Dense Assembly of Soluble Acene Crystal Ribbons and Its Application to Organic Transistors

Nanocrystal Array Engineering and Optoelectronic Applications of Organic Small-Molecule Semiconductors

Organic small-molecule semiconductor materials have attracted extensive attention because of their excellent properties. Due to the randomness of crystal orientation and growth location, however, the preparation of continuous and highly ordered organic small-molecule semiconductor nanocrystal arrays still face more challenges. Compared to organic macromolecules, organic small molecules exhibit better crystallinity, and therefore, they exhibit better semiconductor performance. The formation of organic small-molecule crystals relies heavily on weak interactions such as hydrogen bonds, van der Waals forces, and π-π interactions, which are very sensitive to external stimuli such as mechanical forces, high temperatures, and organic solvents. Therefore, nanocrystal array engineering is more flexible than that of the inorganic materials. In addition, nanocrystal array engineering is a key step towards practical application. To resolve this problem, many conventional nanocrystal array preparation methods have been developed, such as spin coating, etc. In this review, the typical and recent progress of nanocrystal array engineering are summarized. It is the typical and recent innovations that the array of nanocrystal array engineering can be patterned on the substrate through top-down, bottom-up, self-assembly, and crystallization methods, and it can also be patterned by constructing a series of microscopic structures. Finally, various multifunctional and emerging applications based on organic small-molecule semiconductor nanocrystal arrays are introduced.

Recent Advances in Realizing Highly Aligned Organic Semiconductors by Solution-Processing Approaches

Solution-processing approaches are widely used for controlling the aggregation structure of organic semiconductors because they are fast, efficient, and have strong practicability. Effective regulation of the aggregation structure of molecules to achieve highly ordered molecular stacking is key to realizing effective carrier transport and high-performance devices. Numerous studies have achieved highly aligned organic semiconductors using different solution-processing approaches. This article provides a detailed review of the prevalent solution-processing technologies and emerging methods developed over the past few years for the alignment of organic semiconducting materials. These technologies and methods are classified according to the processing principle. This review focuses on the principles of different experimental techniques, improvements upon the conventional methods, and state-of-the-art performance of resulting devices. In addition, a brief discussion of the characteristics and development prospects of various methods is presented.

Engineering Asymmetric Charge Injection/Extraction to Optimize Organic Transistor Performances

The introduction of an appropriate functionality on the electrode/active layer interface has been found to be an efficient methodology to enhance the electrical performances of organic field-effect transistors (OFETs). Herein, we efficiently optimized the charge injection/extraction characteristics of source/drain (S/D) electrodes by applying an asymmetric functionalization at each individual electrode/organic semiconductor (OSC) interface. To further clarify the functionalizing effects of the electrode/OSC interface, we systematically designed five different OFETs: one with pristine S/D electrodes (denoted as pristine S/D) and the remaining ones made by symmetrically or asymmetrically functionalizing the S/D electrodes with up to two different self-assembled monolayers (SAMs) based on thiolated molecules, the strongly electron-donating thiophenol (TP) and electron-withdrawing 2,3,4,5-pentafluorobenzenethiol (PFBT). Both the S and D electrodes were functionalized with TP (denoted as TP-S/D) in one of the two symmetric cases and with PFBT in the other (PFBT-S/D). In each of the two asymmetric cases, one of the S/D electrodes was functionalized with TP and the other with PFBT (to produce PFBT-S/TP-D and TP-S/PFBT-D OFETs). The vapor-deposited p-type dinaphtho[2,3- b:2',3'- f]thieno[3,2- b]thiophene was used as the OSC active layer. The PFBT-S/TP-D case exhibited a field-effect mobility (μ_FET) of 0.86 ± 0.23 cm² V^-1 s^-1, about three times better than that of the pristine S/D case (0.31 ± 0.12 cm² V^-1 s^-1). On the other hand, the μ_FET of the TP-S/PFBT-D case (0.18 ± 0.10 cm² V^-1 s^-1) was significantly lower than that of the pristine case and even lower than those of the TP-S/D (0.23 ± 0.07 cm² V^-1 s^-1) and PFBT-S/D (0.58 ± 0.19 cm² V^-1 s^-1) cases. These results were clearly correlated with the additional hole density, surface potential, and effective work function. In addition, the contact resistance ( R_C) for the asymmetric PFBT-S/TP-D case was 10-fold less than that for the TP-S/PFBT-D case and more than five times lower than that for the pristine case. The results contribute a meaningful step forward in improving the electrical performances of various organic electronics such as OFETs, inverters, solar cells, and sensors.

Directional Solvent Vapor Annealing for Crystal Alignment in Solution-Processed Organic Semiconductors

A unified approach of directional solvent vapor annealing for crystal alignment in solution-processed organic semiconductors is proposed. Highly crystalline molecular self-assembly of the drop-cast technique is further enhanced by postprocessing scheme of the solvent vapor annealing with additional benefit of alignment of the crystalline domains. In this technique, a mixture of carrier gas and solvent vapors are made to flow in a certain direction and in the close proximity of the surface of the substrates carrying the solution. Flow of the carrier gas imparts directionality to the semiconducting crystalline ribbons, whereas the influx of the solvent vapors improves the crystalline order in the semiconducting film. The flow rate of the carrier gas and the position of the substrate in the interaction chamber are the primary regulating factors, which have the ability to provide a semiconducting layer with a well-aligned and interconnected assembly of long ribbons. These favorable film properties further materialize in the form of electrical performance of the corresponding field-effect transistors. The versatility of this technique makes it a viable alternative for the solution processing of organic semiconductors.