Hole Injection Layer Research Articles

Enhancing CdSe/ZnS Quantum-Dot Light-Emitting Diode Performance: The Impact of Thermal Treatment Atmospheres on Fabrication Processes

We explored the thermal treatment impact on the performance of quantum-dot light-emitting diodes (QLEDs). The QLEDs comprised multiple layers: a 2.2-μm thick epoxy buffer layer; a bottom cathode composed of 12-nm MoOx/10-nm Ag/12-nm MoOx; a 20-nm ZnO electron transporting layer (ETL); a 10-nm CdSe/ZnS quantum dot light emission layer (EML); a 40-nm 4,4′,4″-Tris(carbazol-9-yl) triphenylamine hole transporting layer; a 10-nm WOx hole injection layer; and a 100-nm Ag top anode. We applied thermal treatments to the cathode, ETL, and EML separately to assess their effects on the QLEDs. Additionally, we evaluated the impact of the thermal treatment atmosphere. Vacuum thermal treatment on the cathode and EML resulted in minor improvements in QLED performance, whereas treatment of the ETL led to a decline in performance. In contrast, air thermal treatment on the cathode and EML decreased QLED performance but significantly improved it by 15% in current efficiency when applied to the ETL. The performance differences attributable to the thermal treatment atmosphere are likely due to ligand removal and oxidation processes, facilitated by thermal energy and oxygen. Our study highlights that air thermal treatment on the ETL substantially improves QLED performance, offering crucial insights into the significance of thermal treatment in QLED development.

Enhanced Efficiency and Stability of Sky Blue Perovskite Light-Emitting Diodes via Introducing Lead Acetate.

All-inorganic metal halide perovskite is promising for highly efficient and thermally stable perovskite light-emitting diodes (PeLEDs). However, there is still great room for improvement in the film quality, including low coverage and high trap density, which play a vital role in achieving high-efficiency PeLEDs. In this work, lead acetate (Pb(Ac)2) was introduced into the perovskite precursor solution as an additive. Experimental results show that perovskite films deposited from a one-step anti-solvent free solution process with increased surface coverage and reduced trap density were obtained, leading to enhanced photoluminescence (PL) intensity. More than that, the valence band maximum (VBM) of perovskite films was reduced, bringing about a better energy level matching the work function of the hole-injection layer (HIL) poly (3,4-ethylenedioxythiophene)-poly (styrene sulfonate) (PEDOT: PSS), which is facilitated for the hole injection, leading to a decrease in the turn-on voltage (Vth) of PeLEDs from 3.4 V for the control device to 2.6 V. Finally, the external quantum efficiency (EQE) of the sky blue PeLEDs (at 484 nm) increased from 0.09% to 0.66%. The principles of Pb(Ac)2 were thoroughly investigated by using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). This work provides a simple and effective strategy for improving the morphology of perovskite and therefore the performance of PeLEDs.

Fluorinated silane self-assembled monolayer modification of V2O5 for enhanced hole injection and device efficiency

This study introduces a new approach to enhance hole injection in organic light-emitting diodes (OLEDs) by modifying the energy band alignment. This enhancement is achieved by incorporating gap states near the Fermi level through the application of a 1H,1H,2H,2H-perfluorooctyltriethoxysilane self-assembled monolayer (F8SAM) coating on V2O5. Compared to thermally evaporated V2O5, the F8SAM coating induces greater oxygen deficiency, reducing the V2O5 (V5+) content from 84.2 % to 76.9 % and increasing the VO2 (V4+) content from 15.7 % to 23.0 %. This modification causes both the highest occupied molecular orbital and gap state levels to move closer to the Fermi level, facilitated by increased oxygen vacancies, high carrier concentration, and delocalized free electrons. The alteration in the gap state provides a more accessible pathway for efficient hole injection, as confirmed in optoelectronic green light devices, which demonstrate a wavelength of 530 ± 5 nm. Significantly, devices incorporating a double-layer hole-injection layer exhibit a 5-fold increase in maximum luminance, a 1.6-fold increase in current efficiency, and a 1.7-fold increase in power efficiency compared to devices with an unmodified V2O5 film. The enhanced performance is attributed to the well-aligned F8SAM on V2O5, which increases the density of hole states and thereby improves device performance. This study demonstrates a practical method for modulating the cation valence state of V2O5, offering a pathway toward improved hole injection and, consequently, more efficient and functional OLED devices.

Enhancement of Perovskite Photodetector Using MAPbI3 with Formamidinium Bromide

In this study, a perovskite-based mixed cation/anion ultraviolet photodetector with an added halide material is fabricated using perovskite combined with an ABX_3 structure. Mixed cation/anion perovskite thin films of MAPbI3/FABr are manufactured through a relatively simple solution process and employed as light-absorption layers. In the produced thin film, SnO2–sodium dodecylbenzenesulfonate acts as an electron transport layer and spiro-OMeTAD acts as a hole injection layer. Compared to a single cation/anion perovskite, the fabricated device exhibits phase stability and optoelectronic properties, and demonstrates a responsivity of 72.2 mA/W and a detectability of 4.67 × 1013 Jones. In addition, the films show an external quantum efficiency of 56%. This suggests that mixed cation/anion films can replace single cation/anion perovskite films. Thus, photodetectors based on lead halides that can be applied in various fields have recently been manufactured.

Thermally Induced Phase Separation of the PEDOT:PSS Layer for Highly Efficient Laminated Polymer Light-Emitting Diodes.

Among various conductive polymers, the poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) film has been studied as a promising material for use as a transparent electrode and a hole-injecting layer in organic optoelectronic devices. Due to the increasing demand for the low-cost fabrication of organic light-emitting diodes (OLEDs), PEDOT:PSS has been employed as the top electrode by using the coating or lamination method. Herein, a facile method is reported for the fabrication of highly efficient polymer light-emitting diodes (PLEDs) based on a laminated transparent electrode (LTE) consisting of successive PEDOT:PSS and silver-nanowire (AgNW) layers. In particular, thermally induced phase separation (TIPS) of the PEDOT:PSS film is found to depend on the annealing temperature (Tanneal) during preparation of the LTE. At Tanneal close to the glass transition temperature of the PSS chains, a PSS-rich phase with a large number of PSS- molecules enhances the work function of the PEDOT:PSS on the glass-side surface relative to the air side. By using the optimized LTEs, bidirectional laminated PLEDs are obtained with a total external quantum efficiency of 2.9% and a turn-on voltage of 2.6 V, giving a comparable performance to that of the reference bottom-emitting PLED based on a costly evaporated metal electrode. In addition, an analysis of the angular characteristics, including the variation in the electroluminescence spectra and the change in luminance according to the emission angle, indicates that the laminated PLED with the LTE provides a more uniform angular distribution regardless of the direction of emission. Detailed optical and electrical analyses are also performed to evaluate the suitability of LTEs for the low-cost fabrication of efficient PLEDs.

Spray pyrolysis–deposited NiO film as a hole-injection layer for CsPbBr3 nanocrystal-based light-emitting diodes

In perovskite-based light-emitting diodes (LEDs), employing an inorganic material for the hole-injection layer instead of an organic material is crucial for attaining optimal performance and device stability. This layer needs to exhibit [Formula: see text] -type conductivity with high carrier mobility while being fabricated with a simple and industrially compatible method. In this study, uniform and thin [Formula: see text] -type NiO films were deposited through spray pyrolysis in an air atmosphere. The as-deposited film showed poor crystallinity, and high resistance reduced hole-injection performance. Subsequent thermal treatment at 500∘C enhanced the crystallinity of the layer, reducing resistance and improving LED performance. The film exhibited [Formula: see text] -type semiconductor characteristics, and its carrier mobility, carrier concentration and resistivity were 53.0 × [Formula: see text]cm3, 116cm2⋅ [Formula: see text] and 0.98[Formula: see text] ⋅ [Formula: see text], respectively. Moreover, a compact [Formula: see text] nanocrystal (NC) film as an emitting layer was deposited with a centrifugal coating technique. The resulting LED component featuring a fluoride-doped tin oxide (FTO)/NiO/[Formula: see text] NCs/tris-(1-phenyl-1H-benzimidazole)/LiF/Au structure and demonstrating high luminance, current efficiency and external quantum efficiency (EQE) reached 12,000 cd⋅[Formula: see text], 8.8 cd ⋅ [Formula: see text] and 3.56%, respectively, at 5.5 V. Our work offers a simple and unique approach for accelerating the development of advanced interfacial materials and circumventing major interfacial problems in solution-processed perovskite NC films for high-performance LEDs.

Enhancing the performance of CdSe/ZnS quantum-dot light-emitting diodes by utilizing hole injection layers fabricated with a solution-processed mixture of vanadium oxide and molybdenum oxide

We developed a high-performance CdSe/ZnS quantum-dot light-emitting diode (QLED) incorporating a hole injection layer (HIL) composed of a solution-processed vanadium oxide (VO) and molybdenum oxide (MoO) mixture. The QLED with a VO:MoO ratio of 1:1 in the HIL demonstrated a current efficiency of 62.0 cd/A, representing a 60% improvement over the QLED utilizing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the HIL. Besides improved performance, the substitution of the highly reactive PEDOT: PSS with chemically stable metal oxides in HIL fabrication is anticipated to enhance device reliability. The highest occupied molecular orbital level of the VO:MoO ratio of 1:1 HIL was higher than that of HILs with other mixing ratios. Additionally, the VO:MoO ratio of 1:1 HIL exhibited higher oxidation number states compared to HILs with other mixing ratios. These characteristics contributed to the superior performance of the QLED with the VO:MoO ratio of 1:1 HIL. Our approach, which employs chemically stable metal oxides as HIL materials for high-performance QLEDs, can advance future QLED technology.

Nickel Oxide Hole Injection Layers for Balanced Charge Injection in Quantum Dot Light-Emitting Diodes.

Quantum dot (QD) light-emitting diodes (QLEDs) are promising for next-generation displays, but suffer from carrier imbalance arising from lower hole injection compared to electron injection. A defect engineering strategy is reported to tackle transport limitations in nickel oxide-based inorganic hole-injection layers (HILs) and find that hole injection is able to enhance in high-performance InP QLEDs using the newly designed material. Through optoelectronic simulations, how the electronic properties of NiOx affect hole injection efficiency into an InP QD layer, finding that efficient hole injection depends on lowering the hole injection barrier and enhancing the acceptor density of NiOx is explored. Li doping and oxygen enriching are identified as effective strategies to control intrinsic and extrinsic defects in NiOx, thereby increasing acceptor density, as evidenced by density functional theory calculations and experimental validation. With fine-tuned inorganic HIL, InP QLEDs exhibit a luminance of 45200cdm-2 and an external quantum efficiency of 19.9%, surpassing previous inorganic HIL-based QLEDs. This study provides a path to designing inorganic materials for more efficient and sustainable lighting and display technologies.

Direct Optical Patterning of MoO3 Nanoparticles and Their Application as a Hole Injection Layer for Solution-Processed Quantum Dot Light-Emitting Diodes

Direct Optical Patterning of MoO₃ Nanoparticles and Their Application as a Hole Injection Layer for Solution-Processed Quantum Dot Light-Emitting Diodes

Double Passivation Using Ultra‐Thin Insulators for Improved Stability and Efficiency of Quasi‐2D Perovskite Light Emitting Diodes

AbstractInterface engineering is a critical parameter for optimal optoelectronic device performance. In quasi‐2D perovskite light emitting diodes (PeLEDs), traditional poly (3,4‐ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) hole injection layer (HIL) is replaced by self‐assembled monolayer (SAM) [2‐(3,6‐dibromo‐9H‐carbazol‐9‐yl) ethyl] phosphonic acid (Br‐2PACz) HIL. The deep highest occupied molecular orbital (HOMO @ −5.73 eV) of Br‐2PACz facilitates the efficient hole‐injection along with a reduction in leakage current by 4–5 orders of magnitude as compared to PEDOT:PSS (HOMO @ −5.00 eV) HIL based optimized PeLEDs. Ultra‐thin SAM‐based Br‐2PACz HIL presents two challenges: i) device reproducibility is hindered by spatial inhomogeneity, and ii) it fails to regulate nonradiative recombination at the perovskite/HIL interface. To address these challenges, the ultra‐thin interlayer of atomic layer deposited aluminum oxide (Al2O3) is utilized between Br‐2PACz, and perovskite which aids in passivating the interfacial states at the HIL/perovskite interface. With the optimized thickness of Al2O3 interlayer (3 nm), the device's external quantum efficiency (EQE) improved from 9.47% to 13.14%, accompanied by luminous efficiency of 42.8 cd A−1. An ultra‐thin interlayer of lithium fluoride (LiF) (2 nm) is implemented on the top side of the perovskite, which can provide 1.5 times increase in the device's LT50 stability.

Enhanced hole transport of nonpolar InGaN-based light-emitting diodes with lateral p-type superlattice doping structure

In this work, we propose a novel structure for nonpolar (10–10)-plane InGaN-based light-emitting diode (LED) using a lateral p-type Al0.2Ga0.8N/GaN superlattice structure as the hole injection layer. The main objective is to increase the hole concentration and facilitate vertical hole injection. The nonpolar InGaN-based LED lacks polarization along the growth plane (10–10), but the lateral direction along [0001] exhibits strong polarization. Therefore, the Al0.2Ga0.8N/GaN superlattice structure, which is periodic along the [0001] direction, induces net polarization charges at the GaN/Al0.2Ga0.8N interface, resulting in increased ionization rates of the acceptors. Additionally, the high-density two-dimensional hole gases formed at the Al0.2Ga0.8N/GaN interfaces along the [0001] direction can efficiently inject vertically into the quantum wells. Based on the numerical simulation results, the proposed LED structure offers improved electrical characteristics, effective hole injection, and enhanced optical performance compared to the nonpolar LED with conventional p-type doping structure.

High-performance blue TADF OLED using two-step heat plasma-treated MoTe2 as a hole-injection layer

In the pursuit to overcome the limitations posed by traditional hole injection layers (HILs), such as PEDOT: PSS, researchers are focusing on innovative strategies to modify electrode/organic interfaces to facilitate charge-carrier injection and reduce the turn-on voltage, particularly in the context of high-efficiency organic light-emitting diodes (OLEDs). Two-dimensional materials show great potential in addressing the energy barrier challenges at electrode/organic interfaces owing to their exceptional optoelectronic properties and robust chemical stability. However, their implementation in OLEDs has been hindered by complex fabrication processes and work function (WF) mismatches. This study presents a novel approach by introducing a magnetron-sputtered MoTe2 as HIL via two-step O2 doping. This strategy enhances the crystallinity of MoTe2 at a relatively low annealing temperature (350 °C), combined with a plasma-treated anode possessing high WF (approximately 5.05 eV), high transmittance (93 % at 478 nm), and low sheet resistance (33 Ω/sq). Consequently, compared with the conventional blue thermally activated delayed-fluorescence OLED using MoO3 as a HIL, the external quantum efficiency of the manufactured device using MoTe2 as a HIL was improved by 57 % and turn-on voltage was reduced to 2.6 V. This study provides a new pathway for overcoming the limitations of conventional solution-based HILs and chemical vapor deposition techniques to industrialize the large-scale manufacturing and commercialization of OLEDs.

External electric field and UV-ozone treated Ti3C2T x -MXene nanosheets and their hole injection tailoring in UV organic light-emitting diodes

Solution-processed 2D nanosheets of Ti3C2T x -MXene have been facilely synthesized with ultrasonic-assisted liquid phase exfoliation. X-ray diffraction, scanning electron microscope, transmission electron microscope, atomic force microscopy and x-ray photoelectron spectroscopy measurements confirm that the Ti3C2T x -MXene nanosheets have been successfully synthesized, showing good film morphology and exceptional electronic properties. The comprehensive properties of Ti3C2T x -MXene nanosheets are systematically optimized by combination with appropriate sizes of Ti3C2T x -MXene nanosheets, external electric field and ultraviolet (UV)-ozone treatment. With Ti3C2T x -MXene nanosheets as hole injection layer and PBD as emissive molecule, UV organic light-emitting diodes (OLEDs) have been configurated and shown superior performance with maximum radiance of 1.47 mW cm−2, maximum external quantum efficiency of 1.084% and electroluminescent peak around 400 nm. The effects of external electric field and UV-ozone treatment on hole injection ability of Ti3C2T x -MXene nanosheets are investigated in details and further analyzed with impedance spectroscopy. Our experiments pave an alternative approach for advancing Ti3C2T x -MXene nanosheets and UV OLEDs.

Charge balance in OLEDs: Optimization of hole injection layer using novel p‐dopants

AbstractCharge balance is one of the most important factors for realizing high performance organic light emitting devices (OLEDs). In this work, we provide a novel strategy to improve the charge balance in OLEDs by optimizing the hole injection layer (HIL) as well as the electron transporting layer (ETL) and thereby controlling the charge carrier supplies in the device. First, we develop a p‐dopant material (PD02), with a lowest unoccupied molecular orbit (LUMO) of −4.63 eV, much shallower than that of the commercial material (PD01) of which the LUMO is −5.04 eV. Nevertheless, this enables us to modulate the supply of holes to the emissive layer through tuning doping concentration. We demonstrate that device performances are significantly improved by employing such a scheme. With a 23% molar doping of PD02, a bottom emission red OLED achieves an external quantum efficiency (EQE) of over 30%, an operating voltage of 3.4 V and a LT95 ~15,000 h at 10 mA/cm2, with a Digital Cinema Initiative P3 (DCI‐P3) chromaticity of CIE (X, Y) = (0.68, 0.32). Moreover, the efficiency roll‐off is suppressed up till ~3500 cd/m2, a desirable feature in display applications. The lateral conductivity of by using such HIL is also found to be much lower than that of PD01, resulting in reduced crosstalk among RGB pixels. Next, a new electron transporting material (ETM‐02) with a deep LUMO of −2.86 eV is also introduced to further optimize the charge balance. Although devices with ETM‐02 shows lower voltage and higher EQE, lifetime is compromised. In order to improve lifetime, additional fine tuning of the charge balance is essential. Finally, a second p‐dopant PD03 with a LUMO of −4.91 eV is added to the HIL to further extend the modulation flexibility in the hole injection. A double‐layer HIL consisting of 8 nm of HTM:16% PD02 and 2 nm of HTM:3% PD03, where the former is in contact with anode, is adopted in the device structure. The bottom emission deep red device achieve EQE over 30%, an operating voltage of 3.2 V and an improved LT95 ~13,000 h at 10 mA/cm2 with a BT.2020 range chromaticity of CIE (X, Y) = (0.701, 0.299). In the double HIL configuration, the introduction of PD03 provides one more parameter for tuning and therefore improves the overall device performances.

Progress and Outlook of 10% Efficient AlGaN‐Based (290–310 nm) Band UVB LEDs

Eco‐friendly and low‐cost aluminum gallium nitride (AlGaN) for the epitaxial growth of ultraviolet‐B (UVB) light‐emitting diodes (LEDs) on c‐Sapphire has the possibility of high external‐quantum efficiency (EQE). In this review paper a special growth techniques for 50% relaxed and 4 μm thick AlGaN buffer layer underneath the multi‐quantum wells (MQWs) are challenged to achieve a maximum internal‐quantum efficiency of 50–57% in 310–290 nm band UVB LEDs. The influence of a thin “Valley” layer in p‐type multi‐quantum barrier electron‐blocking layer on 2D hole generation and injection via intraband tunneling was attempted. Finally, the influence of soft polarized Mg‐doped p‐type Al‐graded AlGaN hole injection layer assisted by excimer laser annealing for better hole injection toward the MQWs was investigated and quite high hole concentration of 2 × 1016 cm−3 and resistivity of 24 Ω‐cm at room temperature was achieved. Consequently, the EQE of transparent 310 and 304 nm UVB LEDs, respectively, reached to a confirmed world record values of ≈5% and ≈10% with light powers of 29 and 40 mW on wafer. This EQE value can surpass 21% if flip‐chip, nanoPSS, photonic crystal, and lens with highly reflective p‐electrodes are incorporated in LED.

Record high external quantum efficiency of 20% achieved in fully solution-processed quantum dot light-emitting diodes based on hole-conductive metal oxides

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been widely used as a hole injection material in quantum dot (QD) light-emitting diodes (QLEDs). However, it degrades the organic materials and electrodes in QLEDs due to its strong hydroscopicity and acidity. Although hole-conductive metal oxides have a great potential to solve this disadvantage, it is still a challenge to achieve efficient and stable QLEDs by using these solution-processed metal oxides. Herein, the state-of-the-art QLEDs fabricated by using hole-conductive MoOx QDs are achieved. The α-phase MoOx QDs exhibit a monodispersed size distribution with clear and regular crystal lattices, corresponding to high-quality nanocrystals. Meanwhile, the MoOx film owns an excellent transmittance, suitable valence band, good morphology and impressive hole-conductivity, demonstrating that the MoOx film could be used as a hole injection layer in QLEDs. Moreover, the rigid and flexible red QLEDs made by MoOx exhibit peak external quantum efficiencies of over 20%, representing a new record for the hole-conductive metal oxide based QLEDs. Most importantly, the MoOx QDs afford their QLEDs with a longer T95 lifetime than these devices made by PEDOT:PSS. As a result, we believe that the MoOx QDs could be used as efficient and stable hole injection materials used in QLEDs.

The Optimization of Hole Injection Layer in Organic Light-Emitting Diodes.

Organic light-emitting diodes (OLEDs) are widely recognized as the forefront technology for displays and lighting technology. Now, the global OLED market is nearly mature, driven by the rising demand for superior displays in smartphones. In recent years, numerous strategies have been introduced and demonstrated to optimize the hole injection layer to further enhance the efficiency of OLEDs. In this paper, different methods of optimizing the hole injection layer were elucidated, including using a suitable hole injection material to minimize the hole injection barrier and match the energy level with the emission layer, exploring new preparation methods to optimize the structure of hole injection layer, and so on. Meanwhile, this article can help people to understand the current research progress and the challenges still faced in relation to the hole injection layer in OLEDs, providing future research directions to enhance the properties of OLEDs.

Multilayered inverted polymer-based light emitting diodes fabricated by meniscus coating and transfer-printing technique

We have developed polymer-based inverted organic LEDs (iOLED) with a multilayered structure using orthogonal solvent, transfer-printing, improved meniscus-coating method that moves back and forth. Reciprocating the glass rod back and forth repeatedly, the material usage of electron-injection layers, polymeric light-emitting layer and polymeric hole-transporting layers were decreased to 1/10 for the film deposited onto the glass slide and to ca. 1/20 for the film deposited onto the elastomer stamp compared with the conventional spin-coating. We obtained higher external quantum efficiency (EQE) and lower operation voltage with solution processable metal oxide hole-injection layer and silver anode. The improvements in the threshold voltage and the maximum EQE were observed for the device with electron-transporting EIL, such as alcohol soluble phenanthroline derivative, which is an effective way to reduce the number of multilayers and the tact time for fabricating iOLEDs.

Diluted PEDOT:PSS for high-performance organic light-emitting devices with thermally activated delayed fluorescence emitters.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a widely used conductive polymer in organic light-emitting devices. However, its strong acidity and fluorescence quenching effect seriously affect the overall device performance. We report a cost-effective method to address the above concerns by diluting PEDOT:PSS with deionized water, which effectively reduced the film thickness and the acidity. Therefore, the fluorescence quenching occurring at the interface was alleviated. Using the modified PEDOT:PSS as the hole injection layer, the external quantum efficiency of the device could be effectively improved by a factor of 81%, reaching a considerably higher value of 23.5%, compared with the device consisting of the original PEDOT:PSS solution used as received.

Advanced HIL strategies in QLEDs: V2O5 and PEDOT:PSS dual-layer for charge balance and electron leakage prevention

Control of crystallinity and electrical properties in V2O5 prevents electron leakage and minimizes electron trapping, ensuring efficient hole injection and long-term device stability.