Compared with one-component organic single crystals, organic cocrystals with unique packing structures and aggregate states show a variety of novel optoelectronic properties through multi-component synergistic and collective effects, paving the way to the development of high-performance or multifunctional optoelectronic devices, particularly in electrical conductors, ferroelectricity, ambipolar charge transporting, photoconductivity and luminescence. Moreover, the charge transfer pathway between donor (D) and acceptor (A) in organic cocrystal is also fundamentally interesting. Therefore, studies on organic cocrystals have gained much attentions in recent years. In this paper, firstly, we have classified organic cocrystals into three categories by the driving forces: charge transfer (CT), π-π interaction and halogen/hydrogen bond; secondly, we have introduced common organic donor materials, using 7,7,8,8-tetracyanoquinodimethane (TCNQ), 1,2,4,5-tetracyanobenzene (TCNB) and fullerene (C₆₀) as the typical acceptors; Thirdly, we have listed eight popular methods used to prepare these organic cocrystals and discussed the relationship between molecule packing and performance of organic cocrystal; Finally, we have covered the applications of these novel cocrystals in organic optoelectronics. We believe that the study on the organic cocrystals is an effective way to achieve organic multi-functional single crystal materials and devices and to promote the development of organic solid state optoelectronics.

Contents
1 Introduction
2 Classification of organic cocrystals
2.1 Charge transfer complex crystals
2.2 Crystals formed by π-π interactions
2.3 Crystals formed by halogen and hydrogen bonds
2.4 Other crystals
3 Common donor/acceptor materials
3.1 TCNQ and its derivatives as acceptor
3.2 TCNB as acceptor
3.3 C₆₀ as acceptor
4 Methods for preparation of crystals
4.1 Solution methods
4.2 Vapor phase method
4.3 Mechanochemical preparation
5 Relationship between molecular packing and function
6 Applications of organic cocrystals
6.1 Unipolar field effect transistors
6.2 Ambipolar field effect transistors
6.3 Photoresponse devices
6.4 Luminescent materials
6.5 Ambipolar field-effect and solid emitting crystals
6.6 Ferroelectric crystals
7 Conclusion and outlook