Universal strategy for Ohmic hole injection into organic semiconductors with high ionization energies

Inverted device architecture for high efficiency single-layer organic light-emitting diodes with imbalanced charge transport

Many wide-gap organic semiconductors exhibit imbalanced electron and hole transport, therefore efficient organic light-emitting diodes require a multilayer architecture of electron- and hole-transport materials to confine charge recombination to the emissive layer. Here, we show that even for emitters with imbalanced charge transport, it is possible to obtain highly efficient single-layer organic light emitting diodes (OLEDs), without the need for additional charge-transport and blocking layers. For hole-dominated emitters, an inverted single-layer device architecture with ohmic bottom-electron and top-hole contacts moves the emission zone away from the metal top electrode, thereby more than doubling the optical outcoupling efficiency. Finally, a blue-emitting inverted single-layer OLED based on thermally activated delayed fluorescence is achieved, exhibiting a high external quantum efficiency of 19% with little roll-off at high brightness, demonstrating that balanced charge transport is not a prerequisite for highly efficient single-layer OLEDs.

An Organic Optoelectronic Synapse with Multilevel Memory Enabled by Gate Modulation

Artificial synaptic devices are emerging as contenders for next-generation computing systems due to their combined advantages of self-adaptive learning mechanisms, high parallel computation capabilities, adjustable memory level, and energy efficiency. Optoelectronic devices are particularly notable for their responsiveness to both voltage inputs and light exposure, making them attractive for dynamic modulation. However, engineering devices with reconfigurable synaptic plasticity and multilevel memory within a singular configuration present a fundamental challenge. Here, we have established an organic transistor-based synaptic device that exhibits both volatile and nonvolatile memory characteristics, modulated through gate voltage together with light stimuli. Our device demonstrates a range of synaptic behaviors, including both short/long-term plasticity (STP and LTP) as well as STP-LTP transitions. Further, as an encoding unit, it delivers exceptional read current levels, achieving a program/erase current ratio exceeding 105, with excellent repeatability. Additionally, a prototype 4 × 4 matrix demonstrates potential in practical neuromorphic systems, showing capabilities in the perception, processing, and memory retention of image inputs.

Single-Layer Organic Light-Emitting Diode with Trap-Free Host Beats Power Efficiency and Lifetime of Multilayer Devices

Organic light-emitting diodes (OLEDs) employing a single active layer potentially offer a number of benefits compared to multilayer devices; reduced number of materials and deposition steps, potential for solution processing, and reduced operating voltage due to the absence of heterojunctions. However, for single-layer OLEDs to achieve efficiencies approaching those of multilayer devices, balanced charge transport is a prerequisite. This requirement excludes many efficient emitters based on thermally activated delayed fluorescence (TADF) that exhibit electron trapping, such as the green-emitting bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)methanone (DMAC-BP). By employing a recently developed trap-free large band gap material as a host for DMAC-BP, nearly balanced charge transport is achieved. The single-layer OLED reaches an external quantum efficiency (EQE) of 19.6%, which is comparable to the reported EQEs of 18.9-21% for multilayer devices, but achieves a record power efficiency for DMAC-BP OLEDs of 82 lm W-1, clearly surpassing the reported multilayer power efficiencies of 52.9-59 lm W-1. In addition, the operational stability is greatly improved compared to multilayer devices and the use of conventional host materials in combination with DMAC-BP as an emitter. Next to the obvious reduction in production costs, single-layer OLEDs therefore also offer the advantage of reduced energy consumption and enhanced stability.

Effects of Electron-Withdrawing Strengths of the Substituents on the Properties of 4-(Carbazolyl-R-benzoyl)-5-CF3-1H-1,2,3-triazole Derivatives as Blue Emitters for Doping-Free Electroluminescence Devices

The synthesis of four 4-(carbazolyl-R-benzoyl)-5-CF3-1H-1,2,3-triazoles with extra groups ((3-methyl)-phenyl-, 4-fluorophenyl-, quinolinyl-, or (3-trifluoromethyl)-phenyl-) in the acceptor fragment has been reported. The effects of substituents with different electron-withdrawing strengths on the thermal, electrochemical, photophysical, and electroluminescence properties of the synthesized compounds are discussed. The results of X-ray analyses and density functional theory (DFT) calculations support unusual molecular packing and electronic properties. The compounds are capable of glass formation with glass transition temperatures ranging from 54-84 °C. Ionization potentials of the compounds are in the range of 5.98-6.22 eV and electron affinities range from 3.09 to 3.35 eV. Under ultraviolet excitation, the neat films of the compounds exhibit blue emission with photoluminescence quantum yields ranging from 18 to 27%. The films of selected compounds are used for the preparation of host-free light-emitting layers of organic light-emitting diodes with very simple device structures and an external quantum efficiency of 4.6%.

Polarity and orbital driven reduction in contact resistance in organic devices with functionalized electrodes

Interlayers at electrode interfaces have been shown to reduce contact resistance in organic devices. However, there still needs to be more clarity regarding the role of microscopic properties of interlayer functionalized interfaces on device behavior. Here, we show that the impact of functionalized electrodes on device characteristics can be predicted by a few critical computationally derived parameters representing the interface charge distribution and orbital interactions. The significant influences of interfacial orbital interactions and charge distribution over device and interface properties are exhibited. Accordingly, a function is developed based on these parameters that capture their effect on the interface resistance. A strong correlation is observed, such that enhanced orbital interactions and reduced charge separation at the interface correspond to low resistance regardless of the individual molecules utilized as the interlayer. The charge distribution and orbital interactions vary with the molecular structure of the interlayer, allowing the tuning of device characteristics. Hence, the proposed function serves as a guideline for molecular design and selection for interlayers in organic devices.

Elimination of charge-carrier trapping by molecular design

A common obstacle of many organic semiconductors is that they show highly unipolar charge transport. This unipolarity is caused by trapping of either electrons or holes by extrinsic impurities, such as water or oxygen. For devices that benefit from balanced transport, such as organic light-emitting diodes, organic solar cells and organic ambipolar transistors, the energy levels of the organic semiconductors are ideally situated within an energetic window with a width of 2.5 eV where charge trapping is strongly suppressed. However, for semiconductors with a band gap larger than this window, as used in blue-emitting organic light-emitting diodes, the removal or disabling of charge traps poses a longstanding challenge. Here we demonstrate a molecular strategy where the highest occupied molecular orbital and lowest unoccupied molecular orbital are spatially separated on different parts of the molecules. By tuning their stacking by modification of the chemical structure, the lowest unoccupied molecular orbitals can be spatially protected from impurities that cause electron trapping, increasing the electron current by orders of magnitude. In this way, the trap-free window can be substantially broadened, opening a path towards large band gap organic semiconductors with balanced and trap-free transport.

The complex permittivity of PEDOT:PSS

High permittivity materials are required for efficient organic photovoltaic devices, and the addition of the conjugated polymer composite poly(3,4-ethylenedioxythiophen) polystyrene sulfonate (PEDOT:PSS) to dielectric polymers has been shown to significantly heighten their permittivity. The permittivity of PEDOT:PSS at the optical and microwave frequencies has been investigated, but PEDOT:PSS layers are mainly used for low-frequency device applications, where accurate dielectric property measurements are hindered by their high electrical conductivity and the problems arising from the metal-polymer interfaces. Here, we determine the complex relative permittivity (εr*=εr'-jεr″) of PEDOT:PSS layers perpendicular to the layer plane in the 10-2-106 Hz range by combining data from the reactive energy estimations and electrochemical impedance spectroscopy, and discover that: εr' at <1 Hz is ultra-high (∼106) decreasing with frequency to ∼5 at 106 Hz; the experimental data fit the Cole-Cole dielectric relaxation model by considering multiple relaxation mechanisms; PEDOT:PSS polarizes nonlinearly and εr' increases with the intensity of the applied external field; low frequency εr' increases with both thickness and temperature of the layer, opposite trend of temperature-dependence prevails at >103 Hz; the dielectric properties of PEDOT:PSS are highly anisotropic and the in-plane εr' at 1.0 kHz is three orders of magnitude higher than the vertical εr'; and that the εr'' decreases proportional to the reciprocal of frequency (1/f). The latter finding provides an explanation for the ubiquitous pink noise accompanying signals transmitted through organic conductor links. The described methodology can be adopted for investigations on other conjugated polymers.

Interface Engineering in Perylene Diimide-Based Organic Photovoltaics with Enhanced Photovoltage

The introduction of nonfullerene acceptors (NFA) facilitated the realization of high-efficiency organic solar cells (OSCs); however, OSCs suffer from relatively large losses in open-circuit voltage (V_OC) as compared to inorganic or perovskite solar cells. Further enhancement in power conversion efficiency requires an increase in V_OC. In this work, we take advantage of the high dipole moment of twisted perylene-diimide (TPDI) as a nonfullerene acceptor (NFA) to enhance the V_OC of OSCs. In multiple bulk heterojunction solar cells incorporating TPDI with three polymer donors (PTB7-Th, PM6 and PBDB-T), we observed a V_OC enhancement by modifying the cathode with a polyethylenimine (PEIE) interlayer. We show that the dipolar interaction between the TPDI NFA and PEIE─enhanced by the general tendency of TPDI to form J-aggregates─plays a crucial role in reducing nonradiative voltage losses under a constant radiative limit of V_OC. This is aided by comparative studies with PM6:Y6 bulk heterojunction solar cells. We hypothesize that incorporating NFAs with significant dipole moments is a feasible approach to improving the V_OC of OSCs.

Electrode Doping and Dielectric Effect in Hole Injection into Organic Semiconductors through High Work-Function Oxides

High work-function metal oxides are common for enhancing hole injection into organic semiconductors. However, the current understanding of the electrostatic mechanism needs to be more consistent with materials' electronic properties. Here, we study the electrostatic profile of high work-function oxides by considering their dielectricity and energetic disorder. Using MoO₃ as an example, we first show that the significant vacuum-level change at the electrode-oxide interface originates from electrode doping rather than the conventionally assumed interface dipole. Moreover, electrode doping is enough to explain the Fermi-level shift, so MoO₃'s characteristic n-type property is not necessarily due to intrinsic donors. This conclusion also applies to the n-type oxides with reduced work functions, like WO₃, V₂O₅, and p-type NiO. Finally, the dielectricity of the oxide, either n-type or p-type, reduces the surface p-doping of the further deposited organic layer. Increasing the oxide's metallicity and energetic disorder facilitates the hole injection.

Self-Doping Naphthalene Diimide Conjugated Polymers for Flexible Unipolar n-Type OTFTs

The development of high-performance organic thin-film transistor (OTFT) materials is vital for flexible electronics. Numerous OTFTs are so far reported but obtaining high-performance and reliable OTFTs simultaneously for flexible electronics is still challenging. Herein, it is reported that self-doping in conjugated polymer enables high unipolar n-type charge mobility in flexible OTFTs, as well as good operational/ambient stability and bending resistance. New naphthalene diimide (NDI)-conjugated polymers PNDI2T-NM17 and PNDI2T-NM50 with different contents of self-doping groups on their side chains are designed and synthesized. The effects of self-doping on the electronic properties of resulting flexible OTFTs are investigated. The results reveal that the flexible OTFTs based on self-doped PNDI2T-NM17 exhibit unipolar n-type charge-carrier properties and good operational/ambient stability thanks to the appropriate doping level and intermolecular interactions. The charge mobility and on/off ratio are fourfold and four orders of magnitude higher than those of undoped model polymer, respectively. Overall, the proposed self-doping strategy is useful for rationally designing OTFT materials with high semiconducting performance and reliability.

Single-Layer Blue Organic Light-Emitting Diodes With Near-Unity Internal Quantum Efficiency

Efficient organic light-emitting diodes (OLEDs) commonly comprise a multilayer stack including charge-transport and charge- and exciton-blocking layers, to confine charge recombination to the emissive layer. Here, a highly simplified single-layer blue-emitting OLED is demonstrated based on thermally activated delayed fluorescence with the emitting layer simply sandwiched between ohmic contacts consisting of a polymeric conducting anode and a metal cathode. The single-layer OLED exhibits an external quantum efficiency of 27.7% with minor roll-off at high brightness. The internal quantum efficiency approaches unity, demonstrating that highly simplified single-layer OLEDs without confinement layers can achieve state-of-the-art performance, while greatly reducing the complexity of the design, fabrication, and device analysis.

Unlocking the Full Potential of Electron-Acceptor Molecules for Efficient and Stable Hole Injection

Understanding the hole-injection mechanism and improving the hole-injection property are of pivotal importance in the future development of organic optoelectronic devices. Electron-acceptor molecules with high electron affinity (EA) are widely used in electronic applications, such as hole injection and p-doping. Although p-doping has generally been studied in terms of matching the ionization energy (IE) of organic semiconductors with the EA of acceptor molecules, little is known about the effect of the EA of acceptor molecules on the hole-injection property. In this work, the hole-injection mechanism in devices is completely clarified, and a strategy to optimize the hole-injection property of the acceptor molecule is developed. Efficient and stable hole injection is found to be possible even into materials with IEs as high as 5.8 eV by controlling the charged state of an acceptor molecule with an EA of about 5.0 eV. This excellent hole-injection property enables direct hole injection into an emitting layer, realizing a pure blue organic light-emitting diode with an extraordinarily low turn-on voltage of 2.67 V, a power efficiency of 29 lm W^-1 , an external quantum efficiency of 28% and a Commission Internationale de l'Eclairage y coordinate of less than 0.10.

Ultralow contact resistance in organic transistors via orbital hybridization

Organic field-effect transistors (OFETs) are of interest in unconventional form of electronics. However, high-performance OFETs are currently contact-limited, which represent a major challenge toward operation in the gigahertz regime. Here, we realize ultralow total contact resistance (R_c) down to 14.0 Ω ∙ cm in C₁₀-DNTT OFETs by using transferred platinum (Pt) as contact. We observe evidence of Pt-catalyzed dehydrogenation of side alkyl chains which effectively reduces the metal-semiconductor van der Waals gap and promotes orbital hybridization. We report the ultrahigh performance OFETs, including hole mobility of 18 cm² V^-1 s^-1, saturation current of 28.8 μA/μm, subthreshold swing of 60 mV/dec, and intrinsic cutoff frequency of 0.36 GHz. We further develop resist-free transfer and patterning strategies to fabricate large-area OFET arrays, showing 100% yield and excellent variability in the transistor metrics. As alkyl chains widely exist in conjugated molecules and polymers, our strategy can potentially enhance the performance of a broad range of organic optoelectronic devices.

Enhanced Organic Thin-Film Transistor Stability by Preventing MoO3 Diffusion with Metal/MoO3/Organic Multilayered Interface Source-Drain Contact

The source-drain electrode with a MoO₃ interfacial modification layer (IML) is considered the most promising method to solve electrical contact issues impeding organic thin-film transistors (OTFTs) from commercialization. However, this method raises many concerns because MoO₃ might diffuse into organic materials, which causes device instability. In this work, we observed a significant device stability degradation by damaging on/off switching performance caused by MoO₃ diffusion. To prevent the MoO₃ diffusion, a source-drain electrode with a multilayered interface contact (MIC) consisting of a top-down stack of metal, MoO₃ IML, and organic buffer layer (OBL) is proposed. In the MIC device, the MoO₃ IML serves well for its intended functions of reducing contact resistance and suppressing minority carrier injection to the OTFT channel. The inclusion of OBL to the MIC helps block MoO₃ diffusion and thereby leads to better device stability and an increased on/off ratio. Through combinations with several organic compounds as a buffer layer, the MoO₃ diffusion related electrical behaviors of OTFTs are systematically studied. Key parameters related to MoO₃ diffusion such as the Fick coefficient and bias-stress stability such as carrier trapping time are extracted from numerical device analysis. Finally, we summarize a general rule of material selection for making robust source-drain contact.

Self-Assembled 4-Aminopyridine Monolayer as a Nucleation-Inducing Layer for Transparent Silver Electrodes

The role of a self-assembled monolayer obtained by vacuum deposition of 4-aminopyridine (4-AP), a small organic molecule having amine and pyridine groups, as a metal nucleation inducer and adhesion promoter was verified, and the applicability was evaluated. 4-AP deposited to an extremely thin thickness effectively changed the substrate surface properties, increasing the nucleation density of silver (Ag) more than 3 times and eventually forming a more transparent, low-resistance Ag thin film. The optical transmittance of the Ag thin film, which was less than 60% when 4-AP was not applied, could be increased to about 77% by simply applying 4-AP, and the electrical resistance could be lowered from 37 to 14 Ω/square at the same time. Transmittance could be further improved to higher than 90% by depositing an antireflection layer for use as a transparent Ag electrode. It was also verified that 4-AP not only serves as a nucleation inducer but also contributes to improving interfacial adhesion. The Ag transparent electrode using 4-AP provided the improved performance of the organic light-emitting device due to higher transmittance, lower resistance, and surface roughness. Small organic molecules including functional groups that can be vacuum deposited, such as 4-AP, are expected to be used as surface pretreatment materials for various depositions because they can be easily patterned and can efficiently modify the surface even with extremely thin thickness.

Revealing and Controlling Energy Barriers and Valleys at Grain Boundaries in Ultrathin Organic Films

In organic electronics, local crystalline order is of critical importance for the charge transport. Grain boundaries between molecularly ordered domains are generally known to hamper or completely suppress charge transfer and detailed knowledge of the local electronic nature is critical for future minimization of such malicious defects. However, grain boundaries are typically hidden within the bulk film and consequently escape observation or investigation. Here, a minimal model system in form of monolayer-thin films with sub-nm roughness of a prototypical n-type organic semiconductor is presented. Since these films consist of large crystalline areas, the detailed energy landscape at single grain boundaries can be studied using Kelvin probe force microscopy. By controlling the charge-carrier density in the films electrostatically, the impact of the grain boundaries on charge transport in organic devices is modeled. First, two distinct types of grain boundaries are identified, namely energetic barriers and valleys, which can coexist within the same thin film. Their absolute height is found to be especially pronounced at charge-carrier densities below 10¹² cm^{- 2} -the regime at which organic solar cells and light emitting diodes typically operate. Finally, processing conditions by which the type or energetic height of grain boundaries can be controlled are identified.

Virtual Screening for Organic Solar Cells and Light Emitting Diodes

The field of organic semiconductors is multifaceted and the potentially suitable molecular compounds are very diverse. Representative examples include discotic liquid crystals, dye-sensitized solar cells, conjugated polymers, and graphene-based low-dimensional materials. This huge variety not only represents enormous challenges for synthesis but also for theory, which aims at a comprehensive understanding and structuring of the plethora of possible compounds. Eventually computational methods should point to new, better materials, which have not yet been synthesized. In this perspective, it is shown that the answer to this question rests upon the delicate balance between computational efficiency and accuracy of the methods used in the virtual screening. To illustrate the fundamentals of virtual screening, chemical design of non-fullerene acceptors, thermally activated delayed fluorescence emitters, and nanographenes are discussed.

A Self-Assembled 3D Penetrating Nanonetwork for High-Performance Intrinsically Stretchable Polymer Light-Emitting Diodes

The emergence of wearable technology can significantly benefit from electronic displays fabricated using intrinsically stretchable (is-) materials. Typically, an improvement in the stretchability of conventional light-emitting polymers is accompanied by a decrease in charge transportability, thus resulting in a significant decrease in device efficiency. In this study, a self-assembled 3D penetrating nanonetwork is developed to achieve increased stretchability and mobility simultaneously, based on high-molecular-weight phenylenevinylene (L-SY-PPV) and polyacrylonitrile (PAN). The mobility of L-SY-PPV/PAN increases by 5-6 times and the stretchability increases from 20% (pristine L-SY-PPV film) to 100%. A high current efficiency (CE) of 8.13 cd A^-1 is observed in polymer light-emitting diodes (PLEDs) fabricated using 40% stretched L-SY-PPV/PAN. Furthermore, using a polyethyleneimine ethoxylated (PEIE), an 1,10-phenanthroline monohydrate (pBphen), and a reduced Triton X-100 (TR) chelated Zn-based is- electron-injection layer of Zn-PEIE-pBphen-TR, an is-PLED is realized with a turn-on voltage of 6.5 V and a high CE of 2.35 cd A^-1 . These results demonstrate the effectiveness of using the self-assembled 3D penetrating nanonetwork for the fabrication of is-PLEDs.

Centimeter-scale hole diffusion and its application in organic light-emitting diodes

In conventional organic light-emitting diodes (OLEDs), current balance between electron and hole transport regions is typically achieved by leakage of the major carrier through the devices or by accumulation of the major carrier inside the devices. Both of these are known to reduce performances leading to reduction of efficiency and operation stability due to exciton-polaron annihilation, etc. We found that hole diffusion in a centimeter-scale can be achieved in a PEDOT:PSS layer via composition and interface engineering. This ultralong distance hole diffusion enables substantially enhanced hole diffusion current in the lateral direction perpendicular to the applied electric field in typical organic optoelectronic devices. By introducing this lateral hole diffusion layer (LHDL) at the anode side of OLEDs, reduced carrier accumulation, improved efficiency, and enhanced operation stability are demonstrated. The application of the LHDL provides a third strategy for current balancing with much reduced harmful effects from the previous two approaches.

Efficient flexible quantum-dot light-emitting diodes with unipolar charge injection

The exfoliation between the electrode film and the adjacent functional layer is still a big challenge for the flexible light emitting diodes, especially for the devices dependent on the direct charge injection from the electrodes. To address this issue, we design a flexible quantum-dot light-emitting diodes (QLEDs) with a charge-generation layer (CGL) on the bottom electrode as the electron supplier. The CGL consisting of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ZnO can provide sufficient electron injection into the QDs, enabling a balanced charge injection. As a result, the CGL-based QLED exhibits a peak external quantum efficiency 18.6%, over 25% enhancement in comparison with the device with ZnO as the electron transport layer. Moreover, the residual electrons in the ZnO can be pulled back to the PEDOT:PSS/ZnO interface by the storage holes in the CGL, which are released and accelerates the electron injection during the next driving voltage pulse, hence improving the electroluminescence response speed of the QLEDs.

In situ-formed tetrahedrally coordinated double-helical metal complexes for improved coordination-activated n-doping

In situ coordination-activated n-doping by air-stable metals in electron-transport organic ligands has proven to be a viable method to achieve Ohmic electron injection for organic optoelectronics. However, the mutual exclusion of ligands with high nucleophilic quality and strong electron affinity limits the injection efficiency. Here, we propose meta-linkage diphenanthroline-type ligands, which not only possess high electron affinity and good electron transport ability but also favour the formation of tetrahedrally coordinated double-helical metal complexes to decrease the ionization energy of air-stable metals. An electron injection layer (EIL) compatible with various cathodes and electron transport materials is developed with silver as an n-dopant, and the injection efficiency outperforms conventional EILs such as lithium compounds. A deep-blue organic light-emitting diode with an optimized EIL achieves a high current efficiency calibrated by the y colour coordinate (0.045) of 237 cd A^-1 and a superb LT95 of 104.1 h at 5000 cd m^-2.

Improving Charge Injection at Gold/Conjugated Polymer Contacts by Polymer Insulator-Assisted Annealing for Transistors

The poor chemical miscibility between metal and organic materials usually leads to both structural and energetic mismatches at gold/organic interfaces, and thereby, high contact resistance of organic electronic devices. This study shows that the contact resistance of organic field-effect transistors is significantly reduced by one order of magnitude, by reforming the contact interface between gold electrodes and conjugated polymers upon a polymer insulator-assisted thermal annealing. Upon an optimized solution process, the conjugated polymer is homogenously distributed within the amorphous polymer insulator matrix with relatively low glass transition temperature, and thus, even a moderate annealing temperature can induce sufficient motion of conjugated polymer chains to simultaneously adjust the polymer orientation and improve the packing of gold atoms. Consequently, gold/conjugated polymer contact is reorganized after annealing, which improves both charge transport from bulk gold to interface and charge injection from gold into conjugated polymers. This method, with appropriate insulator matrix, is effective for improving the injection of both holes and electrons, and widely applicable for many unipolar and ambipolar conjugated polymers to optimize the device performance and simultaneously increase the optical transparency (over 80%). A frequency doubler and a phase modulator are demonstrated, respectively, using the ambipolar transistors with optimized charge injection properties.

Two-Dimensional Violet Phosphorus: A p-Type Semiconductor for (Opto)electronics

The synthesis of novel two-dimensional (2D) materials displaying an unprecedented composition and structure via the exfoliation of layered systems provides access to uncharted properties. For application in optoelectronics, a vast majority of exfoliated 2D semiconductors possess n-type or more seldom ambipolar characteristics. The shortage of p-type 2D semiconductors enormously hinders the extensive engineering of 2D devices for complementary metal oxide semiconductors (CMOSs) and beyond CMOS applications. However, despite the recent progress in the development of 2D materials endowed with p-type behaviors by direct synthesis or p-doping strategies, finding new structures is still of primary importance. Here, we report the sonication-assisted liquid-phase exfoliation of violet phosphorus (VP) crystals into few-layer-thick flakes and the first exploration of their electrical and optical properties. Field-effect transistors based on exfoliated VP thin films exhibit a p-type transport feature with an I_on/I_off ratio of 10⁴ and a hole mobility of 2.25 cm² V^-1 s^-1 at room temperature. In addition, the VP film-based photodetectors display a photoresponsivity (R) of 10 mA W^-1 and a response time down to 0.16 s. Finally, VP embedded into CMOS inverter arrays displays a voltage gain of ∼17. This scalable production method and high quality of the exfoliated material combined with the excellent optoelectronic performances make VP an enticing and versatile p-type candidate for next-generation more-than-Moore (opto)electronics.

Comment on "Enhanced Charge Selectivity via Anodic-C60 Layer Reduces Nonradiative Losses in Organic Solar Cells"

Understanding interface-related phenomena is important for improving the performance of thin-film solar cells. In ACS Appl. Mater. Interfaces 2021, 13, 12603-12609, Pranav et al. report that incorporating a thin C₆₀ interlayer at the MoO₃ anode results in reduced surface recombination of electrons, which is ascribed to a decreased electron accumulation near the anode on account of an increased built-in voltage. Here, we offer an alternative explanation: the introduction of a C₆₀ interlayer renders the MoO₃ contact Ohmic. The reduced anode barrier simultaneously increases the built-in voltage, minimizes nonradiative voltage losses upon the extraction of majority carriers (holes), and suppresses minority-carrier (electron) surface recombination, the latter being the result of hole accumulation and associated band bending near the Ohmic hole contact. We therefore argue that Ohmic contact formation suppresses both majority- and minority-carrier surface recombination losses, whereas the built-in voltage per se does not play a major role in this respect.

Nanospike electrodes and charge nanoribbons: A new design for nanoscale thin-film transistors

To scale down thin-film transistor (TFT) channel lengths for accessing higher levels of speed and performance, a redesign of the basic device structure is necessary. With nanospike-shaped electrodes, field-emission effects can be used to assist charge injection from the electrodes in sub-200-nm channel length amorphous oxide and organic TFTs. These designs result in the formation of charge nanoribbons at low gate biases that greatly improve subthreshold and turn-off characteristics. A design paradigm in which the gate electric field can be less than the source-drain field is proposed and demonstrated. By combining small channel lengths and thick gate dielectrics, this approach is also shown to be a promising solution for boosting TFT performance through charge focusing and charge nanoribbon formation in flexible/printed electronics applications.

A Critical Outlook for the Pursuit of Lower Contact Resistance in Organic Transistors

To take full advantage of recent and anticipated improvements in the performance of organic semiconductors employed in organic transistors, the high contact resistance arising at the interfaces between the organic semiconductor and the source and drain contacts must be reduced significantly. To date, only a small portion of the accumulated research on organic thin-film transistors (TFTs) has reported channel-width-normalized contact resistances below 100 Ωcm, well above what is regularly demonstrated in transistors based on inorganic semiconductors. A closer look at these cases and the relevant literature strongly suggests that the most significant factor leading to the lowest contact resistances in organic TFTs so far has been the control of the thin-film morphology of the organic semiconductor. By contrast, approaches aimed at increasing the charge-carrier density and/or reducing the intrinsic Schottky barrier height have so far played a relatively minor role in achieving the lowest contact resistances. Herein, the possible explanations for these observations are explored, including the prevalence of Fermi-level pinning and the difficulties in forming optimized interfaces with organic semiconductors. An overview of the research on these topics is provided, and potential device-engineering solutions are discussed based on recent advancements in the theoretical and experimental work on both organic and inorganic semiconductors.

Doping Approaches for Organic Semiconductors

Electronic doping in organic materials has remained an elusive concept for several decades. It drew considerable attention in the early days in the quest for organic materials with high electrical conductivity, paving the way for the pioneering work on pristine organic semiconductors (OSCs) and their eventual use in a plethora of applications. Despite this early trend, however, recent strides in the field of organic electronics have been made hand in hand with the development and use of dopants to the point that are now ubiquitous. Here, we give an overview of all important advances in the area of doping of organic semiconductors and their applications. We first review the relevant literature with particular focus on the physical processes involved, discussing established mechanisms but also newly proposed theories. We then continue with a comprehensive summary of the most widely studied dopants to date, placing particular emphasis on the chemical strategies toward the synthesis of molecules with improved functionality. The processing routes toward doped organic films and the important doping-processing-nanostructure relationships, are also discussed. We conclude the review by highlighting how doping can enhance the operating characteristics of various organic devices.

Rational Design of Chrysene-Based Hybridized Local and Charge-Transfer Molecules as Efficient Non-Doped Deep-Blue Emitters for Simple-Structured Electroluminescent Devices

Herein, we present a molecular design of chrysene-based deep-blue emissive materials (TC, TpPC, TpXC, and TmPC), in which chrysene as a core is functionalized with different triphenylamine moieties to realize a fine-tuning deep-blue fluorescence with superior electroluminescent (EL) performance. The photophysical analyses and density functional theory (DFT) calculations disclose that TC, TpPC, and TpXC possess HLCT characteristics with intense deep-blue emission in the solid-state, good hole-transporting ability, and high thermal and electrochemical stabilities. They are successfully employed as non-doped emitters in simple structured OLEDs (ITO/PEDOT : PSS : NF/emitter/TPBi/LiF : Al). In particular, TC-based device emits a deep-blue light with an emission peak at 446 nm and CIE color coordinates of (0.148, 0.096), a maximum external quantum efficiency (EQE_max ) of 4.31%, and a low turn-on voltage of 2.8 V.

High efficiency blue organic light-emitting diodes with below-bandgap electroluminescence

Blue organic light-emitting diodes require high triplet interlayer materials, which induce large energetic barriers at the interfaces resulting in high device voltages and reduced efficiencies. Here, we alleviate this issue by designing a low triplet energy hole transporting interlayer with high mobility, combined with an interface exciplex that confines excitons at the emissive layer/electron transporting material interface. As a result, blue thermally activated delay fluorescent organic light-emitting diodes with a below-bandgap turn-on voltage of 2.5 V and an external quantum efficiency (EQE) of 41.2% were successfully fabricated. These devices also showed suppressed efficiency roll-off maintaining an EQE of 34.8% at 1000 cd m⁻². Our approach paves the way for further progress through exploring alternative device engineering approaches instead of only focusing on the demanding synthesis of organic compounds with complex structures.

Thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) rely on high triplet energy interlayers to confine excitons, which results in reduced performance. Here, the authors report high-performance blue TADF-OLEDs with below bandgap electroluminescence.

Reviving the "Schottky" Barrier for Flexible Polymer Dielectrics with a Superior 2D Nanoassembly Coating

The organic insulator-metal interface is the most important junction in flexible electronics. The strong band offset of organic insulators over the Fermi level of electrodes should theoretically impart a sufficient impediment for charge injection known as the Schottky barrier. However, defect formation through Anderson localization due to topological disorder in polymers leads to reduced barriers and hence cumbersome devices. A facile nanocoating comprising hundreds of highly oriented organic/inorganic alternating nanolayers is self-coassembled on the surface of polymer films to revive the Schottky barrier. Carrier injection over the enhanced barrier is further shunted by anisotropic 2D conduction. This new interface engineering strategy allows a significant elevation of the operating field for organic insulators by 45% and a 7× improvement in discharge efficiency for Kapton at 150 °C. This superior 2D nanocoating thus provides a defect-tolerant approach for effective reviving of the Schottky barrier, one century after its discovery, broadly applicable for flexible electronics.

Red to orange thermally activated delayed fluorescence polymers based on 2-(4-(diphenylamino)-phenyl)-9H-thioxanthen-9-one-10,10-dioxide for efficient solution-processed OLEDs

Most highly efficient thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) are multi-layer devices fabricated by thermal vacuum evaporation techniques, which are unfavorable for real applications. However, there are only a few reported examples of efficient solution-processed TADF OLEDs, in particular TADF polymer OLEDs. Herein, a series of solution-processable TADF conjugated polymers (PCTXO/PCTXO-Fx (x = 25, 50 and 75)) were designed and synthesized by copolymerization of 2-(4-(diphenylamino)-phenyl)-9H-thioxanthen-9-one-10,10-dioxide (TXO-TPA) as a red/orange emissive TADF unit, 9,9′-((fluorene-9,9-diyl)-bis(octane-8,1-diyl))-bis(3,6-di-tert-butylcarbazole) as host/hole-transporting unit and 2,7-N-(heptadecan-9-yl)carbazole as a conjugated linker and solubilizing group. They possessed a conjugated backbone with donor TPA-carbazole/fluorene moieties and a pendent acceptor 9H-thioxanthen-9-one-10,10-dioxide (TXO) forming a twisted donor–acceptor structure. These polymers in neat films displayed red/orange color emissions (601–655 nm) with TADF properties, proved by theory calculations and transient PL decay measurements. Their hole-transporting capability was improved when the content of 9,9′-((fluorene-9,9-diyl)-bis(octane-8,1-diyl))-bis(3,6-di-tert-butylcarbazole) within the polymers increased. All polymers were successfully employed as emitters in solution-processed OLEDs. In particular, the doped OLED fabricated with PCTXO exhibited an intense deep orange emission at 603 nm with the best electroluminescence performance (a maximum external quantum efficiency 10.44%, a maximum current efficiency of 14.97 cd A⁻¹ and a turn-on voltage of 4.2 V).

TADF conjugated polymers having 2-(4-(diphenylamino)-phenyl)-9H-thioxanthen-9-one-10,10-dioxide as a TADF unit showed red/orange color emissions and enabled OLED devices with a maximum external quantum efficiency of 10.44% and a maximum current efficiency of 14.97 cd A⁻¹

Optical Distance Measurement Based on Induced Nonlinear Photoresponse of High-Performance Organic Near-Infrared Photodetectors

Extraction barriers are usually undesired in organic semiconductor devices since they lead to reduced device performance. In this work, we intentionally introduce an extraction barrier for holes, leading to nonlinear photoresponse. The effect is utilized in near-infrared (NIR) organic photodetectors (OPDs) to perform distance measurements, as delineated in the focus-induced photoresponse technique (FIP). The extraction barrier is introduced by inserting an anodic interlayer with deeper highest occupied molecular orbital (HOMO), compared to the donor material, into a well-performing OPD. With increasing irradiance, achieved by decreasing the illumination spot area on the OPD, a higher number of holes pile up at the anode, counteracting the built-in field and increasing charge-carrier recombination in the bulk. This intended nonlinear response of the photocurrent to the irradiance allows determining the distance between the OPD and the light source. We demonstrate fully vacuum-deposited organic NIR optical distance photodetectors with a detection area up to 256 mm² and detection wavelengths at 850 and 1060 nm. Such NIR OPDs have a high potential for precise, robust, low-cost, and simple optical distance measurement setups.

Suppressing Energetic Disorder Enables Efficient Indoor Organic Photovoltaic Cells With a PTV Derivative

Indoor organic photovoltaics (IOPVs) cells have attracted considerable attention in the past few years. Herein, two PTV-derivatives, PTVT-V and PTVT-T, were used as donor materials to fabricate IOPV cells with ITCC as the acceptor. The preferred orientation of the crystals changed from edge-on to face-on after replacing the ethylene in the backbones of PTVT-V by the thiophene in that of PTVT-T. Besides, it was found that, the energetic disorder of the PTVT-T:ITCC based system is 58 meV, which is much lower than that of PTVT-V:ITCC-based system (70 meV). The lower energetic disorder in PTVT-T:ITCC leads to an efficient charge transfer, charge transport, and thus the weak charge recombination. As a result, a PCE of 9.60% under AM 1.5 G and a PCE of 24.27% under 1,000 lux (LED 2,700 K) with a low non-radiative energy loss of 0.210 eV were obtained based on PTVT-T:ITCC blend. The results indicate that to improve the PTV-derivatives photovoltaic properties by suppressing the energetic disorder is a promising way to realize low-cost IOPV cells.

Improving organic photovoltaic cells by forcing electrode work function well beyond onset of Ohmic transition

As electrode work function rises or falls sufficiently, the organic semiconductor/electrode contact reaches Fermi-level pinning, and then, few tenths of an electron-volt later, Ohmic transition. For organic solar cells, the resultant flattening of open-circuit voltage (V_oc) and fill factor (FF) leads to a ‘plateau’ that maximizes power conversion efficiency (PCE). Here, we demonstrate this plateau in fact tilts slightly upwards. Thus, further driving of the electrode work function can continue to improve V_oc and FF, albeit slowly. The first effect arises from the coercion of Fermi level up the semiconductor density-of-states in the case of ‘soft’ Fermi pinning, raising cell built-in potential. The second effect arises from the contact-induced enhancement of majority-carrier mobility. We exemplify these using PBDTTPD:PCBM solar cells, where PBDTTPD is a prototypal face-stacked semiconductor, and where work function of the hole collection layer is systematically ‘tuned’ from onset of Fermi-level pinning, through Ohmic transition, and well into the Ohmic regime.

Both open-circuit voltage and fill factor of organic solar cells are affected by the metal-organic semiconductor interface. Here, the authors demonstrate that the voltage can continue to rise when the Fermi level is forced up to the semiconductor density-of-states tail.

Enhanced Charge Selectivity via Anodic-C60 Layer Reduces Nonradiative Losses in Organic Solar Cells

Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C₆₀ interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO₃) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C₆₀ interlayer on MoO₃, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO₃ as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.

Tailoring the Electronic Landscape of Quantum Dot Light-Emitting Diodes for High Brightness and Stable Operation

The charge injection imbalance into the quantum dot (QD) emissive layer of QD-based light-emitting diodes (QD-LEDs) is an unresolved issue that is detrimental to the efficiency and operation stability of devices. Herein, an integrated approach to harmonize the charge injection rates for bright and stable QD-LEDs is proposed. Specifically, the electronic characteristics of the hole transport layer (HTL) is delicately designed in order to facilitate the hole injection from the HTL into QDs and confine the electron overflow toward the HTL. The well-defined exciton recombination zone by the engineered QDs and HTL results in high performance with a peak luminance exceeding 410 000 cd/m², suppressed efficiency roll-off characteristics (ΔEQE < 5% between 200 and 200 000 cd/m²), and prolonged operational stability. The electric and optoelectronic analyses reveal the charge carrier injection mechanism at the interface between the HTL and QDs and provides the design principle of QD heterostructures and charge transport layers for high-performance QD-LEDs.

Switching from Electron to Hole Transport in Solution-Processed Organic Blend Field-Effect Transistors

Organic electronics became an attractive alternative for practical applications in complementary logic circuits due to the unique features of organic semiconductors such as solution processability and ease of large-area manufacturing. Bulk heterojunctions (BHJ), consisting of a blend of two organic semiconductors of different electronic affinities, allow fabrication of a broad range of devices such as light-emitting transistors, light-emitting diodes, photovoltaics, photodetectors, ambipolar transistors and sensors. In this work, the charge carrier transport of BHJ films in field-effect transistors is switched from electron to hole domination upon processing and post-treatment. Low molecular weight n-type N,N'-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN₂) was blended with p-type poly[2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene] (PBTTT-C₁₄) and deposited by spin-coating to form BHJ films. Systematic investigation of the role of rotation speed, solution temperature, and thermal annealing on thin film morphology was performed using atomic force microscopy, scanning electron microscopy, and grazing incidence wide-angle X-ray scattering. It has been determined that upon thermal annealing the BHJ morphology is modified from small interconnected PDI8-CN₂ crystals uniformly distributed in the polymer fraction to large planar PDI8-CN2 crystal domains on top of the blend film, leading to the switch from electron to hole transport in field-effect transistors.

A Ladder-like Dopant-free Hole-Transporting Polymer for Hysteresis-less High-Efficiency Perovskite Solar Cells with High Ambient Stability

Perovskite solar cells (PSCs) have received high attention in the past few years due to their terrific photovoltaic performance and potentially low production cost. However, the use of hole transport materials (HTMs) with hygroscopic dopants, which cause the inevitable instability of device performance, has hampered commercialization. Herein, a dopant-free polymeric HTM with functional aromatic rings was used to optimize the HTM/perovskite interface and employed in a planar n-i-p configuration. Poly(1,4-(2,5-bis((2-butyloctyloxy)phenylene)-2,7-(5,5,10,10-tetrakis(4-hexylphenyl)-5,10-dihydro-s-indaceno[2,1-b:6,5-b']dithiophene)) (IDTB) co-polymer constructed with indaceno[1,2-b:5,6-b']dithiophene and bis(alkyloxy)benzene units adopts an S⋅⋅⋅O intramolecular bond linked ladder-like planar conjugated polymer backbone. Without any dopant, the hole mobility of IDTB is in the same order of magnitude as a doped 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD). Also, the hydrophobic nature of IDTB facilitated the long-term stability of the perovskite underneath. The unencapsulated PSC devices made of IDTB-based HTM achieved a power conversion efficiency of 19.38 % with a high moisture stability, retaining above 80 % of initial power conversion efficiency at 65 % relative humidity for more than 10 days. The superior passivation effect to perovskite surface made a hysteresis of 0.44 % was almost the least reported for regular planar undoped polymer HTM PSCs.

Crystallized Monolayer Semiconductor for Ohmic Contact Resistance, High Intrinsic Gain, and High Current Density

The contact resistance limits the downscaling and operating range of organic field-effect transistors (OFETs). Access resistance through multilayers of molecules and the nonideal metal/semiconductor interface are two major bottlenecks preventing the lowering of the contact resistance. In this work, monolayer (1L) organic crystals and nondestructive electrodes are utilized to overcome the abovementioned challenges. High intrinsic mobility of 12.5 cm² V^-1 s^-1 and Ohmic contact resistance of 40 Ω cm are achieved. Unlike the thermionic emission in common Schottky contacts, the carriers are predominantly injected by field emission. The 1L-OFETs can operate linearly from V_DS  = -1 V to V_DS as small as -0.1 mV. Thanks to the good pinch-off behavior brought by the monolayer semiconductor, the 1L-OFETs show high intrinsic gain at the saturation regime. At a high bias load, a maximum current density of 4.2 µA µm^-1 is achieved by the only molecular layer as the active channel, with a current saturation effect being observed. In addition to the low contact resistance and high-resolution lithography, it is suggested that the thermal management of high-mobility OFETs will be the next major challenge in achieving high-speed densely integrated flexible electronics.

Understanding coordination reaction for producing stable electrode with various low work functions

The realisation of a cathode with various work functions (WFs) is required to maximise the potential of organic semiconductors that have various electron affinities. However, the barrier-free contact for electrons could only be achieved by using reactive materials, which significantly reduce the environmental stability of organic devices. We show that a stable electrode with various WFs can be produced by utilising the coordination reaction between several phenanthroline derivatives and the electrode. Although the low WF of the electrode realised by using reactive materials is specific to the material, the WF of the phenanthroline-modified electrode is tunable depending on the amount of electron transfer associated with the coordination reaction. A phenanthroline-modified electrode that has a higher electron injection efficiency than lithium fluoride has been demonstrated. The observation of various WFs induced by the coordination reaction affords strategic perspectives on the development of stable cathodes unique to organic electronics.

Despite recent studies aimed at realizing efficient charge injection and collection in organic electronics, developing stable organic injection materials remains a challenge. Here, the authors report phenanthroline derivative-based materials for improved charge injection in organic devices.

Modulating the luminance of organic light-emitting diodes via optical stimulation of a photochromic molecular monolayer at transparent oxide electrode

Self-assembled monolayers (SAMs) deposited on bottom electrodes are commonly used to tune charge carrier injection or blocking in optoelectronic devices. Beside the enhancement of device performance, the fabrication of multifunctional devices in which the output can be modulated by multiple external stimuli remains a challenging target. In this work, we report the functionalization of an indium tin oxide (ITO) electrode with a SAM of a diarylethene derivative designed for optically control the electronic properties. Following the demonstration of dense SAM formation and its photochromic activity, as a proof-of-principle, an organic light-emitting diode (OLED) embedding the light-responsive SAM-covered electrode was fabricated and characterized. Optically addressing the two-terminal device by irradiation with ultraviolet light doubles the electroluminescence. The original value can be restored reversibly by irradiation with visible light. This expanded functionality is based on the photoinduced modulation of the electronic structure of the diarylethene isomers, which impact the charge carriers' confinement within the emissive layer. This approach could be successfully exploited in the field of opto-communication technology, for example to fabricate opto-electronic logic circuits.

Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?

The field of organic electronics has been prolific in the last couple of years, leading to the design and synthesis of several molecular semiconductors presenting a mobility in excess of 10 cm² V^-1 s^-1 . However, it is also started to recently falter, as a result of doubtful mobility extractions and reduced industrial interest. This critical review addresses the community of chemists and materials scientists to share with it a critical analysis of the best performing molecular semiconductors and of the inherent charge transport physics that takes place in them. The goal is to inspire chemists and materials scientists and to give them hope that the field of molecular semiconductors for logic operations is not engaged into a dead end. To the contrary, it offers plenty of research opportunities in materials chemistry.

High Vertical Carrier Mobilities of Organic Semiconductors Due to a Deposited Laid-Down Herringbone Structure Induced by a Reduced Graphene Oxide Template

High vertical carrier mobilities in organic semiconductor films are a challenging issue for fundamentally improving the performance of vertical devices. To achieve improvement in the vertical direction, a reduced graphene oxide (rGO) template is used with pentacene and DNTT having a herringbone structure enabling two-dimensional (2D) transport in comparison with CuPc having a slipped-stack structure. A thin-film structure and the optoelectrical properties of the oriented films are investigated with respect to molecular structures and packing modes. The rGO template induces a "laid-down" herringbone structure for pentacene and DNTT with a face-on orientation. Our results reveal that intermolecular dispersion energy is an additional important factor to form face-on states of molecules and influences face-on ratios in the films on rGO. Vertical charge mobilities of the films are significantly enhanced by the rGO template. Particularly, the DNTT film with a laid-down herringbone structure produces a vertical mobility as high as 0.27 cm² V^-1 s^-1, one of the highest values for ordinary thin films with several hundred nanometer thickness. These findings suggest that 2D transport is advantageous for vertical carrier transport also.

Role of Schottky Barrier and Access Resistance in Organic Field-Effect Transistors

Despite the increasing understanding of charge transport in organic field-effect transistors (OFETs), charge injection from source/drain electrodes into organic semiconductors remains crucial for improving device performance and lowering power consumption. The analysis of contact resistance is generally carried out without clearly distinguishing the Schottky barrier and access resistance. Here we show that the access resistance through the organic semiconductor bulk can significantly influence the Schottky barrier evaluation and affect the charge-transport exploration. Indeed, access resistance plays a leading role in the contact resistance, whereas the Schottky barrier (expressed as the interface resistance) determines the charge injection at the metal/semiconductor interface. The Schottky barrier evaluation strongly depends on the access resistance and bias voltage. After eliminating the access resistance effect, the intrinsic Schottky barrier appears to be very coincident and weakly dependent on the work function of the contact metal. This work provides clues to understanding the Schottky barrier and charge injection in OFETs to optimize OFETs for high-performance and advanced applications.

Tailoring the growth and electronic structures of organic molecular thin films

In the rapidly developing electronics industry, it has become increasingly necessary to explore materials that are cheap, flexible and versatile which have led to significant research efforts towards organic molecular thin films. Organic molecules are unique compared to their inorganic atomic counterparts as their properties can be tuned drastically through chemical functionalization, offering versatility, though their extended shape and weak intermolecular interactions bring significant challenges to the control of both the growth and the electronic structures of molecular thin films. In this paper, we will review the self-assembly process and how to establish long-range ordered organic molecular thin films. We will also discuss how the electronic structures of thin films are impacted by the molecule's local electrostatic environment and its interaction with the substrate, within the context of controlling interfacial energy level alignment between organic semiconductors and electrodes in electronic devices.

Revealing the Cooperative Relationship between Spin, Energy, and Polarization Parameters toward Developing High-Efficiency Exciplex Light-Emitting Diodes

Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge-transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex light-emitting diodes are reported. Magneto-photoluminescence studies reveal that the triplet-to-singlet conversion in exciplexes involves an artificially generated spin-orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge-transfer occurs with forming electric dipoles (D^+• →A^-• ), providing the ionic polarization to generate SOC in exciplexes. By having different singlet-triplet energy differences (ΔE_ST ) in 9,9'-diphenyl-9H,9'H-3,3'-bicarbazole (BCzPh):3',3'″,3'″″-(1,3,5-triazine-2,4,6-triyl)tris(([1,1'-biphenyl]-3-carbonitrile)) (CN-T2T) (ΔE_ST = 30 meV) and BCzPh:bis-4,6-(3,5-di-3-pyridylphenyl)-2-methyl-pyrimidine (B3PYMPM) (ΔE_ST = 130 meV) exciplexes, the SOC generated by the intermolecular charge-transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQE_max (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔE_ST , and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D^+• →A^-• ) size, enhanced SOC, increased orbital polarization, and decreased ΔE_ST can simultaneously occur to cooperatively operate the triplet-to-singlet conversion.

A window to trap-free charge transport in organic semiconducting thin films

Organic semiconductors, which serve as the active component in devices, such as solar cells, light-emitting diodes and field-effect transistors¹, often exhibit highly unipolar charge transport, meaning that they predominantly conduct either electrons or holes. Here, we identify an energy window inside which organic semiconductors do not experience charge trapping for device-relevant thicknesses in the range of 100 to 300 nm, leading to trap-free charge transport of both carriers. When the ionization energy of a material surpasses 6 eV, hole trapping will limit the hole transport, whereas an electron affinity lower than 3.6 eV will give rise to trap-limited electron transport. When both energy levels are within this window, trap-free bipolar charge transport occurs. Based on simulations, water clusters are proposed to be the source of hole trapping. Organic semiconductors with energy levels situated within this energy window may lead to optoelectronic devices with enhanced performance. However, for blue-emitting light-emitting diodes, which require an energy gap of 3 eV, removing or disabling charge traps will remain a challenge.