Charge Injection Layers Research Articles

Ambipolar Charge Injection and Bright Light Emission in Hybrid Oxide/Polymer Transistors Doped with Poly(9‐Vinylcarbazole) Based Polyelectrolytes

AbstractLight‐emitting transistors (LETs) are a remarkable, emerging class of electronic devices that combine the switching function of field‐effect transistors (FETs) and the light‐emitting function of light‐emitting diodes (LEDs). In order to achieve efficient light emission, effective electron and hole injection from source and drain electrodes is necessary. Various strategies have been introduced to accomplish this, such as incorporating asymmetric electrodes or charge injection layers during device fabrication. These approaches have inevitably introduced complexity in the device fabrication process. Herein, light‐emitting electrochemical transistors (LECTs) are demonstrated that combine principles of electrochemistry and optoelectronics to achieve multi‐functionality in a simple device architecture. Hybrid polyelectrolytes, poly(9‐vinylcarbazolesulfonate)‐ lithium and copper (II) salts (PVK‐Li and PVK‐Cu) incorporating Li+ ion and Cu2+ ions are added at variable concentrations to the organic emitting layer of LECTs to effect electrochemical p‐type doping. This electrochemical doping approach yielded improvements in electrical and optical performances including mobilities, brightnesses, and external quantum efficiency of the LECTs. The dynamics of how charges including ions, electrons, and holes move and interact are discussed in the device to facilitate emissive charge carrier recombination and light emission. This investigation provides valuable insights into the realms of both electrochemistry and optoelectronics.

Architectural design, fabrication techniques, characteristics parameters and different applications for OLED along with some OTFT driven OLEDs: A review

In consumer electronics, Organic LED (OLED) has become mainstream display technology. Using organic materials, opto-electronics devices have become extensively desirable for various reasons. One of the fundamental properties i.e., flexibility permits to fabricate electronic circuits on flexible substrates to make these devices bendable and stretchable. This paper provides a review on various terms of OLED like fabrication methods, operation of OLED, its categorization, few OTFT driven OLEDs, stability issues of white OLED and various applications of OLED based on sensors, display, and lighting. Different lighting devices like incandescent bulb, tube light, CFL, LED and OLED are compared on the basis of their efficiency and lifetime. The comparison highlights that the LED provides good lifetime, however, for OLED it depends on the organic semiconducting materials responsible for emission. Different layers such as charge injection layers, transport layers and blocking layer to refine the properties of organic LEDs are also studied and compared. Addition to this, a low-cost methodology is also incorporated for the fabrication of flexible devices.

Robust Dipolar Layers between Organic Semiconductors and Silver for Energy-Level Alignment.

The interface between a metal electrode and an organic semiconductor (OS) layer has a defining role in the properties of the resulting device. To obtain the desired performance, interlayers are introduced to modify the adhesion and growth of OS and enhance the efficiency of charge transport through the interface. However, the employed interlayers face common challenges, including a lack of electric dipoles to tune the mutual position of energy levels, being too thick for efficient electronic transport, or being prone to intermixing with subsequently deposited OS layers. Here, we show that monolayers of 1,3,5-tris(4-carboxyphenyl)benzene (BTB) with fully deprotonated carboxyl groups on silver substrates form a compact layer resistant to intermixing while capable of mediating energy-level alignment and showing a large insensitivity to substrate termination. Employing a combination of surface-sensitive techniques, i.e., low-energy electron microscopy and diffraction, X-ray photoelectron spectroscopy, and scanning tunneling microscopy, we have comprehensively characterized the compact layer and proven its robustness against mixing with the subsequently deposited organic semiconductor layer. Density functional theory calculations show that the robustness arises from a strong interaction of carboxylate groups with the Ag surface, and thus, the BTB in the first layer is energetically favored. Synchrotron radiation photoelectron spectroscopy shows that this layer displays considerable electrical dipoles that can be utilized for work function engineering and electronic alignment of molecular frontier orbitals with respect to the substrate Fermi level. Our work thus provides a widely applicable molecular interlayer and general insights necessary for engineering of charge injection layers for efficient organic electronics.

Effects of charge generation layers on multiple guest/host bilayer-based tandem OLEDs

Compared to OLEDs with a guest/host system as the light emitting layer (EML), in which guest molecules are doped or mixed into the host molecule layer, those with multiple guest/host bilayers as the EML has the advantage of uniform distribution of guest-to-host molecule ratio throughout the entire emission layer thickness. Herein, we report on the tandem OLED based on multiple guest/host bilayers as the EML, and the effects of charge generation layers (CGL) on device performance for the first time. With four pairs of vacuum evaporated host of 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) and guest of bis(1-phenylisoquinoline) (acetylacetonate) iridium(III) (Ir(piq)2(acac)) as the EML, two types of tandem OLEDs with 8-Hydroxyquinolinolato-lithium (Liq)/MoO3 or Liq/2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT-CN) bilayer as the charge generation layer, respectively, were investigated. The results showed that the device with Liq/MoO3 as CGL (T1) exhibited higher current efficiency but lower power efficiency than the one with Liq/HAT-CN as the CGL (T2). Single carrier devices with either Liq/MoO3 or Liq/HAT-CN as the charge injection layer were fabricated to investigated charge injection ability of the CGLs. The results show that Liq/MoO3 CGL exhibit both lower electron and hole injection ability than Liq/HAT-CN, which is the origin of higher turn-on volage and lower power efficiency of T1 than T2. However, the balance degree of electron and hole injection of Liq/MoO3 is better than Liq/HAT-CN, resulting higher current efficiency of T1 than T2. These results provide a novel strategy towards fabricating high performance tandem OLEDs.

Organic- inorganic nanoparticle composite as an electron injection/hole blocking layer in organic light emitting diodes for large area lighting applications

The charge carrier balance in organic light-emitting diodes (OLEDs) is crucial for optimizing external quantum efficiency (EQE). This is typically achieved by using charge injection and blocking layers deposited by vacuum deposition, which increases the fabrication time and cost. This study investigates the potential of polyethylenimine ethoxylated (PEIE), Zinc oxide (ZnO) nanoparticles and a PEIE-ZnO nanocomposite as functional layers for OLEDs with improved electron injection and hole blocking properties fabricated by solution processing. The PEIE-ZnO nanocomposite improves OLED performance: (a) a 28% increase in current density and enhanced electron mobility, (b) two times increase in electron mobility of the electron injection/transport layer, and (c) a reduction in surface and interface trap density. Consequently, the PEIE-ZnO nanocomposite shows an 84% increase in electroluminescence, 52.5% increase in luminous efficacy, 35.5% increment in external quantum efficiency, and increased stability as measured by degradation studies. Employing these interlayers fabricated by the novel and roll-to-roll compatible Spray-on-Screen deposition technique show a comparable OLED performance as that of spin coated and vacuum deposited electron injection layers, which makes the large area fabrication of optoelectronic devices with high efficiencies, high output, low cost, and long lifetime a possible reality.

Experimental and Theoretical Evidence of Charge Injection Barrier Control by Small-Molecular Charge Injection Layer and Its Effects on Organic–Inorganic Complementary Inverters

Introducing a contact charge injection layer at the interface between the contact electrode and active materials is considered crucial to obtain efficient charge injection behaviors in thin-film transistors (TFTs). Here, we investigate the effect of a small molecule contact charge injection layer through experimental results and theoretical calculations. We found limited electrical characteristics derived from contact resistance in C8-BTBT TFTs with C8-BTBT as the channel and Au as the electrode. We experimentally demonstrated that the limited electrical characteristics of C8-BTBT TFT can be improved by inserting dinaphtho[2,3-b: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2^{\prime} $ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3^{\prime} $ </tex-math></inline-formula> -f]thieno [3,2-b] thiophene (DNTT), which has a low contact resistance at the junction with Au, as a charge injection layer. The results of improved contact properties through the DNTT charge injection layer were also consistent with the results of energy structural simulations. Furthermore, we verified the effect of the DNTT charge injection layer at the circuit level through improved noise margin characteristics in the complementary inverter composed of asymmetric charge injection layer TFT (ACIL-TFT) and a-indium gallium zinc oxide (IGZO) TFT.

Investigation of quantum dots light emitting diodes with different transition metal oxide as charge injection layers

In this paper, various transition metal oxides serve as charge injection layers being prepared by solution process for the fabrication of quantum dot light emitting diodes (QLEDs) is demonstrated. Magnesium doped zinc oxide (MgZnO) nanoparticles is used as electron injection and transport layer, copper oxide (CuO x ), nickel oxide (NiO x ), cobalt oxide (CoO x ) and molybdenum oxide (MoO x ) are used as hole injection layer (HIL) for the comparison with Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), PEDOT:PSS. The fabricated red QLEDs with PEDOT:PSS possess highest maximum luminance of 88,713 cd/m 2 and current efficiency of 4.96 cd/A, whereas QLEDs with MoO x and NiO x HILs demonstrate lower turn on voltage of 1.5V and 2.1V and maintain decent maximum luminance of 59,699 and 64,244 cd/m 2 , respectively. The sub-bandgap turn-on voltage of QLED with MoO x can be attributed to thermal-assisted energy up-conversion effect due to its lower series resistance. • Various solution process metal oxides are used as QLEDs charge injection layers. • QLEDs with NiO x and MoO x HILs demonstrate lower turn on and forward voltage. • MoO x device achieve a sub-bandgap turn-on voltage of 1.5 V for red QLEDs.

Tunable Energy-Level Alignment in Multilayers of Carboxylic Acids on Silver

Precise energy-level alignment between a metal electrode and an organic semiconductor is required to reduce contact resistance and enhance the efficiency of organic-semiconductor-based devices. Here, we introduce monolayer-thick charge-injection layers (CILs) based on aromatic carboxylic acids that can induce an energy-level shift in subsequent layers by up to 0.8 eV. Through a gradual chemical transformation of the as-deposited molecules, we achieve a highly tunable energy-level shift in the range of 0.5 eV. We reveal that the work function and energy-level positions in the CIL increase linearly with the density of induced dipoles. The energy-level position of subsequent layers follows the changes in the CIL. Our results thus connect the energy-alignment quantities, and the high tunability would allow precise tuning of the active layers deposited on the CIL, which marks a path towards efficient charge-injection layers on metal electrodes.

High‐Performance Inverted Tandem OLEDs with the Charge Generation Layer based on MoO x and Ag Doped Planar Heterojunction (Advanced Optical Materials 16/2022)

A high-performance inverted tandem organic light-emitting diode (OLED) based on a charge generation layer integrated with MoOx− and Ag-doped planar heterojunction is reported by Wei Shi, Bin Wei, Wai-Yeung Wong and co-workers in article number 2200984. The charge injection layer enhances the concentration of the charge carrier and balances the charge injection. The work provides a simple strategy to fabricate inverted tandem OLEDs with high performance.

Fluorinated Graphene Contacts and Passivation Layer for MoS2 Field Effect Transistors

AbstractRealizing a future of 2D semiconductor‐based devices requires new approaches to channel passivation and nondestructive contact engineering. Here, a facile one‐step technique is shown that simultaneously utilizes monolayer fluorinated graphene (FG) as the passivation layer and contact buffer layer to 2D semiconductor transistors. Monolayer graphene is transferred onto the MoS2, followed by fluorination by XeF2 gas exposure. Metal electrodes for source and drain are fabricated on top of FG‐covered MoS2 regions. The MoS2 transistor is perfectly passivated by insulating FG layer and, in the contacts, FG layer also acts as an efficient charge injection layer, leading to the formation of Ohmic contacts and high carrier mobility of up to 64 cm2 V−1 s−1 at room temperature. This work shows a novel strategy for simultaneous fabrication of passivation layer and low‐resistance contacts by using ultrathin functionalized graphene, which has applications for high performance 2D semiconductor integrated electronics.

Chiral Optoelectronic Functionalities via DNA-Organic Semiconductor Complex.

We fabricate the bio-organic field-effect transistor (BOFET) with the DNA-perylene diimide (PDI) complex, which shows unusual chiroptical and electrical functionalities. DNA is used as the chirality-inducing scaffold and the charge-injection layer. The shear-oriented film of the DNA-PDI complex shows how the large-area periodic molecular orientation and the charge transport are related, generating drastically different optoelectronic properties at each DNA/PDI concentration. The resultant BOFET reveals chiral structures with a high charge carrier mobility, photoresponsivity, and photosensitivity, reaching 3.97 cm2 V-1 s-1, 1.18 A W-1, and 7.76 × 103, respectively. Interestingly, the BOFET enables the definitive response under the handedness of circularly polarized light with a high dissymmetry factor of approximately +0.14. This work highlights the natural chirality and anisotropy of DNA material and the electron conductivity of organic semiconducting molecules to be mutually used in significant chiro-optoelectronic functions as an added ability to the traditional OFET.

Inkjet‐Printed Ternary Oxide Dielectric and Doped Interface Layer for Metal‐Oxide Thin‐Film Transistors with Low Voltage Operation

AbstractAdditive solution process patterning, such as inkjet printing, is desirable for high‐throughput roll‐to‐roll and sheet fabrication environments of electronics manufacturing because it can help to reduce cost by conserving active materials and circumventing multistep processing. This paper reports inkjet printing of YxAl2−xO3 gate dielectric, In2O3 semiconductor, and a polyethyleneimine‐doped In2O3 interfacial charge injection layer to achieve a thin‐film transistor (TFT) mobility (μsat) of ≈1 cm2 V−1 s−1 at a low 3 V operating voltage. When the dielectric material is annealed at 350 °C, plasma treatment induces low‐frequency capacitance instability, leading to overestimation of mobility. On the contrary, films annealed at 500 °C show stable capacitance from 1 MHz down to 0.1 Hz. This result highlights the importance of low‐frequency capacitance characterization of solution‐processed dielectrics, especially if plasma treatment is applied before subsequent processing steps. This study progresses metal‐oxide TFT fabrication toward fully inkjet‐printed thin‐film electronics.

Critical role of additive-induced molecular interaction on the operational stability of perovskite light-emitting diodes

Summary Despite rapid improvements in efficiency and brightness of perovskite light-emitting diodes (PeLEDs), the poor operational stability remains a critical challenge hindering their practical applications. Here, we demonstrate greatly improved operational stability of high-efficiency PeLEDs, enabled by incorporating dicarboxylic acids into the precursor for perovskite depositions. We reveal that the dicarboxylic acids efficiently eliminate reactive organic ingredients in perovskite emissive layers through an in situ amidation process, which is catalyzed by the alkaline zinc oxide substrate. The formed stable amides prohibit detrimental reactions between the perovskites and the charge injection layer underneath, stabilizing the perovskites and the interfacial contacts and ensuring the excellent operational stability of the resulting PeLEDs. Through rationally optimizing the amidation reaction in the perovskite emissive layers, we achieve efficient PeLEDs with a peak external quantum efficiency of 18.6% and a long half-life time of 682 h at 20 mA cm−2, presenting an important breakthrough in PeLEDs.

Tuning hole transport layers and optimizing perovskite films thickness for high efficiency CsPbBr3 nanocrystals electroluminescence light-emitting diodes

Metal halide perovskite nanocrystals (NCs) have sparked considerable attentions in the area of light-emitting diodes (LED) by virtue of their remarkable color purity and spectral tunability (400–700 nm). However, the optoelectronic performance of LED devices based on perovskite NCs is severely limited by the problem of charge injection since (i) the charge injection layers exhibit significant differences in energy levels and charge mobility, (ii) one thick NCs emitting layer can obstruct the electrons transmission and one thin NCs emitting layer can change the charge recombination region, inducing series of troubles in fabrication and application of LED devices. Herein, a series of LED devices based on cesium lead bromide (CsPbBr3) green-emitting perovskite NCs are reported by adopting Poly-N-vinylcarbazole (PVK)/Poly [9,9-dioctylfluoreneco-N-[4-(3-methylpropyl)]diphenylamine] (TFB) and Poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) as the hole transport layer (HTL) materials. With the increase of applied voltage, the CsPbBr3 NCs LED device with PTAA as HTL displays better transport characteristics, such as a lower turn-on voltage and a higher luminance, than that of the device with TFB and PVK as HTL. By optimizing the spin-coating craft parameter of the CsPbBr3 NCs layer, an efficient green-emitting CsPbBr3 NCs LED device with maximum luminance of 4531 cd m−2, current efficiency of 14.4 cd A−1, power efficiency of 14.1 lm W−1 and maximum external quantum efficiency of 4.28% was obtained. These results provide an approach for the practical applications of CsPbBr3 NCs electroluminescence LED devices.

Prominent Heat Dissipation in Perovskite Light-Emitting Diodes with Reduced Efficiency Droop for Silicon-Based Display.

Solution-processed perovskite light-emitting diodes (LEDs) possess outstanding optoelectronic properties for potential solid-state display applications. However, poor device stability results in significant efficiency droop partly being ascribed to Joule heating when LEDs are operated at high current densities. Herein, we used monocrystal silicon (c-Si) as the substrate and a charge injection layer to alleviate the thermal affection in perovskite LED (PeLED). By incorporating silicon oxide (SiOx) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB) layers to tune the charge injection balance in a c-Si-based device, a PeLED achieves an external quantum efficiency of 2.12% with a current efficiency of 6.06 cd A-1. Benefiting from excellent heat dissipation of c-Si, the PeLEDs display reduced efficiency droop and extended operational lifetime. Furthermore, both electroluminescent (EL) dynamic information and static pattern displays of a c-Si-based PeLED have been successfully demonstrated. These results reveal the feasibility of potential practical c-Si-based PeLEDs with reduced efficiency droop for EL display applications.

Light‐Emitting Electrochemical Cells Based on Color‐Tunable Inorganic Colloidal Quantum Dots

AbstractLarge‐area, ultrathin light‐emitting devices currently inspire architects and interior and automotive designers all over the world. Light‐emitting electrochemical cells (LECs) and quantum dot light‐emitting diodes (QD‐LEDs) belong to the most promising next‐generation device concepts for future flexible and large‐area lighting technologies. Both concepts incorporate solution‐based fabrication techniques, which makes them attractive for low cost applications based on, for example, roll‐to‐roll fabrication or inkjet printing. However, both concepts have unique benefits that justify their appeal. LECs comprise ionic species in the active layer, which leads to the omission of additional organic charge injection and transport layers and reactive cathode materials, thus LECs impress with their simple device architecture. QD‐LEDs impress with purity and opulence of available colors: colloidal quantum dots (QDs) are semiconducting nanocrystals that show high yield light emission, which can be easily tuned over the whole visible spectrum by material composition and size. Emerging technologies that unite the potential of both concepts (LEC and QD‐LED) are covered, either by extending a typical LEC architecture with additional QDs, or by replacing the entire organic LEC emitter with QDs or perovskite nanocrystals, still keeping the easy LEC setup featured by the incorporation of mobile ions.

Improving the performance of dye-sensitized solar cells by incoporating a novel ZnO:Al charge injection layer

In this study, the effect of integrating a ZnO:Al (AZO) charge injection layer into dye-sensitized solar cells (DSSCs) is investigated. The AZO charge injection layer was deposited using a low-cost sol-gel deposition method directly onto fluorine-doped tin oxide (FTO) conducting glass just before TiO2 photoanode coating. The structural and optoelectronic properties of AZO charge injection layer are then investigated as a function of the precursor concentration. The integration of AZO charge injection layer leads to the improvement of power conversion efficiency (PCE) by preventing the electrolyte coming in contact with FTO and improving charge injection from the charge AZO layer. It was found that there is an optimum AZO precursor concentration for charge injection layer deposition and DSSCs fabricated using higher concentration resulted in reduced photovoltaic performance due to reduced light transparency, whilst charge injection layer deposited at lower precursor concentration were found to reduce DSSC performance due to insufficient converage.

A Guide for Realizing Efficient Polymer Light‐Emitting Electrochemical Cells in a Single Active Layer Device Structure

AbstractHere, we systematically examine the impact of electrode work function, ionic species, and charge injection layers on the operation of polymer light‐emitting electrochemical cells (PLECs) based on p‐type Super Yellow. To achieve PLECs yielding a more intensive luminance compared to what can be achieved from the non‐ionic light‐emitting device counterpart, i. e., the polymer light‐emitting diode (PLED), in an analogous device structure, the experimental results here guide the following lessons. First, the use of charge injection layer does not necessarily yield a PLEC that outperforms its PLED counterpart but rather complexes the device structure. Second, injection of minority electrons is critical for enhancing the luminescence characteristics of p‐type polymer based LECs, and the enhanced electron injection is achieved more effectively by modifying the work function of the cathode than by changing the cations. Third, for p‐type PLECs with a low‐work function cathode, careful selection of anions can drastically change the device performance, which are involved in the p‐type electrochemically doping process.

General bis(fluorophenyl azide) photo-crosslinkers for conjugated and non-conjugated polyelectrolytes

Photo-patterning and crosslinking of polymer semiconductors, charge injection and/or transport layers are essential to improve the performance and/or functionality of organic semiconductor devices.

Analytical modelling and parameters extraction of multilayered OLED

This research study investigates electrical performance of the multilayered organic light-emitting diode (OLED) with a focus on the role of charge injection, transport and emission layers. Device parameters; luminescence and current density are extracted using Silvaco ATLAS numerical device simulator and validated through the fabricated experimental results with a minor deviation of 3%. Furthermore, a mathematical model is applied to extract other device parameters such as electric field, charge carrier mobility, concentration and current density. Additionally, the multilayered device architecture is critically analysed through cut line methodology to better comprehend the internal device physics in terms of hole-electron mobility, concentration and their recombination. Subsequently, the performance parameters extracted using analytical model are compared with the results of internal analysis and a close match is observed. These results prove the Poole-Frenkel mobility-dependent behaviour in the OLEDs that varies following electric field. Analyses also highlight high electron and hole concentrations in the vicinity of the emission layer as a reason of high luminescence in the multilayered OLED, directly following the Langevin's theory of recombination in organic semiconductors. These analyses highlight the impact of different layers in the OLEDs and thus open up new horizons to further performance improvement in these devices.