The liquid-phase methods are the most frequently used methods for preparing cocrystals owing to the advantages of low cost and easy preparation (Yan et al., 2014; Li and yan, 2018b; Lu et al., 2018). By adjusting some factors such as solvent type, temperature, and concentration, cocrystals of different morphologies and sizes can be obtained easily (Wang et al., 2021). Here, we mainly introduced three common liquid-phase methods: slow evaporation, drop-casting, and diffusion method.

In the slow evaporation method, the mixture of donors and acceptors is dissolved in the organic solvent and then kept at room temperature (Figure 1A). As the solvent evaporates, raw components aggregate and crystallize as a result of the intermolecular interaction. The donors and acceptors should have similar and good solubility in the same solvent to avoid the precipitation of a single component (Sun et al., 2019; Wu et al., 2021). Since the solubility of raw components highly depends on the solvent type, the selection of solvent is very crucial. When changing the solvent type, the morphology and composition of cocrystal can be quite different. For instance, by using CH₂Cl₂ and tetrahydrofuran (THF) as the solvent, respectively, Wang et al. obtained a binary NDI-Cor (NDI, napthalenetetracarboxylic diimide; Cor, coronene) with ribbon structure and a ternary (NDI-Cor)·THF with block structure (Wang et al., 2020a). Slow cooling evaporation is based on the slow evaporation method, which is a method for growing cocrystals by controlling the temperature condition (Figure 1B). With this method, more components dissolve in the solution as the temperature increases, the raw materials crystallize as the temperature decreases. This method is more suitable for materials with moderate solubility at room temperature (Zhang et al., 2017a).

While the slow evaporation method is used to grow cocrystals with big sizes, the drop-casting method is used to prepare micro/nano cocrystals for constructing optoelectronic devices (Sun et al., 2018; Yan, 2015). By dropping an amount of solution on the prepared substrate, raw components gradually nucleate and crystallize with the volatilization of solvent in the droplet (Figure 1D). In this method, the solution concentration is a crucial factor affecting the micro-/nanostructures of cocrystals. Liu et al. revealed that the DMAQ (DMAQ, 4-(4-Dimethylaminostyryl)quinoline) and FDIB (FDIB, 1,4-diiodotetrafluorobenzene) with high concentration formed an M-DFC cocrystal with a two-dimension (2D) hexagonal microplate shape, whereas the low concentration formed a T-DFC cocrystal with 2D rhomboid-shaped microplate morphology (Liu Y. et al., 2019). Injecting a solution of raw materials into the nonvolatile solvent before drop-casting can induce cocrystals with unique morphologies. For example, after injecting a solution of pyrene and TCNB into an ethanol/water mixture, microtubes of pyrene-TCNB were collected on the quartz substrate (Sun et al., 2017).

The process of diffusion method is more complex, in which the raw materials are dissolved in a good solvent, and then a poor solvent (methanol, ether, or triethylamine) is diffused into the solution. The solubility of the solution gradually decreases as the poor solvent diffuses, and then the solution becomes saturated for crystallization (Figure 1C). The slow diffusion process guarantees the good quality and large size of cocrystal (Huang et al., 2019). Wang et al. assembled NDI-Δ with coronene (NDI-Δ, an organic naphthalenediimide-based triangle) by the diffusion method, obtained two bulk cocrystals of CNC-T and CNC-Q with good quality for the X-ray single-crystal structure characterization (Wang et al., 2020b).

Compared with the liquid phase methods, vapor phase methods are unrelated to materials’ solubility, which are suitable for materials with low solubility (Wang et al., 2021; Fang et al., 2017). The physical vapor transport (PVT) method is most popular (Figure 1E), using equipment consisting of a vacuum pump, a tubular furnace, a quartz tube, temperature controllers, and a gas path device. Under a flowing atmosphere of inert gas or in a vacuum, the original components in the high-temperature region sublimate and are subsequently transported to the low-temperature zone to form cocrystals. There are two types of PVT methods according to the sublimation points of the constituents (Figure 1E). The components are placed in the same sublimation region when the sublimation temperatures of the donors and acceptors are similar (Figure 1E-i). For example, two sizes of coronene-HAT(CN)₆ (HAT(CN)₆, 1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile) were prepared by coevaporation in argon gas or vacuum (Liang et al., 2019). Another type of PVT method is appropriate for the constituents with significantly different sublimation points, in which the donors and acceptors are placed in two furnace regions (Figure 1E-ii). By placing the donors and acceptors in two furnace regions of 155°C and 190°C, respectively, the micro cocrystals of TMB-TCNQ (TMB, 3,3′,5,5′-tetramethylbenzidine) were obtained (Mezzadri et al., 2018). However, the products are difficult to separate, which is an inevitable problem when using this method to prepare cocrystals with different phases (Wang et al., 2021).

The PVT method requires a vacuum environment and a long time, resulting in high cost and time-consuming (Sun et al., 2019). To solve this problem, Tao’s group proposed a microspacing in-air sublimation (MAS) method to grow a series of PAH-TCNB (PAH, polycyclic aromatic hydrocarbon) cocrystals, which exhibited (one-dimension) 1D needle-like (Figure 2A) or 2D plate-like morphologies (Figure 2B) (Ye et al., 2019).

Solid-phase methods produce fewer organic cocrystals than the methods mentioned above. However, in recent years, these methods being commonly employed to prepare cocrystals due to the advantages of vacuum/heat-free conditions and a minimal amount of solvent or no solvent. The solid-phase methods can be divided into plain grinding and liquid-assisted grinding (LAG) methods. In the plain grinding method, raw materials are mixed according to a certain molar ratio in a mortar for grounding. This method is suitable for raw materials with poor solubility (Sun et al., 2018). As an example, in the grounding process, the yellow BQ and IP (BQ, p-benzoquinone; IP, 4-iodophenol) powders converted into red in several seconds, forming a 1:1 BQ-IP cocrystal (Carstens et al., 2020). Although the grinding method is fast and has a higher yield, the products always have small sizes and irregular morphologies. The other grinding method is liquid-assisted grinding (LAG). By adding a small amount of solvent during the grinding process, the interaction between donors and acceptors becomes stronger as the friction between the two substances increases, contributing to the cocrystallization of the components (Sun et al., 2018). Huang et al. successfully prepared TC-OFN (TC, tetracene; OFN, octafluoronaphthalene) by adding the THF solvent twice in a two-step LAG process (Figure 2C). This method produces cocrystals with better crystallinity and more controllable polymorphs (Huang et al., 2020).

At present, researchers have made great progress in the synthesis of organic semiconductor materials with ambipolar properties (Zhang J. et al., 2018; Mandal et al., 2019a; Mandal et al., 2019b). Regardless, there are few high-performance and stable ambipolar materials in the ambient atmosphere because of the complexity and uncertainty of the synthesis route. It is inspiring that the cocrystal strategy can effectively integrate donors and acceptors into a single crystal system to obtain hole or electron carriers channels (Sun et al., 2019). This “molecular level heterojunction” provides an alternative approach to realize ambipolar transport through an easy-to-process method of low cost and high efficiency. Therefore, the cocrystals are considered promising active elements to construct ambipolar OFETs with high performance. Herein, we introduce the latest achievements in OFETs based on cocrystals and discuss the influencing factors on the adjustable ambipolar properties, including energy level, molecular stacking pattern, and molecule structure, from theoretical and experimental perspectives.

Compared with single-component materials, the electronic properties of cocrystals can be easily regulated by altering the donors or acceptors (Wang et al., 2021; Sun et al., 2018). Using molecules with increasing F atoms as acceptors is a typical method for regulating the charge transport properties of cocrystals (Liu H. et al., 2019; Wei et al., 2020). The increasing electron affinity of acceptors usually results in enhanced CT degree, which has a significant impact on the molecular stacking pattern and the energy levels, further influencing the charge transport properties of cocrystals. In comparison to DPTTA-TCNQ that had no CT between D-A molecules, DPTTA-F_xTCNQ (FxTCNQ, fluorinated derivatives of 7,7,8,8,-tetracyanoquinodimethane, X = 2, 4) exhibited enhanced CT features with almost identical overlap patterns between D-A molecules along the stacking direction (Liang et al., 2020). Furthermore, Liang et al. proved that the CT degree of DPTTA-F_xTCNQ (X = 1, 2, 4) increased as F atoms of the acceptor molecules increased. The calculated transfer integrals displayed an increasing tendency, indicating that the electronic coupling improved from DPTTA-F₁TCNQ, DPTTA-F₂TCNQ to DPTTA-F₄TCNQ. The relatively strong intermolecular electronic couplings led to more dispersed valence bands and conducting bands, as well as narrower band gaps (Zheng et al., 2015). OFETs based on these cocrystals all exhibited ambipolar transport characters. The mobilities were 0.15 cm² V⁻¹ s⁻¹ (μh) and 0.24 cm² V⁻¹ s⁻¹ (μe) for DPTTA-F₁TCNQ, respectively; 1.01 cm² V⁻¹ s⁻¹ (μh), 0.27 cm² V⁻¹ s⁻¹ (μe) for DPTTA-F₂TCNQ; and 0.11 cm² V⁻¹ s⁻¹ (μh), 0.46 cm² V⁻¹ s⁻¹ (μe) for DPTTA-F₄TCNQ. It should be noted that the predominant carrier in DPTTA-F₄TCNQ were electrons, while that in DPTTA-F₁TCNQ were holes (Figures 3A–C). The n-doping in the DPTTA-F₄TCNQ was contributed to the deepest conducting band minimum (CBM) level caused by the strongest electron affinity of F₄TCNQ. On the contrary, the F₁TCNQ complex preferred to be p-type doped because of the highest valence band maximum (VBM) level (Figure 3D) (Liang et al., 2020). This study shed light on the design of cocrystals with ambipolar transport behaviors.

In addition, Yu et al. also achieved the ambipolar charge transport in cocrystals by assembling acceptors with donors of different aromatic conjugated backbones. With the aromatic conjugated backbone of donors increased, the energy levels of supramolecular hybrid orbitals in D/A pairs were higher, contributing to the CT interaction (Zhang X. et al., 2018; Dasari et al., 2019). They synthesized four cocrystals using PDICNFN (PDICNF, N′-bis(perfluorobutyl)-1,7- dicyanoperylene-3,4:9,10-bis (dicarboximide) as the acceptor and anthracene, pyrene, perylene, and DPTTA as the donors (Figure 4A). The theoretical calculation of density functional theory (Jiang et al., 2018) suggested that in the range of -3.82 eV to -4.07 eV, the cocrystals maintained similar lowest unoccupied molecular orbitals (LUMOs), slightly higher than PDICNF. The highest occupied molecular orbitals (HOMOs) of the cocrystals increased from −5.75 eV (anthracene-PDICNF) to −4.84 eV (DPTTA-PDICNF), higher than the corresponding donors. Meanwhile, the extended π-conjugated system of the donor molecule DPTTA further promoted electronic coupling. Therefore, the DPTTA-PDICNF was hypothesized to have better charge transport properties, which were confirmed by well-balanced field-effect mobilities of 2.0 × 10^–2 cm² V⁻¹ s⁻¹ for the holes and 1.7 × 10^–2 cm² V⁻¹ s⁻¹ for the electrons (Figures 4B,C). The anthracene-PDICNF, pyrene-PDICNF, and perylene-PDICNF only showed n-transport properties. Notably, pyrene-PDICNF had carrier mobility of 0.19 cm² V⁻¹ s⁻¹, the highest value ever found in PDI-based cocrystals (Yu et al., 2021). This research also provided a guide for synthesizing cocrystals with ambipolar transport properties.

Considering that the electrical properties of cocrystals highly rely on the molecular structures (Zhu et al., 2014; Ai et al., 2017), selecting D-A molecules with matching structures as constituents is another strategy to achieve the ambipolar properties. For example, DTTCNQ [DTTCNQ, 4,8-bis(dicyanomethylene)-4,8-dihydrobenzo(1,2-b:4,5-b')-dithiophene] with the extended π-conjugated system may better match the donor molecule than TCNQ. The increasing conjugated system and partial charge-transfer character in DPTTA-DTTCNQ enhanced D-A interactions by shortening the D-A distance and formed a quasi-2D ambipolar transport network. There were both superexchange and indirect paths for charge transport. Thus, high charge-transport properties could be expected by extending the π-conjugated systems despite the weak electron-accepting ability of DTTCNQ (Qin et al., 2014). In addition to applying the similar structures of D-A molecules, complementary geometry also facilitates charge transport. Recently, Gao et al. synthesized diindeno (4,3,2,1-fgh i:4′,3′,2′, 1′-Opqr) perylene, which was a subunit of C₇₀. This buckybowl skeleton was functionalized at the meta-positions with triethylsilyl-ethynyl (TES-ethynyl) (1), ensuring the solubility and stability of the buckybowl skeleton and forming 1D concave-in-convex stacking columns with a hole mobility of 0.31 cm² V⁻¹ s⁻¹. Considering the potential shape complementarity, one was blended with the C₇₀ acceptor to obtain a novel cocrystal. The TES-ethynyl helped form buckybowls arrangement with strong concave-convex interactions. As shown in Figure 5A, each C₇₀ molecule made contact with six bowl molecules, forming 2D cocrystals and facilitating the effective transmission of charge carriers through curved surfaces. The OFET measurements demonstrated that the cocrystal possessed ambipolar property, with electron and hole mobilities of 0.40 cm² V⁻¹ s⁻¹ and 0.07 cm² V⁻¹ s⁻¹, respectively (Figure 5B) (Gao et al., 2020), indicating that the complementary structures were promising for the ambipolar transport of cocrystals.

All in all, cocrystal engineering provides a practical and simple strategy for systematically controlling the operation mode (ambipolar, or p-/n-type) of the transistor by modifying the components. Through co-crystallization, the band gaps of the semiconductors can be adjusted to facilitate the energy matching between the cocrystal Frontier orbitals and the work function of the injected electrodes, which is beneficial to efficient charge injection to improve the OFETs performance.

Photoresponse materials play an important role in the organic optoelectronics field, which can transfer optical signals into electrical signals, have wide applications in photodetectors (Altaqui et al., 2021), photoswitches (Kellner and Berlin, 2020), phototransistors (Gelinck et al., 2010; Dong et al., 2012), and optical imaging (Chen et al., 2021). An idea photoresponse device should ensure the processes of photon absorption, exciton dissociation, and charge carrier transport (Najafov et al., 2010; Ostroverkhova, 2016). The features of modulating absorption, special D-A molecular interfaces engender cocrystals serving as outstanding candidates for photoresponse (Wu et al., 2014b; Wang et al., 2016b; Wang et al., 2020d; Singha et al., 2021). In this section, besides the superiorities, we will discuss the structure-property relationship of cocrystals in photoresponse and introduce recent high-performance photoresponse devices based on micro/nano cocrystals.

In CT cocrystals, a new CT state generates between donors and receptors because of the intermolecular interaction, allowing for redshift absorption (Siegmund et al., 2017). When strong CT interaction occurs, the CT absorption band moves to the long-wavelength region (Sun et al., 2019; Tang et al., 2021). By virtue of this phenomenon, photoresponse in the infrared or near-infrared region can be achieved. Wakahara et al. fabricated an OFET, in which 3,5-TPP/C₆₀ [3,5-TPP, 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)porphyrin] served as the semiconductor layer (Figures 6A,B). The four dimethoxyphenyl substitutions endowed the 3,5-TPP with a strong electron-donating ability that enhanced the CT interaction with C₆₀. A new CT absorption band emerged at 600–800 nm. The channel current (ID) increased as the light intensity (Elight) increased when light-emitting diodes with emission peaks in the visible-to-NIR region (450, 590, 660, 810, and 940 nm) were used to illuminate the OFET (Figure 6C). The increasing current at 810 nm was attributed to the CT state in the C₆₀/3,5-TPP cocrystals were excited to generate excitons that subsequently separated. Due to the CT absorption band and component bands, the phototransistor based on the C₆₀/3,5-TPP cocrystal exhibited a strong photoresponse at 660 nm, and the measured photosensitivity was 4.5 (0.05 mW/cm²) (Figure 6D) (Wakahara et al., 2020).

In addition to the modulating absorption, the plenty of D-A interfaces in cocrystals ensure efficient exciton dissociation, contributing to the photoelectric conversion (Wang et al., 2017; Sun et al., 2019). The CT excitons in cocrystals are considered a highly localized excitation-pair state and then relax to the ground state or dissociate into free carriers (Hubig and Kochi, 1995; Sun et al., 2019). Meanwhile, the hybrid molecular orbital (MOs) at D-A interfaces hinder the reversed charge-transfer process, which prevents the exciton recombination, ultimately affects the photoresponse. Zhang’s group selected TMIQ (TMIQ, 8,8,18,18-tetramethyl-8,18-dihydroindolo(1,2,3-fg)indolo(3′,2′,1′:8,1)quinolino[2, 3-b]acridine) as the donor and synthesized it with acceptors of CA, FA, and TCNQ (CA, p-chloranil; FA, p-fluoranil). Under photoexcitation, the charge was redistributed between D-A molecules, which enhanced the density of charge carrier and thus induced the photocurrent. The large energy barriers in TMIQ-CA and TMIQ-FA were 0.4 and o.96 eV, which hindered the reversed charge-transfer processes, while the energy barrier was lost in TMIQ-TCNQ. Therefore, only the TMIQ-CA and TMIQ-FA exhibited photoresponse properties. However, the result appeared that the TMIQ-CA showed the best photoresponse despite having a smaller energy barrier than TMIQ-FA. It may be attributed to that the CH … C bonds network of donors in TMIQ-CA further promoted the excitons separation and carrier transport. Under ultraviolet (UV) illumination, the phototransistor based on TMIQ-CA had a maximum photocurrent on/off ratio of 353, photoresponsivity of 3.0 × 10³ A W⁻¹, detectivity of 1.4 × 10¹⁴ Jones, and external quantum efficiency of 2.4 × 10⁶%, which were the best values among all reported organic cocrystals (Wang et al., 2020d).

It is worth noting that the optoelectronic properties of cocrystals are also closely related to the molecular stacking structure (Park et al., 2013; Zhang et al., 2017b). Cocrystals with identical components but different stacking structures exhibit different photoresponse properties (Goetz et al., 2016). It was proposed that the (perylene)₁-TCNQ with segregated-stacking mode had better photoresponse properties than the (perylene)₃-TCNQ with mixed-stacking mode, which was unfavorable for the exciton dissociation (Zhu et al., 2015). A recent study reported that the cocrystals with different phases also showed different photoresponse properties. Jin et al. synthesized α-phase and β-phase cocrystals composed of perylene and DTTCNQ through homogeneous and heterogeneous nucleation, respectively. Thereinto, the α-phase cocrystal exhibited ambipolar transporting, but the semiconducting feature and photoresponse were low (Figure 7A). Compared to the brick-type α-cocrystal, β-cocrystal had a 20.5° rotation angle between D-A molecules, more like a columniform type (Figures 7B,C). This packing mode avoided steric hindrance but caused the vanish of the p-type channel (Figure 7D). Nonetheless, the photocurrent of the device based on β-cocrystal increased sharply under the illumination. The photosensitivity reached 1.5 × 10⁵ at V_G of 1 V, and the photoresponsivity was 28.2 mA W⁻¹ (Figures 7E,F) (Jin et al., 2020).

Thanks to the advantages in photoelectric conversion, cocrystals have been widely used in photoresponse. Nowadays, novel ways for synthesizing cocrystals with photoresponse properties are being developed (Dong et al., 2012; Wang C. et al., 2018). For instance, molecule-level heterojunction cocrystal thin films, which promote the migration and separation of excitons, display great potential in achieving photoresponse. Yang et al. assembled AD with IPA, IPB, and TMA (AD, acridine; IPA, isophthalic acid; IPB, 5-bromoisophthalic acid; TMA, trimesic acid) to obtain three cocrystal thin films of AD-IPA, AD-IPB, and AD-TMA (Figure 8A). Among the three cocrystal thin films, the AD-TMA thin film exhibited the best photoresponse. The high crystallinity of the AD-TMA thin film benefited the transfer of charge carriers. Besides, the TMA anions layer and AD cation layer formed an internal electric field that promoted the efficient charge carriers separation. In a three-electrode system, the photocurrent density of the AD-TMA thin film electrode rapidly increased to 27.79 μA/cm² (I_light) under the on-off cycle’s illumination (30 s). After switching off the irradiation, the low photocurrent density is 0.002 μA/cm² (I_dark) (Figure 8B). The maximum current on/off ratio of the AD-TMA cocrystal thin film was 13,895 (I_light/I_dark), much higher than that of carbon nitride nanotube membranes, metal-organic framework materials in electrolytes, and the optoelectronic devices composed of inorganic perovskite and organic single crystal, indicating the exceptional sensitivity to light. Furthermore, the incident photon-to-current efficiency of the AD-TMA thin film was highest (Figure 8C). The fast CT rate was also confirmed by the lowest CT resistance (Figure 8D) (Yang et al., 2020). Recently, Wang et al. successfully fabricated a vertical photodetector device based on the 2D cocrystal film of ZnTPP (ZnTPP, 5,10,15,20-tetraphenyl-21H,23H-porphine Zinc) and C₆₀. The photoresponsivity of this large-area cocrystal film was as high as 2,424 mAW⁻¹ at 800 nm, combined with fast response times and high external quantum efficiency of 376%, further proving the superiority of cocrystal film in photoresponse (Wang et al., 2020c).

The cocrystal strategy provides a fascinating avenue for constructing materials with photoresponse by rationally selecting the donors and acceptors. The features of strong intramolecular interaction and unique structure facilitate an efficient photoelectric conversion. Nevertheless, the ultimate goal is to achieve more cocrystals with high-performance photoresponse, which requires further exploration and expansion of the co-crystalline system.