Organic semiconductors have attracted a lot of attention since the discovery of highly doped conductive polymers, due to the potential application in field-effect transistors (OFETs), light-emitting diodes (OLEDs) and photovoltaic cells (OPVs). Single crystals of organic semiconductors are particularly intriguing because they are free of grain boundaries and have long-range periodic order as well as minimal traps and defects. Hence, organic semiconductor crystals provide a powerful tool for revealing the intrinsic properties, examining the structure-property relationships, demonstrating the important factors for high performance devices and uncovering fundamental physics in organic semiconductors. This review provides a comprehensive overview of the molecular packing, morphology and charge transport features of organic semiconductor crystals, the control of crystallization for achieving high quality crystals and the device physics in the three main applications. We hope that this comprehensive summary can give a clear picture of the state-of-art status and guide future work in this area.

Electroluminescence in light-emitting devices relies on the encounter and radiative recombination of electrons and holes in the emissive layer. In organometal halide perovskite light-emitting diodes, poor film formation creates electrical shunting paths, where injected charge carriers bypass the perovskite emitter, leading to a loss in electroluminescence yield. Here, we report a solution-processing method to block electrical shunts and thereby enhance electroluminescence quantum efficiency in perovskite devices. In this method, a blend of perovskite and a polyimide precursor dielectric (PIP) is solution-deposited to form perovskite nanocrystals in a thin-film matrix of PIP. The PIP forms a pinhole-free charge-blocking layer, while still allowing the embedded perovskite crystals to form electrical contact with the electron- and hole-injection layers. This modified structure reduces nonradiative current losses and improves quantum efficiency by 2 orders of magnitude, giving an external quantum efficiency of 1.2%. This simple technique provides an alternative route to circumvent film formation problems in perovskite optoelectronics and offers the possibility of flexible and high-performance light-emitting displays.

Unresolved problems associated with the production of graphene materials include the need for greater control over layer number, crystallinity, size, edge structure and spatial orientation, and a better understanding of the underlying mechanisms. Here we report a chemical vapor deposition approach that allows the direct synthesis of uniform single-layered, large-size (up to 10,000 μm 2 ), spatially self-aligned, and single-crystalline hexagonal graphene flakes (HGFs) and their continuous films on liquid Cu surfaces. Employing a liquid Cu surface completely eliminates the grain boundaries in solid polycrystalline Cu, resulting in a uniform nucleation distribution and low graphene nucleation density, but also enables self-assembly of HGFs into compact and ordered structures. These HGFs show an average two-dimensional resistivity of 609 AE 200 Ω and saturation current density of 0.96 AE 0.15 mA∕μm, demonstrating their good conductivity and capability for carrying high current density.atomic crystal | electronic materials G raphene has attracted considerable attention because of its extraordinary physical properties and potential electronic and spintronic applications (1-3). It is critical to find ways of precisely controlling the graphene layer number (4-6), crystallinity, size, edge structure, and even spatial orientation. The chemical vapor deposition (CVD) approach is a powerful and cost-effective technique for the production of high-quality and large-scale graphene films. In spite of the complexity of CVD procedures involving different catalysts, carbon sources, and other variables, the physical principles underlying this method are relatively simple. It is widely accepted that CVD mainly involves either surface catalytic reaction (7,8) or bulk carbon precipitation onto the surface during cooling (9, 10) for catalysts with low-carbon and high-carbon solubility, respectively. In both cases, graphene nucleation on a catalyst surface is one of the critical steps in the growth process. Various factors affect the initiation of the graphene nucleation process, including the type (11, 12) or surface microstructure of the catalyst, carbon source (13), carbon segregation from metal-carbon melts (14), processing history, and parameters in CVD growth (15)(16)(17). In general, nucleation densities on substrates such as Cu or Ni are nonuniform. This nonuniformity causes a large dispersion of both nucleus density and size distribution of graphene, representing a general problem in graphene CVD growth systems.It has been found that low-pressure CVD synthesis of graphene on Cu foil provides a good way of fabricating uniform single-layer graphene films (7). Studies have shown that the continuous films were formed by connecting randomly oriented, irregular-shaped, and micrometer-sized graphene flakes, resulting in the presence of a large amount of both low-and high-angle grain boundaries composed of pentagons and heptagons, which leads to a dramatic degradation in electronic properties compared with those of pristine graphene (7...