Electron Injection Material Research Articles

The design of push-pull substituted coronene molecules for optoelectronic applications

The optoelectronic and electronic properties of coronene ([6]circulene) and push-pull coronene molecules were predicted by using different computational chemistry programs. For this aim, the electron/hole reorganization energies, the adiabatic/vertical ionization potentials, the adiabatic/vertical electron affinities, the chemical hardness values, the frontier orbital shapes and energy levels of the coronene and its derivatives were obtained at B3LYP/6-31G(d) level. Then the energies of the studied molecules in S0, S1, and T1 states were calculated at PBE0/TZP level. From the obtained results, the emission values and TADF parameters of the investigated compounds were determined. Based on the theoretically obtained results, it was found that 6ethynyl-coronene and 12ethynyl-coronene molecules can be used as electron transfer materials and 6cyano-coronene and 12cyano-coronene compounds can be utilized as hole transfer materials. Additionally, it was noted that 6NO2-coronene and 12NH2-coronene derivatives can be good candidates for electron blocking materials, while 6NH2-coronene and 12NH2-coronene molecules can be utilized as hole blocking materials. Furthermore, it was emphasized that both 6NH2-coronene and 12NH2-coronene molecules can be good candidates as both electron injection and hole injection materials. Lastly, it was reported that 6ethynyl-coronene and 12cyano-coronene structures can be considered the most suitable candidates for near infrared organic emitting diodes.

Enhancing the Light Output‐Coupling of Inverted Top‐Emitting Organic Light‐Emitting Diodes by Using the Localized Surface Plasmon Resonance of Ag Nanoparticles

AbstractIn this study, a light out‐coupling layer (LOCL) composed of Ag nanoparticles (NPs) and MoOx is introduced on top of the inverted top‐emitting organic light‐emitting diode (OLED). The full width at half maximum (FWHM) is narrowed to 50 nm due to the micro‐cavity effect of the top‐emitting structure and the turn‐on voltage is as low as 2.6 V. The AgNPs in the LOCL significantly improves the light output‐coupling of the OLED owing to the localized surface plasmon resonance, and the MoOx in the LOCL effectively protects the water‐ and oxygen‐sensitive electron injection material. As a result, the performance of the OLED with the LOCL is significantly enhanced. The maximum current efficiency and external quantum efficiency of the device are increased by 53% and 30% to 28.27 cd A−1 and 13.09%. The enhanced electroluminescent performance of the OLED, together with the narrowed FWHM and the low turn‐on voltage, provides a new way to realize ultra‐high‐definition display and micro‐display.

Ytterbium oxide electron injection interface in organic light-emitting diode

The ytterbium oxide (Yb3+) is found to have an extremely low work function of 2.42 eV, which is even lower than that of its metallic form Yb0 (2.64 eV). The stability of oxides makes Yb3+ an ideal electron injection material for both top-emitting and bottom-emitting organic light-emitting diodes (TOLED and BOLED). The device test data indeed show that at 1000 nit luminance, the TOLED has a 94 cd/A current efficiency and 70 lm/W power efficiency, and BOLED has a 76 cd/A and 60 lm/W efficiency, respectively. X-ray and ultraviolet photoemission spectroscopical studies indicate that the Fermi level of the metal oxide is pinned to the lowest unoccupied molecular orbital of the electron transport layer, leading to the formation of a cathode interface with an ultra-low electron injection barrier.

Characteristics of electron injection at the oxide electrode/polyethylenimine ethoxylated/Alq3 interface

Inverted organic light-emitting diodes (inverted OLEDs) require electron injection to an organic semiconductor from a transparent oxide electrode. Polyethylenimine ethoxylated (PEIE) has attracted considerable attention as an electron injection material. An injection mechanism has been suggested; however, the barrier height of electron injection has not been determined. In this paper, we present the experimental values for the electron injection barrier height at the transparent oxide electrode/PEIE/organic semiconductor interface. Electron-only devices, consisting of indium-tin-oxide (ITO)/PEIE/tris(8-hydroxyquinolinato) aluminum (Alq3)/Al, are fabricated. The temperature dependence of the current–voltage curves is measured corresponding to the electron injection of the ITO/PEIE/Alq3 interface. The current–voltage curves are found to be independent of the measurement temperature, which is explained by the tunneling model. The tunneling injection barriers height are calculated, and the experimental injection barrier height will be important for the development of inverted OLED devices.

Green-synthesized, low-cost tetracyanodiazafluorene (TCAF) as electron injection material for organic light-emitting diodes

Two electron-deficient azaacenes including di- and tetra-cyanodiazafluorene (DCAF and TCAF) with the advantages of deep lowest unoccupied molecular orbital (LUMO), green-synthesis, low-cost, simply purification method, excellent yields have been obtained, characterized and used as electron injection materials (EIMs) in three groups of electroluminescence devices. Device B with TCAF as EIM exhibited the best performance including turn-on voltage of 5.0 V, stronger maximum luminance intensity of 31,549 cd/m2, higher luminance efficiency of 62.34 cd/A and larger power efficiency of 21.74 lm/W which are 0.53, 6.7, 9.3 and 15.3 times than that of device A with DCAF as EIMs, respectively. The enhanced interfacial electron injection ability of TCAF than that of DCAF is supported by its better electron mobility in electron-only device, deeper LUMO (-4.52 eV), and stronger electronic affinity. Best external quantum efficiency of 16.56% was achieved with optimized thicknesses of TCAF as EIM and TPBi as electron transporting layer. As a new comer of acceptor family, TCAF would push forward organic electronics with more fascinating and significant applications.

Perovskite light-emitting diodes based on n-type nanocrystalline silicon oxide electron injection layer

Organometal halide perovskites featuring solution-processable characteristics, high photoluminescence quantum yield (PLQY), and color purity, are an emerging class of semiconductor with considerable potential applications in optoelectronic devices. Electron injection layer is an important component of perovskite light-emitting device, which determines the growth of perovskite film directly. In this paper, the perovskite light-emitting diodes (PeLEDs) based on n-type nanocrystalline silicon oxide (n-nc-SiO<sub><i>x</i></sub>:H) electron injection layer are designed and realized. This novel electron injecting material is prepared by the plasma enhanced chemical vapor deposition (PECVD), and its smooth surface and matched energy band result in superior perovskite crystallinity and low electron injection barrier from the electron injecting layer to the emissive layer, respectively. However, the external quantum efficiency (EQE) of PeLED is as low as 0.43%, which relates to defects and leakage current due to the incomplete surface coverage of perovskite film. The fast exciton emission decay (< 10 ns) stems from strong non-radiative energy transfer to the trap states, and represents a big challenge in fabricating high-efficiency PeLEDs. In order to obtain desirable perovskite film morphology, an excessive proportion of methylammonium bromide (MABr) is incorporated into the perovskite solution, and a volume of benzylamine (PMA) is added into the chlorobenzene antisolvent. The perovskite films suffer low PLQY and short PL lifetime if only MABr or PMA is introduced. When the molar ratio of MABr is higher than 60%, the luminescence quenching arising from Joule heating is depressed by employing PMA, contributing to a higher PLQY (> 30%) and a longer carrier lifetime. The synergistic effect of MABr and PMA increase the coverage and reduce the trap density of perovskite film, inhibit the luminescence quenching in the annealing process, and thus facilitating the perovskite film with higher quality. Finally, the n-i-p PeLED exhibits green-light emission with a maximum current efficiency of 7.93 cd·A<sup>-1</sup> and a maximum EQE up to 2.13% is obtained. These facts provide a novel electron injecting material and a feasible process for implementing the PeLEDs. With further optimizing the perovskite layer and device configuration, the performance of n-i-p type PeLEDs will be improved significantly on the basis of this electron injection material.

Silver-induced activation of 8-hydroxyquinolinato lithium as electron injection material in single-stack and tandem OLED devices

Abstract The 8-hydroxyquinolinato lithium (Liq)|Ag system has been evaluated as the electron-injection layer for the MoOx| N,N′-di (1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) charge-generation layer in tandem organic light-emitting diode (OLED) devices. From current-voltage studies of single-stack devices, it is determined that Liq is a relatively poor electron-transport material. However, Liq is found to be useful as an electron-injection layer with activation by Ag. Efficient tandem devices have been obtained using Liq|Ag as the electron-injection layer adjacent to the MoOx|NPB charge-generation layer, where the optimal Ag thickness is about 20–40 A and the activation of Liq can be seen with as little as 1 A of Ag.

New Class of Wide Energy Gap Benzotriimidazole Optical Materials

A new class of wide energy gap benzotriimidazole materials have been synthesized by a two-step condensation reaction. All of the benzotriimidazole compounds have π-π* absorption bands in the range of 250–400 nm. The photoluminescence (PL) quantum efficiency of each benzotriimidazole depends strongly on the presence of electron withdrawing groups. PL quantum efficiencies of benzotriimidazoles without electron withdrawing groups were less than desirable (40–43%), while molecules with electron withdrawing groups displayed much stronger PL with efficiencies in the range of 73–75%. The electron withdrawing groups shift the emission to a longer wavelength, towards a more “true blue” color. This new class of benzotriimidazole optical materials could be used as electron-injecting and electron-transporting blue luminescence materials for potential organic light-emitting diode (OLED) applications.

Organic Radicals Outperform LiF as Efficient Electron-Injection Materials for Organic Light-Emitting Diodes.

One of the key issues for organic light-emitting diodes (OLEDs) is to achieve high electroluminescence efficiency and high power efficiency, which requires extremely efficient electron injection and thus low driving voltage. Here, we design a series of precursors for reactive organic radicals according to theoretical calculations and achieve efficient electron injection by using a highly reducing radical on the surface of the electron injection layer to reduce the electron injection barrier through an interface charge-transfer process. In contrast to bulk charge transfer in electron-transporting material, interface charge transfer allows us to make efficient electron injection at contact without introducing any structural and electronic disorder to electron-transporting material. 2-(2,4,6-Trimethoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium (R3), with the strongest electron-donating ability, could largely reduce the electron injection barrier and outperform the previously reported organic radical (2-(2-methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium, o-MeO-DMBI or R1) and the widely used electron injection material (LiF) to boost device performance.

Synthesis, Structural Characterization, and Field-Effect Transistor Properties of n-Channel Semiconducting Polymers Containing Five-Membered Heterocyclic Acceptors: Superiority of Thiadiazole Compared with Oxadiazole.

Five-membered 1,3,4-oxadiazole (OZ) and 1,3,4-thiadiazole (TZ) heterocycle-based copolymers as active layer have long been ignored in solution-processable n-channel polymer field-effect transistors (PFETs) despite the long history of using OZ or TZ derivatives as the electron-injecting materials in organic light-emitting devices and their favorable electron affinities. Herein, we first report the synthesis and PFETs performance of two n-channel conjugated polymers bearing OZ- or TZ-based acceptor moieties, i.e., PNOZ and PNTZ, where simple thiophene units are utilized as the weak donors and additional alkylated-naphthalenediimides units are used as the second acceptors. A comparative study has been performed to reveal the effect of different heterocyclic acceptors on thermal properties, electronic properties, ordering structures, and carrier transport performance of the target polymers. It is found that both polymers possess low-lying LUMO values below -4.0 eV, indicating high electron affinity for both heterocycle-based polymers. Because of strong polarizable ability of sulfur atom in TZ heterocycle, PNTZ exhibits a red shift in maximal absorption and stronger molecular aggregation even in the diluted chlorobenzene solution as compared to the OZ-containing PNOZ. Surface morphological study reveals that a nodule-like surface with a rough surface morphology is observed clearly for PNOZ films, whereas PNTZ films display highly uniform surface morphology with well interconnected fiber-like polycrystalline grains. Investigation of PFETs performance indicates that both polymers afford air-stable n-channel transport characteristics. The uniform morphological structure and compact π-π stacking endow PNTZ with a high electron mobility of 0.36 cm2 V-1 s-1, much higher than that of PNOZ (0.026 cm2 V-1 s-1). These results manifest the feasibility in improving electron-transporting property simply by tuning heteroatom substitutes in n-channel polymers; further demostrate that TZ derivatives possess much superior potential for developing high-performance n-channel polymers compared to OZ derivatives.

Significant Performance Enhancement in Asymmetric Supercapacitors with Redox Additive Aqueous Electrolyte and Trade-Off Between Capacitance/Cycling at Elevated Temperatures

Supercapacitors (SCs) are now competing with Li-ion batteries for large scale use in various niche technologies owing to their distinctive merits of rapid charging-discharging process, long lifespan, superior durability, high specific power and low maintenance. As we develop better understanding of the thermodynamics, kinetics and ion transport mechanism in electrode materials used in supercapacitors, rapid growth in this energy storage technology is envisaged. In addition to electrode materials, supercapacitor geometries and configurations will also have to be investigated to bring step change in device performance. For example, development of asymmetric supercapacitors (ASCs) has seen tremendous growth in recent times. Most used SCs today are mostly fabricated using nano-structured transition metal oxides (TMOs). TMOs such as MoO3, V2O5 and WO3 with higher work function or electron chemical potential act as hole-injection materials and hold great promise for application as negative electrode materials. In comparison, TMOs like ZrO2, MnO2 and SnO2, etc with low work function or electron chemical potential behave like electron-injection materials and are mostly suitable for positive electrodes. Such TMOs have rich redox chemistry (oxidation/reduction, intercalation/de-intercalation, chemisorption, etc.) but overcoming their limited specific power remains a challenge. As a result, composite of TMOs with multiwall carbon nanotubes (MWCNTs) is becoming popular. The operating voltage window of an asymmetric cell is a convoluted effect of overpotential provided by the electrolytes and the difference of work functions of negative (Φn) and positive (Φp) electrodes i.e., Φn-Φp. Therefore, ASCs fabricated using TMOs with a large difference in their respective work functions and neutral aqueous electrolytes (having highly solvated ions) may be operated up to voltages as high as 2.2 V. The methodology of carefully unbalancing the device has also been recently proposed to increase the operating voltage window. In this paper, we show a novel strategy i.e., use of optimized concentration of redox additive electrolyte to bring significant enhancement in the specific energy whilst maintaining power of asymmetric supercapacitors. Very few studies have been undertaken to explore the use of redox additives in the 3-electrode or symmetric cells. It is shown that the charge-balanced ASCs can be operated up to 2.2 V leading to specific energy and power as high as ~65 Wh kg-1 and ~950 W kg-1, respectively. The specific energy value is significantly enhanced on addition of the optimized quantity of redox additive viz., potassium iodide (KI). More specifically, increase of ~105% in the specific energy value was observed with good cyclic stability even after 3,000 charge-discharge operations. With such high specific energy and power values, the proposed ASCs have the capacity for large scale integration in applications such as portable electronics devices, back-up power supplies, hybrid electric vehicles and energy harvesting devices. In most of these applications, temperature effect of the storage device will be important. The fabricated ASCs show stable performance upto 70oC, which make them ideal for above mentioned applications. It is shown that the temperature mainly affects the capacitance fade during galvanostatic charging-discharging. The improvement in the specific capacitance at elevated temperatures is strongly governed by the activation energy of diffusions for ionic species and chemical potential of ionic species. Besides, the capacitance fade at higher temperatures is primarily attributed to (a) the total “time spent” at a given temperature during cycling and (b) a reduced kinetic barrier for iodine/iodide redox pairs at the positive electrode/electrolyte interface. This report provides significant information about the effect of temperature on the electrochemical performance of ASCs based on redox additive aqueous electrolytes.

Performance Enhancement of Organic Light–Emitting Diodes Using Electron–Injection Materials of Metal Carbonates

Abstract Performance of organic light-emitting diodes was investigated depending on the electron-injection materials of metal carbonates (Li2CO3 and Cs2CO3 ); and number of layers. In order to improve the device efficiency, two types of devices were manufactured by using the hole-injection material (Teflon-amorphous fluoropolymer -AF) and electron-injection materials; one is a two-layer reference device ( ITO/Teflon-AF/Alq3/Al ) and the other is a three-layer device (ITO/Teflon-AF/Alq3/metal carbonate/Al). From the results of the efficiency for the devices with hole-injection layer and electron-injection layer, it was found that the electron-injection layer affects the electrical properties of the device more than the hole-injection layer. The external-quantum efficiency for the three-layer device with Li2CO3 and Cs2CO3 layer is improved by approximately six and eight times, respectively, compared with that of the two-layer reference device. It is thought that a use of electron-injection layer increases recombination rate of charge carriers by the active injection of electrons and the blocking of holes.

31-4: Novel Inorganic Electron Injection and Transport Materials Enabling Large-Sized Inverted OLEDs Driven by Oxide TFTs

Inverted OLEDs are fabricated using two novel transparent amorphous oxide semiconductors, C12A7 electride with work function of 3.0eV for EIL and ZnO-based materials with 3.5eV for ETL. Both materials form an Ohmic contact with conventional cathode (ITO or Al). The performance of inverted devices employing these two materials is superior to that of normal type with conventional LiF/Al cathode. This material combination enables fabrication processes involving an exposure to ambient atmosphere.

Li2CO3 As an n-Type Dopant in Enhancing the Electron Injection of Organic Light-Emitting Devices

In this article, we demonstrated the OLED incorporating a transport layer composed of lithium carbonate (Li2CO3) doped into the 2-methyl-9, 10-di (2-naphthyl) anthracene (MADN) as electron injection material. In our devices, an OLED with n-doping structure ITO/NPB (40nm)/Alq3 (40nm)/Li2CO3: MADN (5nm, 25%)/LiF (1nm)/Al (140nm) accomplished maximum luminance of 20760 cd/m2 and current efficiency of 4.878 cd/A. By contrast, the device without doping Li2CO3 ITO/NPB(40nm)/Alq3 (40nm)/ MADN (5nm)/LiF (1nm)/Al (140nm) achieved a maximum luminance of 15530 cd/m2 and current efficiency of 3.922 cd/A. The device of the standard one ITO/NPB(40nm)/Alq3 (40nm)/LiF (1nm)/Al (140nm) only achieved a maximum luminance of 18450 cd/m2 and current efficiency of 3.134 cd/A. The experiment results show that the transport layer with appropriate doping can efficiently enhance the electron injection, leading to more balanced concentration of electrons and holes. Compared with the standard sample, the maximum luminance and current efficiency are increased by12.5% and 55.6%, respectively. Figure 1

Improvement of Electron Injection by a Ramp-Shape N-Doped Architecture for High-Efficiency Organic Light Emitting Diodes

Over the past decades, organic light-emitting diodes (OLEDs) have drawn a lot of attention due to its numerous advantages over light-emitting diodes (LEDs) and liquid crystal display (LCDs). These characteristics make OLED become one of the most promising candidates for commercial applications such as solid state lighting and active matrix organic light-emitting diode (AMOLED) display. However, device properties such as lifetime, efficiency, power consumption are still the main issues needed to overcome for their practical uses. In order to fulfill a highly power-efficient light source or display with OLEDs, the device energy loss during electron–photon conversion should be low. Recent years, there are plenty of approaches proposed to improve the electron or hole injection from the electrodes and reduce the ohmic losses. The most typical way is inserting a chemically p- or n-doped layer between the hole transporting layer (HTL) and anodes or between the electron transporting layer (ETL) and the cathodes, respectively. In our work, we report OLED with a novel n-doped ramp-shape electron injection structure composed of rubidium carbonate (Rb2CO3) doped 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) which is prepared by thermal co-evaporation. We increase the doping concentration from 17.5 mol% to 84 mol% while depositing the electron injection materials approaching the cathode. By increasing the doping concentration, the activation energy of the material decreases as the Fermi level shifting toward the lowest unoccupied molecular orbital (LUMO). Under thermal equilibrium conditions, a ramp-shape architecture is formed and gradually distribute the energy barrier from cathode to electron transport layer in the device therefore achieve significant electron injection improvement. As a result, a OLED with the ramp-shape electron injection structure ITO/NPB (40nm)/Alq3 (40nm)/Rb2CO3:MADN (14nm, 17.5~84 mol%)/LiF (1nm)/Al (140nm) accomplished maximum luminance of 33370 cd/m2 and current efficiency of 5.292 cd/A. By contrast, the device without the ramp-shape architecture (the normal device) ITO/NPB (40nm)/Alq3 (40nm)/LiF (1nm)/Al (140nm) only achieved a maximum luminance of 17030 cd/m2 and current efficiency of 3.41 cd/A. The total improvement (55% by current efficiency and 95% by luminance) made by adding ramp-shape structure to device indicated a remarkable electron injection facilitation and its performance is much more efficient than the device with typical electron injection layer ITO/NPB (40nm)/Alq3 (40nm)/Rb2CO3:MADN (5nm, 64 mol%)/LiF (1nm)/Al (140nm). Figure 1

Using an organic radical precursor as an electron injection material for efficient and stable organic light-emitting diodes

Materials with strong reducibility have been used as electron injection layers (EILs) to lower the work function of cathodes and reduce the driving voltage of organic light-emitting diodes (OLEDs). However, the most prominent electron injection materials presented so far are high-temperature-evaporable inorganic salts based on alkaline metals, which suffer from a high tendency of metal diffusion throughout the organic layer and thus reduce the device efficiency and stability. Here, we introduce a new kind of EIL based on a stable precursor of a strongly reducing organic radical. By using an organic precursor, we are able to take the advantage of the low-evaporation-temperature and avoid the problem of metal diffusion, thus improving the device efficiency and stability. Ultraviolet photoelectron spectroscopy (UPS) study indicates that inserting a thin layer of organic radical between the electron transport layer and cathode could greatly reduce the electron injection barrier due to the strong interaction of radical with cathode and the electron transporting material. As a result, OLEDs with an organic radical as the EIL showed a 25.2% higher efficiency and 2.2 times longer lifetime than the control device with conventional LiF as the EIL.

Simple pyridine hydrochlorides as bifunctional electron injection and transport materials for high-performance all-solution-processed organic light emitting diodes

Simple hydrochloric acid treatment of pyridine-containing precursors enable the resulting pyridine hydrochlorides to act as solution-processed bifunctional electron injection/transport materials.

Poly(pyridinium iodide ionic liquid)-based electron injection layers for solution-processed organic light-emitting devices

Solution-processable poly(pyridinium iodide ionic liquid)-based electron-injection materials achieved more effective electron injection than the conventional Li-complex in OLEDs.

QM/MD simulations on the role of SiO2in polymeric insulation materials

SiO2is an efficient electron-injection material and can help stabilize polyethylene in electric insulation materials.

Fully solution-processed and multilayer blue organic light-emitting diodes based on efficient small molecule emissive layer and intergrated interlayer optimization

Efficient and fully solution-processed blue organic light-emitting diodes (OLEDs) based on fluorescent small-molecule and methanol/water soluble conjugated polymer as electron-injection material are reported. The emitting layer is 3,6-bis(9,9,9′,9′-tetrakis (6-(9H-carbazol-9-yl)hexyl)-9H,9′H-[2,2′-bifluoren]-7-yl)dib-nzo[b, d]thiophene 5, 5-dioxide (OCSoC) with a blue-fluorescent small-molecule, and a methanol/water soluble polymer poly[(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)] (PFN) acted as electron-injection layer (EIL). All the organic layers are spin-coated from solution. The multilayer device structure with emitting layer/electron-injection layer is achieved by solution-processed method without the dissolution problem between layers. The performances of the devices show that the maximum luminous efficiency of the multilayer device is increased about 43%, compared to the single-layer device. PFN acting as the EIL material plays a key role in the improvement of the device performance when used in solution-processed small-molecule OLEDs.