Perylene-3,4,9,10-tetracarboxylic acid diimides (perylene diimides, PDIs) have been used as industrial pigments for many years. More recently, new applications for PDI derivatives have emerged in ...areas including organic photovoltaic devices and field-effect transistors. This Perspective discusses the synthesis and physical properties of PDI derivatives and their applications in organic electronics.
The performance of lead‐halide perovskite light‐emitting diodes (LEDs) has increased rapidly in recent years. However, most reports feature devices operated at relatively small current densities ...(<500 mA cm−2) with moderate radiance (<400 W sr−1 m−2). Here, Joule heating and inefficient thermal dissipation are shown to be major obstacles toward high radiance and long lifetime. Several thermal management strategies are proposed in this work, such as doping charge‐transport layers, optimizing device geometry, and attaching heat spreaders and sinks. Combining these strategies, high‐performance perovskite LEDs are demonstrated with maximum radiance of 2555 W sr−1 m−2, peak external quantum efficiency (EQE) of 17%, considerably reduced EQE roll‐off (EQE > 10% to current densities as high as 2000 mA cm−2), and tenfold increase in operational lifetime (when driven at 100 mA cm−2). Furthermore, with proper thermal management, a maximum current density of 2.5 kA cm−2 and an EQE of ≈1% at 1 kA cm−2 are shown using electrical pulses, which represents an important milestone toward electrically driven perovskite lasers.
Several thermal management strategies are proposed and applied to perovskite light‐emitting diodes (LEDs), resulting in greatly reduced efficiency roll‐off, record high radiance, and significantly improved operational lifetime. Furthermore, these thermal management strategies enable pulsed operation of perovskite LEDs in the kA cm‐2 range with much improved device performance, an important step toward electrically driven perovskite lasers.
Here we report that a covalent organic framework (COF), which contains 2,5-di(imine)-substituted 1,4-dihydroxybenzene (diiminol) moieties, undergoes color changes in the presence of solvents or ...solvent vapor that are rapid, passive, reversible, and easily detectable by the naked eye. A new visible absorption band appears in the presence of polar solvents, especially water, suggesting reversible conversion to another species. This reversibility is attributed to the ability of the diiminol to rapidly tautomerize to an iminol/cis-ketoenamine and its inability to doubly tautomerize to a diketoenamine. Density functional theory (DFT) calculations suggest similar energies for the two tautomers in the presence of water, but the diiminol is much more stable in its absence. Time-dependent DFT calculations confirm that the iminol/cis-ketoenamine absorbs at longer wavelength than the diiminol and indicate that this absorption has significant charge-transfer character. A colorimetric humidity sensing device constructed from an oriented thin film of the COF responded quickly to water vapor and was stable for months. These results suggest that tautomerization-induced electronic structure changes can be exploited in COF platforms to give rapid, reversible sensing in systems that exhibit long-term stability.
Organic electron‐transporting materials are essential for the fabrication of organic p‐n junctions, photovoltaic cells, n‐channel field‐effect transistors, and complementary logic circuits. Rylene ...diimides are a robust, versatile class of polycyclic aromatic electron‐transport materials with excellent thermal and oxidative stability, high electron affinities, and, in many cases, high electron mobilities; they are, therefore, promising candidates for a variety of organic electronics applications. In this review, recent developments in the area of high‐electron‐mobility diimides based on rylenes and related aromatic cores, particularly perylene‐ and naphthalene‐diimide‐based small molecules and polymers, for application in high‐performance organic field‐effect transistors and photovoltaic cells are summarized and analyzed.
Electronic devices based on organic materials offer the potential of low‐cost processing on flexible substrates when compared to traditional inorganic semiconductors. Rylene diimides have recently shown promise as potential alternatives to the fullerenes as acceptors in photovoltaic devices, exhibiting power conversion efficiencies that are amongst the highest achieved for all‐polymer fullerene‐free systems. This article reviews their development for both transistor and solar cell applications.
Molecular doping—the use of redox‐active small molecules as dopants for organic semiconductors—has seen a surge in research interest driven by emerging applications in sensing, bioelectronics, and ...thermoelectrics. However, molecular doping carries with it several intrinsic problems stemming directly from the redox‐active character of these materials. A recent breakthrough was a doping technique based on ion‐exchange, which separates the redox and charge compensation steps of the doping process. Here, the equilibrium and kinetics of ion exchange doping in a model system, poly(2,5‐bis(3‐alkylthiophen‐2‐yl)thieno(3,2‐b)thiophene) (PBTTT) doped with FeCl3 and an ionic liquid, is studied, reaching conductivities in excess of 1000 S cm−1 and ion exchange efficiencies above 99%. Several factors that enable such high performance, including the choice of acetonitrile as the doping solvent, which largely eliminates electrolyte association effects and dramatically increases the doping strength of FeCl3, are demonstrated. In this high ion exchange efficiency regime, a simple connection between electrochemical doping and ion exchange is illustrated, and it is shown that the performance and stability of highly doped PBTTT is ultimately limited by intrinsically poor stability at high redox potential.
An extremely efficient ion‐exchange doping process for conjugated polymers which enables conductivities exceeding 1000 S cm−1 is demonstrated. Factors which affect ion exchange, such as electrolyte concentration, doping solvent, and film crystallinity are discussed. When exchange is efficient there is a direct correspondence between ion exchange electrochemical doping, which is used to reveal the detrimental impact of off‐target oxidation reactions.
Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We ...show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low-work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.
Developing processes to controllably dope transition‐metal dichalcogenides (TMDs) is critical for optical and electrical applications. Here, molecular reductants and oxidants are introduced onto ...monolayer TMDs, specifically MoS2, WS2, MoSe2, and WSe2. Doping is achieved by exposing the TMD surface to solutions of pentamethylrhodocene dimer as the reductant (n‐dopant) and “Magic Blue,” N(C6H4‐p‐Br)3SbCl6, as the oxidant (p‐dopant). Current–voltage characteristics of field‐effect transistors show that, regardless of their initial transport behavior, all four TMDs can be used in either p‐ or n‐channel devices when appropriately doped. The extent of doping can be controlled by varying the concentration of dopant solutions and treatment time, and, in some cases, both nondegenerate and degenerate regimes are accessible. For all four TMD materials, the photoluminescence intensity; for all four materials the PL intensity is enhanced with p‐doping but reduced with n‐doping. Raman and X‐ray photoelectron spectroscopy (XPS) also provide insight into the underlying physical mechanism by which the molecular dopants react with the monolayer. Estimates of changes of carrier density from electrical, PL, and XPS results are compared. Overall a simple and effective route to tailor the electrical and optical properties of TMDs is demonstrated.
Four monolayer transition‐metal dichalcogenides (TMDs), MoS2, WS2, MoSe2, and WSe2, are chemically doped using molecular reductants and oxidants. Electrical measurements, photoluminescence, Raman spectroscopy, and X‐ray photoelectron spectroscopy are used to study the doping effect and to understand the underlying mechanism. This work presents a simple and effective route to tailor the electrical and optical properties of atomically thin TMDs.
Benzimidazoline radical dimers that can be handled in air but that function as powerful reductants are reported and evaluated as n‐dopants by solution‐ and vacuum processing. In several host ...materials, one of these dimers is found to have a more‐consistent doping effect than a hydride‐donor dopant analog. Notably, a record high room‐temperature conductivity of 12.0 S cm−1 is obtained for doped C60.
The fabrication of multilayer organic light-emitting diodes through solution processing presents challenges, especially regarding dissolution of the first layer during deposition of a second layer. ...One possible approach to this problem is to insolubilize the first layer using cross-linking. Cross-linking has also been used to control the morphological stability and aggregation phenomena of the active organic materials. In this short review, we discuss the alternative chemically, thermally, and photochemically promoted cross-linking chemistries that have been examined in the context of organic light-emitting diodes including: the hydrolysis of silicon compounds to form siloxanes; the polymerization of styrene, acrylate, and oxetane groups; and the dimerization of trifluorovinyl ethers, benzocyclobutenes, and cinnamates.
Transparent conducting oxides (TCOs), such as indium tin oxide and zinc oxide, play an important role as electrode materials in organic-semiconductor devices. The properties of the inorganic–organic ...interfacethe offset between the TCO Fermi level and the relevant transport level, the extent to which the organic semiconductor can wet the oxide surface, and the influence of the surface on semiconductor morphologysignificantly affect device performance. This review surveys the literature on TCO modification with phosphonic acids (PAs), which has increasingly been used to engineer these interfacial properties. The first part outlines the relevance of TCO surface modification to organic electronics, surveys methods for the synthesis of PAs, discusses the modes by which they can bind to TCO surfaces, and compares PAs to alternative organic surface modifiers. The next section discusses methods of PA monolayer deposition, the kinetics of monolayer formation, and structural evidence regarding molecular orientation on TCOs. The next sections discuss TCO work-function modification using PAs, tuning of TCO surface energy using PAs, and initiation of polymerizations from TCO-tethered PAs. Finally, studies that examine the use of PA-modified TCOs in organic light-emitting diodes and organic photovoltaics are compared.
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