Solution-Processed, Large-Area, Two-Dimensional Crystals of Organic Semiconductors for Field-Effect Transistors and Phototransistors.

Organic electronics with π-conjugated organic semiconductors are promising candidates for the next electronics revolution. For the conductive channel, the large-area two-dimensional (2D) crystals of organic semiconductors (2DCOS) serve as useful scaffolds for modern organic electronics, benefiting not only from long-range order and low defect density nature but also from unique charge transport characteristic and photoelectrical properties. Meanwhile, the solution process with advantages of cost-effectiveness and room temperature compatibility is the foundation of high-throughput print electrical devices. Herein, we will give an insightful overview to witness the huge advances in 2DCOS over the past decade. First, the typical influencing factors and state-of-the-art assembly strategies of the solution-process for large-area 2DCOS over sub-millimeter even to wafer size are discussed accompanying rational evaluation. Then, the charge transport characteristics and contact resistance of 2DCOS-based transistors are explored. Following this, beyond single transistors, the p-n junction devices and planar integrated circuits based on 2DCOS are also emphasized. Furthermore, the burgeoning phototransistors (OPTs) based on crystals in the 2D limits are elaborated. Next, we emphasized the unique and enhanced photoelectrical properties based on a hybrid system with other 2D van der Waals solids. Finally, frontier insights and opportunities are proposed, promoting further research in this field.

Introduction

Organic field-effect trann class="Chemical">sistors (OFETs) and photoelectric devices un class="Chemical">sing π-conjugated organic semiconductors as active layers have attracted increasing attention for next-generation flexible and printable electronics.[1−3] Although intensive research efforts, from material design to process engineering, have been devoted to achieving progress in performance, traditional bulk polycrystalline or amorphous thin films still face vital challenges due to intrinsic properties as well as serious device performance impediments in high-density integrated circuits.[4,5] Because of their lack of grain boundaries and extremely low defect densities, organic single crystals, in which organic semiconductor molecules are periodically arranged by relatively weak noncovalent bonds (such as van der Waals forces and hydrogen bonds), have been regarded as model systems for improving performance and revealing structure–property relationships.[6−8] The charge mobility has been optimized over 10 cm2 V–1 s–1,[9−13] and studies of anisotropic charge transport typically reveal the dominant crystal orientation.[14−16] However, because of their weak intermolecular interactions, organic semiconductor molecules are commonly assembled into micro/nano single crystals with disordered distribution and random orientation with respect to the substrate, limiting their practical applications.

Large-area, two-dimenn class="Chemical">sional (n class="Chemical">2D) crystals of organic semiconductors, named 2DCOS, whose thicknesses scale down to the monolayer or few monolayers over sub-millimeter to wafer sizes, are promising candidates to satisfy the expected demand for modern organic field-effect devices.[17] Benefiting from long-range ordered molecular packing and suppressed bulk contact resistance, large-area 2DCOS could serve as useful scaffolds for logic circuits, with encouraging carrier transport and high-speed switching characteristics.[16,18,19] In addition, compared to thick crystals, 2DCOS with controllable numbers of layers have significant advantages in studies of basic problems, such as the question of the intrinsic conduction mechanism, as well as in the construction of photoelectric devices that achieve groundbreaking functionalities.[20,21] Nonetheless, traditional epitaxial growth methods for 2D materials not only affect the molecular packing closest to the interface through van der Waals forces, but also limit the size of the crystals. Strict growth conditions and high costs are also obstacles to practical commercial applications. Alternatively, assembly treatments in solution allow large-scale production of semiconductor crystals in ambient conditions without a vacuum, representing an important way to reduce industrialization costs and promote the development of crystal engineering.

As early as in 2011, the first millimeter-sized monolayer n class="Chemical">2DCOS were prepared successfully by the Hu group via a solution self-assembly process, and their mobility already exceeded 1 cm2 V–1s –1.[22] The need for performance modulation has greatly promoted the development of various solution assembly processes to obtain large-area 2DCOS persistently in the past decade. Although novel features of ultrathin single crystals have been described in many outstanding reviews,[23−26] the 2DCOS over millimeter to wafer size that have flourished in recent years are rarely mentioned. Meanwhile, the solution-processed, large-area crystals are often mentioned as subsections in many comprehensive reviews covering 2D functional materials,[27] organic crystalline semiconductors in electronics, and flexible devices.[7,28,29] Therefore, with the rapid development of this emerging field, a systematic and specialized review on 2DCOS with a large lateral size is imperative. In this outlook, we will focus on the fabrication of large-area 2DCOS via the solution assembly process, as well as their optoelectrical field-effect devices (Figure ). We start in Section 2 with the critical factors affecting the solution process and discuss representative self-assembly and confined-assembly techniques for large-area 2DCOS. Next, we emphasize the electrical properties of 2DCOS approaching millimeter to wafer sizes, including charge transport characteristics, carrier injection behavior of individual transistors, and integration of devices beyond single OFETs. In Section 4, the most recent organic phototransistors (OPTs) based on ultrathin 2DCOS are illustrated, mainly focusing on peculiar single-crystal characteristics in the 2D limit. In Section 5, interfacing 2DCOS with other 2D van der Waals solids (vdWs) will be emphasized according to the unique and enhanced photoelectrical properties. Finally, on the basis of current progress, we predict challenges to be encountered and the orientation of future development in this promising field.

Figure 1

An overview concept, including the solution-processed self-assembly and confined-assembly for 2DCOS, as well as their photoelectrical devices in the main text.

An overview concept, including the solution-processed self-assembly and confined-assembly for n class="Chemical">2DCOS, as well as their photoelectrical devices in the main text.

Solution-Based Techniques for Fabricating Ultrathin, Large-Area 2DCOS

Driven by the potential applications for next-generation electronics, a number of solution-based techniques concerning large-n class="Chemical">size n class="Chemical">2DCOS with mono- or a few molecular layers have been recently developed. These π-conjugated organic small molecules, with excellent self-organizing behavior and good solubility in a variety of organic solvents, are particularly appropriate for efficient fabrication of high-performance, ultrathin 2DCOS by solution methods.[30] This section is tripartite: it introduces (i) the solution-based assembly processing parameters, (ii) the self-assembly techniques for growing large-area 2DCOS, and (iii) the confined-assembly techniques toward fabricating ultrathin large-area 2DCOS. It is our hope that the methods described here can inspire controlled assembly of large-scale 2DCOS.

Solution-Based Assembly Processing Parameters

In n class="Chemical">simple solution-based processes, it is worth pointing out that the aggregation, crystallinity, and grain n class="Chemical">size of the solution-processed crystals are sensitively affected by the external conditions. Undoubtedly, they are all key parameters for tuning the kinetics of crystal assembly, affecting the final morphology of 2DCOS. In this section, we summarize the influence of each parameter on the solution assembly process, which would help to guide the assembly of 2DCOS using the solution-processing techniques.

Concentration Effect

The concentration of solution can influence the nucleation and growth rate, which in turn influences the likelihood for the molecules to be packed with each other via intermolecular interactions.[31] Low nucleation denn class="Chemical">sities, when couple with pronounced growth rates, play a crucial role in reducing the denn class="Chemical">sity of the grain boundaries and ultimately lead to large crystalline domain sizes.[32] More interestingly, the thickness of the 2DCOS can also be controlled by adjusting the concentration of the solution. For instance, Jiang and co-workers reported that the 2DCOS thickness can be tuned from 3.5 to 14.5 nm (corresponding to 1–4 layers) by varying the concentration of 1,4-bis((5′-hexyl-2,2′-bithiophen-5-yl)ethynyl)benzene (HTEB).[22]

Temperature Effect

The substrate temperature can influence the molecular thermodynamics by changing the interactions between the molecule and the substrate.[33] For example, by exploiting the van der Waals interactions (vdW) at the molecule–substrate interface, the Li group successfully controlled the morphology of n class="Chemical">2DCOS with a clear number of layers in a self-assembled-growth manner by tuning the annealing temperature.[34] Because the upper-layer molecules are less thermodynamically stable than the bottom molecules, the monolayer n class="Chemical">2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) can be more accurately controlled than bilayer C8-BTBT from the perspective of molecular dynamics.

Solvent Effect

In solution-based self-assembly processes, molecular aggregation occurs when the molecule–molecule interactions are stronger than the solvent–molecule interactions.[35,36] The typical “good” solvents are n class="Chemical">benzene-type reagents, such as n class="Chemical">toluene, chlorobenzene, and dichlorobenzene, while the “poor” solvents include methanol, water, and ethanol. Poor solvents have weak molecule–solvent interactions, which could help molecular aggregation and promote crystallization.[37] By increasing the crystallization driving force at the gas/solution interface, use of “poor solvents” result in higher lateral growth rates at the gas/solution interface. For example, Hiromi et al. developed a method that combined a poor solvent technique with inkjet printing to produce highly crystalline 2DCOS.[38] The authors used 1,2-dichlorobenzene (DCB) as the good solvent and N,N-dimethylformamide (DMF) as the “poor solvent”, triggering the controlled formation of exceptional uniform single-crystal thin films at that grew the liquid/air interfaces.

Substrate Effect

Substrates for the solution-processed assembly can be categorized into two categories: solid substrates (e.g., n class="Chemical">silicon) and liquid substrates (e.g., n class="Chemical">water). Liquid substrates will be discussed in detail in Section 2.2.2. For solid substrates, surface tension and roughness significantly affect the crystallization process and determine the assembly of morphology. Self-assembled monolayers (SAMs) are regarded as an effective surface modification strategy for changing surface roughness and wettability to affect the crystallization process.[39] For example, the modification of SiO2 substrate with PTS-SAMs increased the substrate surface energy and reduce the contact angle, thereby making the substrate hydrophilic and improving the crystallinity of the deposited organic films.[40]

Surfactant Effect

In the past few decades, surfactant-assisted self-assembly has been successfully applied to morphology control in the solution assembly process for organic semiconductors.[41] Typically, the surfactant possesses hydrophilic and hydrophobic structures, which enhance the spread area of the solution on the substrate.[42] The surfactant limits the diffusion of molecules in a direction perpendicular to the lamellar plane, which provides in effect a limited 2D space for the growth of large-area continuous 2DCOS.

Annealing Effect

The thermal annealing and solvent vapor annealing are the n class="Chemical">simple yet highly efficient postprocesn class="Chemical">sing route to improve the crystalline degree of the film following vapor deposition or spin coating.[43] The thermal annealing could give energy to the molecules, leading to enlarging the single crystalline domains caused by the reorganization of the organic molecules in the thin films via self-assembly.[44] Most of the reported solvent vapor annealing is carried out at the room temperature.[45] During the solvent vapor annealing, predeposited thin films are partially dissolved in the solvent vapor and reorganized to increase crystalline grain sizes,[46] which could prevent thermal damage of the organic semiconductors.

Self-Assembly Techniques for Growing Large-Area 2DCOS

Self-assembly, which is a spontaneous assembly process driven by molecular interactions, has been developed into one of the most efficient ways to grow n class="Chemical">2DCOS.[47] It is generally conn class="Chemical">sidered that 2DCOS obtained using self-assembly techniques will be of high quality since the self-assembly is free from external forces.[26,28] At present, high-quality self-assembly techniques are performed primarily on two substrates. The first one focuses on “phase separation” molecular structure engineering toward 2DCOS by the drop-casting method on the solid substrate. The second part deals with the solution epitaxy method, which is a facile, general, and effective method for growing millimeter- or centimeter-sized 2DCOS on the solution surface.

“Phase Separation” Molecular Engineering toward 2DCOS by Drop-Casting

In a drop-casting process, organic crystals are held together by weak noncovalent interactions, which implies that the morphologies and dimenn class="Chemical">sions of the crystals are directly controlled by the molecule structure (Figure a).[48,49] At present, “phase separation” molecular engineering has been widely applied in the assembly of n class="Chemical">ultrathin 2DCOS, for two primary reasons.[50] (i) The rigid core of the molecule provides strong in-plane π–π overlap that forms a well-balanced 2D network, ideal for expansion into large-area 2D crystal films. (ii) The soft alkyl chains on sides of the rigid core foster weak alkyl···alkyl interactions do duty for the separation layer, resulting in 2DCOS with mono- to few-layers thickness. To date, there are three types of organic semiconductors with “phase separation” molecular structures: planar p-type semiconductors, planar n-type semiconductors, and nonplanar p-type semiconductors, as shown in Figure , top.

Figure 2

Representative semiconductors with a “phase separation” molecule structure discussed herein include planar p-type semiconductor, planar n-type semiconductor, and nonplanar p-type semiconductor. (a) Schematic diagram of a drop-casting technique for growing 2DCOS with the “phase separation” molecule structure. (b, c) POM of the 3Se-2N 2DCOS on SiO2/Si substrate. Reproduced with permission from ref (50). Copyright 2019, Wiley-VCH.

Representative semiconductors with a “phase separation” molecule structure discussed herein include planar p-type semiconductor, planar n-type semiconductor, and nonplanar p-type semiconductor. (a) Schematic diagram of a drop-casting technique for growing n class="Chemical">2DCOS with the “phase separation” molecule structure. (b, c) POM of the 3Se-2N n class="Chemical">2DCOS on SiO2/Si substrate. Reproduced with permission from ref (50). Copyright 2019, Wiley-VCH.

In 2011, Jiang et al. applied the n class="Chemical">HTEB molecule to produce n class="Chemical">ultrathin 2DCOS with millimeter-scale on various substrates by a simple drop-casting method.[22,51] The key to success was the HTEB molecule structure, in which π–π and C–H···π interactions ensured strong laminar layer interactions for efficient self-assembled 2D growth, and in which the long alkyl substituents drive separate out-of-plane interlayer interactions. In the subsequent reports, a variety of organic molecules were prepared into large-area ultrathin 2DCOS, including 2,6-bis(4-hexylphenyl)anthracene (C6-DPA) and dioctyl-substituted dibenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophenes (C8-DBTDT).[50,52−54] In order to further promote the development of materials for large-area 2DCOS, a novel “phase separation” molecular design strategy was recently reported by Fu and co-workers.[50] As shown in Figure b,c, the extended π-conjugated network (3Se-2N) increased exchange (e.g., π–π stacking, C–H···π interactions, and hydrogen bonding forces) beyond the bowl-shaped molecule using only dense convex–concave π–π stacking, which offered 2D network interactions and enabled millimeter-sized single crystals.

Attractively, drop-casting methods toward n class="Chemical">2DCOS are also suitable for fabricating n-type semiconductors with “phase separated” molecular structures. Recently, Weitz and co-workers reported that n class="Chemical">ultrathin (2–5 nm), large-area (hundreds of microns) 2DCOS of N,N′-di((S)-1-methylpentyl)-1,7(6)-dicyano-perylene-3,4:9,10-bis(dicarboximide) (PDI1MPCN2) were successfully prepared by drop-casting techniques.[55] Furthermore, derivatives of naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile (NDI3HU-DTYM2) and branched alkyl chains 5-alkyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione moieties (2DQTT-o-B) also confirmed the effectiveness of the method.[50,56,57]

An class="Chemical">side from the planar organic semiconductors, nonplanar aromatic backbones can be exploited to obtain a “phase separation” molecule structure, representing an innovative molecular engineering strategy for fabricating n class="Chemical">ultrathin 2DCOS. In the Zhang group, various long alkyl chains were attached to fullerene materials to afford the separation layer, resulting in 2DCOS with thicknesses of 5–6 nm.[58] Other typical examples of nonplanar organic semiconductors with brickwork arrangements are hexabenzoperylene derivatives (CR-HBPs).[59] The molecules are closely packed with two-dimensional π–π interactions, and the propyl group enables the phase separation interactions, helping to form the ultrathin 2DCOS.

In short, the “phase separation” molecule structure offers a facile method for the growth of large-scale n class="Chemical">ultrathin 2DCOS by n class="Chemical">simple drop-casting. However, since the crystallization relies on the molecular structure, this method is more sensitive to external conditions, and the crystals usually grow randomly on the substrate with uncontrolled positions and orientations. Moreover, because the insulating soft alkyl chains may depress the semiconducting properties of organic semiconductors, designing the appropriate alkyl chain length to allow for the large-area 2DCOS self-assembly while simultaneously guaranteeing excellent electrical performance is still a challenge.

Solution-Epitaxy

Epitaxy technology is one of the most effective methods to achieve high-quality crystal growth in inorganic semiconductors.[60] Recently, this method has been extended to the assembly of controllable, high-quality, and n class="Chemical">ultrathin 2DCOS.[21,61] A fundamental prerequisite for epitaxy is accurate lattice matching with the substrate. However, it is hard to find crystalline substrates that match organic lattices, limiting the possibilities for the 2DCOS fabrication. Considering this, Xu et al. developed a novel approach for assembling 2DCOS at the water/air interface, named “solution epitaxy”, schematically depicted in Figure a.[62] This method has two pivotal operations. (i) The water surface acts as an ideal growth substrate because the coffee-ring effect is effectively suppressed at the water surface by elimination of the pinned three-phase contact line, resulting in a uniform 2DCOS.[63] (ii) Small 2DCOS as seed crystals is another pivotal operation. The small-sized 2DCOS were first obtained on the water surface by drop-casting and then they serve as seed crystals for further epitaxy growth by dropping the additional solution onto the water surface. Encouraged by this approach, a wide range of organic semiconductors were effectively assembled into high-quality large-sized 2DCOS by epitaxy growth. As a typical example, the characterization over an entire centimeter-sized dihexyl-substituted dibenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophenes (C6-DBTDT) 2DCOS indicated the ultrathin single-crystalline nature shown in Figure b–g.[62] Besides the C6-DBTDT molecule, many p-type organic semiconductors have been grown into large-area 2DCOS, including C6-DPA, dihexyl-substituted pentathienoacene (C6-PTA), perylene, and 1,4-bis(4-methylstyryl)benzene (p-MSB).[62,64] Soon after their initial report, the same group further extended this method to the growth of n-type semiconductor with a large lateral size and low defect density. They successfully prepared millimeter-sized n-type furan-thiophene quinoidal compound (TFT-CN) 2DCOS with only 2–3 molecular layers (Figure h–m). Impressively, a competitive field-effect electron mobility (1.36 cm2 V–1 s–1) and high responsivity of near-infrared organic phototransistors (OPTs) based on TFT-CN were obtained.[65]

Figure 3

(a) Schematic illustration of growing large-area 2DCOS by “solution epitaxy” method and (b, c) morphology characterization and (d–g) structure analysis of p-type C6-DBTDT 2DCOS. Reproduced with permission from ref (62). Copyright 2016, Wiley-VCH. (h, i) morphology characterization and (j–m) structure analysis of a 2DCOS of n-type TFT-CN 2DCOS via “solution epitaxy” method. Reproduced with permission from ref (65). Copyright 2018, Wiley-VCH.

Because of the free-standing nature of the fabricated n class="Chemical">2DCOS by this outstanding method, it offers further opportunities for manufacturing of integrated devices. Wang et al. successfully fabricated a p–n junction based on n class="Chemical">C8-BTBT (p-type) and a furan-thiophene quinoidal compound (TFT-CN, n-type), via secondary transfer technology.[66] Through a simple drop-casting method, single crystal 2DCOS films of p- and n-type semiconductors, respectively, were grown on the water surface. Then, the as-prepared 2DCOS were sequentially transferred to dielectric substrates, and the p–n junction was formed by the alignment.

That said, despite numerous reports on the assembly of n class="Chemical">2DCOS, disadvantages remain with the solution epitaxy method in its current form. (i) n class="Chemical">Ultrathin 2DCOS with a defined layer still remain to grow challenging due to the viscosity of water. (ii) The quasi-equilibrium condition needs several hours to be established. (iii) The transfer of 2DCOS from water can hamper the effectiveness of water-soluble substrates (i.e., water-soluble dielectric layers).

Confined-Assembly toward Ultrathin Large-Area 2DCOS

Compared with self-assembly methods, which are mainly controlled via molecular organization, confined-assembly processes control the assembly kinetics via external forces. When un class="Chemical">sing confined-assembly to grow crystals, the number of spatial degrees of freedom for the solute molecules is reduced, affecting the rates of solute molecules diffun class="Chemical">sion and precipitation from solution.[67] In this section, we will discuss two aspects of confined-assembly: (i) how to promote the 2D growth of organic crystals by space-confined assembly, which makes use of solution droplets in 2D confined spaces, and (ii) how to develop 2DCOS from directional-confined assembly, which forces the crystal to grow along a given direction.

Space-Confined Assembly

Organic molecules can move freely in solution, and this behavior makes crystals grow randomly on the substrate. At present, in typical space-confined environments, the assembly is concentrated along the solid/solid or air/n class="Chemical">water interfaces. Inpan> the solid/solid case, Nakayama and co-workers achieved monolayer or few-layer large n class="Chemical">2DCOS by 2D space-confined assembly.[11] A piece of plastic or silicon was placed onto a silicon substrate with an angle between them of less than 30°, resulting in a 2D confined space.

The semiconductor/dielectric interface with a low-energy/hydrophobic is advantageous in minimizing the number of electron traps producing high electronic performance.[68] However, it is difficult to assembly n class="Chemical">2DCOS on the low-energy surface by solution assembly methods n class="Chemical">since dewetting of surface occurs. Considering this, Shi and co-workers. developed a modified solid/solid 2D space-confined assembly method, named a gravity-assisted space-confined, to overcome this problem so that dewetting dielectrics could be used for the crystal growth, as shown in Figure a–e.[69] Because of the force of gravity on the top substrate, a 2D space was formed between the two substrates. Because the chlorobenzene solvent has better wetting properties solvent on the benzocyclobutene (BCB)-modified top substrate than on the hydrophobic octadecyltrichlorosilane (OTS)-treated bottom substrate, monolayer molecular crystals to grew on the top BCB-treated substrates over centimeter-sized areas.

Figure 4

(a) Schematic illustration of growing large-area 2DCOS by the solid/solid space-confined assembly method. The abbreviation of MMC represents monolayer molecular crystals. (b) Optical microscopy image and (c) corresponding AFM and (d, e) HR-AFM image of the 2DCOS on BCB-treated SiO2/Si substrate. Reproduced with permission from ref (69). (f) Sketches of the air/water 2D space-confined assembly method for the growth and transfer of 2DCOS. Picture showing the spreading area of organic solution on (g) deionized water or (h) water with a phase transfer surfactant. (i, j) Optical microscopy images of 2DCOS of C6-DPA. Reproduced with permission from ref (70). Copyright 2018, American Chemical Society.

(a) Schematic illustration of growing n class="Gene">large-area 2DCOS by the solid/solid space-confined assembly method. The abbreviation of MMC represents monolayer molecular crystals. (b) Optical microscopy image and (c) corresponding AFM and (d, e) HR-AFM image of the 2DCOS on BCB-treated SiO2/Si substrate. Reproduced with permission from ref (69). (f) Sketches of the air/water 2D space-confined assembly method for the growth and transfer of 2DCOS. Picture showing the spreading area of organic solution on (g) deionized water or (h) water with a phase transfer surfactant. (i, j) Optical microscopy images of 2DCOS of C6-DPA. Reproduced with permission from ref (70). Copyright 2018, American Chemical Society.

Ben class="Chemical">sides the solid/solid space-confined method, space-confined assembly can also be realized at the air/n class="Chemical">water interface. In 2018, Wang and co-workers demonstrated that the phase transfer surfactants could control the interfacial tension between the water/solution and the spreading area of the solvent, leading to the desired 2D space-confinement for growth of 2DCOS as shown in Figure f–j.[70] However, the dynamical movement of the whole solution on the water surface, caused by the low viscosity of water, posed challenges to the fixed position and layer-defined growth of 2DCOS. To overcome this difficulty, a modified air/liquid interface was reported by the same group.[71] They used high viscosity glycerol, instead of water, which served as a viscous liquid substrate, leading the organic solution to have a fixed position. More interestingly, by adjusting the ratio of glycerol in the liquid matrix, the formation of crystals from bulk sizes down to the monolayer was precisely controlled.

Orientation-Force-Directed Assembly

Orientation-force-directed assembly refers to large-area fabrication tactics which adn class="Chemical">opt an oriented external force on the substrates for controlling the orientation as den class="Chemical">sired. In this strategy, external forces such as gravitational forces, shearing forces, and capillary forces are utilized to guide the movement direction of the solution realizing high crystal quality with a high degree of orientation. Here, in a typical orientation-force example, the assembly is divided into the vertical gravity and horizontal orientation-force-directed assembly.

n class="Chemical">Dip-coating is a popular approach for fabricating well-alignpan>ed molecular crystals.[72−74] Inpan> a typical process, a substrate is immersed into the solution and then is withdrawnpan>, accompanied by precipitation and crystallization as the substrate is withdrawnpan>. Li and co-workers used the solvent, solution concentration, and recesn class="Chemical">sion rate as the main parameters to balance the pulling speed, nucleation and assembly rate, which played critical roles in the crystallization process, as shown in Figure a.[75] By externally controlling the pulling speed of the substrate from solution, Gelinck and co-workers produced 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) 2D crystals which exhibited uniform morphology up to size of a centimeter.[76] In addition, preparation of large-scale ultrathin 2DCOS requires not only control of the crystallization process, but also the selection of organic semiconductors with appropriate molecular structures. Tian et al. reported an asymmetric molecule, a novel thienoacene-based conjugated oligomer (BTTT-T-C12) as a candidate semiconductor to prepare 2DCOS with 2–6 mm lateral dimensions via a dip-coating method (Figure b–c).[77] The asymmetric molecule may suppress the crystal growth along the normal to the substrate surface, forming much larger grains than its symmetric counterparts[78] and is therefore beneficial for growing ultrathin 2DCOS.

Figure 5

(a) Schematic illustration of the dip-coating processes and the key factors influencing the growth of ultrathin film in dip-coating. Reproduced with permission from ref (75). Copyright 2016, American Chemical Society. (b) Optical micrographs and (c) crossed polarizers of the BTTT-T-C12 2DCOS. Reproduced with permission from ref (77). Copyright 2012, The Royal Society of Chemistry. (d) Schematic illustration of solution-shearing method. Reproduced with permission from ref (79). Copyright 2020, Wiley-VCH. (e) Optical microscopy characterization of the HTEB films and (f) its corresponding AFM image, and the Δ5 representation the thickness of 2DCOS. Reproduced with permission from ref (90). Copyright 2014, The Royal Society of Chemistry. (g) Sketches illustration of the fabrication process by the floating-coffee-ring method. (h) Optical microscopy image and (i) its high-resolution AFM images of 2L C8-BTBT 2DCOS. (g–i) Reproduced with permission from ref (85). Copyright 2016, Wiley-VCH.

Compared with the vertical gravity-directed orientation force of n class="Chemical">dip-coating methods, the solution-shearing[79−84] and floating coffee ring-driven assembly[85] methods utilize movement of the orientation force in the horizontal direction to assembly the organic semiconductor. Generally, the solution-shearing method involves dragging a solution meniscus that forms between the substrate and the top element, such as a bar or a blade, via the movement of the top element or the substrate, as seen in Figure d.[86,87] Recently, Akifumi et al. successfully prepared n class="Chemical">2DCOS with lateral dimensions of wafer-scales by a modified solution-shearing method, using 3,11-dioctyldinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C8-DNBDT-NW), which has good solubility.[19] Solution-shearing methods are also beneficial for assembly ultrathin 2DCOS,[88,89] which respond quickly to external stimulation, providing a promising platform for sensing applications. In a typically example, Hu et al. reported a monolayer HTEB 2DCOS via solution-shearing method for highly sensitive and reversible gas sensors application (Figure e,f).[90] Because of external forces, the solution-shearing speed can also modify the molecular packing through lattice strain, which is used to improve the charge transport properties by increasing intermolecular interactions.[91] Giri et al. fabricated 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) thin-film crystallites by a solution-shearing method, which showed an increase in charge carrier mobility from 0.8 to 4.6 cm2 V–1 s–1 along with the decreased the π–π stacking distance from 3.33 to 3.08 Å, aiding a high-mobility device.[92]

Another effective method utilizing the orientation-force-directed assembly in the horizontal direction is called a floating-coffee-ring-driven assembly.[85,93] Usually, the lateral crystalline domain n class="Chemical">size is suppressed due to the coffee-ring effect. However, the coffee-ring phenomenon can also be used subtly to induce mass transport of dispersed organic molecules to the edge of the solution, achieving large-area n class="Chemical">2DCOS growth. Wang et al. proposed a method that combined the technique of “poor solvent” crystallization with floating-coffee-ring-driven assembly technology to produce 2D molecular crystalline semiconductors (Figure g–i).[85] The key to success was that, near the solvent edge, the evaporation-driven flow of the good solvent resulted in a floating-coffee-ring effect in the poor solvent; it also to create positions for the crystallization of the organic semiconductor that supported high crystal growth rate.

Electrical Characteristics Based on 2DCOS

Single OFETs: Intrinsic Charge Transport, Optimized Contact Resistance, and High Mobility

OFETs are three-terminal switching electronic components modulated by an electric field applied to a gate terminal, which use organic semiconductors as the conducting channel. It is understandable that the gate-induced carrier concentration in bulk channels decays exponentially with increan class="Chemical">sing vertical distance from the dielectric layer, and charge transport occurs in the first few molecular layers near the dielectric interface, as shownpan> in Figure a, top.[17] Hence, carrier transport studies in the n class="Chemical">2D limit provide a platform to directly probe intrinsic carrier transport properties at the interface.

Figure 6

(a) Schematics of transistors based on HTEB bulk (top, left) and monolayer (top, right), as well as their corresponding band structures and density of states (bottom). The embedded molecules represent only a schematic of the stack, and the alkyl side chains are omitted. The purple dots stand for the omitted multilayer molecules. Reproduced with permission from ref (69). (b) Contact resistance diagram of bulk device (top), and resistance evaluation for different molecular layers (bottom). The Rc is extracted by the transmission-line model according to the intercept of the function of the width-normalized total device resistance (RtotalW) versus the channel length. Reproduced with permission from ref (104). Copyright 2019, Royal Society of Chemistry. (c) A histogram summary of charge carrier mobility for representative large-area 2DCOS in the past few years. It should be stated that the values in the histogram are the average mobility of the devices. The μm-sized represents the size smaller than 1 mm2; the mm-sized represents the size range from 1 mm2 to 1 cm2; the cm-sized represents the crystal size over 1 cm2.

(a) Schematics of trann class="Chemical">sistors based on n class="Chemical">HTEB bulk (top, left) and monolayer (top, right), as well as their corresponding band structures and density of states (bottom). The embedded molecules represent only a schematic of the stack, and the alkyl side chains are omitted. The purple dots stand for the omitted multilayer molecules. Reproduced with permission from ref (69). (b) Contact resistance diagram of bulk device (top), and resistance evaluation for different molecular layers (bottom). The Rc is extracted by the transmission-line model according to the intercept of the function of the width-normalized total device resistance (RtotalW) versus the channel length. Reproduced with permission from ref (104). Copyright 2019, Royal Society of Chemistry. (c) A histogram summary of charge carrier mobility for representative large-area 2DCOS in the past few years. It should be stated that the values in the histogram are the average mobility of the devices. The μm-sized represents the size smaller than 1 mm2; the mm-sized represents the size range from 1 mm2 to 1 cm2; the cm-sized represents the crystal size over 1 cm2.

n class="Chemical">Since organic molecules bind by weak van der Waals forces and many grain boundaries exist in polycrystalline films, thermally activated transport characteristics are often observed, in which mobility is pon class="Chemical">sitively correlated with temperature.[94] Nevertheless, it has been verified that intrinsic extended-state conduction (band-like transport) is possible with highly crystalline films without static disorder.[12,95] The realization of intrinsic transport in monolayer crystals is a challenging task because the degree of the disorder and the number of surface trap states are typically larger than in the bulk. To investigate the factors that influence transport physics, Chan et al. fabricated monolayer and multilayer 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) 2DCOS, which had the expected molecular arrangements.[84] Band-like transport took place in the multilayer channel, and thermal activation behavior was observed in the monolayer under cryogenic conditions. With the exception of molecular packing, it can be deduced that the closest interfacial conducting layer may also be perturbed by polar functional groups, such as hydroxyl groups on SiO2 dielectric. Impressively, Jiang et al. suggested the possibility of band-like transport in organic materials close to room temperature even at the monolayer limit using the BCB dielectric.[69] The initial increase of mobility upon cooling between 200 and 300 K indicated band-like transport in monolayer dicyanomethylene-substituted fused tetrathienoquinoid (CMUT). By way of contrast, similar band-like charge transport characteristics were not found in the device based on bare SiO2 dielectric. In addition, as shown in Figure a, bottom, the simulated band structure of the monolayer CUMT was more disperse than that of bulk crystals, which was also consistent with band-like intrinsic transport.

Contact ren class="Chemical">sistance (Rc, which represents width-normalized contact ren class="Chemical">sistance to offset the influence of the channel width) is another property that affects the performance of OFETs, which not only hinders the operation speeds of the device, but also obstructs the optimization of device power consumption. In the commonly used bottom-gate/top-contact configuration, the typical contact resistance ranges from 104 to 106 Ω·cm, following the nonlinearity of the current–voltage relationship after the linear region.[96] Apparently, as shown in Figure b, top, the contact resistance can be divided into the following two parts. One part is the interfacial injection resistance (Rc,int) caused by the carrier injection potential barrier between the metal electrode and semiconductor, and the other is the bulk injection resistance (Rc,bulk), which refers to the interlayer injection resistance in the vertical direction of the semiconductor layer.[97] The former, as reported in the literature, can be optimized using many effective tactics, including contact metal work function adjustment by a self-assembled monolayer[98,99] and efficient interfacial doping between the channel and the electrode.[100−103] For the latter, it is accepted that the thinner channel offers reduced Rc,bulk, but the continuity of the channel should be maintained. Thanks to the long-range coherent arrangement and the effective transport of oriented molecules in the 2D limit, the 2DCOS can be regarded as a candidate channel for transistor fabrication and a medium to explore the relationship between channel thickness and contact resistance.[18] Jiang and co-workers assessed the resistance of devices based on monolayer, 4-layer, and 15-layer 2DCOS, and a gradually increasing trend was found, as shown in Figure b, bottom.[104] Furthermore, when combined with the interfacial layer F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), bilayer C8-BTBT 2DCOS-based devices exhibited total resistance down to 46.9 Ω·cm, providing a platform for subsequent high-frequency transistors (see Section 3.2.2).

As summarized in Figure c, in the past few years, solution-processed large-area 2DCOS with excellent charge transport properties have been developed rapidly from micrometer-sized crystals to centimeter-sized and even wafer-sized crystalline films. Hu et al. first reported that devices based on millimeter-sized monolayer crystals of HTEB exhibited mobility superior to bulk channels (beyond 1 cm2 V–1 s–1) and long-term stability over 300 days in the atmosphere.[22] Afterward, free-standing centimeter-sized C6-DBTDT 2DCOS fabricated by “solution epitaxy” methods showed average mobilities as high as 2.8 cm2 V–1 s–1.[62] Takeya et al. achieved wafer-sized bilayer C8-DNBDT-NW single crystals with an excellent, highest mobility of 13 cm2 V–1 s–1, providing a foundation for subsequent integrated applications.[19]

Compared with n class="Species">p-type 2DCOS, the development of n-type trann class="Chemical">sistors lags behind and is still struggling to catch up.[55,105] Wang et al. fabricated millimeter-sized n-type 2DCOS by combining the molecular design strategy with an effective free-standing preparation method.[65] A competitive average electron mobility (1.04 cm2 V–1 s–1) was obtained in air. However, it has been verified that conventional dielectrics with hydroxyl groups, for which electrochemical trapping of electrons by silanol groups at the interface occurs, will hinder the electron transport.[68] In the Jiang group, the hydroxyl-free BCB-modified SiO2 showed reduced electron traps at the semiconductor/dielectric interface and dipole-induced disorder. As a result, the monolayer 2DCOS were estimated to possess a maximum electron mobility of 1.24 cm2 V–1 s–1, which is one of the highest values in monolayer n-type transistors.[69]

Though great successes around planar charge transport characteristics, vertical ren class="Chemical">sistance characteristics, and charge mobility have been achieved in OFETs based on n class="Chemical">ultrathin large-area 2DCOS, further progress is needed. The surface charge accumulated thickness and charge trap states in OFETs are still confounding. Compared with inorganic materials, organic materials are hobbled by innately poor stability and especially serious intolerance to water, oxygen, and organic solvents. Because of their large volume-to-surface ratios, the stability of 2D single crystals is more inviting.

Beyond Single OFETs Based on 2DCOS

The previous discusn class="Chemical">sion has mostly been focused on unique and exceptional electronic properties of n class="Chemical">single-component solution-processed 2DCOS. It is nonetheless essential to probe the deeper device physics and explore functional integrated circuits beyond a single OFET. Integration applications can be divided into heterogeneous junction devices, in which p- and n-type semiconductors stack directly, and integrated circuits, in which an OFET connects to other components or interconnections. For the former, ultrathin 2DCOS can be used as the bottom layer to construct a smooth junction for the novel devices. In the latter, because of their excellent carrier transport abilities, high-frequency complementary circuits based on 2DCOS can be expected.

p–n Junctions

The p–n junctions, joining a p-type semiconductor for holes and an n-type semiconductor together for electron transport, plays a pivotal, fundamental role in contemporary electronics.[106] As a built-in electric field is induced at the interface between the p- and n-type materials, novel electronic and n class="Chemical">optoelectronic properties far beyond the corresponding n class="Chemical">single components are expected. That said, the thick underlying bulk of the p–n junction will screen the electrostatic field, making an inferior gate field regulation on the upper semiconductor. This drawback can be compensated for by using ultrathin 2D materials.[107] Benefiting from their ultrathin characteristics, which allow good electrostatic control over the junction, and from their single-crystal nature, which allows exploration of intrinsic charge transport properties, monolayer or few-layer 2DCOS can be considered as candidates for effective p–n junction devices, including gate controllable rectifiers, balanced ambipolar OFETs, and enhanced light-controlled transistors (see Section 4).

The first use of a p–n junction that we will introduce is a gate-controllable current rectifier. Devices based on n class="Chemical">2DCOS are usually constructed un class="Chemical">sing lateral junctions, in which two 2DCOS are joined face-to-face to create partial overlap, and the source and drain electrodes are connected, respectively, to the p- and n-type materials. A typical example of a p–n junction gate-controlled rectifier was reported by the Jiang group.[69] The 2,6-diphenylanthracene (DPA) crystal as a p-type component was transferred onto a monolayer n-type CMUT by a mechanical transfer method, and thus a lateral p–n junction was successfully fabricated, as shown in Figure a. In comparison, thick bottom semiconductors (larger than 30 nm) caused cracks in the upper semiconductor at the edge of the junction and failed in constructing lateral p–n junctions. In the intermediate voltage region near |Vg| ≈ |Vd|/2, the current peak appeared with maximum rectification ratios of 4 × 105. The peak of the current in the intermediate region can be explained by an increase of the interlayer recombination rate under a gate field in which the holes and electrons are simultaneously induced in the p-type and n-type materials, respectively, and its dependence on a gate field or any other external stimulus.

Figure 7

(a) The device structure diagram (left), the fluorescence microscope photograph (middle) and electrical properties of gate-controlled rectifier (right). Reproduced with permission from ref (69). (b) The schematic of ambipolar transistor based on 2DCOS (left) and transfer curves for ambipolar transport (middle and right). Reproduced with permission from ref (66). Copyright 2019, Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature. (c) The device diagram (left) and high-frequency rectifying characteristics of a diode-connected 2DCOS-based OFET circuit (middle and right). Reproduced with permission from (19).

(a) The device structure diagram (left), the fluorescence microscope photograph (middle) and electrical properties of gate-controlled rectifier (right). Reproduced with permisn class="Chemical">sion from ref (69). (b) The schematic of ambipolar trann class="Chemical">sistor based on 2DCOS (left) and transfer curves for ambipolar transport (middle and right). Reproduced with permission from ref (66). Copyright 2019, Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature. (c) The device diagram (left) and high-frequency rectifying characteristics of a diode-connected 2DCOS-based OFET circuit (middle and right). Reproduced with permission from (19).

Another important application is so-called ambipolar OFETs, in which one of the n class="Chemical">2DCOS completely covers the other, and the source and drain electrodes are n class="Chemical">simultaneously mounted above the p–n junction region. In the Hu group, high-quality p–n junctions based on two kinds of 2DOCS were successfully fabricated by the facile process known as the “secondary transfer method’’. These p–n junctions realized ambipolar transport of both electrons and holes, as shown in Figure b.[66] Furthermore, in order to obtain well-balanced hole and electron mobilities, Li et al. controlled two factors that affect the performance of the device, which were contact resistance and charge carrier scattering, by tuning the thicknesses of the p- and n-type layers.[108] When the bottom TFT-CN (n-type) and top C6-DPA (p-type) 2DCOS were tuned simultaneously, well-balanced ambipolar charge characteristics were obtained, with hole mobilities up to 0.87 cm2 V–1 s–1 and electron mobilities up to 0.82 cm2 V–1 s–1, proving the layered crystal engineering strategy and achieving balanced ambipolar double-channel OFETs.

Despite the promin class="Chemical">sing experiments discussed in this section, which show the differentiating properties and uniqueness of n class="Chemical">2DCOS p–n junction devices, the extent and mechanisms of charge transfer at the junction interface still need further study. In addition, the preparation of p–n junctions still remain at the size range of micrometers to millimeters. In order to achieve high performance in integration applications, the production of wafer-sized crystalline p–n junctions with well-controlled interfaces is required.

High-Speed Circuits

Functional electronics, such as radio frequency identification (RFID) tags and digital logic circuits, require several trann class="Chemical">sistors to connect with each other to achieve data communication and exchange. As a fundamental prerequin class="Chemical">site, such devices require the OFETs to have high enough operating speeds to drive the entire circuit. For example, near-field communication RFID tags should respond at 13.56 MHz, and thus for OFETs, it is a meaningful, significant goal to realize operating speeds of at least tens of MHz.[109] To realize high-speed transistors with high cutoff frequencies, it is essential to improve the effective mobilities in short-channel devices.[110] However, as the channel length is reduced, device performance is suppressed by the contact resistance, which raises the injection barrier, as well as by the poor long-range order of organic semiconductors.[111] Low contact resistance and coherent carrier transport characteristics in ultrathin 2DCOS make them promising candidates for short-channel high-frequency devices.[112] In Takeya’s recent research, the effective mobility, extracted from a transfer curve based on C8-DNBDT-NW 2DCOS with bilayers (2L) in 3 μm short-channel transistors, was estimated to be 2.7 cm2 V–1 s–1, as shown in Figure c.[19] Together with the ultralow contact resistance (46.9 Ω·cm, as mentioned in section 3.1), the VDS-normalized cutoff frequency reached 20 MHz, which is among the highest values for solution-processed transistors. Furthermore, a high-speed rectifier with diode-connected 2L-2DCOS OFETs, exhibited operation up to 29 MHz, which is two times higher than that needed for near-field communication RFID tags.

The second requirement for OFETs in high-density circuits is excellent noise properties, which are responn class="Chemical">sible for further improving the circuit stability and durability when signals propagate between devices. Among the various sources of electronic noise, the flicker noise (1/f noise) or excess noise, which is caused by carrier density fluctuations in the active devices, can be regarded as the main contributor.[113,114] Many pioneering studies have been executed on the flicker noise feature of the polymer[115] or small molecular polycrystalline films[116] and organic single crystals from physical vapor sublimation.[117] These results have verified that (i) lower flicker noise is beneficial for charge transport and stability and that (ii) enhanced crystallinity and flatness (microstructure) have positive impacts on device noise. However, the hopping transport mechanism commonly explored in traditional disordered polycrystalline films hinders the study of noise sources and the development of low-noise devices. Recently, Watanabe et al. made a breakthrough in using ultrathin C8-DNBDT-NW 2DCOS as semiconductor layers to study their noise characteristics.[118] Thanks to its low trap-state density (8.3 × 1018 eV–1 cm–3), fitting by the McWorther model, and the high hole mobility (13.6 cm2 V–1 s–1) arising from its coherent band-like transport nature, the amplitude of 1/f was demonstrated to be as low as 6 × 10–6 μm2 Hz–1, which is lower than that for any other solution-processed semiconductors and 2–3 orders of magnitude lower than that for inorganic van der Waals materials.[119]

Furthermore, solution-processed high-quality n class="Chemical">2DCOS over large area is capable of practical use in organic logic circuits because of their satisfying reproducibility and good uniformity. Uno et al. realized parallel-serial delay-type flip-flop (DFF) organic circuits by connecting p-type (C10-DNBDT-NW) and n-type (BASF Gn class="Chemical">SID104031-1) 2DCOS, which operated with extremely fast response times of about 37 μs.[120] Yamamura et al. fabricated digital temperature sensor circuits composed of DFF circuits and wireless RFID tags, using alternating p-type and n-type line-shaped crystalline films combined in particular orientations.[121] These excellent results will accelerate the development of organic logic circuits using solution-process techniques and promote practical applications of organic electronic products. In the future, large-area patterned 2DCOS with controllable orientations and well-defined positions need further study in order to reduce interference and variability between devices, as well as achieve more complex circuits.

Emerging Insights in Phototransistors in the 2D Limits

OPTs are receiving much attention for their wide range of uses in imaging, motion monitoring, night vin class="Chemical">sion equipment, remote sensing, and so on. In OPTs, the photogenerated electron–hole pairs in the photoactive channel are separated by an external vertical electric field and eventually collected by the drain electrodes, thereby affecting the electrical channel properties of the transistors. The figures of merit for evaluating the OPTs are available in other comprehensive reviews.[122,123] Herein, we will focus on a few novel effects of the photoelectrical response that benefit from the ultrathin characteristics of OPTs and their long-range ordered molecular arrangements.

First, we note that the charge transport properties of the channels in OPTs greatly affect the separation, transmisn class="Chemical">sion, and collection of photogenerated carriers. In a typical example, Ji et al. obtained a series of OPTs with different mobilities by controlling the thickness of DPA films from 10 to 30 nm, as shown in Figure a.[124] Under the same lighting conditions, the changes in photosensitivity (P) and photoresponsivity (R) were consistent with the trend in mobility, confirming that higher mobility contributes to higher optical performance. Moreover, compared to the hopping modes seen with small localization lengths,[125] longer exciton diffusion lengths and greater separation efficiencies are expected due to band-like transport with stronger electron cloud overlap,[126] which provides more photogenerated free carriers and enhanced light modulation.

Figure 8

(a) The device scheme (left, top) based on DPA films from 10 to 30 nm and their photoresponse characteristics (right) with different light intensities and photoresponse mapping (left, bottom). Reproduced with permission from ref (124). (b) The photogating effect of the devices based on monolayer (top) or bilayer (bottom) 2DCOS. Reproduced with permission from ref (50). Copyright 2019, Wiley-VCH. (c) The carrier distribution and dark current diagram in both bulk and ultrathin channel. Reproduced with permission from ref (65). Copyright 2018, Wiley-VCH. (d) The energy level diagram of PTCDA/pentacene 2DCOS junction (left, top), absorption spectrum (right, top) and their photoresponse properties (bottom). Reproduced with permission from ref (128). Copyright 2017, American Chemical Society.

(a) The device scheme (left, top) based on n class="Chemical">DPA films from 10 to 30 nm and their photoresponse characteristics (right) with different light intenn class="Chemical">sities and photoresponse mapping (left, bottom). Reproduced with permission from ref (124). (b) The photogating effect of the devices based on monolayer (top) or bilayer (bottom) 2DCOS. Reproduced with permission from ref (50). Copyright 2019, Wiley-VCH. (c) The carrier distribution and dark current diagram in both bulk and ultrathin channel. Reproduced with permission from ref (65). Copyright 2018, Wiley-VCH. (d) The energy level diagram of PTCDA/pentacene 2DCOS junction (left, top), absorption spectrum (right, top) and their photoresponse properties (bottom). Reproduced with permission from ref (128). Copyright 2017, American Chemical Society.

The second effect to discuss is the photogating effect. Photogating, a conductance modulation through a photoinduced gate voltage, is considered a way to improve the n class="Chemical">optical response of 2D organic OPTs, especially responsivity. Different from the conventional photoconductive effect, the photogating effect is caused by photogenerated minority carriers (holes for n-type semiconductors or electrons for p-type semiconductors) stationed at the traps on the crystal surface or at crystal/insulator interface, which create an additional gate voltage and result in a threshold voltage shift (ΔVth, more negative for n-type channels or more positive for p-type channels).[127] Intuitively, in the ultrathin limit, the low light absorption of 2DCOS makes them unsuitable as direct replacements for thin-film semiconductors. However, the large surface-to-volume ratio and the reduction in screening improve the trapping capabilities of 2DCOS, resulting in excellent electrical tunability by localized fields and enhanced performance through applied gate voltages. For example, as shown in Figure b, Fu et al. successfully fabricated an ultrasensitive OPT based on 2DCOS with few monolayers.[50] The OPTs based on the 2L-2DCOS showed photosensitivity (P, the value of which is equal to Iph divided by Idark) up to 2.58 × 107 and responsivities reaching 1.91 × 104 A W–1. In comparison, the threshold voltages of devices based on 1L-2DCOS shifted to the positive side of those of 2L-2DCOS, indicating stronger doping effects in the thinner crystals.

Furthermore, in order to meet the requirements for high specific detectivity (D*) and detection at low noise levels, suppresn class="Chemical">sion of the dark current and improvement of the on/off ratio are critical. The charge transport dilemma for n class="Chemical">2D channels and thick channels is described in Figure c. As discussed in the previous section, the carrier density in the channel decreases exponentially with increasing distance from the semiconductor/insulator interface in the “on” state of the device. Similarly, the gate field mainly depletes a few-nanometer-long region near the interface in the “off” state.[71] As a result, 2DCOS-based devices usually exhibit high on/off ratios and weak signals in the “off” state due to their thin channels and can be operated in full depletion.[123] For example, Hu et al. recently fabricated near-infrared-sensitive OPTs based on high-quality TFT-CN 2DCOS with thicknesses of 4.8 nm.[65] Because of efficient carrier injection and intrinsic, defect-free carrier transport, the device showed excellent responsivity (9 × 104 A W–1) and EQE (106 %). Impressively, compared with thick crystals (thickness of 32 nm), the dark current of the 2DCOS-based device was 1 order of magnitude lower (0.3 pA vs 3 pA), leading to an ultrahigh detectivity of 6 × 1014 Jones, which was among the best performances in current studies.

Finally, as mentioned above, n class="Chemical">single-component devices dominated by the photogating effect suffer from trade-offs in the photoresponn class="Chemical">sivity and response time. Because of the lack of a built-in electric field, the high responsivity generally arises from long-lived trap-state-induced excitons (usually lasting in the range of hundreds of milliseconds to a few seconds).[30] Cao et al. demonstrated that interfacing 2DCOS with other high-mobility materials can counterbalance this contradiction.[20] As the thickness of the 2DCOS is equivalent to the exciton diffusion length in organic semiconductors, an effective separation of photogenerated electron pairs can be achieved at the interface. Furthermore, Wang et al. fabricated OPTs using a perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and pentacene 2D crystalline heterostructure as the light-absorbing layer (Figure d).[128] The device achieved a good balance between a high responsivity of 105 A W–1 and a response time as low as 28 μs, which are 1 and 2 orders of magnitude better, respectively, than those of other solution-processed organic/graphene OPTs.[129,130]

Although many novel devices and excellent performance characteristics have been demonstrated, there are still many challenges in photoelectric control of n class="Chemical">2D organic crystals. (i) Because of their nearly transparent nature, high-mobility n class="Chemical">2DCOS with efficient light absorption should be broadly explored, and the device structures and interfaces need further optimization to obtain practical devices with good trade-offs. (ii) The underlying photophysical questions, including the length of the exciton diffusion between different molecular layers of organic single crystals, are still not clear. The separation and transmission mechanisms of photogenerated excitons are only inferred theoretically and lack accurate experimental data.

Hybrid System with Other 2D Van Der Waals Solids

n class="Chemical">2D vdWs, such as semin class="Chemical">metallic graphene, semiconductive transition metal dichalcogenides (TMDs), insulated few-layer crystals of hexagonal boron nitride (hBN), possess new opportunities for device concepts and applications due to their unique properties beyond bulk materials. The hybrid system of organic molecules and 2D vdWs appears to design many predictable functions successfully, which have been summarized in some professional reviews.[131,132] However, the organic counterparts are mainly concentrated on amorphous films and micro-/nanocrystals. In parallel with the efforts on 2DCOS, interfacing ultrathin 2DCOS with other 2D vdWs solids have recently emerged, which provide a molecular level pristine heterogeneous interface for exploring the charge transfer,[20] tunneling effect,[133] and other transport physics.[134,135] Moreover, most reports have focused on the assembly of organic molecules directly on the surface of 2D vdWs. In this process, organic molecules can form self-assembled ordered structures to construct controllable long-range ordered organic–inorganic van der Waals heterojunctions.

In the Wang group, a series of works on the heterogeneous interface between n class="Chemical">ultrathin molecular crystals and n class="Chemical">2D vdWs have been reported.[18,20,101,136] Typically, the interfacial layer of molecules that do not contribute to conductance is clearly characterized, lying prone on the substrate in a different arrangement from the bulk packing.[101] Above the interfacial layer, the first monolayer C8-BTBT standing on hBN insulation could reach ∼10 cm2 V–1 s–1, indicating the pristine, ultrasmooth interface between C8-BTBT and hBN. Different from graphene and hBN, MoS2 was not the π-conjugated system so that the first molecular layer was tilted at a greater angle to the MoS2.[133] Gate-controlled vertical 2D C8-BTBT/MoS2 heterostructures exhibited electrical properties distinct from those of a single-component channel. In the range of gate that permitted both hole and electron injection, an interfacial recombination resulted in the current peak; as the forward gate voltage continued to increase, the electrons accumulated in MoS2 and crossed ultrathin molecular crystals through the tunneling effect, which was rarely reported in bulk heterojunction. In addition, the interactions between the 2D vdWs and molecules could adjust by controlling the electronic state of 2D vdWs templates. Cho et al. demonstrated that the linear dependence nucleation sites of organic molecules on carrier densities[137] and first realized the controlled layer-by-layer growth of fullerenes on electrical-doped graphene in the case of suppressing charge transfer.[138] As a result, the Fermi-level pinning in heterojunction disappeared, and the charge injection in barristors of fullerene/graphene approached ideal Schottky–Mott limits.

For the phototrann class="Chemical">sistors, n class="Chemical">graphene with an excellent charge transport property can make up for the short exciton diffusion length of organic semiconductors in a hybrid system. Liu et al. fabricated monocrystalline films with high order and controllable layers on graphene and explored the photoelectric characteristics.[20] In this C8-BTBT/graphene heterojunction device, electrons spontaneously move from graphene to molecular layers, and a built-in electric field is directed from the graphene to molecules. Under the illumination, photogenerated electrons moved toward graphene to form n-doped channel. Despite the monolayer thickness, responsivity reached up to 104 A W–1 and short response time of 25 ms. The recovery time after turning off the light and photoconductive gain of the device increased as the film thickened to seven layers with a linear relationship, which was due to the increasing barrier of charge transport between layers resulting in a prolonged recombination process.

Future Directions and Perspectives

Over the past decade, explon class="Chemical">sive progress has been achieved in the area of solution-processed large-n class="Chemical">size ultrathin 2DCOS, from fundamental research to next-generation printable electronics. In this outlook, we are committed to provide overall progress in this emerging field around effective fabricated protocols for large-area 2DCOS via solution assembly process, electrical performance in gate field-controlled device, their burgeoning photoelectric device, and their hybrid system with other 2D vdWs. Despite the remarkable advances achieved in large-area 2DCOS and their applications in OFET-based electronic and optoelectronic devices, representing a new research frontier in the evolution of technology, the current research on large-area crystals in the 2D limit is far from mature.

Because of their definite molecular weights and clear conformations, high-mobility small-molecule n class="Chemical">2DCOS are playing vital roles in revealing carrier transport characteristics at interfaces and in aiding fundamental investigations of emerging functional photoelectronic devices. However, because of the severe entanglement between molecular chains, the preparation of conjugated n class="Chemical">polymer-based 2DCOS remains a major challenge, limiting abundant basic research of physical properties. Fortunately, micrometer-sized quasi-2D polymer semiconductor crystals have been successfully fabricated by topochemical polymerization in a previously reported advance.[16] Successful strategies are urgently needed to fabricate high-quality 2D conjugated polymer single crystals over large areas because of their excellent charge transport properties along conjugated molecular chains (as compared to intermolecular orbit coupling), which may provide more opportunities for high-speed logic circuits.

Additionally, most n class="Chemical">single-crystal circuits are currently implemented with an entire crystal, leading to intersections and crosstalk between adjacent components as well as lumped leakage current. Thus, high-precin class="Chemical">sion patterned arrays of organic crystals represent a vital procedure for realizing accurate data acquisition and low power consumption in organic circuits. However, because of their intolerance toward high temperatures and organic solvents, the 2DCOS are incompatible with traditional solvents in multistep solvent treatment photolithography techniques. Up to this point, template-assisted methods have been explored to control the pattern and growth of organic crystals.[139] The challenge lies in further processing the 2DCOS to obtain the desired large-area patterns for functional circuits without affecting the carrier transport.

Finally, the most serious obstacles to practical applications of n class="Chemical">2DCOS-based devices are stability and long-term durability. The relatively low stability results from several aspects: (i) the nonpern class="Chemical">sistence of organic materials during the storage and transportation, (ii) the sensitivity of most organic semiconductors, especially n-type organic semiconductors, to water and oxygen in air, leading to limited electron transport and performance decays over time, and (iii) the greater susceptibility of ultrathin 2DCOS to interfacial trap states compared with thick single crystals. The induced charge applied by the dielectric layers can fill the traps, causing bias instability that is harmful to the stable operation of the circuit. Therefore, one of the key challenges in this field is to develop environmentally stable organic materials and explore reliable methods for encapsulation, so as to significantly extend the materials’ stability.