n‐Type Organic Semiconductors in Organic Electronics

Organic semiconductors have been the subject of intensive academic and commercial interest over the past two decades, and successful commercial devices incorporating them are slowly beginning to enter the market. Much of the focus has been on the development of hole transporting, or p‐type, semiconductors that have seen a dramatic rise in performance over the last decade. Much less attention has been devoted to electron transporting, or so called n‐type, materials, and in this paper we focus upon recent developments in several classes of n‐type materials and the design guidelines used to develop them.     

Thiazole‐Based Organic Semiconductors for Organic Electronics

Over the past two decades, organic semiconductors have been the subject of intensive academic and commercial interests. Thiazole is a common electron‐accepting heterocycle due to electron‐withdrawing nitrogen of imine (C=N), several moieties based on thiazole have been widely introduced into organic semiconductors, and yielded high performance in organic electronic devices. This article reviews recent developments in the area of thiazole‐based organic semiconductors, particularly thiazole, bithiazole, thiazolothiazole and benzobisthiazole‐based small molecules and polymers, for applications in organic field‐effect transistors, solar cells and light‐emitting diodes. The remaining problems and challenges, and the key research direction in near future are discussed.     

Controlling Molecular Doping in Organic Semiconductors

The field of organic electronics thrives on the hope of enabling low‐cost, solution‐processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution‐processed p‐type doped polymeric semiconductors. Highlighted topics include how solution‐processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication—applications beyond those directly analogous to inorganic doping.     

Organic Semiconductors based on Dyes and Color Pigments

Organic dyes and pigments constitute a large class of industrial products. The utilization of these compounds in the field of organic electronics is reviewed with particular emphasis on organic field‐effect transistors. It is shown that for most major classes of industrial dyes and pigments, i.e., phthalocyanines, perylene and naphthalene diimides, diketopyrrolopyrroles, indigos and isoindigos, squaraines, and merocyanines, charge‐carrier mobilities exceeding 1 cm² V^?1 s^?1 have been achieved. The most widely investigated molecules due to their n‐channel operation are perylene and naphthalene diimides, for which even values close to 10 cm² V^?1 s^?1 have been demonstrated. The fact that all of these π‐conjugated colorants contain polar substituents leading to strongly quadrupolar or even dipolar molecules suggests that indeed a much larger structural space shows promise for the design of organic semiconductor molecules than was considered in this field traditionally. In particular, because many of these dye and pigment chromophores demonstrate excellent thermal and (photo‐)chemical stability in their original applications in dyeing and printing, and are accessible by straightforward synthetic protocols, they bear a particularly high potential for commercial applications in the area of organic electronics.     

Wearable Electronics Based on Stretchable Organic Semiconductors

Wearable electronics are attracting increasing interest due to the emerging Internet of Things (IoT). Compared to their inorganic counterparts, stretchable organic semiconductors (SOSs) are promising candidates for wearable electronics due to their excellent properties, including light weight, stretchability, dissolubility, compatibility with flexible substrates, easy tuning of electrical properties, low cost, and low temperature solution processability for large-area printing. Considerable efforts have been dedicated to the fabrication of SOS-based wearable electronics and their potential applications in various areas, including chemical sensors, organic light emitting diodes (OLEDs), organic photodiodes (OPDs), and organic photovoltaics (OPVs), have been demonstrated. In this review, some recent advances of SOS-based wearable electronics based on the classification by device functionality and potential applications are presented. In addition, a conclusion and potential challenges for further development of SOS-based wearable electronics are also discussed.     

Anharmonic Lattice Vibrations in Small-Molecule Organic Semiconductors

The intermolecular lattice vibrations in small-molecule organic semiconductors have a strong impact on their functional properties. Existing models treat the lattice vibrations within the harmonic approximation. In this work, polarization-orientation (PO) Raman measurements are used to monitor the temperature-evolution of the symmetry of lattice vibrations in anthracene and pentacene single crystals. Combined with first-principles calculations, it is shown that at 10 K, the lattice dynamics of the crystals are indeed harmonic. However, as the temperature is increased, specific lattice modes gradually lose their PO dependence and become more liquid-like. This finding is indicative of a dynamic symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Pentacene also shows an apparent phase transition between 80 and 150 K, indicated by a change in the vibrational symmetry of one of the lattice modes. These findings lay the groundwork for accurate predictions of the electronic properties of high-mobility organic semiconductors at room temperature.     

Bioinspired Multifunctional Organic Transistors Based on Natural Chlorophyll/Organic Semiconductors

Inspired by the photosynthesis process of natural plants, multifunctional transistors based on natural biomaterial chlorophyll and organic semiconductors (OSCs) are reported. Functions as photodetectors (PDs) and light-stimulated synaptic transistors (LSSTs) can be switched by gate voltage. As PDs, the devices exhibit ultrahigh photoresponsivity up to 2 × 10⁶ A W⁻¹, detectivity of 6 × 10¹⁵ Jones, and I_photo/I_dark ratio of 2.7 × 10⁶, which make them among the best reported organic PDs. As LSSTs, important synaptic functions similar to biological synapses are demonstrated, together with a dynamic learning and forgetting process and image-processing function. Significantly, benefiting from the ultrahigh photosensitivity of chlorophyll, the lowest operating voltage and energy consumption of the LSSTs can be 10⁻⁵ V and 0.25 fJ, respectively. The devices also exhibit high flexibility and long-term air stability. This work provides a new guide for developing organic electronics based on natural biomaterials.     

Organic Superconductors as Stages for Exploration of Superconductivity in High Magnetic Fields

The low dimensionality of organic superconductors leads to pronounced anisotropy in the upper critical field. In the vicinity of the field direction parallel to the superconducting plane, the critical field shoots out due to the suppression of the orbital pair-breaking effect. Organic superconductors are suitable for the study of the high-field state related to the spin effect under an aligned field, since they are of a high crystalline quality. The reported experimental results covering the behavior at low temperatures are reviewed first, and the breakthrough of the BCS Pauli paramagnetic limit is discussed. The potential of the low-dimensional organic superconductor for study of the effect of electronic spectrum quantization is argued.