Good Charge Transport Properties Research Articles

Heteroatom‐Embedded Spiro[Fluorene‐9,9′‐Xanthene]‐Based Electron Transport Materials for Improved Driving Voltage and Efficiency in Phosphorescent Organic Light‐Emitting Diodes

AbstractThe organic material 9,9‐spirobifluorene (SBF) is a useful building block for the construction of organic light‐emitting diodes owing to its rigid molecular structure, good charge transport properties, and high triplet energy. However, the electron transport properties of the SBF‐derived electron transport materials (ETMs) are currently not good enough and should be improved. In this work, two ETMs derived from spiro[fluorene‐9,9′‐xanthene] (SFX) and spiro[cyclopenta[1,2‐b:5,4‐b′] dipyridine‐5,9′‐xanthene] (SPX) is developed to enhance the electron transport properties and triplet energy of the SBF‐derived ETMs. The SFX core can increase the triplet energy by disconnecting the conjugation on the xanthene part and SPX can improve the electron transport properties through the incorporation of two nitrogen atoms in the fluorene part of the SFX core. The SFX and SPX cores are substituted with diphenyltriazine to produce high mobility and high triplet energy ETMs. As a result, the SFX and SPX‐based ETMs showed high triplet energy of over 2.95 eV compared with 2.53 eV of the SBF‐derived ETM; in addition, the SPX‐derived ETM demonstrated higher mobility than the SBF‐derived ETM. As a result, the SPX‐derived ETM achieved lowered driving voltage and increased external quantum efficiency compared to other ETMs derived from SBF and SFX.

Intense and Stable Blue Light Emission From CsPbBr3/Cs4PbBr6 Heterostructures Embedded in Transparent Nanoporous Films

AbstractLead halide perovskite nanocrystals are attractive for light emitting devices both as electroluminescent and color‐converting materials since they combine intense and narrow emissions with good charge injection and transport properties. However, while most perovskite nanocrystals shine at green and red wavelengths, the observation of intense and stable blue emission still remains a challenging target. In this work, a method is reported to attain intense and enduring blue emission (470–480 nm), with a photoluminescence quantum yield (PLQY) of 40%, originating from very small CsPbBr3 nanocrystals (diameter < 3 nm) formed by controllably exposing Cs4PbBr6 to humidity. This process is mediated by the void network of a mesoporous transparent scaffold in which the zero‐dimensional Cs4PbBr6 lattice is embedded, which allows the fine control over water adsorption and condensation that determines the optimization of the synthetic procedure and, eventually, the nanocrystal size. The approach provides a means to attain highly efficient transparent and stable blue light‐emitting films that complete the palette offered by perovskite nanocrystals for lighting and display applications.

All-in-one organic ligand for emitting perovskite nanocrystals: Efficient dispersion, photocurable and charge transporting capability

Perovskite nanocrystals (PNCs) have attracted considerable interest as next-generation display materials owing to their outstanding photoluminescence quantum yield (PLQY), wide color gamut, and narrow emission bandwidth. However, because of their weak stability against solvents, the patterning of PNCs remains a challenge. In this study, we developed functional organic ligands (AzL1-Th and AzL2-Th) for the fine pixelation of PNC displays. Functional ligands containing photocurable azide moieties exhibit good charge transport properties and fast and efficient photocrosslinking performance, while maintaining a high PLQY. Moreover, the crosslinked PNC light emitting diodes were successfully demonstrated using AzL1-Th. These results suggest the high potential of photocurable ligands for the micro-patterning of PNC films without film damages.

Influence of molecular structure on the coupling strength to a plasmonic nanoparticle and hot carrier generation.

Strong coupling between metal nanoparticles and molecules mixes their excitations, creating new eigenstates with modified properties such as altered chemical reactivity, different relaxation pathways or modified phase transitions. Here, we explore excited state plasmon-molecule coupling and discuss how strong coupling together with a changed orientation and number of an asymmetric molecule affects the generation of hot carriers in the system. We used a promising plasmonic material, magnesium, for the nanoparticle and coupled it with CPDT molecules, which are used in organic optoelectronic materials for organic electronic applications due to their facile modification, electron-rich structure, low band gap, high electrical conductivity and good charge transport properties. By employing computational quantum electronic tools we demonstrate the existence of a strong coupling mediated charge transfer plasmon whose direction, magnitude, and spectral position can be tuned. We find that the orientation of CPDT changes the nanoparticle-molecule gap for which maximum charge separation occurs, while larger gaps result in trapping hot carriers within the moieties due to weaker interactions. This research highlights the potential for tuning hot carrier generation in strongly coupled plasmon-molecule systems for enhanced energy generation or excited state chemistry.

Incorporation of 2D pyreneammonium iodide for enhancing the efficiency and stability of perovskite solar cells.

Despite the excellent performance of three-dimensional (3D) perovskite-based solar cells (PSCs), their poor stability under moisture and heating conditions limits their commercial application. To address this issue, a new pyreneammonium iodide (named TAPPyI), in which the pyrene-based compound 4,4',4'',4'''-(1,8-dihydropyrene-1,3,6,8-tetrayl)tetraaniline (named TAPPy) acts as the 2D cation, is introduced into 3D perovskite precursor solution for forming a 2D/3D heterostructured perovskite, which improves the quality of the perovskite film and enhances the stability of the perovskite film against moisture and heating. The planar pyrene endows TAPPyI with good charge transport properties, while the iodide on the arylamine side group effectively passivates the perovskite defects, thereby suppressing non-radiative recombination losses. Finally, the power conversion efficiency (PCE) of the TAPPyI-modified PSC is increased from 20.51% in the reference PSC to 22.73%. Furthermore, the stability of the TAPPyI-modified PSC is greatly improved, retaining 86% of the initial PCE after 360 hours in an environment of 85 °C and 85% humidity (ISOS-D-3), whereas the reference PSC only retains 2%. This work demonstrates that the conjugated planar molecule as a 2D cation to construct a 2D/3D heterostructured perovskite, which combines the good stability of 2D perovskite with the excellent carrier transport properties of 3D perovskite, can greatly enhance the efficiency and stability of PSCs.

Synergetic mixed ionic liquid strategy for comprehensive defect passivation and increased carrier mobility in high-performance green perovskite light-emitting diodes

Metal halide perovskites have recently garnered significant attention owing to their potential in light-emitting diode applications. The performance of perovskite light-emitting diodes (PeLEDs) can be significantly influenced by mitigating the halide vacancy-related defects and optimizing the morphology of the perovskite layer. Achieving defect passivation in perovskites often involves the incorporation of electrically insulating long-chain ligands, which can result in the transformation of the three-dimensional perovskite structure to lower-dimensional structures. This enhances its optoelectronic properties, but reduces the interparticle charge transport, thus limiting the performance of the PeLEDs. This study demonstrates a novel approach for passivating defects by utilizing mixed ionic liquid (IL) additives, namely 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) and 1-benzyl-3-methylimidazolium tetrafluoroborate (BZIMBF4). The BMIMBF4 enhances carrier mobility, whereas BZIMBF4 enhances the film morphology and improves defect passivation within the perovskite active layer. Consequently, the mixed IL strategy enables improved control over the crystal growth kinetics, alteration of crystallographic orientation, superior defect passivation, and adjustment of the energy band levels. Additionally, this IL-based approach maintains the three-dimensional structure of the perovskite, thus preserving good charge transport properties compared to organic ligands. These combined effects result in high-performance green PeLEDs with a maximum external quantum efficiency of 20.03% and a brightness of 57599.36 cd/m2.

Neutron Irradiation Effects on Boron Nitride-Based Ceramics for Use in X-ray Detection.

X-ray detectors based on conventional semiconductors with large atomic numbers are suffering from the poor stability under a high dose rate of ionizing irradiation. In this work, we demonstrate that a wide band gap ceramic-boron nitride with small atomic numbers could be used for sensitive X-ray detection. Boron nitride samples showed excellent resistance to ionizing radiation, which have been systematically studied with the neutron- and electron-aging experiments. Then, we fully analyzed the influence of these aging effects on the fundamental properties of boron nitride. Interestingly, we found that the boron nitride samples could maintain relatively good charge transport properties even after large dose of neutron irradiation. The fabricated X-ray detectors showed decent performance metrics, and the neutron-aged boron nitride even showed improved operational stability under continuous X-ray irradiation, suggesting the great potential for real applications.

Aromatic self-assembled monolayers with pentafluoro-λ6-sulfanyl (−SF5) termination: Molecular organization and charge transport properties

A series of molecules with oligophenylene backbone, thiolate anchoring group, and pentafluoro-λ6-sulfanyl (−SF5) tail group was synthesized and used as precursors to form self-assembled monolayers (SAMs) on Au(111) substrates. The resulting SAMs feature dense molecular packing, upright molecular orientation, and chemically homogeneous SAM-ambient interface, comprised entirely of the −SF5 moieties. These SAMs exhibit exceptional wetting and electrostatic properties, showing advancing water contact angles up to 103° and work function values up to 5.96 eV—probably the highest values reported for any aromatic monolayers on gold. They also feature a comparably low value of the tunnelling decay coefficient (0.38 ± 0.07 Å−1), typical of oligophenylene backbone, which is not affected by the introduction of the −SF5 group. The latter also hardly affects the current densities at a specific bias compared to analogous monolayers with other electronegative tail groups. The superior electrostatic and good charge transport properties of the designed, SF5-terminated SAMs make them potentially useful for interface engineering in organic electronics and photovoltaics.

Cross-dipole π-π stacking of crossed perylene diimides with D-A-D structures by crystal engineering towards ambipolar charge transport performance and aggregation-induced emission

A cross-dipole π-π close-stacking of 1,7-diphenylazophenol (AZO) perylene diimides (PDI, AZO-PDIs) formed in a single-crystal integrating good ambipolar charge transport and aggregation-induced emission (AIE) behaviors has been unveiled. The weak electron donating and moderate steric hindrance of AZOs endows AZO-PDIs with crossed Donor-Acceptor-Donor (D-A-D) structures, which facilitates the charge injection of both electrons and holes and a distinct cross-dipole π-π close-packing with the distance of only 3.29 Å. Accordingly, the wave function overlaps and electronic coupling of PDI cores are significantly enhanced to provide a quasi-1D charge transport channel, resulting in ambipolar performance with the hole and electron mobility of 6.74 cm2 V−1 s−1 and 5.96 cm2 V−1 s−1 respectively. The average conductivity of 0.5 S m−1 has further confirmed the good charge transport properties of the single crystals. Moreover, it also exhibits AIE behaviors due to the weak transition dipole resonance induced by the cross-dipole stacking of a large rotation angle (79.8°) and restriction of intramolecular rotation of the bulk AZOs. This work initially provides a promising strategy for achieving both charge transport and AIE performance on account of D-A-D molecules with crossed conformations at the molecular level and efficient π-overlaps induced by cross-dipole stacking at the supramolecular level.

Addressing the Role of 2D Domains in High‐Dimensionality Ruddlesden–Popper Perovskite for Solar Cells

High‐dimensionality Ruddlesden–Popper (RP) perovskites, with general formula R2An−1BnX3n+1 and high n values (n ≥ 5), are regarded as viable materials for photovoltaics because they feature higher stability if compared to the 3D perovskite, i.e., ABX3, still maintaining good charge absorption and transport properties. When integrated into the actual solar cells, however, scattered, sometimes contradictory results are reported among different deposition procedures and different cations, especially for higher n resulting in not uniform morphology and mixed composition. Herein, high‐dimensionality RP perovskites with n = 1, 4, 10, 20, and 40 values are systematically investigated considering the interplay between the formation of 2D domains, their distribution along the active layer, the active layer thickness, and the solar cells’ performance. Given the complexity of the investigated system, combined advanced structural/morphological analyses are performed to explain solar cells’ performance, finding that the 2D phase segregates at the interface with the top electrode, acting as a barrier for charge extraction, overall decreasing the short‐circuit current (Jsc). Reducing the relative amount of bulky alkylammonium cation with respect to the methylammonium, the 2D perovskite overlayer is intentionally decreased leading to a recovery of the Jsc values, corroborating the hypothesis.

Ultra-bright, efficient and stable perovskite light-emitting diodes.

Metal halide perovskites are attracting a lot of attention as next-generation light-emitting materials owing to their excellent emission properties, with narrow band emission1-4. However, perovskite light-emitting diodes (PeLEDs), irrespective of their material type (polycrystals or nanocrystals), have not realized high luminance, high efficiency and long lifetime simultaneously, as they are influenced by intrinsic limitations related to the trade-off of properties between charge transport and confinement in each type of perovskite material5-8. Here, we report an ultra-bright, efficient and stable PeLED made of core/shell perovskite nanocrystals with a size of approximately 10 nm, obtained using a simple in situ reaction of benzylphosphonic acid (BPA) additive with three-dimensional (3D) polycrystalline perovskite films, without separate synthesis processes. During the reaction, large 3D crystals are split into nanocrystals and the BPA surrounds the nanocrystals, achieving strong carrier confinement. The BPA shell passivates the undercoordinated lead atoms by forming covalent bonds, and thereby greatly reduces the trap density while maintaining good charge-transport properties for the 3D perovskites. We demonstrate simultaneously efficient, bright and stable PeLEDs that have a maximum brightness of approximately 470,000 cd m-2, maximum external quantum efficiency of 28.9% (average = 25.2 ± 1.6% over 40 devices), maximum current efficiency of 151 cd A-1 and half-lifetime of 520 h at 1,000 cd m-2 (estimated half-lifetime >30,000 h at 100 cd m-2). Our work sheds light on the possibility that PeLEDs can be commercialized in the future display industry.

Conjugated Three-Dimensional High-Connected Covalent Organic Frameworks for Lithium-Sulfur Batteries.

Developing conjugated three-dimensional (3D) covalent organic frameworks (COFs) still remains an extremely difficult task due to the lack of enough conjugated 3D building blocks. Herein, condensation between an 8-connected pentiptycene-based D2h building block (DMOPTP) and 4-connected square-planar linkers affords two 3D COFs (named 3D-scu-COF-1 and 3D-scu-COF-2). A combination of the 3D homoaromatic conjugated structure of the former building block with the 2D conjugated structure of the latter linking units enables the π-electron delocalization over the whole frameworks of both COFs, endowing them with excellent conductivities of 3.2-3.5 × 10-5 S cm-1. In particular, the 3D rigid quadrangular prism shape of DMOPTP guides the formation of a twofold interpenetrated scu 3D topology and high-connected permanent porosity with a large Brunauer-Emmett-Teller (BET) surface area of 2340 and 1602 m2 g-1 for 3D-scu-COF-1 and 3D-scu-COF-2, respectively, ensuring effective small molecule storage and mass transport characteristics. This, in combination with their good charge transport properties, renders them promising sulfur host materials for lithium-sulfur batteries (LSBs) with high capacities (1035-1155 mA h g-1 at 0.2 C, 1 C = 1675 mA g-1), excellent rate capabilities (713-757 mA h g-1 at 5.0 C), and superior cycling stability (71-83% capacity retention at 2.0 C after 500 cycles), surpassing the most of organic LSB cathodes reported thus far.

P‐135: New Hole‐Transport Materials Composed of Indenocarbazole‐Based Copolymer for Ultra‐High‐Efficiency Solution‐Processed OLED

New hole transport indenocarbazole based copolymers, KHU‐HTL1 and KHU‐HTL2, were found to have good solubility, intermediate HOMO levels, especially good charge transport properties due to low hole reorganization energy. Subsequently, the device with KHU‐HTL1 showed a maximum current efficiency up to 100.4 cd/A. To the best of our knowledge, the efficiency value we achieved from the solution processed green phosphorescent OLEDs with this material is one of the best efficiency realized from solution‐processed OLED.

Solution‐Processed Wafer‐Scale Ag2S Thin Films: Synthesis and Excellent Charge Transport Properties

AbstractMonoclinic α‐Ag2S is an intriguing member of transition metal sulfides with great potential for all‐inorganic flexible optoelectronics and thermoelectrics. Fabrication of large‐area, high‐quality α‐Ag2S thin films and understanding their charge transport properties are critical for device operations yet have remained largely unexplored. Here, a novel two‐step, the solution‐processed approach is reported to produce wafer‐sized, highly crystalline α‐Ag2S thin films. Ultrafast terahertz (THz) conductivity measurements reveal that photogenerated charge carriers undergo efficient band transport, with room‐temperature mobility of ≈150 cm2 V‐1 s‐1 and a diffusion length exceeding 500 nm. Furthermore, introducing poly(vinyl alcohol) (PVA) as the rigid component into the aqueous silver thiolate precursor enables the synthesis of free‐standing α‐Ag2S thin films with a mobility of ≈35 cm2 V‐1 s‐1, demonstrating their potential for flexible optoelectronics. This study provides a facile synthesis route for high‐quality, large‐area α‐Ag2S thin films with good charge transport properties, relevant for their integration into optoelectronics and wearable electronics.

Polycrystalline Formamidinium Lead Bromide X-ray Detectors

We have investigated the performance of formamidinium lead bromide (FAPbBr3) perovskite X-ray detectors fabricated from polycrystalline material that is pressed into a pellet at high pressures. FAPbBr3 has been shown to exhibit a remarkable combination of electrical and physical properties, such that mechanically-formed polycrystalline pellets exhibit good charge transport properties suitable for use as X-ray detectors. We characterise the morphology and structure of FAPbBr3 pellets using photoluminescence (PL), electron microscopy (SEM) and X-ray diffraction (XRD), and demonstrate an improvement in the microstructure, density, and charge transport performance of the material as the pressure is increased from 12 MPa to 124 MPa. The use of annealing of the pellets after pressing also improves the stability and charge transport performance of the devices. Using a 40 kV X-ray beam, a maximum X-ray sensitivity of 169 µC Gy−1 cm−2 was measured, and the fast time response of the devices was demonstrated using a chopped X-ray beam.

Influence of π-Endcaps on the Performance of Functionalized Quinolines for p-Channel OFETs.

This study investigates the influence of aryl and ethynyl linkers as well the effect of various pi-end-groups on the performance of the quinoline-based organic field-effect transistors. A series of new functionalized quinolines with D-π-A-π-D and A-π-A-π-A architectures are designed and synthesized via the Sonagashira cross-coupling reaction. All the new compounds are well characterized and their photophysical properties are studied. The bottom gate-top contact-organic field-effect transistors devices are fabricated using the spin-coating technique. By employing the pre and post-annealing technique, films with uniform surface coverage are obtained. The variation in the end-groups results in versatile packing arrangements which determine their good charge transport properties. The p-channel transistor behavior is observed for all the new compounds. Among the molecules studied, methoxyphenyl and thiophen-2-yl terminal functionalized with D-π-A-π-D architecture exhibit the higher p-channel transistor characteristics with hole mobilities of 1.39 and 1.33 cm2 V-1 s-1 , respectively. The good charge carrier mobilities are supported by an electron-donating methoxy group and thiophene as the end-groups with high highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) levels, extensive π-conjugation, and better self-assembly.

Replacing the Electron-Hole Transport Layer by Doping: Optimization of Tin-Based Perovskite Solar Cells from a Simulation Perspective

Due to their non-toxic nature and good charge transport properties, tin-based perovskite solar cells are gaining attention. Here, we used SCAPS-1D (Solar Cell Capacitance Simulator) to simulate and analyze constructed devices. We then created n-MASnI3 and p-MASnI3 layers that facilitate electron-hole transport by changing the doping concentration of MASnI3 in the perovskite absorber layer, replacing the conventional electron transport layer (ETL) and hole transport layer (HTL). This simplifies the manufacturing process to reduce cost and realize the conversion of lead-free perovskite solar cells. By studying the thickness of n-MASnI3 and p-MASnI3, and the acceptor and donor doping concentrations (ND and NA), we explored the effects on each device performance parameter in terms of energy band distribution, recombination rate variation, and external quantum efficiency. The first optimization of the electron transport layer and hole transport layer was thereby achieved. A second optimized analysis of the doping concentration of i-MASnI3 absorber layer was achieved. Finally, we optimized the device performance parameters as V OC = 0.8827 V, J SC = 23.1720 mA cm−2, FF = 63.1390% and PCE = 12.9144%.

PAMAM‐containing semiconducting polymers: Effect of dendritic side chains on optoelectronic and solid‐state properties

AbstractDendronized polymers are a particularly interesting platform for the preparation of advanced semiconductors given their high degree of functionalization, monodispersity, and bulkiness. Despite advantageous features, the incorporation of dendritic moieties in semiconducting polymers is still relatively underexplored, and the impact on the optoelectronic, thermomechanical, and solid‐state properties are difficult to predict. This work focuses on the incorporation of polyamidoamine (PAMAM) dendritic side chains to semicrystalline polymers based on diketopyrrolopyrrole. Using a versatile synthetic strategy based on the azide‐alkyne Huisgen 1,3‐dipolar cycloaddition, dendronized semiconducting polymers were prepared and the effect of the dendritic side chains on different properties were carefully characterized using different techniques. The dendritic side chains were found to reduce aggregation and crystallinity of the polymers in thin films. PAMAM‐containing semiconducting polymers were also shown to have good charge transport properties in organic field‐effect transistors, within the same order of magnitude to that of diketopyrrolopyrrole‐based polymers bearing branched alkyl chains. This new design approach is particularly interesting to develop advanced semiconducting polymers given its synthetic versatility and the structural diversity of the dendronized moieties. Furthermore, the utilization of dendritic moieties in semiconducting polymers is a promising approach to fine‐tune the thermomechanical properties toward semiconducting polymers for next‐generation organic electronics.

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Switching the electrical characteristics of TiO2 from n-type to p-type by ion implantation

For many applications, as for example tandem dye-sensitized solar cells (DSSCs), the development of transparent p-type semiconductor with good charge transport properties is crucial. In this work, a nitrogen-doped TiO2 material is transformed by ion implantation into an efficient hole transport layer that could reduce the electron-hole recombination processes in the devices. Theoretical calculations allowed to demonstrate that the position of nitrogen species as well as their respective ratio impact the optical and electrical properties of N-doped TiO2 materials. The chemical composition was therefore tuned by the ion implantation parameters, achieving a high transparency in the UV–visible region while generating delocalized states near the top of the valence band that do not act as traps. In terms of electrical properties, beyond the possibility to tune the electrical behavior of TiO2 from n-type to p-type upon nitrogen doping, we succeed to increase the conductivity by a higher electron mobility without change in their density, which is of prime interest for charge transport application.