Carrier Injection Barrier Research Articles

Highly-efficient low-cost polarization-sensitive organic photodetectors based on laminated self-assembly planar and bulk heterojunctions

To overcome the severe problems arising from the insufficient light absorption of ultrathin self-assembly active layers and the high cost use of atomic force deposition (ALD)-grown low-leakage-current transport layers, we successfully developed a low-cost, simple and facile strategy of floating-film transfer and multilayer lamination (FFTML) for constructing highly-efficient ALD-free broadband polarization-sensitive organic photodetectors (OPDs) with the two commonly used structures of donor/acceptor planar heterojunction (PHJ) and donor:acceptor multilayer bulk heterojunction (BHJ). It was found that the PHJ-based polarization-sensitive OPD by FFTML possesses a low dark current due to the high carrier injection barrier, indicating it is more suitable to be applied in low polarized light detection scenarios. In contrast, the BHJ-based device by FFTML has a higher spectral responsivity in the whole wavelength due to more photo-excitons transferred to the donor:acceptor interface and dissociated into photoexcited carrirers. Furthermore, the film thickness, which is tuned by increasing lamination number of BHJ layers, has a big effect on the polarization-sensitive photodetection performance. The polarization-sensitive 4-BHJ OPD by FFTML finally achieved a high specific detectivity of 8.33 × 1010 Jones, which was much higher than 2.72 × 1010 Jones for the 2-BHJ device at 0 V. This work demonstrates that layer-by-layer lamination of self-assembly films can effectively improve the polarized-light detection performance, contributing significantly to the rapid development of the field.

28‐5: Late‐News Paper: Investigation on Enhanced Performance of All†solution Inverted Quantum Dot Light Emitting Diode via Changing a Solvent

In inverted quantum‐dot light‐emitting‐diodes (QD‐LED), it is important to use orthogonal solvent for hole‐transport layer(HTL). In addition, modulating work function of each layer is good way to enhance carrier injection barrier. In this research, we confirmed the viability of usage of a solvent, named gamma‐valerolactone (GVL) for forming hole transport layer (HTL). By using the solvent, current density, luminance, roll‐off characteristics were enhanced, and highest occupied molecular orbital (HOMO) level was changed, which can enhance the hole injection.

Drag force shear manipulating ligand distribution at perovskite buried interface enables efficiently suppressed EQE roll-off of perovskite light-emitting diodes

Effectively manipulating the ligand distribution at the perovskite buried interface is critical for achieving high-performance nanocrystal-based perovskite light-emitting diodes (PeLEDs). However, it is challenging to realize it due to the non-exposed feature of the buried interface. Here, the drag force shear (DFS) is utilized to successfully regulate the ligand distribution at the perovskite buried interface which can effectively suppress the EQE roll-off of devices. By manipulating the ligand distribution at the perovskite buried interface, it is revealed that a more ligand preservation could result in an improved defect passivation leading to higher EQE, while the carrier injection barrier is increased simultaneously making devices suffer from severe EQE roll-off and decreased luminance, and vice versa. With the help of optimizing the ligand distribution by manipulating the DFS, the balance between carrier injection and interfacial defect passivation is successfully achieved. Resultantly, an enhanced EQE of 17.24 % with substantially suppressed EQE roll-off is reached, culminating in a remarkable luminance of 61,900 cd/m2. This work presents an efficient approach to control ligand distribution at the perovskite buried interface and offers valuable insights for realizing high luminance and low EQE roll-off of PeLEDs.

Combined resonant tunneling and rate equation modeling of terahertz quantum cascade lasers

Terahertz (THz) quantum cascade lasers (QCLs) are technologically important laser sources for the THz range but are complex to model. An efficient extended rate equation model is developed here by incorporating the resonant tunneling mechanism from the density matrix formalism, which permits to simulate THz QCLs with thick carrier injection barriers within the semi-classical formalism. A self-consistent solution is obtained by iteratively solving the Schrödinger–Poisson equation with this transport model. Carrier–light coupling is also included to simulate the current behavior arising from stimulated emission. As a quasi-ab initio model, intermediate parameters, such as pure dephasing time and optical linewidth, are dynamically calculated in the convergence process, and the only fitting parameters are the interface roughness correlation length and height. Good agreement has been achieved by comparing the simulation results of various designs with experiments, and other models such as density matrix Monte Carlo and non-equilibrium Green's function method that, unlike here, require important computational resources. The accuracy, compatibility, and computational efficiency of our model enable many application scenarios, such as design optimization and quantitative insights into THz QCLs. Finally, the source code of the model is also provided in the supplementary material of this article for readers to repeat the results presented here, investigate, and optimize new designs.

A 3D simulation model to study all-inorganic CsPbX3 (X = Br and I) perovskites-based light-emitting diodes with different hole-transporting layers

The development of numerical models is essential for optimizing perovskite light-emitting diodes (PeLEDs) and explaining their physical mechanism for further efficiency improvement. This study reports, for the first time, on a detailed device modelling of an all-inorganic perovskite LED consisting of CsPbX3 (X = Br and I) as light emitting layer (LEL) with different hole transporting layers (HTLs), employing COMSOL Multiphysics simulation package. Therefore, a 3D simulation model is served to investigate the appropriate HTLs that meet the design requirements of a PeLED in terms of band off-set engineering. For this purpose, a series of all-inorganic halide perovskites with different HTLs such as PEDOT: PSS, CuSCN and MoO3 are simulated under the same theoretical settings, and the performances of LEDs are compared with each other. This is done through studying their electronic properties using current density–voltage (J-V) curves and internal quantum efficiency (IQE) measurements. The results obtained from the J-V curves reveal that all the CsPbBr3-based samples with different HTLs exhibit the same turn-on voltage (V on) of approximately 4.2 V, while this value increases to 5.8 V for the CsPbI3-based samples. Compared with the PeLEDs based on CsPbI3, the PeLEDs based on CsPbBr3 indicate lower V on due to the formation of shorter charge carrier injection barriers at their interfaces. Furthermore, among the various simulated structures, the highest IQE is obtained for perovskite CsPbI3-based LED with MoO3 HTL (5.21%). The effect of different parameters on the performance of the proposed configurations are also investigated, and it turns out that the thickness of LELs and lifetime of charge carriers have a decisive role to play in the efficiency of PeLEDs. This theoretical study not only successfully explains the working principle of PeLEDs but also clearly shows researchers how to produce high-performance LEDs in the laboratory by knowing the physical properties of materials and accurately adjusting energy band alignments.

Design, synthesis, and properties of a novel red asymmetric luminescent material incorporating diphenylfumaronitrile, carbazole and acridine units

Herein, a deep red luminescent material BCZ-DBFN-AD containing asymmetric structure with both aggregation-induced emission (AIE) and hot exciton properties has been designed and synthesized through bilateral substitution molecular design strategy by introducing 9,9-dimethyl-9,10-dihydroacridine and carbazole electron donors with rotor effect on both sides of diphenylfumanitrile. The distorted molecular configuration and the introduction of larger rigid electron donors endow BCZ-DBFN-AD obvious AIE properties and high solid-state photoluminescence quantum yield (PLQY). Additionally, the distorted molecular configuration can also reduce the carrier injection barrier, activate the conversion channel from triplet exciton to singlet exciton, effectively take into account the locally excited (LE) state component and charge transfer (CT) state component in the hybrid excited state, and effectively improve the efficiency of the device. The PLQY of BCZ-DBFN-AD in the amorphous thin film is 31 %, and the maximum emission wavelength of the non-doped OLED prepared based on BCZ-DBFN-AD is 672 nm with Commission Internationale de L'Eclairage (CIE) coordinates of (0.64, 0.34). The Lmax, CEmax, PEmax, EQEmax are 2489 cd m−2, 1.01 cd A−1, 0.82 lm W−1, 1.83 %, respectively, the efficiency roll-off is only 3 %, and the exciton utilization rate is 29 %, which exceeds the upper limit of 25 % of traditional fluorescent materials.

High‐Efficiency and Narrow‐Band Near‐Ultraviolet Emitters with Low CIEy of 0.03 by Incorporating Extra Weak Charge Transfer Channel into Multi‐Resonance Skeleton

AbstractHybridized local and charge‐transfer (HLCT) excited‐state emitters can effectively utilize non‐radiative triplet excitons through high‐lying reverse intersystem crossing (hRISC), but they are mostly limited to the donor‐π‐acceptor type molecules. It is a great challenge to develop high‐performance near‐ultraviolet (NUV) emitters with narrow‐band emission by HLCT due to the large carrier injection barriers and high triplet energy. In this work, by incorporating planar multi‐resonance (MR) skeleton of oxygen‐bridged triphenylborane (BO) and weak electron‐donating unit of tetraphenyl‐silane (TPS), two NUV emitters of tBOSi and tBOSiCz are developed, where an extra weak charge transfer (CT) channel between BO skeleton and peripheral phenyl in TPS is observed to increase CT component and activate hot exciton channel of hRISC. As a result, tBOSi and tBOSiCz show an outstanding narrow‐band NUV emission at 414 nm with a FWHM of about 32 nm in solution‐processed organic light‐emitting diodes (OLEDs), even at a heavy doping ratio. The best NUV electroluminescent properties are further achieved in the optimal tBOSi‐doped OLEDs with a record efficiency of 9.15% and a low CIEy of 0.034. This work provides a profound guidance for developing high‐performance narrow‐band NUV emitters by an extra weak CT channel into MR skeleton to activate HLCT emission.

Energy-Efficient Stable Hyperfluorescence Organic Light-Emitting Diodes with Improved Color Purities and Ultrahigh Power Efficiencies Based on Low-Polar Sensitizing Systems.

Multi-resonance (MR) molecules with thermally activated delayed fluorescence (TADF) are emerging as promising candidates for high-definition displays because of their narrow emission spectra. However, the electroluminescence (EL) efficiencies and spectra of MR-TADF molecules are highly sensitive to hosts and sensitizers when applied to organic light-emitting diodes (OLEDs), and the highly polar environments in devices often lead to significantly broadened EL spectra. In this study, a proof-of-concept TADF sensitizer (BTDMAC-XT) with low polarity, high steric hindrance, and concentration-quenching free feature is constructed, which acts as a good emitter in doped and non-doped OLEDs with high external quantum efficiencies (ηext s) of 26.7% and 29.3%, respectively. By combining BTDMAC-XT with conventional low-polarity hosts, low-polarity sensitizing systems with a small carrier injection barrier and full exciton utilization are constructed for the MR-TADF molecule BN2. Hyperfluorescence (HF) OLEDs employing the low-polar sensitizing systems successfully improve the color quality of BN2 and afford an excellent ηext of 34.4%, a record-high power efficiency of 166.3lmW-1 and a long operational lifetime (LT50 =40309h) at an initial luminance of 100cdm-2 . These results provide instructive guidance for the sensitizer design and device optimization for energy-efficient and stable HF-OLEDs with high-quality light.

Tuning the carrier injection barrier of hybrid metal-organic interfaces on rare earth-gold surface compounds.

Magnetic hybrid metal-organic interfaces possess a great potential in areas such as organic spintronics and quantum information processing. However, tuning their carrier injection barriers on-demand is fundamental for the implementation in technological devices. We have prepared hybrid metal-organic interfaces by the adsorption of copper phthalocyanine CuPc on REAu2 surfaces (RE = Gd, Ho and Yb) and studied their growth, electrostatics and electronic structure. CuPc exhibits a long-range commensurability and a vacuum level pinning of the molecular energy levels. We observe a significant effect of the RE valence of the substrate on the carrier injection barrier of the hybrid metal-organic interface. CuPc adsorbed on trivalent RE-based surfaces (HoAu2 and GdAu2) exhibits molecular level energies that may allow injection carriers significantly closer to an ambipolar injection behavior than in the divalent case (YbAu2).

Ambipolar Nickel Dithiolene Complex Semiconductors: From One- to Two-Dimensional Electronic Structures Based upon Alkoxy Chain Lengths.

Air-stable single-component ambipolar organic semiconductors that conduct both holes and electrons are highly desired but have been rarely realized. Neutral nickel bis(dithiolene) complexes are promising candidates that fulfill the stringent electronic requirements of shallow HOMO levels and deep LUMO levels, which can reduce the carrier injection barrier to overcome the work function of gold electrodes and ensure air stability. However, most nickel bis(dithiolene) analogs that have been characterized as ambipolar semiconductors have twisted molecular structures that hinder the effective intermolecular interactions required for carrier conduction. To address this issue, we synthesized planar alkoxy-substituted nickel bis(dithiolene) analogs that facilitate dense packing with effective intermolecular interactions. Remarkably, changing the methoxy substituents to ethoxy or propoxy groups led to a dramatic change in the packing mode, from one-dimensional to herringbone-like, while maintaining effective intermolecular interactions. These materials overcome the usual trade-off between crystallinity and solubility; they are highly crystalline, even in their film forms, and are highly soluble in organic solvents. They are therefore readily solution-processable to form semiconducting layers with well-defined and well-ordered structures in field-effect transistors. Devices based on these compounds exhibited efficient ambipolar characteristics, even after several months of exposure to air, achieving high carrier mobilities of up to 10-2 cm2 V-1 s-1 and large on/off ratios of up to 105, which are the top-class performances achieved for a single-component ambipolar semiconductor material driven in air.

Efficient Selective Sorting of Semiconducting Carbon Nanotubes Using Ultra-Narrow-Band-Gap Polymers.

Conjugated polymers with narrow band gaps are particularly useful for sorting and discriminating semiconducting single-walled carbon nanotubes (s-SWCNT) due to the low charge carrier injection barrier for transport. In this paper, we report two newly synthesized narrow-band-gap conjugated polymers (PNDITEG-TVT and PNDIC8TEG-TVT) based on naphthalene diimide (NDI) and thienylennevinylene (TVT) building blocks, decorated with different polar side chains that can be used for dispersing and discriminating s-SWCNT. Compared with the mid-band-gap conjugated polymer PNDITEG-AH, which is composed of naphthalene diimide (NDI) and head-to-head bithiophene building blocks, the addition of a vinylene linker eliminates the steric congestion present in head-to-head bithiophene, which promotes backbone planarity, extending the π-conjugation length and narrowing the band gap. Cyclic voltammetry (CV) and density functional theory (DFT) calculations suggest that inserting a vinylene group in a head-to-head bithiophene efficiently lifts the highest occupied molecular orbital (HOMO) level (−5.60 eV for PNDITEG-AH, −5.02 eV for PNDITEG-TVT, and −5.09 eV for PNDIC8TEG-TVT). All three polymers are able to select for s-SWCNT, as evidenced by the sharp transitions in the absorption spectra. Field-effect transistors (FETs) fabricated with the polymer:SWCNT inks display p-dominant properties, with higher hole mobilities when using the NDI-TVT polymers as compared with PNDITEG-AH (0.6 cm2 V–1 s–1 for HiPCO:PNDITEG-AH, 1.5 cm2 V–1 s–1 for HiPCO:PNDITEG-TVT, and 2.3 cm2 V–1 s–1 for HiPCO:PNDIC8TEG-TVT). This improvement is due to the better alignment of the HOMO level of PNDITEG-TVT and PNDIC8TEG-TVT with that of the dominant SWCNT specie.

Enhancement in operational current of PTB7 based ammonia gas sensor utilizing F4-TCNQ as P-type dopant

We demonstrate a (poly[(4,8-bis(2-ethylhexyloxy)-benzo-(1,2-b:4,5-b)dithiophene)− 2,6-diyl-alt-(4-(2-ethylhexyl)− 3-fluorothieno[3,4-b]thiophene-)− 2-carboxylate-2,6-diyl)]) (PTB7) based ammonia gas sensor with high operational current up to 10−4 A. Using (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) (F4-TCNQ) as a p-type dopant for PTB7, the operation current under driving voltage of 5 V was effectively increased to 1.5×10−4 A, while that of a standard device was only 3×10−6 A. According to the study of space-charge-limited current and ultraviolet photoemission spectroscopy (UPS), the enhanced operational current can be attributed both to the higher mobility and lower carrier injection barrier of the F4-TCNQ doped PTB7. Detailed comparative analysis between the F4-TCNQ doped sensor and the non-doped sensor was also studied. The increased operational current and the current drop during sensing of the F4-TCNQ doped sensors are both beneficial to the realization of low-cost circuitry sensing system, which demonstrates the potential of the strategy of utilizing p-type doping in organic gas sensors for the developing of affordable point-to-care (POC) devices.

Superior Optoelectronic Performance of N-Polar GaN LED to Ga-Polar Counterpart in the “Green Gap” Range

The low luminous efficiency of indium gallium nitride (InGaN) light-emitting diodes (LED) in the “green gap” range has been a long unsettled issue confounding the researchers. One of the main obstacles comes from the intrinsic polarization field in the incumbent Ga-polar LEDs (Ga-LEDs), where the polarization field will bend the energy band thus reducing the radiative recombination efficiency. The scenario will become different when we reverse the polarization field with the adoption of N-polar GaN, which should be a promising candidate to obtain LEDs with high luminous efficiency in the “green gap” range. In this study, the optical and electronic performances of InGaN LEDs in the “green gap” range with N- and Ga-polar have been numerically investigated. The results demonstrate that the light-output power of N-polar LED (N-LED) is ~1.69-fold higher than that of Ga-LED at a current density of 1250 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , thus leading to a significantly improved internal quantum efficiency. Meanwhile, the turn-on voltage of N-LED is lowered by ~17.3% compared to that of Ga-LED. As revealed by the energy band diagram, the superior optoelectronic performance of N-LED is mainly attributed to the stronger carrier confinement in the active region and the lower carrier injection barriers. This study suggests the prospective realization of high luminous efficiency InGaN LEDs in the “green gap” range by the implementation of N-LEDs.

Surface state-induced barrierless carrier injection in quantum dot electroluminescent devices

The past decade has witnessed remarkable progress in the device efficiency of quantum dot light-emitting diodes based on the framework of organic-inorganic hybrid device structure. The striking improvement notwithstanding, the following conundrum remains underexplored: state-of-the-art devices with seemingly unfavorable energy landscape exhibit barrierless hole injection initiated even at sub-band gap voltages. Here, we unravel that the cause of barrierless hole injection stems from the Fermi level alignment derived by the surface states. The reorganized energy landscape provides macroscopic electrostatic potential gain to promote hole injection to quantum dots. The energy level alignment surpasses the Coulombic attraction induced by a charge employed in quantum dots which adjust the local carrier injection barrier of opposite charges by a relatively small margin. Our finding elucidates how quantum dots accommodate barrierless carrier injection and paves the way to a generalized design principle for efficient electroluminescent devices employing nanocrystal emitters.

Engineering of Printable and Air‐Stable Silver Electrodes with High Work Function using Contact Primer Layer: From Organometallic Interphases to Sharp Interfaces

AbstractContact engineering is an important issue for organic electronics as it allows to reduce charge carrier injection barriers. While the use of molecular contact primer layers to control the energy level alignment is demonstrated in many concept studies, mainly using (single crystalline) model substrates, the processability of electrodes and their robustness must also be considered in real devices. Although silver electrodes can be printed using silver ink, their low work function and sensitivity to oxidation severely limits their use for printable organic electronics. The present study demonstrates that monolayers of F4TCNQ and F6TCNNQ provide a reliable approach to engineer high work function silver electrodes, which is examined for Ag(111) as well as polycrystalline and silver ink substrates. Notably, upon multilayer growth, a pronounced intercalation of silver into the molecular adlayer occurs, yielding thermally stabilized organometallic interphases extending over the entire adlayer. It is shown that heating allows their controlled desorption leaving behind a well‐defined monolayer that is further stabilized by additional charge transfer. Especially F6TCNNQ contact primer layers can also be prepared on oxidized silver electrodes yielding work functions of 5.5–5.6 eV, which can even withstand air exposure. Such contact primers show no interdiffusion into subsequently deposited layers of the prototypical p‐type organic semiconductor pentacene, hence validating their use for organic electronic devices.

Effects of morphology and charge transport of PDIF-CN2 /graphene TFT

A two dimensional form of carbon, Graphene has captured great interest in many fields due to its attractive physical and electrical properties. Recently, graphene based transistors are produced widely, which becomes in place of silicon in the coming years. Here, we studied the effect of integration n-type molecule with graphene to develop thin film transistors. Morphology and transfer characteristics of N,N′-1H,1H-perfluorobutyldicyanoperylene-carboxydi-imide(PDIF-CN2) islands on graphene are studied and compared the transfer characteristics of graphene TFT and PDIF-CN2/graphene TFT. CVD graphene is used to examine the growth and charge transport of PDIF-CN2/graphene TFT. 6.3 times increase in mobility has noticed after depositing PDIF-CN2 islands on graphene. ION/OFF ratio of PDIF-CN2/graphene TFT is 2 and Threshold voltage is 6.5 ± 1 V. The low ION/OFF ratio is due to the high quantity of PDIF-CN2 molecules. Observed linear behavior of ID at VDS =1 V, shows the absence of interface energy barrier for charge carrier injection, where gold electrodes were deposited on PDIF-CN2/ Graphene. Gold-PDIF-CN2 layer behave as ohmic contact, where the current-voltage (I-V) curve is linear as with Ohm's law. Morphological studies were conducted using Atomic Force Microscopy and Transmission Electron Microscopy. AFM studies reveal that size of PDIF-CN2 islands on graphene. The height of these islands is 35 nm and width is 350 nm. Different crystalline orientation has noticed by Transmission Electron Microscopy.

Performance comparison of CuPc and 2T-NATA in organic light-emitting devices

Using CuPc as the hole buffer material, the effect of 2T-NATA and CuPc as hole buffer materials in similar structure on the performance of devices is studied. For this reason, the ITO/CuPc (20 nm)/NPBX/DPVBi (20 nm) (15 nm)/Alq3: Rubrene (10, X nm)/Alq3 (40 nm)/LIF/Al (5 nm) light-emitting device is designed. Referring to the previous papers on similar structure devices, the best effect is that the thickness of doped rubrene is 20 nm. In this paper, the thickness of Alq3 doping is 10 nm, the thickness of rubrene is adjusted to 5 nm, 10 nm, 15 nm, 20 nm and 25 nm, and the buffer thickness of CuPc is selected at 20 nm according to the previous experimental results. The experimental results show that when the thickness of rubrene is 20 nm, the yellow light and blue light of the light-emitting materials tend to balance, and the device is close to the white light device. The brightness and efficiency of the device are the highest, reaching and 21050 cd/m2 5.527 cd/A respectively. Compared with 2T-NATA as the buffer material, its luminous efficiency and brightness are improved. Through the analysis of the experimental results, we find that CuPc as a hole buffer material reduces the hole carrier injection barrier, improves the hole injection ability, and enhances the recombination probability of hole and electron, improves the luminous efficiency and brightness of the device. It is concluded that 2T-NATA is more suitable than CuPc in the aspect of practical application and energy saving. The advantage of CuPc is that it can inhibit the diffusion of chemical components in ITO to the hole transport layer, which is beneficial to the improvement on device performance and life.

Reduced contact resistance in organic field-effect transistors fabricated using floating film transfer method

This paper presents an in-depth performance-based comparison of organic field-effect transistors (OFETs) prepared using the conventional spin coating (SC) technique and a recently developed floating film transfer method (FTM). A remarkable improvement in the performance of transistors fabricated using FTM was achieved in comparison to their SC counterparts. The estimated value of width-normalized contact resistance in FTM-based OFETs was an order lower in comparison to that of transistors prepared using SC technique. The observed results were credited to a significant enhancement in the length of π-conjugation due to the presence of edge-on oriented polymer chains of active layer deposited using FTM, leading to the lowering of carrier injection barrier at the Au/P3HT interface. These results were well supported through absorption, photoluminescence and Raman measurements as well as the anisotropy measurements using polarized absorption spectra, which also pointed towards the improvement in the polymer chain alignment of thin films prepared by FTM over that prepared by the conventional SC technique. The results indicate thin film morphology as a key towards reducing the contact resistance in OFETs.

Long‐Lived Efficient Inverted Organic Light‐Emitting Diodes Developed by Controlling Carrier Injection Barrier into Emitting Layer

AbstractInverted organic light‐emitting diodes (iOLEDs), where electrons are injected without the use of reactive metals such as alkali metals, have attracted extensive attention owing to their higher air stability than conventional OLEDs (cOLEDs). Although iOLEDs are promising for use in applications such as flexible displays, little is known about the emitting layer most suitable for improving their operational lifetime, which is one of the most important parameters for practical applications. Here, it is shown that the operational lifetimes of both iOLEDs and cOLEDs strongly depend on the hole and electron injection barriers of the emitting layer, which is clarified by synthesizing novel emitting hosts having similar molecular structures but different energy levels. The cOLED employing the novel emitting host with the largest hole injection barrier exhibits the shortest operational lifetime, whereas the iOLED employing the same emitting host exhibits the longest operational lifetime owing to the smallest electron injection barrier. The appropriate energy levels of the emitting layer can differ between iOLEDs and cOLEDs since the management conditions of the carriers, excitons, and polarons in iOLEDs are different from those in cOLEDs. A green iOLED with performance equivalent to that of state‐of‐the‐art green cOLEDs reported in the literature is first realized.

Self-Assembled Monolayers with Distributed Dipole Moments Originating from Bipyrimidine Units

The concept of distributed dipoles in molecular self-assembly on solid substrates was tested for the example of thiolate self-assembled monolayers (SAMs) on Au(111) containing dipolar 2,5′-bipyrimidine units. These were attached to a thiol anchoring group either directly or via a phenylene–methylene spacer, with the spacer decoupling the dipolar moiety from the substrate and promoting layer formation. As expected, the SAMs containing the spacer groups exhibited a higher quality, including a higher packing density and nearly upright molecular orientation. The electrostatic effects of the dipolar bipyrimidine moieties were tested through C 1s and N 1s photoemission spectra, where electrostatic core-level shifts impact the shapes of the spectra. Additionally, changing the orientation of the dipoles allows a variation of the work function over a range of ∼1.35 eV. The experiments were complemented by density-functional theory calculations. The work function tuning range was reasonably high, but smaller than expected considering that for SAMs with a single embedded pyrimidine group per molecule work function changes already amounted to ∼1.0 eV. This behavior is rooted in an asymmetry of the studied SAMs: For dipoles pointing away from the substrate, the expected doubling of the work function change between monopyrimidine and bipyrimidine SAMs essentially occurs. Conversely, for the downward-oriented pyrimidine dipoles, the second polar ring has hardly any effect. Consistent observations were made for the core-level shifts. We discuss several factors, which are potentially responsible for this asymmetry, like disorder, depolarization, or Fermi-level pinning. Of these, the most likely explanation is the adsorption of airborne contaminants interacting with the nitrogen atoms in the immediate vicinity of the outer surface. These are present only in films with downward oriented dipoles. In spite of these complications, some of the introduced distributed dipole SAMs serve as important model systems for understanding electrostatic effects at interfaces. They are also of interest for controlling carrier-injection barriers in organic (opto) electronic devices.