Coexistent Integer Charge Transfer and Charge Transfer Complex in F4-TCNQ-Doped PTAA for Efficient Flexible Organic Light-Emitting Diodes

PEDOT:PSS-Free Quantum-Dot Light-Emitting Diode with Enhanced Efficiency and Stability

Although high-performance quantum-dot light-emitting diodes (QLEDs) have been achieved, their stability is still limited due to the use of unstable PEDOT:PSS as the hole injection layer (HIL). Here, we developed a PEDOT:PSS-free QLED by using a binary PTAA:F4-TCNQ HIL. Because the PTAA, with a highest occupied molecular orbital (HOMO) level of ∼5.20 eV, can facilitate hole injection from ITO to the hole transport layer, and the F4-TCNQ can act as the electron acceptor dopant to improve the hole density and hole mobility of PTAA, the PTAA:F4-TCNQ HIL can exhibit excellent hole injection capability. As a result, the PEDOT:PSS-free QLED can exhibit a high EQE of 24.19% and an impressive brightness of 367,200 cd/m2, which are significantly higher than those of conventional QLEDs. Moreover, due to the improvement of device performance and the removal of PEDOT:PSS, the PEDOT:PSS-free QLED can also exhibit a high T95 operational lifetime of 4206 h at 1000 cd/m2 and an excellent T80 shelf lifetime of 207.41 h at 136400 cd/m2, which are about 1.6- and 3.3-fold those of conventional QLEDs, respectively. We believe that the demonstrated PEDOT:PSS-free QLED, with higher performance and stability, will promote the practical application of QLEDs in displays.

Charge-Transfer Complexes: Fundamentals and Advances in Catalysis, Sensing, and Optoelectronic Applications

Supramolecular assemblies, formed through electronic charge transfer between two or more entities, represent a rich class of compounds dubbed as charge-transfer complexes (CTCs). Their distinctive formation pathway, rooted in charge-transfer processes at the interface of CTC-forming components, results in the delocalization of electronic charge along molecular stacks, rendering CTCs intrinsic molecular conductors. Since the discovery of CTCs, intensive research has explored their unique properties including magnetism, conductivity, and superconductivity. Their more recently recognized semiconducting functionality has inspired recent developments in applications requiring organic semiconductors. In this context, CTCs offer a tuneable energy gap, unique charge-transport properties, tailorable physicochemical interactions, photoresponsiveness, and the potential for scalable manufacturing. Here, an updated viewpoint on CTCs is provided, presenting them as emerging organic semiconductors. To this end, their electronic and chemical properties alongside their synthesis methods are reviewed. The unique properties of CTCs that benefit various related applications in the realms of organic optoelectronics, catalysts, and gas sensors are discussed. Insights for future developments and existing limitations are described.

Development of High Performance Thermoelectric Polymers via Doping or Dedoping Engineering

It is of great significance to develop high-performance thermoelectric (TE) materials, because they can be used to harvest waste heat into electricity and there is abundant waste heat on earth. The conventional TE materials are inorganic semimetals or semiconductors like Bi2Te3 and its derivatives. However, they have problems of high cost, scarce/toxic elements, high thermal conductivity, and poor mechanical flexibility. Organic TE materials emerged as the next-generation TE materials because of their merits including solution processability, low cost, abundant element, low intrinsic thermal conductivity, and high mechanical flexibility. Organic TE materials are mainly conducting polymers because of their high conductivity. Both the conductivity and Seebeck coefficient depend on the doping level, and they are interdependent. Hence, the TE properties of polymers can be improved through doping/dedoping engineering. There are three types of doping forms, oxidative (or reductive) doping, protonic acid doping, and charge transfer doping. Accordingly, they can be dedoped by different approaches. In this article, we review the methods to dope and dedope p-type and n-type TE polymers and the combination of doping and dedoping to optimize their TE properties. Secondary doping is also covered, since it can significantly enhance the conductivity of some TE polymers.

Organic Donor-Acceptor Complexes As Potential Semiconducting Materials

In this review article, the synthesis, characterization and physico-chemical properties of the organic donor-acceptor complexes are highlighted and a special emphasis has been placed on developing them as semiconducting materials. The electron-rich molecules, i. e., donors have been broadly grouped in three categories, namely polycyclic aromatic hydrocarbons, nitrogen heterocycles and sulphur containing aromatic donors. The reactions of these classes of the donors with the acceptors, namely tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), tetracyanobenzene (TCNB), benzoquinone, pyromellitic dianhydride and pyromellitic diimides, fullerenes, phenazine, benzothiadiazole, naphthalimide, DMAD, maleic anhydride, viologens and naphthalene diimide are described. The potential applications of the resulting DA complexes for physico-electronic purposes are also included. The theoretical investigation of many of these products with a view to rationalise their observed physico-chemical properties is also discussed.

Anion-Counterion Strategy toward Organic Cocrystal Engineering for Near-Infrared Photothermal Conversion and Solar-Driven Water Evaporation

An anion-counterion strategy is proposed to construct organic mono-radical charge-transfer cocrystals for near-infrared photothermal conversion and solar-driven water evaporation. Ionic compounds with halogen anions as the counterions serve as electron donors, providing the necessary electrons for efficient charge transfer with unchanged skeleton atoms and structures as well as the broad red-shifted absorption (200-2000 nm) and unprecedented photothermal conversion efficiency (~90.5 %@808 nm) for the cocrystals. Based on these cocrystals, an excellent solar-driven interfacial water evaporation rate up to 6.1±1.1 kg ⋅ m-2  ⋅ h-1 under 1 sun is recorded due to the comprehensive evaporation effect from the cocrystal loading in polyurethane foams and chimney addition, such performance is superior to the reported results on charge-transfer cocrystals or other materials for solar-driven interfacial evaporation. This prototype exhibits the great potential of cocrystals prepared by the one-step mechanochemistry method in practical large-scale seawater desalination applications.

Pulse-Induced Dynamics of a Charge-Transfer Complex from First Principles

The ultrafast dynamics of charge carriers in organic donor-acceptor interfaces are of primary importance to understanding the fundamental properties of these systems. In this work, we focus on a charge-transfer complex formed by quaterthiophene p-doped by tetrafluoro-tetracyanoquinodimethane and investigate electron dynamics and vibronic interactions also at finite temperatures by applying a femtosecond pulse in resonance with the two lowest energy excitations of the system with perpendicular and parallel polarization with respect to the interface. The adopted ab initio formalism based on real-time time-dependent density-functional theory coupled to Ehrenfest dynamics enables monitoring the dynamical charge transfer across the interface and assessing the role played by the nuclear motion. Our results show that the strong intermolecular interactions binding the complex already in the ground state influence the dynamics, too. The analysis of the nuclear motion involved in these processes reveals the participation of different vibrational modes depending on the electronic states stimulated by the resonant pulse. Coupled donor-acceptor modes mostly influence the excited state polarized across the interface, while intramolecular vibrations in the donor molecule dominate the excitation in the orthogonal direction. The results obtained at finite temperatures are overall consistent with this picture, although thermal disorder contributes to slightly decreasing interfacial charge transfer.

Mechanochemistry toward Organic "Salt" via Integer-Charge-Transfer Cocrystal Strategy for Rapid, Efficient, and Scalable Near-Infrared Photothermal Conversion

Inspired by the concept of ionic charge-transfer complexes for the Mott insulator, integer-charge-transfer (integer-CT) cocrystals are designed for NIR photo-thermal conversion (PTC). With amino-styryl-pyridinium dyes and F4TCNQ (7,7',8,8'-Tetracyano-2,3,5,6-tetrafluoroquinodimethane) serving as donor/acceptor (D/A) units, integer-CT cocrystals, including amorphous stacking "salt" and segregated stacking "ionic crystal", are synthesized by mechanochemistry and solution method, respectively. Surprisingly, the integer-CT cocrystals are self-assembled only through multiple D-A hydrogen bonds (C-H⋅⋅⋅X (X=N, F)). Strong charge-transfer interactions in cocrystals contribute to the strong light-harvesting ability at 200-1500 nm. Under 808 nm laser illumination, both the "salt" and "ionic crystal" display excellent PTC efficiency beneficial from ultrafast (∼2 ps) nonradiative decay of excited states. Thus integer-CT cocrystals are potential candidates for rapid, efficient, and scalable PTC platforms. Especially amorphous "salt" with good photo/thermal stability is highly desirable in practical large-scale solar-harvesting/conversion applications in water environment. This work verifies the validity of the integer-CT cocrystal strategy, and charts a promising path to synthesize amorphous PTC materials by mechanochemical method in one-step.

Permittivity Threshold and Thermodynamics of Integer Charge-Transfer Complexation for an Organic Donor-Acceptor Pair

Molecular charge doping involves the formation of donor-acceptor charge-transfer complexes (CTCs) through integer or partial electron transfer; understanding how local chemical environment impacts complexation is important for controlling the properties of organic materials. We present steady-state and temperature-dependent spectroscopic investigations of the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) complexed with the electron donor and hole transport material N,N'-diphenyl-N,N'-di-p-tolylbenzene-1,4-diamine (MPDA). Equilibrium formation constants (K_CT) were determined for donor-acceptor pairs dissolved in a series of solvents covering a range of values of permittivity. A threshold for highly favorable complex formation was observed to occur at ϵ ∼ 8-9, with large (>10⁴) and small (<10³) values of K_CT obtained in solvents of higher and lower permittivity, respectively, but with chloroform (ϵ = 4.81) exhibiting an anomalously high formation constant. Temperature-dependent formation constants were determined in order to evaluate the thermodynamics of complex formation. In 1,2-dichloroethane (ϵ = 10.36) and chlorobenzene (ϵ = 5.62), complex formation is both enthalpically and entropically favorable, with higher enthalpic and entropic stabilization in the solvent with higher permittivity. Complexation in chloroform is exothermic and entropically disfavored, indicating that specific, inner-shell solvent-solute interactions stabilize the charge-separated complex and result in a net increase in local solution structure. Our results provide insight into how modification to the chemical environment may be utilized to support stable integer charge transfer for molecular doping applications and requiring only modest changes in local permittivity.