Organic spintronics

Organic semiconductors are emerging materials in the field of spintronics. Successful achievements include their use as a tunnel barrier in magnetoresistive tunnelling devices and as a medium for spin-polarized current in transport devices. In this paper, we give an overview of the basic concepts of spin transport in organic semiconductors and present the results obtained in the field, highlighting the open questions that have to be addressed in order to improve devices performance and reproducibility. The most challenging perspectives will be discussed and a possible evolution of organic spin devices featuring multi-functional operation is presented.     

Organic ferroelectrics

Ferroelectricity results from one of the most representative phase transitions in solids, and is widely used for technical applications. However, observations of ferroelectricity in organic solids have until recently been limited to well-known polymer ferroelectrics and only a few low-molecular-mass compounds. Whereas the traditional use of dipolar molecules has hardly succeeded in producing ferroelectricity in general, here we review advances in the synthesis of new organic materials with promising ferroelectric properties near room temperature, using design principles in analogy to inorganic compounds. These materials are based on non-covalent molecules formed by two or more components, in which ferroelectricity arises either from molecular displacements or from the collective transfer of electrons or protons. The principle of using multi-component molecular compounds leads to a much broader design flexibility and may therefore facilitate the development of future functional organics.     

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.     

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.     

Organic photovoltaic films

Organic electronic materials are of interest for future applications in solar cells. Although results for single layer organic materials have been disappointing, high photocurrent quantum efficiencies can be achieved in composite systems including both electron donating and electron accepting components. Efficiencies of over 2% have now been reported in four different types of organic solar cell. Performance is limited by the low red absorption of organic materials, poor charge transport, and low stability. These problems are being tackled by the synthesis of new materials, the use of new material combinations, and optimisation of molecular design, self assembly and processing conditions to control morphology. Power conversion efficiencies of over 5% are within reach, but the fundamental physics of organic donor–acceptor solar cells remains poorly understood. Within the last 18 months, power conversion efficiencies of over 2% have been achieved in four different types of organic solar cells. All are composite systems including electron donating and electron accepting components. Performance is limited by weak absorption in the red, poor charge transport, and low stability, but improvements are available through optimisation of materials and device structures.     

Quasi-Two-Dimensional Organic Superconductors

A review on selected normal-state and superconducting properties of the quasi-two-dimensional organic superconductors is given. Thereby, the focus is laid on the charge-transfer salts based on bisethylenedithio-tetrathiafulvalene, or ET for short, the building block of most of the to-date known organic superconductors. Some basic features of the crystallographic structure, the highly anisotropic electronic band structure for some materials, as well as unusual electronic-transport properties are highlighted. The principal phase diagram of the κ-phase salts and the fundamental difficulties of a purely electronic explanation of the observed features are discussed. A brief overview on the anisotropic superconducting properties as well as the still very controversial notion on the nature of the superconducting state is presented.      

Organic Microcavity Photodiodes

The interaction of light and matter lies at the heart of the principle of optoelectronic devices. By tuning the strength of the electric field component of the light wave, one can gain control over this interaction. A simple way of achieving this is by employing microcavities, which are one‐dimensional photonic structures. These give rise to an effective quantization of the light field in one direction. The largest enhancements in the strength of light–matter coupling are achieved for cavities with dimensions on the order of the effective wavelength of light. As organic materials have the very large oscillator strengths required for light–matter coupling, as well as excellent thin film forming properties, they are ideal materials with which to exploit tunable electron–photon coupling. We demonstrate the influence of the optical field strength in organic microcavity photodiodes. Besides allowing tunability of the response spectrum by varying the effective resonator thickness, a large increase in the photocurrent sensitivity is observed below the absorption threshold of the optically active material. The microcavity induced field enhancement plays a particularly important role under two‐photon excitation. In this case we observe a 500‐fold increase in the photocurrent response with respect to a non‐cavity device. This opens up a range of applications for organic microcavity photodiodes as nonlinear detector elements.