Highly ordered organic crystalline materials provide an ideal platform for next-generation flexible electronics.

materials, [9][10][11][12] which are crystalline singlephase materials composed of two or more different compounds generally in a stoichiometric ratio, [13] providing a platform for unveiling the structure-property relationships at a molecular level. [14] The building blocks are assembled via noncovalent interactions, offering the opportunity to achieve noncovalent synthesis of functional molecules. [15] Compared with the traditional covalent synthesis, cocrystal engineering offers numerous advantages, including: 1) avoiding complicated synthesis procedures, and cocrystal assemblies have been successfully fabricated by the vapor methods and solution-processing methods in a facile and low-cost way; 2) manipulating intermolecular interactions through selecting suitable conformers from a plentiful supply of raw materials, resulting in tunable structures, morphologies, and sizes; 3) achieving rare and multifunctional properties through a collaborative strategy in distinct constituent units, which are difficult to realize for individual components. [16][17][18] Cocrystal engineering began to draw much attention since the exploration of the metal conductive tetrathiafulvalene-7,7,8,8-tetracyanoquinodimethane (TTF-TCNQ) cocrystals in 1973. [19] The peculiar conductivity in organic materials makes it possible for cocrystals to serve as organic electrodes, which can effectively lower the contact resistance and further improve device performance. Encouraged by this, cocrystals can serve as a breakthrough and provide an opportunity to discover novel physicochemical properties, which are far beyond the performance of single materials. In this respect, dielectric response [20] and nonlinear optics [21,22] are also realized through rational design of each component, which is absent in their constitute components. Even more, the bottom-up supramolecular assembly provides the opportunity to equip them with the individual properties of the donor or acceptor to create multifunctional materials. [23] Typically, ambipolar charge transport can be generated by coassembling p-type semiconductors and n-type ones, and the charge-carrier mobility is now up to 1.57 cm 2 V −1 s −1 for holes and 0.47 cm 2 V −1 s −1 for electrons. [24] To date, these exciting results accelerate the investigations of organic functional cocrystals, such as room-temperature ferroelectricity, [25][26][27] optical waveguide properties, [28,29] pure organic room-temperature phosphorescent properties, [30][31][32] stimulusresponse (e.g., mechanical stress, [33,34] heat, [35,36] or solvent [37] ) characteristics, photovoltaics, [38] and near-infrared photothermal (PT) conversion and imaging, [39] etc.Cocrystal engineering with a noncovalent assembly feature by simple constituent units has inspired great interest and has emerged as an efficient and versatile route to construct functional materials, especially for the fabrication of novel and multifunctional materials, due to the collaborative strategy in the distinct constituent units. Meanwhile, the precise crystal a...

Organic cocrystals, formed by a combination of electron-rich donors and electron-poor acceptors, play an important role in tailoring the optoelectronic properties of molecular materials. Charge transfer interactions in cocrystals not only endow them with an ordered three-dimensional (3D) supramolecular network in different constituent units, but also render them ideal scaffolds to control the intermolecular interactions in multicomponent solids. In this perspective, we firstly introduce preparation methods, molecular packing modes and charge transfer in organic cocrystals. Then, we focus on the novel and promising optoelectronic properties of organic cocrystals based on charge transfer interactions. Finally, we briefly discuss the outlook for the future development of these multicomponent crystalline materials.