel can be a better option for deposition of HTL.84 Although there have been far fewer reports of sol–gel derived p-type materials and the authors claim that for the first time p-NiO was used in LED. Based on the resistivity measurements, the NiO resistivity was 5–10  cm which is comparable to previous reports of conductive NiO films made via spray pyrolysis but somewhat higher than the sputtered NiO films.38 82 The roughness of spin coated NiO film was (RMS roughness 1.4 nm) which is relatively smooth but less smooth as compared to PEDOT:PSS (RMS roughness of less than 0.9 nm).85 Therefore the purpose of planarize the ITO surface is still not well appreciated in the case of NiO. It is likely that further optimization of this parameter will be needed before very high-performance QD-LEDs are achieved. The devices showed very pure quantum light emission and low turn-on voltages. The technique for fabricating sol–gel derived NiO described here provides a facile route to a chemically robust p-type and highly conductive layer, with an energy offset suitable for hole injection into CdSe/ZnS QDs. However, there still remains significant work to optimize the performance of these devices, where as this design holds potential with regards to its inherent simplicity and scalability. Another report is worth considerable, where the QDLED was reported to have output 1600 cd/m2 having the turn-on voltage of 5.5 V.86 The nanocrystals used in this study were 460 nm with a narrow FWHM bandwidth of 20 nm and high brightness and emit pure blue color. It was also pointed out that the residual organic emission of this LED is minimized due to the lower luminescence efficiency of poly-TPD as compared to QDs. A similar study was also carried out that the QD-LED without Alq3 also minimize the residual organic emission.87 The main idea was based on the use of PEDOT:PSS as the buffer layer on the anode to increase the work function of anode (ITO) from 4.7 eV to 5.0 eV and to reduce the surface roughness of anode for stable and pin-holefree electrical conduction across the device.88 In this struggle various color (orange, green, yellow and red) LEDs were obtained with maximum luminances of 3,200 cd/m2 (1.8 cd/A), 3,700 cd/m2 (1.1 cd/A), 4,470 cd/m2 (1.3 cd/A) and 9,064 cd/m2 (2.8 cd/A), respectively. These luminance values were the highest reported until that time. 3179 REVIEW the interface of two dissimilar solvents, i.e., QDs dispersed solution on silicon substrate.72 73 Hence, the injection and recombination efficiencies are increased. The aqueous functionalization step also helped removing the organic trioctyl phosphine oxide (TOPO) layer from the ZnS shell and replace it with a smaller cap of amino ethane thiol which leads to enhanced electroluminescence (EL) due to better exciton transfer to the QDs through the Forster mechanism and/or easier carrier injection into QDs.74 The tu-PBD as ETL and holes blocking layer (HBL) was spin coated from toluene and in this way because of the QD functionalization, none of the solvents had an effect on the previous layer. This was verified by the unchanged photoluminescence (PL) peak intensity before and after the deposition of the next layers. It was claimed that this device showed three-order higher quantum efficiency about 0.2% with less than half of the threshold voltage (about 2.5) compared with the similar device.1 The prepared device showed brightness in excess of 500 cd/m2 and it was increased linearly with the increase in current while no emission was observed in the negative bias until 20 V. This methodology can be used to fabricate multilayer structures, which are most suitable for incorporating different size QDs in organic materials, towards obtaining saturated full-color QDs based lightemitting displays.75 Quantum Dots for Light Emitting Diodes REVIEW Quantum Dots for Light Emitting Diodes By optimizing the thickness of constituent layers in the device they succeeded to build QDs based LEDs with low turn-on voltages (3∼4 V), improved electroluminescent efficiency (1.1∼2.8 cd/A), and long operation lifetimes. It was also reported that the Commission Internationale de l’Enclairage (CIE) coordinates of QD-LEDs emitted lights are (0.653, 0.324), (0.202, 0.676), (0.480, 0.485) and (0.544, 0.418) for red, green, yellow, and orange light, respectively. All these colors are in excess of the highdefinition-television (HDTV) standard 6, 8, and 19 when the coordinates were compared with the color triangle of the National Television System Committee (NTSC). Thus, these efforts represent a clear improvement towards the application of QD based LEDs featuring saturated colors and bright, large area and low cost. However, it suggests that the high-performance, multi-colored QD-LEDs can be achieved by simply controlling the structure of QDs and optimizing the thicknesses of ETL, QDs and HTL layers. Multicolored LED was fabricated by incorporating different sized QDs in-between the organic HTL and ETL and found that 90% of the emission is obtained from the top layer of QDs.89 In this work, a many devices were fabricated with different QDs nanocrystals such as red, orange, yellow–green, and green were placed in different possible positions and the exciton-recombination zone variation was observed. From the device structures (a), (b), and (c) were such that the (a) top QD layer is green and the second, third, fourth, and fifth layers are red. The EL spectrum shows that 90% of total emission is green which is coming from the top QD layer and about 10% of total emission is red attributed to underneath red QD monolayers. In (b), the top QD layer is red, the second is green, and third, fourth, and fifth QD layers are red. Approximately 90% of total emission is red coming from the top red QD layer and about 10% of total emission is green attributed to underneath green QD monolayers. In (c), the top and the second QD layers are red, the third is green, and the fourth and fifth QD layers are red. Approximately 100% of total EL emission is red coming from the top and second red QD layers. Consistent results were observed for other devices with different layers order. This could be because of the direct injection from the ETL or by Forster resonance energy transfer recombined mostly (90%) from the top QD monolayer, adjacent to the ETL (TPBi in this case) and partially (10%) from the second QD monolayer. A similar phenomenon was also previously observed by Sun and his colleagues regarding the optimal QD coverage 1.2 to 1.5 QD monolayers for the best QLED performance.87 90 Up-to-date the best efficient QD-LED is reported with maximum luminance of 4,200 cd/m2 (0.17 lm/W) for blue, 68,000 cd/m2 (8.2 lm/W) for green, and 31,000 cd/m2 (3.8 lm/W) for orange–red.91 This LED was solutions processibly fabricated and ZnO nanoparticle layer was utilized as ETL. The authors state that this high efficiency 3180 Qasim et al. is because of the perfect band matching of the ZnO layer with the electrode and with the QDs layer, which facilitates the efficient injections of electrons from the electrode into the emissive layer as well as the hole confinement due to the valence band offset.92 93 The turn-on voltages for these devices are also lower than the similar nature devices where Alq3 or TiO2 nanoparticles used as ETL.90 94 95 This lower turn-on voltage led to higher power efficiency and better device stability specially to cause the Auger-assisted charge injection, which strongly depends on the level of electron injection into the QD layer, which is the main reason for achieving such lower operating voltages. For the ZnO based LED, more electrons accumulated at the poly-TPD/QD interface, the interfacial recombination rate is much higher than in other similar devices, which also suggests that the Auger process is efficient. Although the same Auger assisted carriers injection process also takes place at the poly-TPD/QD interface in Al-only or Alq3 /Al devices, the voltages required to build up a sufficient concentration of electrons for an efficient Auger process at the polyTPD/QD interface are much higher, presumably due to poor electron transport and/or injection.1 94 96 This efficient electron injection is attributed to the higher electron mobility of ZnO nanoparticles, conduction band alignments of ZnO is with electrode’s Fermi level. Therefore, this band alignment apparently results in low energy barriers for electron injection from the cathode into the EML. 5. FABRICATION TECHNIQUES 5.1. Large-Area LED via Phase Separation Closed packing of QDs in a layer is one of the challenges in the QDs thin film deposition. Therefore different group of researchers are investigating various techniques to achieve the close packed QDs thin films in order to get the 100% EL efficiency. The large area (> cm2  well ordered colloidal QDs monolayers were deposited by phase separation during the spin coating deposition.97 The colloidal QDs used in this study were aliphatically caped in the aromatic organic solvents. Due to the phase separation between the two different materials, the nanocrystals well arrange in close packed monolayer during the solvent drying process. It was stated that this mechanism is capable of deposition of QDs layers at large scale in a 2D sheet and simultaneously yields in QDs monolayers self assembled into hexagonally closed packed arrays and places this monolayer on the top contact. During the drying step, the QDs phase separate and are pushed towards the top contact of the film by the organic underlayer material. When the drying is complete the underlayer organic material is homogenous and coated with QDs layer on the top. It is believed J. Nanosci. Nanotechnol. 13, 3173–3185, 2013 Qasim et al. J. Nanosci. Nanotechnol. 13, 3173–3185, 2013 emission spectrum of -conjugated molecules, typically with FWHM in the range 50 to 100 nm. The fluorescence is caused by the rotational and vibrational motion of atoms inside the -conjugated molecules. So with an organic LED, it is difficult to get pure color (e.g., green light) with high external quantum efficiency. While in case of QD-LEDs it is possible to get the pure color with very narrow FWHM spectrum. But a disadvantage of these LEDs is that they have not yet gained the device durability or external emission efficiency required for practical display applications.100 101 In QDs nanocrystals the charge is confined within small volume of the crystal in such a way that the device can give high quantum efficiency exceeding 50%. However, electroluminescence has been observed by mixing -conjugated polymers with inorganic nanocrystals, but the quantum emission efficiency was found much lower than that of conventional polymer based LEDs.63 67 For the optimal structure of QD-LEDs one of the challenging aspects is how to bring the holes and electrons together in small region of the nanocrystal layer so that they can recombine efficiently to emit photons without dissipating or escaping from the system. If there is a single layer instead of the emissive layer consisting of molecules, holes and electrons may be transferred directly from the surfaces of the HTL and the ETL, and high recombination efficiency is expected. For example a molecular-size emissive layer has been reported in a molecular bilayer of rubrene dye layer and cyanine dye each sandwiched between HTL and ETL of 50 nm thicknesses.102 103 Polymers used as fluorescent material in conventional LEDs by means of statistical mechanics restrictions; fewer than half of the electron–hole recombinations result in light emission. This is the main reason for the low quantum efficiency in such devices.104 But in the case of inorganic nanocrystals that has the potentials that every electron–hole recombination can produce a photon giving much better performance than the conventional LEDs. 5.2. Contact Printing and Inkjet Printing and Transfer Printing Multicolored and patterned thin films of QDs were deposited by print-deposition method. The device was AC operated and claimed that this display was well patterned, bright, robust, and full-color.105 The basic device structure was such that the multi colored QDs were sandwiched between the ITO and phosphorus thin layer. However, in order to minimize wave-guiding behavior of the QD emission in ITO (RI = 20) layer, lower indexing materials such as Al2 O3 (RI = 16) was inserted between the ITO and the QDs layer. To improve the color saturation and purity of the top-surface QD emission, ZnO refractive index of approximately 2.0 in the visible–wavelength range, is a desirable buffer layer between the phosphor 3181 REVIEW that before equilibrium is achieved, this film is morphologically well arranged. However, the phase separation and film properties are not critically depend on single parameter, rather it depends on many factors such as, organic underlayer material, solvents, QDs organic capping group, QDs core material, QDs size aspect ratio, QDs diameter, spin-casting parameters, and environmental conditions. There are the very must conditions for the phase separation compatibility, the solvent or solvent system must dissolve both QDs and organic material the solvent must wet the substrate surface. Keeping the spin casting parameters constant, the thickness of the organic film can be varied by varying the solution concentrations such as, with the increasing TPD concentration from 2 mg/mL to 94 mg/ml the thickness of the underlayer organic film increased from 12 nm to 440 nm. Due to the increase in concentration there is no effect on the QDs coverage. In the same way, by increasing the QDs concentration in the solution, the QDs density increased resulting into the increased film thickness. The size distribution standard deviation also greatly affects the morphology during the phase separation. The high degree of periodicity and smaller grain size and grain boundary can be achieved by varying the size standard deviation sd > 10% to standard deviation sd ≤ 5%. By careful controlling these parameters the QDs monolayers can be controlled which make them suitable for application in LED. The device fabricated using this technique showed highest external quantum efficiency until that time (EQE 2%). It was stated that electrons and holes can be efficiently injected into the QDs nanocrystal layers and so can emit light efficiently. Due to the phase separation organic underlayer becomes the HTL and the top layer of QDs is the responsible for EL. It was believed that the phase separation technique is robust and easy to deal with, which can lead to the optimized QD-LED and other optoelectronic devices. The structure of the device was elaborated by Tsutsui with QDs sandwiched between the hole-blocking-layer and hole-transporting-layer as reported earlier. In this case the nanocrystals layer is sandwiched between the layers of HTL and ETL.98 99 An applied electric field causes holes and electrons to move into the nanocrystal layer, where they are captured in the quantum dots, re-combine, and emitting photons. The spectrum of these photons emission is narrow, characterized by its FWHM value of the peak/spectrum. Emission is caused through fluorescence when electron transitions take place between the orbital states of -conjugated organic molecules (the -bond arises in carbon atoms from the overlap of the 2p orbitals). Highly quantum efficient for electron-to-photon conversion in organic LEDs -conjugated molecules has the advantage of color tunability so that they can be used to make full-color displays with red–green–blue (RGB) sub-pixels. However, it has some drawbacks such as the very broad Quantum Dots for Light Emitting Diodes REVIEW Quantum Dots for Light Emitting Diodes and the QDs nanocrystals layer because it waveguides some of the phosphor electroluminescence. To maximize the QDs luminescence the thicknesses of these layers were proposed to be 50 nm thick for ZnO and 160 nm thick for Al2 O3 . The EL efficiency of these devices were approximately 0.1 lm W−1 for the blue phosphor driven with 420 V rms at 31 kHz, which is comparable to commercially available AC-operating electroluminescence devices.106 In order to deposit only-QDs-layer in the devices contact printing method was adopted because of the some limitations of the spin-casting technique.107 These limitations are (1) it cannot be applied to patterning of QD monolayers and, (2) it places solvent compatibility requirements on the device fabrication process. Using the printing method, the researchers demonstrated EQE of 0.65% for green, 2.3% for red, and 0.35% for blue printed QD-LEDs.96 108 It was believed that this method is more efficient for the LED fabrication however the controllability could be a critical issue in this technique. In the beginning of the last year, Kim et al. have presented full color QDs display fabricated by transfer printing. The inability to achieve size-selective quantum dot patterning by conventional methods hinders the realization of full-color quantum dot displays.109 It was reported for the first time demonstration of a large area, full-color QDs display, including in flexible form, using optimized QDs films, and with control of the carrier behavior and nano-interfaces. However, the integration of quantum dots into a full-color LED structure has not been possible to achieve due to the difficulty in patterning red–green–blue (RGB) QDs individually onto the pixilated display panel. The uniformity and thickness of the assembled QD films are crucial factors in the operation of a large-area, fullcolor display, with the best emissive properties of QD layer being strongly dependent on the pattering of QD films.88 90 Transfer printing technique was adapted to fabricate the device in which kinetically controlled nano-transfer process resulted in the QD films with the excellent surface morphology, clearly defined interfaces, and well-ordered structure required to achieve the full-color display over a large area. This approach among one of the promising techniques for the next-generation displays with high color purity and extremely high resolution, and also for the flexible QD-LEDs. By means of a solvent-free ‘inking’ and stamping process QD film transfer from the donor substrate to the display substrate was carried out. The major difference between the transfer-printed and the spin-coated QD films is the degree of ordering in the structure. The wavelengths at the maximum electroluminescence peak for the printed QD-LED were red 615 nm, green 530 nm and blue 480 nm, with maximum brightness values of 16,380, 6,425 and 423 cd/m2 , respectively. The maximum power efficiency of the printed QD-LEDs for red is 71% (4.25 lm/W), better than the 2.49 lm/W recorded for the 3182 Qasim et al. spin-coated QD-LEDs. These results suggest the better applicability of the large-area process, using various functional materials for the LEDs, electronic devices, photovoltaics, and bio-imaging devices in the flexible form. 6. THE RECENT DEVELOPMENTS AND GOLES 6.1. Innovative QDs, Hybridization, and Cross-Linking Mechanisms In the past two years, researchers have tried different QDs and different possible structures to improve the efficiency of the QDs based LEDs.110–114 Some have synthesized PbSe QDs to replace CdSe for the multilayer architecture.115 116 PbSe quantum dot light emitting diodes (QD-LEDs) of multi-layer architecture exhibited high external quantum efficiencies. In these devices, ligand replacement technique was employed to activate PbSe QDs, and ZnO nanoparticles were used for the electron transport layer with efficiency of 0.73% measured at 1412 nm. The high-quality CdE (E = Te, TexSe1 − xSe) nanocrystals were synthesized at relatively low temperatures (180∼200 degrees C).117 They dispersed in trioctylphosphine oxide (TOPO) and Se dissolved in paraffin oil were used as the chalcogen precursors, in which the use of any air-sensitive, toxic, and expensive starting materials such as trioctylphosphine (TOP) and tributylphosphine (TBP) was eliminated. The structural and optical characterizations of these high quality CdE nanocrystals were improved.118 Wang et al. have synthesized highly-fluorescent core/ multi-shell quantum dot CdSe/CdS/ZnS nanophosphors via a phosphine-free and continual precursor injection method in paraffin liquid for the first time.119 120 The asprepared cubic zinc blended QDs showed high photoluminescent quantum yield (PLQY) and excellent optical stability. They also fabricated QDs/epoxy composites by directly dispersing the core/multi-shell QDs into epoxy resin as the encapsulating and light conversion materials for the application on the white light LED, and these new QDs composites possessed even better stability against UV radiation and heating. Moreover, a white light LED was fabricated by combining QDs/epoxy composites incorporated with green, yellow and red emission QDs with blue InGaN LED, and the resulting four-band RYGB QDWLED. The power efficiency of 32 lm W−1 at 100 mA indicating that the combination of highly fluorescent CdSe/ CdS/ZnS core/multi-shell QDs in LEDs can be a promising solution for white light sources. Polymer encapsulated 3C-SiC quantum dots (QDs) showed strong room-temperature photoluminescence in the wavelength range of 400∼540 nm.121 The QD surface is completely passivated by the polymer so that the QD film J. Nanosci. Nanotechnol. 13, 3173–3185, 2013 Qasim et al. 6.2. Updated QD-LED Structure Possibilities Up-to-date the best efficient QD-LED is reported with maximum luminance of 4,200 cd/m2 (0.17 lm/W) for blue, 68,000 cd/m2 (8.2 lm/W) for green, and 31,000 cd/m2 (3.8 lm/W) for orange–red.91 This LED was solution processibly fabricated and ZnO nanoparticle layer was utilized as ETL. The authors state that this high efficiency is because of the perfect band matching of the ZnO layer J. Nanosci. Nanotechnol. 13, 3173–3185, 2013 with the electrode and with the QDs layer, which facilitates the efficient injections of electrons from the electrode into the emissive layer as well as the hole confinement due to the valence band offset. Another latest work reported the inverted device structure of QD-LED using ZnO as ETL with the organic HTL.130 They demonstrated highly bright red, green, and blue QLEDs showing maximum luminances up to 23040, 218800, and 2250 cd m−2 , and external quantum efficiencies of 7.3, 5.8, and 1.7%, respectively. These devices have the very low turn-on voltages comparable to the bandgap energy of each QD and long operational lifetime, mainly attributed to the direct exciton recombination within QDs through the inverted device structure. These results signify a remarkable progress in QD-LEDs for the realization of QD-based full-color lightings and displays. 7. SUMMARY AND OUTLOOK In this article we discussed the overall development carried out in the history of QD based LEDs from the start to the present. Some serious issues can’t be ignored during the fabrication of QD-LEDs such as the quenching of emitted photons and changing of QDs, which tremendously decrease the external quantum efficiency in the devices. Scientists and engineers are trying different topologies and structures to overcome such problems so that these LEDs can be commercialized. Moreover, the newly discovered possibilities of phosphorescent emitters, making QD-LEDs with simple inorganic nanocrystals opens a new route towards the achieving of 100% external quantum efficiency at any desired wavelength.131 Solution processibility and fabrication of multilayered structures is a challenging fact which needs careful selection of solvents with different polarities. Electrons and holes transporting layers need proper energy-band aligned materials. Organic transporting materials have some lifetime limitations for the operation, therefore, it is recommended to use the inorganic materials for this purpose. The last but not the least, electrodes need to improved in order to improve the overall device efficiency such as cost and external quantum efficiency. Acknowledgment: This work was supported in part by the National Key Basic Research Program 973 (2010CB327705), National Natural Science Foundation Project (50872022, 60801002, 60971017, 51120125001), and the Research fund for International Young Scientists from NSFC (51050110142, 51150110160). References and Notes 1. V. L. Colvin, M. C. Schlamp, and A. P. Alivisatos, Nature 370, 354 (1994). 2. M. C. Schlamp, X. G. Peng, and A. P. Alivisatos, J. Appl. Phys. 82, 5837 (1997). 3183 REVIEW possesses voltage-tunable electroluminescence. The electroluminescence spectrum was blueshifted from 490 to 460 nm when the applied voltage is increased from 5 to 10 V. This electroluminescence tuning is attributed to carrier recombination in the core quantum confinement states as a result of Pool-Frenkel emission. Owing to the nontoxicity, environmental friendliness, stability, and easy fabrication, the polymer passivated 3C-SiC QD thin films have promising applications.6 122–124 Quantum dots (QDs) embedded in poly(N -vinyl carbazole) (PVK) hole transport layer (HTL) were fabricated.125 126 Current densities as functions of the voltage showed enhanced hole trapping and decreased hole current in the LEDs containing CdSe and CdSe/ZnS QDs embedded in HTL. The luminance–voltage curve and the electroluminescence spectra showed that the brightness of the blue LEDs fabricated utilizing the HTL based on CdSe and CdSe/ZnS QDs embedded in PVK layer reached over 3,000 cd/m2 and the dominant exciton peak was shifted to longer wavelength. High performance field-induced AC electroluminescence (EL) in a simple ITO/insulator/hybrid emitter/Au structure was demonstrated with efficient control of the brightness and colors based on solution-processed nanohybrids of CdSe–ZnS core–shell colloidal quantum dots and fluorescent polymers.127 It was reported that for the first time the benzenedithiol cross-linking method was employed to produce highquality PbS nanocrystalline films acting both as electroluminescent layer and an ETL within a near-infrared light emitting diode (LED) architecture.128 Using ethanedithiol cross-linking molecules result in low PL and EL efficiet nanocrystalline films as compared to the benzenedithiol cross-linkers. Due to the good carrier-transport properties within the cross-linked nanocrystalline films and the large volume of nanocrystals, this device architecture yields good quantum efficiencies and high light emission. 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