Improved Electron Injection Research Articles

P‐177: Engineering Host Materials for High Efficiency and Long Operational Lifetime in Blue Phosphorescent Organic Light‐emitting Diodes

Two host materials for blue phosphorescent organic light emitting diodes (PhOLEDs) were developed to exhibit high external quantum efficiency (EQE) and long device lifetime, By substituting strong electron deficient CN moiety in different positions, the highest occupied molecular orbital and the lowest unoccupied molecular orbital energy levels were controlled. Although the CN substitution position is different between the two hosts, both of the host materials showed high triplet energy of 2.98 eV. The CN modification in dibenzothiophene was effective to improve electron injection and high EQE. The new host materials also showed long device operational lifetime when CN modified phenylimidazole ligand based blue phosphorescent emitter was used as a blue triplet emitter.

Enhancement of electron injection in inverted bottom-emitting organic light-emitting diodes using Al/LiF compound thin film

The inverted bottom-emitting organic light-emitting devices (IBOLEDs) were prepared, with the structure of ITO/Al (x nm)/LiF (1 nm)/Bphen (40 nm)/CBP: GIr1 (14%):R-4b (2%) (10 nm)/BCP (3 nm)/CBP:GIr1 (14%):R-4b (2%) (20 nm)/TCTA (10 nm)/NPB (40 nm)/MoO3 (40 nm)/Al (100 nm), where the thickness of electron injection layer Al (x) are 0 nm, 2 nm, 3 nm, 4 nm and 5 nm, respectively. In this paper, the electron injection condition and luminance properties of inverted devices were investigated by changing the thickness of Al layer in Al/LiF compound thin film. It turns out that the introduction of Al layer can improve electron injection of the devices dramatically. Furthermore, the device exerts lower driving voltage and higher current efficiency when the thickness of electron injection Al layer is 3 nm. For example, the current efficiency of the device with 3-nm-thick Al layer reaches 19.75 cd·A-1 when driving voltage is 7 V, which is 1.24, 1.17 and 17.03 times larger than those of the devices with 2 nm, 4 nm and 5 nm Al layer, respectively. The device property reaches up to the level of corresponding conventional device. In addition, all inverted devices with electron injection Al layer show superior stability of color coordinate due to the adoption of co-evaporation emitting layer and BCP spacer-layer, and the color coordinate of the inverted device with 3-nm-thick Al layer only changes from (0.580 6, 0.405 6) to (0.532 8, 0.436 3) when driving voltage increases from 6 V to 10 V.

Blue and green light emitting polyarylpyrazolines luminogens containing anthracene and thiophene units

Polyarylpyrazolines are synthesized by Claisen-Schmidt condensation followed by Suzuki cross-coupling polycondensation. Polyarylpyrazolines (PAPF, PTPF) containing various aryl substituents emit with blue and green light makes them suitable for optoelectronics applications. Photophysical and electrochemical properties are changed by varying monomeric units in the polymer backbone. The polymers are designed to serve as difunctional charge carriers useful as both hole and electron transport materials in PLEDs and achieved by modulating the energy levels through changing segments and their emissive nature turn them as tri-functional polymers. The photophysical properties such as absorption and emission along with lifetime analysis were studied. The polymers are soluble in many organic solvents, hence solvatochromism explored and explained in nonpolar to polar solvents along with Lippert-Mataga plots. It is observed that LUMO level is brought down and imparts improved electron injection and better transport ability. One of fascinating characteristics of these polymers is trouble-free synthetic procedure, nontoxic and tuning emission properties. This approach yields a toolbox of molecules to construct π-conjugated polymers to explore better organic light emitting diode performance.

Rutile versus anatase for quantum dot sensitized solar cell

In this paper, we report comparative studies of CdS/CdSe quantum dot sensitized solar cell (QDSSC) using the phase pure anatase and rutile TiO2 nanocrystals as the photoanode materials, which were prepared through a controlled hydrolysis route. Photovoltaic performance investigations demonstrate that the QDSSC made from rutile TiO2 (R-QDSSC) achieved higher shot-circuit photocurrent density (Jsc) and open-circuit photovoltage (Voc), which leads to a higher power conversion efficiency compared to the cell made from anatase TiO2 (A-QDSSC). Our systematic studies have shown that the use of rutile TiO2 to fabricate QDSSC offers two advantages over anatase TiO2: (1) rutile is more favorable for the growth of CdS/CdSe QDs that leads to the enhancement of light harvesting. (2) rutile is of advantage to inhibit the charge recombination and improve electron injection from QDs to TiO2, ascribable to its superior ability to lower the surface/interface defects in photoanode film together with the larger conduction band offset between QD and rutile TiO2.

All-organic bipolar vertical transistor with sulfonated polyaniline base energy barriers favoring recombination emitter-collector current

Abstract We report the development and characterization of an all-organic vertical transistor, with tris(8-hydroxyquinoline) aluminum (Alq3) as emitter, poly(bithiophene) (PBT), as collector material and a sulfonated polyaniline (SPAN) layer as base material in the structure Au/PBT/SPAN/Alq3/CsO/Al. A thin cesium oxide (CsO) layer provides improved electron injection from the Al into the Alq3. The transistor is operated in the usual common-emitter mode were the output characteristic evidences the modulation of the collector current through the applied voltage, showing common-emitter gain of 160 at 4.5 V. The incremented gain is due to an increased recombination current, consequence of the arrangement of the frontier orbitals of PBT, SPAN and Alq3, which favors the confinement of electrons and holes in the SPAN layer.

Improved electron injection and efficiency in blue organic light-emitting diodes using coupled electric field near cathode

Abstract In this paper, we report high-performance blue organic light-emitting diodes (OLEDs) based on evaporated silver nanoclusters (SNCs) located near the aluminum cathode. We investigate the influence of localized surface plasmon resonance (LSPR) and trapping effects of SNCs on the performances of blue OLEDs depending on the size, distribution, and location of the SNCs. The research results reveal when the SNCs are located in the electron transporting layer, the serious trapping effect would cause reduced current density and efficiency of our blue OLEDs. Fortunately, when the SNCs are close to the cathode, owing to the electric field coupling effect between the LSPR of SNCs and the surface polarization of aluminum cathode, the resonant electric field is concentrated near the cathode, thereby enhancing the electron injection. The balanced carrier injection and transport processes improve the performance of blue OLEDs. The optimized device with SNCs located 2 nm away from the aluminum cathode achieves the highest current efficiency of 40.7 cd/A, enhancing by 15% compared to the control device without SNCs.

Novel Solution‐Processed ZnO‐Based Electron Injection Layer for Organic Light‐Emitting Diodes

A novel solution‐processed zinc oxide (ZnO)‐based electron injection layer (EIL) has been developed with an easy method. The inverted organic light‐emitting diode (OLED) with pristine ZnO as EIL shows improved electron injection property compared to that with thermally evaporated Cs2CO3, and a maximum efficiency of 58.5 cd A−1 has been achieved for a green phosphorescent OLED. However, when the ZnO is applied to a solution‐processed OLED using poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) as emissive layer, it is found that the luminance remains at a low level although the current increases very fast. The low luminance is mainly attributed to the inefficient hole blocking property of ZnO, resulting in unbalanced electron–hole pairs in the emissive layer. Hence, a mixed interlayer of ZnO and polyethyleneimine ethoxylated (PEIE) is introduced to modify the ZnO layer. The optimized device shows a maximum luminance of 11 660 cd m−2, which is more than one order of magnitude higher than that of the device without the interlayer (≈775 cd m−2).

Improved Performance in n‐Type Organic Field‐Effect Transistors via Polyelectrolyte‐Mediated Interfacial Doping

AbstractTo enhance electron injection in n‐type organic field‐effect transistors (OFETs), nonconjugated polyelectrolyte (NPE) layers are interposed between a [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) layer and Au electrodes. A series of NPEs based on an ethoxylated polyethylenimine backbone with various counterions, including Cl−, Br−, and I−, improve electron mobilities up to ≈10−2 cm2 V−1 s−1 and yield on–off ratios (Ion/Ioff) of 105 in PCBM OFETs. Ultraviolet photoelectron spectroscopy reveals that all of the NPEs lead to reduced electron injection barriers (φe) at the NPE/metal interface; this reduction in φe is consistent with dipole formation or n‐type doping at the electrode interface. Absorption measurements of PCBM films treated with NPEs are consistent with n‐doping of the PCBM. Regardless of the type of anion, thick NPE layers lead to high conductivity in the films independent of gate bias, whereas thin NPE layers lead to dramatically improved electron injection and performance. These results demonstrate that thin polyelectrolyte layers can be used to achieve controlled interfacial doping in organic semiconductors. Furthermore, this study provides valuable information about the function of NPEs, which may be exploited to improve device performance and to design new materials for future use in optoelectronic devices.

Extending donor size in D-A-π-A organic dye for dye sensitized solar cells: Anti-aggregation and improving electron injection

We have theoretically designed two D-A-π-A dyes 3 and 4 based on the efficient references 1 and 2 by introducing an extra electron donor unit (D2). Via calculating the electronic structures of isolated dyes, we obtain that dyes 3 and 4 possess stronger light-harvesting efficiency imparted by the fluorescence energy transfer of D2 part, maintain comparable lifetime of excited states, and shorten the electron injection time significantly with regard to 1 and 2. Meanwhile, dye 3 positively shifts the edge of virtual states of TiO2 in a larger extent compared to its counterparts. Then after considering the alignment morphology of multiple dyes adsorbed on TiO2 surface, we find that dyes 3 and 4 manifest the capability of anti-aggregation obviously, which is evidenced by the smaller quantity of intermolecular electronic coupling compared to that of dyes 1 and 2, definitively illustrating the prominent performance of novel dyes with the bulky D2 moiety. Finally, dye 3 is screened out as the potential candidate for future application.

Polythiophenylpyrazoline containing fluorene and benzothiadiazole moieties as blue and white light emitting materials

Low band gap polythiophenylpyrazoline containing fluorene and benzothiadiazole moieties were synthesized and characterized in combination with high donor content (PFPF) and high acceptor content (PBPF). The PBPF displays broad emission as white light dual fluorescence via locally excited state and charge transfer state as single polymer, while PFPF exhibits blue light emission. This dual emission can be utilized to reduce problem with forster energy transfer from high energy to low energy excited state. The polymers are soluble in many organic solvents thus solvatochromism explored in nonpolar to polar solvents. The energy state of polymers were compared with small molecule namely, 1,3-diphenyl-5-(9-phenanthrenyl)-2-pyrazoline (DPPhP) under similar condition. It is observed that LUMO level is brought down and imparts improved electron injection and better transport ability. This approach yield a toolbox of molecules that can be used to construct π-conjugated polymers with a variety of composition lead to explore better organic light emitting diode performance.

Synergistic promotion of photoelectrochemical water splitting efficiency of TiO2 nanorods using metal-semiconducting nanoparticles

Highly efficient photoelectrochemical (PEC) water splitting has been achieved by TiO2 nanorods (TNRs) decorated with Au nanoparticles (AuNPs) and graphene quantum dots (GQDs). The experimental analysis has indicated that the AuNPs has contributed to the plasmon resonance energy transfer/surface plasmon resonance-mediated hot electron injection and the GQDs contributed to the improved electron injection. The synergistic effect, which could be due to exciton-plasmon interactions and/or nonresonance energy transfer between the AuNPs and GQDs, is attributed to the superior PEC activity of the TNRs, which lead to a high photocurrent density of 1.75mAcm−2 at 1.23V vs RHE.

Improved electron injection in all-solution-processed n-type organic field-effect transistors with an inkjet-printed ZnO electron injection layer

Interface engineering for the improved injection properties of all-solution-processed n-type organic field-effect transistors (OFETs) arising from the use of an inkjet-printed ZnO electron injection layer were demonstrated. The characteristics of ZnO in terms of electron injection and transport were investigated, and then we employed ZnO as the electron injection layer via inkjet-printing during the fabrication of all-solution-processed, n-type OFETs. With the inkjet-printed ZnO electron injection layer, the devices exhibited approximately five-fold increased mobility (0.0058cm2/Vs to 0.030cm2/Vs), more than two-fold increased charge concentration (2.76×1011cm−2 to 6.86×1011cm−2), and two orders of magnitude reduced device resistance (120MΩcm to 3MΩcm). Moreover, n-type polymer form smoother film with ZnO implying denser packing of polymer, which results in higher mobility.

2-(4-Butoxyphenyl)-N-hydroxyacetamide: An Efficient Preadsorber for Dye-Sensitized Solar Cells.

The effect of chemical modification of mesoporous TiO2 electrodes by 2-(4-butoxyphenyl)-N-hydroxyacetamide (BPHA) before dye adsorption is investigated in dye-sensitized solar cells (DSCs). Two organic dyes, LEG4 and Dyenamo blue, were used in combination with the cobalt (II/III) tris(bipyridine) redox couple. The photovoltaic performance of the DSCs is clearly enhanced by BPHA. Preadsorption of mesoporous TiO2 electrodes with BPHA lowered the amount of adsorbed dye but improved the short-circuit current densities and the power conversion efficiencies by 10–20%, while keeping the open-circuit potential essentially unaffected. Notably, BPHA improved the LEG4 performance, whereas it has been reported for this dye that chenodeoxycholic acid as a coadsorbent lowers solar cell efficiency. Faster dye regeneration was found to be one reason for improved performance, but improved electron injection efficiency may also contribute to the favorable effect of BPHA.

DFT and TD-DFT investigations of organic dye with different π-spacer used for solar cell

The ground-state geometries and electronic properties of two D–π–A dyes (LEG4 and LEG4F) have been studied by using density functional theory (DFT) and time-dependent density functional theory. LEG4 and LEG4F have the same donor (triphenylamine) and acceptor (carboxylic acid) bridged by different π-spacer. We estimated the light harvesting efficiency, energy levels and electron density, electron accepting power, molar extinction coefficient, electron injection free energy, chemical hardness and reorganization energy. The results demonstrate that LEG4F has higher HOMO energy and lower energy gap, which is benefit to electron transfer. For configuration of LEG4F, it was found that it has a widen absorption spectra and larger maximum absorption strength, bigger light harvesting efficiency and improved electron injection ability in comparison with LEG4. The difference of photoelectric parameters for solar cell has been interpreted from the viewpoint of calculation, which is an important clue for understanding the relationship between structure and performance.

Electron Transport and Nanomorphology in Solution‐Processed Polymeric Semiconductor n‐Doped with an Air‐Stable Organometallic Dimer

This study investigates electron transport and distribution of an organometallic dimer‐based dopant (RuCp*Mes)2 in benchmarked P(NDI2OD‐T2) films, in which electron transport is not affected by deep traps originating from atmospheric contaminants. The electron mobility of P(NDI2OD‐T2) can be enhanced by >10‚ in diodes with reduced thermal activation energy using (RuCp*Mes)2 dopants, which is rationalized by the filling up of tail electronic states by doping induced carriers. n‐doping with (RuCp*Mes)2 can also improve electron injection at Schottky contacts in nanoscale transport measurements confirmed by conducting atomic force microscopy. The results suggest that the (RuCp*Mes)2 dopants are homogenously distributed throughout the P(NDI2OD‐T2) film, at least laterally, at moderate doping concentrations. Thus, these results demonstrate an opportunity of using air‐stable molecular n‐doping to modulate charge transport properties for solution‐processed organic optoelectronic devices.

Structure-Property Relations of Methylamine Vapor Treated Hybrid Perovskite CH3NH3PbI3 Films and Solar Cells.

The power conversion efficiency of halide perovskite solar cells is heavily dependent on the perovskite layer being sufficiently smooth and pinhole-free. It has been shown that these features can be obtained even when starting out from rough and discontinuous perovskite film by briefly exposing the film to methylamine (MA) vapor. The exact underlying physical mechanisms of this phenomenon are, however, still unclear. By investigating smooth, MA treated films based on very rough and discontinuous reference films of methylammonium triiode (MAPbI3) and considering their morphology, crystalline features, local conductive properties, and charge carrier lifetime, we unraveled the relation between their characteristic physical qualities and their performance in corresponding solar cells. We discovered that the extensive improvement in photovoltaic performance upon MA treatment is a consequence of the induced morphological enhancement of the perovskite layer together with improved electron injection into TiO2, which in fact compensates for an otherwise compromised bulk electronic quality simultaneously caused by the MA treatment.

Effect of Injection Layer Sub-Bandgap States on Electron Injection in Organic Light-Emitting Diodes.

It is generally considered that the injection of charges into an active layer of an organic light-emitting diode (OLED) is solely determined by the energetic injection barrier formed at the device interfaces. Here, we demonstrate that the density of surface states of the electron-injecting ZnO layer has a profound effect on both the charge injection and the overall performance of the OLED device. Introducing a dopant into ZnO reduces both the energy depth and density of surface states without altering the position of the energy levels-thus, the magnitude of the injection barrier formed at the organic/ZnO interface remains unchanged. Changes observed in the density of surface states result in an improved electron injection and enhanced luminescence of the device. We implemented a numerical simulation, modeling the effects of energetics and the density of surface states on the electron injection, demonstrating that both contributions should be considered when choosing the appropriate injection layer.

Energy Level Alignment at Bebq2/PEI/ITO Interfaces Studied by UV Photoemission Spectroscopy

ABSTRACTA polyethyleneimine (PEI) interlayer has been applied on indium tin oxide (ITO) to improve electron injection in organic devices including inverted organic light-emitting diodes (OLEDs). To understand the improvement effect by PEI insertion, the energy level alignment at bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2)/PEI/ITO interfaces was investigated by UV photoemission spectroscopy (UPS). The deposition of a PEI layer was found to reduce the absolute work function of ITO by 1.4 eV. The vacuum level shifts at Bebq2/ITO and Bebq2/PEI interfaces were also determined as 0.3 eV and 0.1 eV in the direction to reduce the electron injection barrier, respectively. Thus the work function reduction by PEI and downward vacuum level shift at the Bebq2/PEI interface can contribute to the improvement effect. Kelvin probe measurement revealed the weak orientation polarization in Bebq2 film with the bottom side positively polarized. This polarization polarity is also advantageous for electron injection in inverted devices.

Electrochemical Doping of Halide Perovskites with Ion Intercalation.

Halide perovskites have recently been investigated for various solution-processed optoelectronic devices. The majority of studies have focused on using intrinsic halide perovskites, and the intentional incoporation of dopants has not been well explored. In this work, we discovered that small alkali ions, including lithium and sodium ions, could be electrochemically intercalated into a variety of halide and pseudohalide perovskites. The ion intercalation caused a lattice expansion of the perovskite crystals and resulted in an n-type doping of the perovskites. Such electrochemical doping improved the conductivity and changed the color of the perovskites, leading to an electrochromism with more than 40% reduction of transmittance in the 450-850 nm wavelength range. The doped perovskites exhibited improved electron injection efficiency into the pristine perovskite crystals, resulting in bright light-emitting diodes with a low turn-on voltage.

Synergetic Enhancement of the Photocatalytic Activity of TiO2 with Visible Light by Sensitization Using a Novel Push-Pull Zinc Phthalocyanine

A new one-pot synthesis of a novel A3B-type asymmetric zinc phthalocyanine (AZnPc) was developed. The phthalocyanine complex was characterized unambiguously and used to prepare a TiO2 hybrid photocatalyst to enhance its photocatalytic activity in the visible range. Different compositions of the phthalocyanine dye were tested in order to find the optimum amount of sensitizer to get the highest activity during the photocatalytic tests. The hybrid photocatalyst was characterized by UV-Vis diffuse reflectance (DRS) and Fourier transform infrared spectroscopy (FT-IR) and its photocatalytic activity was compared with that of the individual components considering the effects of sensitization on their efficiency to degrade Rhodamine B as a model reaction. A synergic improvement of the photocatalytic activity for the hybrid system was explained in terms of an improved electron injection from the photo-activated phthalocyanine to the TiO2. Considering the structural features of the phthalocyanine sensitizer and their effect on aggregation, some mechanistic aspects of its binding to TiO2 are suggested to account for the photocatalytic activity enhancement. Finally, the inhibitory effect on the sprouting of chia seeds (Salvia hispanica) was evaluated in order to test the toxicity of the water effluent obtained after the photodegradation process. According to our growth inhibition assays, it was found that the Rh-B degradation by-products do not lead to an acute toxicity.