Hole Injection Energy Barrier Research Articles

Alcohol‐Soluble n‐Type Polythiophenes as Cathode Interlayer in Organic Photodetectors for Hole Blocking

AbstractFor organic photodetectors (OPDs), cathode interlayer (CIL) with low‐lying highest occupied molecular orbital (EHOMO) energy level is required to reduce dark current density (Jd). In this study, toward application as CIL in OPDs, an alcohol‐processable n‐type conjugated polymer (n‐PT8) based on imide/cyano‐substituted polythiophene backbone and branched oligo(ethylene glycol) side chain is developed. Owing to the electron‐accepting imide/cyano groups on the backbone, n‐PT8 exhibits deep‐lying EHOMO of −5.80 eV and a high conductivity of 5.12 × 10−6 S cm−1. OPD with n‐PT8 as CIL exhibits dark current density one order of magnitude lower than that with the state‐of‐the‐art CIL material because of the large energy barrier for reverse hole injection. Moreover, the device performance is insensitive to the n‐PT8 layer thickness, which is very desirable for the mass production of OPDs. The near‐infrared (NIR) OPDs based on n‐PT8 exhibit an ultra‐low dark current density of 1.76 × 10−9 A cm−2 and a specific detectivity of 2.88 × 1012 cm Hz1/2 W−1 at −1 V. To the best knowledge, this is the first report on n‐type polythiophene derivatives as CIL in organic opto‐electronic devices.

Double hole transport layers deliver promising-performance in light-emitting diodes based on MAPbBr3 nanocrystals

Perovskite Nanocrystal Light Emitting Diodes (PNC-LEDs) are highly regarded as promising candidates for future solid-state lighting and high-definition display applications. To achieve high-performance LEDs, it is essential to have an emitting layer with a high photoluminescence quantum yield (PLQY) and excellent carrier transport properties, along with a well-optimized device structure. In this work, high performance MAPbBr3 NCs (PLQY>95 %) were synthesized via combination of 4-methoxyphenethylamine and didodecyldimethylammonium bromide, which lead to improved carrier transport of NC emitting layer. Moreover, a double hole transport layers (D-HTL) structure was developed to provide stepwise energy barrier for hole injection, resulting in improved external quantum efficiency (EQE). The green MAPbBr3 PNC-LED with the D-HTL structure exhibited a maximum EQE of 10.7 % and a current efficiency (CE) of 32.5 cd A−1, outperforming the PNC-LED with the conventional single HTL PTAA (EQE 5.8 %, and CE 17.7 cd A−1) by almost 2 times. These findings showcase a rational strategy to enhance PNC-LEDs performance through device structure engineering, particularly when utilizing a high-performance PNC emitting layer.

Utilizing VO2 as a Hole Injection Layer for Efficient Charge Injection in Quantum Dot Light-Emitting Diodes Enables High Device Performance.

Quantum dot light-emitting diodes (QLEDs) are promising devices for display applications. Polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS) is a common hole injection layer (HIL) material in optoelectronic devices because of its high conductivity and high work function. Nevertheless, PEDOT:PSS-based QLEDs have a high energy barrier for hole injection, which results in low device efficiency. Therefore, a new strategy is needed to improve the device efficiency. Herein, we have demonstrated a bilayer-HIL using VO2 and a PEDOT:PSS-based QLED that exhibits an 18% external quantum efficiency (EQE), 78 cd/A current efficiency (CE), and 25,771 cd/m2 maximum luminance. In contrast, the PEDOT:PSS-based QLED exhibits an EQE of 13%, CE of 54 cd/A, and maximum luminance of 14,817 cd/m2. An increase in EQE was attributed to a reduction in the energy barrier between indium tin oxide (ITO) and PEDOT:PSS, caused by the insertion of a VO2 HIL. Therefore, our results could demonstrate that using a bilayer-HIL is effective in increasing the EQE in QLEDs.

Key factors affecting contact resistance in coplanar organic thin-film transistors

We present a comprehensive numerical analysis of contact resistance in coplanar organic thin-film transistors. A large number of hole-transporting organic transistors are investigated through two-dimensional finite-element simulation, by deliberately changing the channel length, source/drain electrode thickness, and hole-injection energy barrier heights. Gate-field-dependent terminal contact resistances of these devices are fully estimated and electrostatic distributions inside the organic semiconductor film are visualized for the understanding of physical mechanisms. It is found that the relationship between source/drain electrode thickness and contact resistance does not follow any simple trend and is also strongly associated with the injection energy barrier. Moreover, the origin of negative contact resistance in organic transistors featuring a minimal charge-injection barrier is elaborated. Finally, a direct impact of the semiconductor charge-carrier mobility on contact resistance is addressed, revealing a linear dependence of contact resistance on inverse mobility over a broad parameter range.

Hole transport and current–voltage characteristics of PEO/PVP/cobalt nitrate polymer blend electrolytes

AbstractHole transport properties and Current–Voltage characteristics of the single‐layer device in an inverted structure were investigated with spin‐coated PEO/PVP/Cobalt nitrate polyblend electrolyte film as an active layer. A strong correlation between morphology and ionic conductivity was confirmed through XRD, DSC‐TGA, SEM and fluorescence spectroscopic techniques. The optical band gap values are in good agreement with the electrochemical band gap. Schottky diode measurements confirmed the formation of p‐type semiconductors with PEO/PVP bulk films with cobalt nitrate as a dopant at room temperature. The I‐V characteristics are strongly dependent on the salt concentration in the polymer blend. The parameters of diodes such as barrier height, saturation current, and charge carrier concentration were calculated from I‐V characteristics. From C‐V characteristics a charge carrier concentration in the range of 1022 cm−3 was obtained for the highest concentration of cobalt nitrate salt in the PEO/PVP blend. A very low energy barrier for hole injection at the anode ΔEh = 0.28 eV was observed.

Cadmium-Doped Zinc Sulfide Shell as a Hole Injection Springboard for Red, Green, and Blue Quantum Dot Light-Emitting Diodes.

A new strategy is developed in which cadmium‐doped zinc sulfide (CdZnS) is used as the outermost shell to synthesize red, green, and blue (RGB) quantum dots (QDs) with the core/shell structures of CdZnSe/ZnSe/ZnS/CdZnS, CdZnSe/ZnSe/ZnSeS/CdZnS, and CdZnSe/ZnSeS/ZnS/CdZnS, respectively. Firstly, the inner ZnS and ZnSe shells confine the excitons inside the cores of QDs and provide a better lattice matching with respect to the outermost shell, which ensures high photoluminescence quantum yields of QDs. Secondly, the CdZnS shell affords its QDs with shallow valence bands (VBs). Therefore, the CdZnS shell could be used as a springboard, which decreases the energy barrier for hole injection from polymers to QDs to be below 1.0 eV. It makes the holes to be easily injected into the QD EMLs and enables a balanced recombination of charge carriers in quantum dot light‐emitting diodes (QLEDs). Thirdly, the RGB QLEDs made by these new QDs exhibit peak external quantum efficiencies (EQEs) of 20.2%, 19.2%, and 8.4%, respectively. In addition, the QLEDs exhibit unexpected luminance values at low applied voltages and therefore high power efficiencies. From these results, it is evident that CdZnS could act as an excellent shell and hole injection springboard to afford high performance QLEDs.

Sequential Growth of 2D/3D Double-Layer Perovskite Films with Superior X-Ray Detection Performance.

Perovskite materials in different dimensions show great potential in direct X‐ray detection, but each with limitations stemming from its own intrinsic properties. Particularly, the sensitivity of two‐dimensional (2D) perovskites is limited by poor carrier transport while ion migration in three‐dimensional (3D) perovskites causes the baseline drifting problem. To circumvent these limitations, herein a double‐layer perovskite film is developed with properly aligned energy level, where 2D (PEA)2MA3Pb4I13 (PEA=2‐phenylethylammonium, MA=methylammonium) is cascaded with vertically crystallized 3D MAPbI3. In this new design paradigm, the 3D layer ensures fast carrier transport while the 2D layer mitigates ion migration, thus offering a high sensitivity and a greatly stabilized baseline. Besides, the 2D layer increases the film resistivity and enlarges the energy barrier for hole injection without compromising carrier extraction. Consequently, the double‐layer perovskite detector delivers a high sensitivity (1.95 × 104 μC Gyair −1 cm−2) and a low detection limit (480 nGyair s−1). Also demonstrated is the X‐ray imaging capacity using a circuit board as the object. This work opens up a new avenue for enhancing X‐ray detection performance via cascade assembly of various perovskites with complementary properties.

Highly Efficient and Super Stable Full-Color Quantum Dots Light-Emitting Diodes with Solution-Processed All-Inorganic Charge Transport Layers.

High performance and super stable all-inorganic full-color quantum dot light-emitting diodes (QLEDs) are constructed by adopting solution-processed Mg-doped NiOx (Mg-NiOx ) nanoparticles as hole transport layer (HTL) and Al-doped ZnO (AZO) as electron transport layer (ETL). Mg-NiOx nanoparticles possess the advantages of low-temperature solution processability, intrinsic stability, and controllable electronic properties. UV-ozone (UVO) treatment is applied to the Mg-NiOx film to modulate its surface composition. By carefully controlling the UVO treating time, favorable energy levels can be achieved to minimize the energy barrier for hole injection. At the cathode side, Al-doping can reduce the conductivity of ZnO ETL and decrease the interface charge transfer, effectively, thus leading to more balanced charge injection and consequent high luminance and efficiency. The maximum luminance and EQE can reach as high as 38444cd m-2 and 5.09% for R-QLEDs, 177825cd m-2 and 10.1% for G-QLEDs, and 3103cd m-2 and 2.19% for B-QLEDs. The luminance values are the highest ever reported for all-inorganic QLEDs. Furthermore, the all-inorganic devices show much better resistance to water and oxygen existing in air. The results show that the ion-doped NiOx and AZO nanoparticles would facilitate the design and development of highly efficient and super stable QLEDs.

A Color Tunable Quantum-Dot Light-Emitting Diode Device Driven by Variable Voltage.

A hole transporting layer (HTL) consisting of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine)] doped with 4,4'-Bis(9H-carbazol-9-yl)biphenyl was designed for high-performance multicolor quantum-dot light-emitting diode (QLED) fabricated by an all-solution process technique. The band structure of the HTL is tailored via small molecule doping to create a cascade of highest occupied molecular orbital levels (HOMO) from the hole injection layer to the quantum-dot emissive layer (EML). This energy band tailoring significantly overcomes the energy barrier for hole injection and enables multicolor emission from a single device, which consists of mixture of red, green, and blue QDs as EML. The color tunability as a function of applied voltage is the most novel feature of our device. The color tunability was observed at a voltage of 2 V for red emission, 3 V for orange, 4 V for yellow and 6 V for greenish white and a high brightness of 15,000 cd m-2 was demonstrated for white emission, which is attributed to the efficient and balanced carriers' injection into the EML. The strategy of using tunable HOMO of the HTL in combination with color tunability of the EML as a function of applied voltage, pushes QLED a step closer to practical multicolor display applications.

All-solution-processed quantum-dot light emitting diodes with nickel oxide nanoparticles as a hole injection layer

Solution-processed nickel oxide (NiO) nanoparticles (NPs) were used as a hole injection layer (HIL) to enhance the performance of quantum dot light-emitting diodes (QLEDs). NiO NP solution was synthesized by a sol-gel method. We synthesized NiO NPs of sizes 10 nm, 20 nm, and 30 nm by controlling the annealing temperature of an NiO NP powder-like precipitate. The different-sized NiO NPs were applied on QLEDs as HILs, and exhibited good hole injection properties. All layers of the QLEDs except the electrodes were fabricated using a solution process. The QLED with 10 nm NiO NPs had the lowest turn-on voltage of 2.5 V, indicating that NiO NP HIL based on 10 nm NPs reduced the energy barrier for hole injection into the hole transport layer (HTL) most. The maximum luminance and luminous efficiency of the QLED with NiO NPs of 10 nm were 17,818 cd/m2 and 7.11 cd/A, respectively.

Mixture interlayer for high performance organic-inorganic perovskite photodetectors

Organic-inorganic perovskites are promising light absorbing active materials for photodetectors; however, the performance of current organic-inorganic perovskite-based photodetectors are limited by the high dark current due to hole injection at the cathode interlayer typically composed of fullerene derivatives. We have developed a mixture interlayer by simply blending polymethyl methacrylate (PMMA) with [6,6]-phenyl-C61-butyric acidmethyl ester (PCBM). Scanning Kelvin probe microscopy imaging reveals that the presence of PMMA reduced the work function of the PCBM:PMMA interlayer, which leads to increased energy barrier for hole injection and better hole-blocking property. Optimized perovskite photodetector with PCBM:PMMA hole-blocking interlayer exhibits a high detectivity of 1.1 × 1013 Jones, a broad linear dynamic range of 112 dB, and a fast response time of 2.2 μs.

Elucidating the Crucial Role of Hole Injection Layer in Degradation of Organic Light-Emitting Diodes

Although the luminous efficiency has been significantly improved in multilayered organic light-emitting diodes (OLEDs), understanding the major factors that influence degradation of OLEDs remains a major challenge due to their complex device structure. In this regard, we elucidate the crucial role of hole injection layer (HIL) in degradation of OLEDs by using systematically controlled hole injection interfaces. To analyze charge injection dependent degradation mechanism of OLEDs, we fabricate multilayered small-molecule OLEDs with molecularly controlled HILs. Although a reduced hole injection energy barrier greatly improves both a luminous efficiency and an operational lifetime (>10 times) of the OLEDs at the same time, large hole injection energy barrier increasingly aggravates its charge injection and transport during device operation. By using various kinds of nondestructive analyses at gradual stages of degradation, we demonstrate that accumulated charges at interfaces due to inefficient charge injection accelerates rate of device degradation.

Magnetoresistance in electrochemically deposited polybithiophene thin films

Magnetoresistance phenomenon on organic semiconductors has been studied mostly on spin-coated or vacuum thermal sublimated materials. In this work, electrochemistry is used as organic deposition technique. We report electrical measurements with an applied magnetic field on devices constructed in the sandwich structure indium tin oxide (ITO)/poly(bithiophene) (PBT)/aluminum (Al). The PBT polymer was electrochemically deposited on ITO. Hole mobility of 2.6 × 10−5 cm2/Vs was measured by impedance spectroscopy measurements, and thermally stimulated current measurements indicated two trapping states with activation energies around E1–3 = 0.49 eV and E4 = 0.51 eV. The interface ITO/PBT was characterized as a tunneling junction with ~0.9 eV energy barrier for hole injection from ITO. The highest magnetoresistance, ~0.6 %, was observed for a 273-nm PBT thickness, independent on direction between current and magnetic field.

Investigations of Charge Migration and Charge Trapping in Fatigued Organic Photoconductors

This article presents the experimental photodischarge kinetics of electrostatically fatigued dual-layer organic photoconductors characterized by an electrophotographic incremental charging technique that reveals the differences in the photoconductor charging profiles. During normal operation, 15% of the holes that migrate to the surface after photodischarge are not neutralized by the negative surface ions. The accumulation of these lingering surface charges (charge transport material radical cations) manifests itself as defects in half-tone images, as either missing or reduced size dots or lines. In the presence of corona gases, these surface holes oxidize and reduce the energy barrier for hole injection from a positively charged surface, e.g., contact with a transfer roller. These injected holes accumulate near the charge generation layer and require twice the amount of negative charge to attain the same surface potential as that of a new organic photoconductor (OPC) drum. The damaged depth of this injection region extends to about 50 nm into the OPC surface and is easily removed by the printer’s abrasion mechanisms (e.g., cleaning blade, toner, paper).

Top-gate staggered poly(3,3″′-dialkyl-quarterthiophene) organic thin-film transistors with reverse-offset-printed silver source/drain electrodes

Here, we report on high-performance top-gated poly(3,3″′-dialkyl-quarterthiophene) (PQT-12) organic thin-film transistors (OTFTs) with reverse-offset-printed (ROP) silver (Ag) source/drain (S/D) electrodes. OTFT devices with ROP S/D electrodes using Ag nanopaste show higher performance (∼0.01 cm2/Vs) than those fabricated by vacuum electron beam evaporation with conventional photolithography and a standard lift-off process (∼1 × 10−3 cm2/Vs). This dissimilarity is attributed to the higher work function (−4.9 eV) of the ROP Ag electrode due to AgO formation on the Ag surface during thermal annealing. This results in a low interfacial hole injection energy barrier between the S/D electrodes and the PQT-12 semiconductor.

Investigation of the gate-bias induced instability for InGaZnO TFTs under dark and light illumination

Mechanism of the instability for indium–gallium–zinc oxide thin film transistors caused by gate-bias stress performed in the dark and light illumination was investigated in this paper. The parallel V t shift with no degradation of subthreshold swing ( S.S) and the fine fitting to the stretched-exponential equation indicate that charge trapping model dominates the degradation behavior under positive gate-bias stress. In addition, the significant gate-bias dependence of V t shift demonstrates that electron trapping effect easily occurs under large gate-bias since the average effective energy barrier of electron injection decreases with increasing gate bias. Moreover, the noticeable decrease of threshold voltage ( V t) shift under illuminated positive gate-bias stress and the accelerated recovery rate in the light indicate that the charge detrapping mechanism occurs under light illumination. Finally, the apparent negative V t shift under illuminated negative gate-bias stress was investigated in this paper. The average effectively energy barrier of electron and hole injection were extracted to clarify that the serious V t degradation behavior comparing with positive gate-bias stress was attributed to the lower energy barrier for hole injection.

Simultaneous Plasma and Thermal Treatments of ITO Surfaces for Organic Solar Cells

Organic solar cells' power conversion efficiency was improved by combined thermal and CF4 plasma surface treatments of indium tin oxide (ITO). This was further enhanced by adding O2 to the CF4 plasma, which increased atomic fluorine concentration. Organic solar cells were produced with layered structures of ITO, spin-cast thin film of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) mixture (100 nm), and Al (100 nm) as top electrode. While CF4 plasma surface treatment alone significantly improved power conversion efficiency (PCE), from 0.34% to 1.99%, further enhancements occurred with the combined thermal treatment, up to 2.24%. 20% O2 addition to the CF4 plasma resulted in greatest improvement (up to 2.34%) due to optimized surface fluorination. Measured work functions of surface treated ITO increased from 4.80 to 5.17 eV with the plasma and thermal treatments. Such treatments reduce the energy barrier for hole injection at the ITO surface by increasing ITO's work function.

A Study on the Phosphorescent Blue Organic Light-Emitting Diodes Using Various Host Materials

We investigated the electrical characteristics of three devices which were fabricated with phosphorescent blue emitter, iridium(III) bis(4,6-difluorophenylpyridinato-N,C2′)picolinate (FIrpic), doped in host materials, 4,4,N,N′-dicarbazolebiphenyl (CBP), N,N′-dicarbazolyl-3,5-benzene (mCP) of the carbazole-type and p-bis(triphenylsilyly)benzene (UGH2) of the wide-energy-gap, respectively. We demonstrated the efficient blue electrophosphorescence using exothermic energy transfer between mCP and FIrpic with triplet energy level of 2.90 eV and 2.65 eV, respectively. Device property for a mCP-based structure was found to be superior to that employing the endothermic host-guest combination consisting of the CBP and FIrpic with triplet energy level of 2.57 eV and 2.65 eV, respectively. In case the device with UGH2, despite of the very wide band gap (4.40 eV), it is inefficient given the large energy barrier (1.70 eV) for hole injection into the highest occupied molecular orbital (HOMO) of UGH2 as the drive voltage increases. The device with mCP host material shows a maximum luminous efficiency and power efficiency of 17.00 cd/A and 10.9 lm/W, respectively.

C60 Modification of Al Anode for Efficient Hole Injection in Organic Light-Emitting Diodes

An Al:C60 composite anode structure was developed to improve hole injection in organic light-emitting diodes. Work function of Al was shifted from 4.3 eV to 5.2 eV by the addition of C60 because of complex formation between C60 and Al. The current density in the device was improved by 10 times by using a composite anode structure because of low hole injection energy barrier between the composite anode and hole transport layer.

A density functional theory study on photophysical properties of red light-emitting materials: Meso-substituted porphyrins

The density functional theory (DFT) has been applied to investigate the photophysical properties of meso-substituted porphyrins: 5,15-difluorenylporphyrin (DFP), 5,10,15,20-tetrafluorenylporphyrin (TFP), Zn-5,10,15,20-tetrafluorenylporphyrin (FPZ) and Zn-5,10,15,20-tetrafluorenoneporphyrin (OPZ). The geometry structures, frontier molecular orbitals, ionization potential (IP), electron affinity (EA) and reorganization energy ( λ) together with the absorption and emission wavelengths of these compounds were investigated. Theoretical calculations indicated that the introduction of meso-substitutions brought about significant effect on the photophysical properties. The TFP molecule has lowest hole injection energy barrier of all the studied compounds, and OPZ owns lowest electron injection energy barrier of them. The equilibrium of the hole and electron transport of FPZ is better than those of porphyrin (FBP), DPZ, TFP, and OPZ. The electron reorganization energies ( λ electron) of FBP, DFP, TFP, FPZ and OPZ are 0.22 eV, 0.20 eV, 0.23 eV, 0.26 eV and 0.13 eV, respectively, which are smaller than tris(8-hydroxyquinolinato) aluminum(III), thus these molecules are potential electron transport materials. Their visible absorption wavelengths are calculated by TD-B3LYP and semi-empirical ZINDO methods, and these results agree with corresponding experimental values. The fluorescence spectra of these complexes predicted by TD-HF method exhibit more consistent results with experimental data than by TD-B3LYP method.