Electroactive covalent organic frameworks: a new choice for organic electronics

Covalent organic frameworks (COFs) with implemented electroactive moieties and coherent conduction channels are found to be competent for the transport of charges, in a similar manner as conventional organic (semi)conductors.Unlike densely packed organic and polymer layers that are used in organic electronics, the crystallinity of COFs is mostly supported by covalent bonds that potentially boost communication and stability.The voids inside the framework structure enable encapsulation and mass transport, allowing space for guest components such as external electron donors/acceptors as well as channels for diffusion-based functions such as sensing and switching. Covalent organic frameworks (COFs) feature covalent bond-supported crystallinity along with high encapsulating and mass transport abilities. Together with the ease of chemical addition of electroactive moieties, these properties have recently raised interest in this class of material in organic electronics. In this review, we systematically summarize the utilization of advantageous characteristics of COFs to fulfill different functions in electronic processes, resulting in various applications. Broadly, COFs have been successfully implemented as conductive or semiconductive components for electronic devices, such as organic photodetector and photovoltaics, organic transistors, organic light-emitting devices, and organic sensor and memory devices. Simultaneously, general tactics to provide electroactive functionalities are discussed, providing open considerations and inspiration for future electronic design. Covalent organic frameworks (COFs) feature covalent bond-supported crystallinity along with high encapsulating and mass transport abilities. Together with the ease of chemical addition of electroactive moieties, these properties have recently raised interest in this class of material in organic electronics. In this review, we systematically summarize the utilization of advantageous characteristics of COFs to fulfill different functions in electronic processes, resulting in various applications. Broadly, COFs have been successfully implemented as conductive or semiconductive components for electronic devices, such as organic photodetector and photovoltaics, organic transistors, organic light-emitting devices, and organic sensor and memory devices. Simultaneously, general tactics to provide electroactive functionalities are discussed, providing open considerations and inspiration for future electronic design. COFs are porous, crystalline polymers with periodically organized networks connected by covalent bonds between light atoms such as C, N, and O. Directed by reticular chemistry, 2D or 3D COFs are constructed by selecting building blocks and bonding linkages in predefined orientations [1.Lyle S.J. et al.Covalent organic frameworks: organic chemistry extended into two and three dimensions.Trends Chem. 2019; 1: 172-184Abstract Full Text Full Text PDF Scopus (124) Google Scholar]. 2D COFs have chemical bonds that extends in two dimensions, forming sheets that stack together by intermolecular interactions, whereas 3D COFs have covalent bonds that reach out in all directions in 3D space therefore forming an isotropic structure. The predesigned bottom-up synthesis (see Glossary) and rigid architecture endows this class of material with ultrahigh surface area, lightweight, precisely controlled pore and atom distribution, and high stability in a wide range of solvents and conditions [2.Liu R. et al.Covalent organic frameworks: an ideal platform for designing ordered materials and advanced applications.Chem. Soc. Rev. 2021; 50: 120-242Crossref PubMed Google Scholar,3.Geng K. et al.Covalent organic frameworks: design, synthesis, and functions.Chem. Rev. 2020; 120: 8814-8933Crossref PubMed Scopus (774) Google Scholar]. Intrigued by their unique features, a race among researchers to explore applications of COF-based materials has started. Organic electronics is a discipline that uses densely packed small organic molecules or organic polymers as functional elements in electronic devices [4.Pron A. et al.Electroactive materials for organic electronics: preparation strategies, structural aspects and characterization techniques.Chem. Soc. Rev. 2010; 39: 2577-2632Crossref PubMed Scopus (407) Google Scholar]. This is an interdisciplinary research area including materials chemistry and device engineering. Focusing on materials chemistry, the organic electroactive materials act as electrically conductive or semiconductive components that transfer charges under certain conditions. Compared with their conventional inorganic equivalents, the organic electroactive materials usually have advantages of being highly diverse, lightweight, and flexible, having low energy consumption when fabricated, and being solution processable [5.Yang Y. et al.The effects of side chains on the charge mobilities and functionalities of semiconducting conjugated polymers beyond solubilities.Adv. Mater. 2019; 311903104Crossref PubMed Scopus (93) Google Scholar]. Organic conductive materials with high electrical conductance can be used for ductile and transparent electrodes for flexible circuits, whereas organic semiconductive materials are applied in organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic lasers, among others. As an emerging organic material, COFs are now being designed for electroactivity, releasing a new choice of functional material for organic electronics (Box 1) [6.Yusran Y. et al.Electroactive covalent organic frameworks: design, synthesis, and applications.Adv. Mater. 2020; 322002038Crossref Scopus (66) Google Scholar,7.Allendorf M.D. et al.Electronic devices using open framework materials.Chem. Rev. 2020; 120: 8581-8640Crossref PubMed Scopus (82) Google Scholar]. Compared to conventional organic electronic materials, electroactive COFs show some unique advantages that remedy traditional limitations and might even lead to device innovations. First, the covalent bond-supported crystallinity of COFs vastly surpasses the intermolecular-force-supported crystallinity of semiconducting molecules/polymers, considering enhanced long-range communication and higher stability. The charge transfer behavior is profoundly affected by crystallinity and COFs allow a stable, long range, and a priori predictable crystallinity. Second, porous COFs enable mass transport within the tunnels of electroactive layers, which is uncommon for traditional conductive/semiconductive materials that are too densely packed to allow high mass diffusion of molecules. Furthermore, the precisely controlled pores of COFs provide a space for guest materials (e.g., dopants), which could interact with the COF host for property modification or to integrate multiple functionalities. Considering these benefits, electroactive COFs are an attractive alternative to traditional materials in a vast number of electronic applications.Box 1The branches of COF-based electronic applications and processesDepending on the different electronic processes (shown in rectangles in Figure I), electroactive COFs can be categorized as and applied to different device applications (shown in colored boxes in Figure I). If the COF can transfer charges between two electrodes under bias voltage, it is electrically conductive and can be defined as a conductive COF. Conductive behavior is the most basic and widely researched aspect of electroactive COF materials. If the COF can be thermally activated at room temperature, the COF is intrinsically conductive. Light and chemicals (dopants) can also be the source of activation, corresponding to light activation and doping activation, respectively. Furthermore, if conduction is triggered by a gate voltage from a third electrode, it belongs to the scope of three-terminal devices, OFETs. For a photoactivated conductive material, if the current change on illumination is large, the material shows photoresponsive characteristics and can be utilized in photodetectors. If the formed excitons can dissociate and accumulate in different phases followed by extraction at the electrodes, the materials can be exploited in organic solar cells. For electroactive COFs showing conduction, if the COFs: (i) reversibly trap ions/charges under voltage switching, the device behaves as a memory device for signal storage; (ii) interact with chemicals causing a current change, the device acts as a sensor for chemical detection; or (iii) change light absorption due to an electrochemical redox reaction, it can be used in an EC device. Furthermore, an electroactive and emissive COF can be integrated as the active layer in an OLED. Depending on the different electronic processes (shown in rectangles in Figure I), electroactive COFs can be categorized as and applied to different device applications (shown in colored boxes in Figure I). If the COF can transfer charges between two electrodes under bias voltage, it is electrically conductive and can be defined as a conductive COF. Conductive behavior is the most basic and widely researched aspect of electroactive COF materials. If the COF can be thermally activated at room temperature, the COF is intrinsically conductive. Light and chemicals (dopants) can also be the source of activation, corresponding to light activation and doping activation, respectively. Furthermore, if conduction is triggered by a gate voltage from a third electrode, it belongs to the scope of three-terminal devices, OFETs. For a photoactivated conductive material, if the current change on illumination is large, the material shows photoresponsive characteristics and can be utilized in photodetectors. If the formed excitons can dissociate and accumulate in different phases followed by extraction at the electrodes, the materials can be exploited in organic solar cells. For electroactive COFs showing conduction, if the COFs: (i) reversibly trap ions/charges under voltage switching, the device behaves as a memory device for signal storage; (ii) interact with chemicals causing a current change, the device acts as a sensor for chemical detection; or (iii) change light absorption due to an electrochemical redox reaction, it can be used in an EC device. Furthermore, an electroactive and emissive COF can be integrated as the active layer in an OLED. Being electrically conductive, meaning that the material can transfer charges under a bias voltage, is the most fundamental property of electroactive materials. Considerable effort has been devoted to construct conductive COFs, and achievements are marked by continuously increasing conductivities (Table 1). From recent research, some design rules for highly conductive COFs can be extracted (Box 2). In the following section, we expand on these strategies.Table 1Comparison of electrical conductivities for COFs reported to date and electrical conductivities of some representative MOFs and polymersMaterial typeMaterialConductivity (S m−1)MethodSample typeRefs2D COFI2@TTF-Ph-COFI2@TTF-Py-COF10−310−4Two probePellet[10.Jin S. et al.Two-dimensional tetrathiafulvalene covalent organic frameworks: towards latticed conductive organic salts.Chem. Eur. J. 2014; 20: 14608-14613Crossref PubMed Scopus (115) Google Scholar]I2@TTF-COF1.8 × 10−4Two probePellet[8.Ding H. et al.A tetrathiafulvalene-based electroactive covalent organic framework.Chem. Eur. J. 2014; 20: 14614-14618Crossref PubMed Scopus (123) Google Scholar]TTF-COFI2@TTF-COF1.2 × 10−40.28Two probeFilm[9.Cai S.-L. et al.Tunable electrical conductivity in oriented thin films of tetrathiafulvalene-based covalent organic framework.Chem. Sci. 2014; 5: 4693-4700Crossref Google Scholar]TTF-DMTA1.8 × 10−4Two probeFilm[25.Cai S. et al.Reversible interlayer sliding and conductivity changes in adaptive tetrathiafulvalene-based covalent organic frameworks.ACS Appl. Mater. Interfaces. 2020; 12: 19054-19061Crossref PubMed Scopus (24) Google Scholar]COF-DC-8I2@COF-DC-82.51 × 10−3~1Four probePellet[12.Meng Z. et al.Two-dimensional chemiresistive covalent organic framework with high intrinsic conductivity.J. Am. Chem. Soc. 2019; 141: 11929-11937Crossref PubMed Scopus (164) Google Scholar]POR-COFI2@POR-COF4.6 × 10−91.52 × 10−5Two probePellet[15.Nath B. et al.A new azodioxy-linked porphyrin-based semiconductive covalent organic framework with I2 doping-enhanced photoconductivity.CrystEngComm. 2016; 18: 4259-4263Crossref Google Scholar]TAPP−TFPP−COFI2@TAPP−TFPP−COF1.12 × 10−81.46 × 10−5Two probePellet[26.Xu X. et al.Semiconductive porphyrin-based covalent organic frameworks for sensitive near-infrared detection.ACS Appl. Mater. Interfaces. 2020; 12: 37427-37434Crossref PubMed Scopus (35) Google Scholar]I2@TANG-COF1Four probePellet[18.Lakshmi V. et al.A two-dimensional poly(azatriangulene) covalent organic framework with semiconducting and paramagnetic states.J. Am. Chem. Soc. 2020; 142: 2155-2160Crossref PubMed Scopus (40) Google Scholar]WBDT[email protected]2.70 × 10−43.67vdPaAbbreviation: vdP, van der Pauw.Pellet[19.Rotter J.M. et al.Highly conducting Wurster-type twisted covalent organic frameworks.Chem. Sci. 2020; 11: 12843-12853Crossref PubMed Google Scholar]1-S1-Se1-Te3.7(± 0.4) × 10−88.4 (± 3.8) × 10−71.3 (± 0.1) × 10−5Two probePellet[20.Duhović S. Dincă M. Synthesis and electrical properties of covalent organic frameworks with heavy chalcogens.Chem. Mater. 2015; 27: 5487-5490Crossref Scopus (77) Google Scholar]BDT-COF1~5 × 10−5Two probeFilm[21.Medina D.D. et al.Directional charge-carrier transport in oriented benzodithiophene covalent organic framework thin films.ACS Nano. 2017; 11: 2706-2713Crossref PubMed Scopus (87) Google Scholar]I2@sp2c-COF7.1 × 10−2Two probePellet[23.Jin E. et al.Two-dimensional sp2 carbon-conjugated covalent organic frameworks.Science. 2017; 357: 673-676Crossref PubMed Scopus (533) Google Scholar]PyVg-COF0.4Two probeFilm[27.Wang L. et al.A highly soluble, crystalline covalent organic framework compatible with device implementation.Chem. Sci. 2019; 10: 1023-1028Crossref PubMed Google Scholar][email protected]110Two probePellet[5.Yang Y. et al.The effects of side chains on the charge mobilities and functionalities of semiconducting conjugated polymers beyond solubilities.Adv. Mater. 2019; 311903104Crossref PubMed Scopus (93) Google Scholar]3D COFI2@JUC-5182.7 × 10−2 (25oC)1.4 (120oC)Two probePellet[11.Li H. et al.Three-dimensional tetrathiafulvalene-based covalent organic frameworks for tunable electrical conductivity.J. Am. Chem. Soc. 2019; 141: 13324-13329Crossref PubMed Scopus (93) Google Scholar][email protected]3.4Two probeFilm[29.Yang Y. et al.A highly conductive all-carbon linked 3D covalent organic framework film.Small. 2021; 17e2103152Crossref PubMed Scopus (2) Google Scholar]2D MOF{[Cu2(6-Hmna)(6-mn)]NH4}n1096Four probeCrystal[33.Pathak A. et al.Integration of a (–Cu–S–)n plane in a metal–organic framework affords high electrical conductivity.Nat. Commun. 2019; 10: 1721Crossref PubMed Scopus (83) Google Scholar]Ni3(HITP)25540Four probePellet[34.Chen T. et al.Continuous electrical conductivity variation in M3(hexaiminotriphenylene)2 (M = Co, Ni, Cu) MOF alloys.J. Am. Chem. Soc. 2020; 142: 12367-12373Crossref PubMed Scopus (76) Google Scholar][Ag5(C6S6)]n25 000Four probePellet[35.Huang X. et al.Highly conducting neutral coordination polymer with infinite two-dimensional silver–sulfur networks.J. Am. Chem. Soc. 2018; 140: 15153-15156Crossref PubMed Scopus (68) Google Scholar]PolymerPEDOT:PSS(H2SO4 treated)438 000Four probeFilm[37.Kim N. et al.Highly conductive PEDOT:PSS nanofibrils induced by solution-processed crystallization.Adv. Mater. 2014; 26: 2268-2272Crossref PubMed Scopus (677) Google Scholar]PBTTT(FeCl3 doped)100 000vdPaAbbreviation: vdP, van der Pauw.Film[38.Jacobs, I.E. et al. High-efficiency ion-exchange doping of conducting polymers. Adv. Mater. Published online August 21, 2021. https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202102988Google Scholar]a Abbreviation: vdP, van der Pauw. Open table in a new tab Box 2Key elements for conductive COFsAs a category of organic materials, COFs usually have a common wide bandgap that is too large to allow electrons to be thermally promoted from the valence band to the conduction band under ambient conditions, and thus show electrical insulating properties. To endow the COF with metallic electrical conductivity, three elements are important: electroactive moieties (Figure IA), activating factors (Figure IB), and conduction channels (Figure IC). Specifically, the electroactive moieties introduced into frameworks act as functional sites ready to produce mobile charge carriers. These functional moieties are further activated by activating factors (e.g., heat, light, dopants) to form active sites that provide mobile charges. Finally, the mobile charges need coherent conduction channels for efficient transportation. The optimization of all three elements cooperatively contributes to the conductivity. As a category of organic materials, COFs usually have a common wide bandgap that is too large to allow electrons to be thermally promoted from the valence band to the conduction band under ambient conditions, and thus show electrical insulating properties. To endow the COF with metallic electrical conductivity, three elements are important: electroactive moieties (Figure IA), activating factors (Figure IB), and conduction channels (Figure IC). Specifically, the electroactive moieties introduced into frameworks act as functional sites ready to produce mobile charge carriers. These functional moieties are further activated by activating factors (e.g., heat, light, dopants) to form active sites that provide mobile charges. Finally, the mobile charges need coherent conduction channels for efficient transportation. The optimization of all three elements cooperatively contributes to the conductivity. The electroactive moieties in COFs endow electrical conductivity to the bulk material. Efficient electroactive moieties are usually conjugated functional building blocks that can be easily oxidized or reduced to form a stable open shell species (i.e., a radical cation or anion), which serves as a source for mobile charge carriers. Due to the predesigned synthesis for the construction of COFs, these functional building blocks can be easily introduced into the framework structure with precisely controlled density and order (see Figure 1 for examples). To be conductive, the synthesized electroactive COFs need to be activated. This involves the removal of electrons in the valence band and/or the addition of electrons to the conduction band. Methods of activation include thermal excitation, photoexcitation, and doping. Among activating factors, doping is the most efficient way to create a large amount of charge carriers. Tetrathiafulvalene (TTF) is a widely utilized electroactive building block to make conductive COFs due to its strong electron-donating ability [8.Ding H. et al.A tetrathiafulvalene-based electroactive covalent organic framework.Chem. Eur. J. 2014; 20: 14614-14618Crossref PubMed Scopus (123) Google Scholar]. When TTF is introduced into COFs, it can be oxidized into highly ordered radical cations by the addition of a dopant [9.Cai S.-L. et al.Tunable electrical conductivity in oriented thin films of tetrathiafulvalene-based covalent organic framework.Chem. Sci. 2014; 5: 4693-4700Crossref Google Scholar,10.Jin S. et al.Two-dimensional tetrathiafulvalene covalent organic frameworks: towards latticed conductive organic salts.Chem. Eur. J. 2014; 20: 14608-14613Crossref PubMed Scopus (115) Google Scholar], resulting in the partially filled band structure that is necessary for a conductive behavior. TTF-based 2D [9.Cai S.-L. et al.Tunable electrical conductivity in oriented thin films of tetrathiafulvalene-based covalent organic framework.Chem. Sci. 2014; 5: 4693-4700Crossref Google Scholar,10.Jin S. et al.Two-dimensional tetrathiafulvalene covalent organic frameworks: towards latticed conductive organic salts.Chem. Eur. J. 2014; 20: 14608-14613Crossref PubMed Scopus (115) Google Scholar] (Figure 1A) and 3D [11.Li H. et al.Three-dimensional tetrathiafulvalene-based covalent organic frameworks for tunable electrical conductivity.J. Am. Chem. Soc. 2019; 141: 13324-13329Crossref PubMed Scopus (93) Google Scholar] COFs (Figure 1B) both show electrical conductivity after doping, indicated by linear I–V characteristics, with conductivity as high as 1.4 S m−1 at 120°C [11.Li H. et al.Three-dimensional tetrathiafulvalene-based covalent organic frameworks for tunable electrical conductivity.J. Am. Chem. Soc. 2019; 141: 13324-13329Crossref PubMed Scopus (93) Google Scholar]. Phthalocyanines and porphyrins both feature cyclic conjugation involving 18e− and belong to an active category of building blocks used in electroactive materials. Phthalocyanine/porphyrin-based 2D COFs can be thermally excited [12.Meng Z. et al.Two-dimensional chemiresistive covalent organic framework with high intrinsic conductivity.J. Am. Chem. Soc. 2019; 141: 11929-11937Crossref PubMed Scopus (164) Google Scholar,13.Wan S. et al.Covalent organic frameworks with high charge carrier mobility.Chem. Mater. 2011; 23: 4094-4097Crossref Scopus (510) Google Scholar], photoexcited [14.Ding X. et al.Synthesis of metallophthalocyanine covalent organic frameworks that exhibit high carrier mobility and photoconductivity.Angew. Chem. Int. Ed. 2011; 50: 1289-1293Crossref PubMed Scopus (387) Google Scholar], or doping activated [15.Nath B. et al.A new azodioxy-linked porphyrin-based semiconductive covalent organic framework with I2 doping-enhanced photoconductivity.CrystEngComm. 2016; 18: 4259-4263Crossref Google Scholar] to gain conductivity. Interestingly, the charge-carrier type is tunable by changing the coordinated central metal of the phthalocyanine/porphyrin (Figure 1C) [16.Feng X. et al.High-rate charge-carrier transport in porphyrin covalent organic frameworks: switching from hole to electron to ambipolar conduction.Angew. Chem. Int. Ed. 2012; 51: 2618-2622Crossref PubMed Scopus (289) Google Scholar,17.Ding X. et al.Conducting metallophthalocyanine 2D covalent organic frameworks: the role of central metals in controlling π-electronic functions.Chem. Commun. 2012; 48: 8952-8954Crossref PubMed Scopus (107) Google Scholar]. There are also other excellent recently introduced building blocks [18.Lakshmi V. et al.A two-dimensional poly(azatriangulene) covalent organic framework with semiconducting and paramagnetic states.J. Am. Chem. Soc. 2020; 142: 2155-2160Crossref PubMed Scopus (40) Google Scholar, 19.Rotter J.M. et al.Highly conducting Wurster-type twisted covalent organic frameworks.Chem. Sci. 2020; 11: 12843-12853Crossref PubMed Google Scholar, 20.Duhović S. Dincă M. Synthesis and electrical properties of covalent organic frameworks with heavy chalcogens.Chem. Mater. 2015; 27: 5487-5490Crossref Scopus (77) Google Scholar, 21.Medina D.D. et al.Directional charge-carrier transport in oriented benzodithiophene covalent organic framework thin films.ACS Nano. 2017; 11: 2706-2713Crossref PubMed Scopus (87) Google Scholar] with strong electron-donating properties that give the TANG (Figure 1D) [18.Lakshmi V. et al.A two-dimensional poly(azatriangulene) covalent organic framework with semiconducting and paramagnetic states.J. Am. Chem. Soc. 2020; 142: 2155-2160Crossref PubMed Scopus (40) Google Scholar] and WBDT (Figure 1E) COFs [19.Rotter J.M. et al.Highly conducting Wurster-type twisted covalent organic frameworks.Chem. Sci. 2020; 11: 12843-12853Crossref PubMed Google Scholar] high electrical conductivities of 1 S m−1 and 3.67 S m−1, respectively, after doping. To improve the doping efficiency for electroactive COFs, the selection of the matching dopant is important. It is generally difficult to predict which dopant will give the best result. Dopant screening is therefore necessary for COF materials. For instance, among dopants such as SbCl5, I2, and F4TCNQ, the latter has the best doping efficiency for WBTD (Figure 1E) [19.Rotter J.M. et al.Highly conducting Wurster-type twisted covalent organic frameworks.Chem. Sci. 2020; 11: 12843-12853Crossref PubMed Google Scholar]. Having a large uptake of dopants is also important to ensure that all active sites are doped. Fortunately, COF materials have the natural advantage of high porosity, exemplified by the TANG-COF showing close-to-complete doping of active sites using I2 [18.Lakshmi V. et al.A two-dimensional poly(azatriangulene) covalent organic framework with semiconducting and paramagnetic states.J. Am. Chem. Soc. 2020; 142: 2155-2160Crossref PubMed Scopus (40) Google Scholar]. In this respect, 3D COFs have the potential to achieve higher dopant capture as they exhibit greater porosity than 2D COFs (Figure 1F) [22.Wang C. et al.A 3D covalent organic framework with exceptionally high iodine capture capability.Chem. Eur. J. 2018; 24: 585-589Crossref PubMed Scopus (151) Google Scholar]. It is worth mentioning that different types of activation sometimes can be overlapped to generate better performance [12.Meng Z. et al.Two-dimensional chemiresistive covalent organic framework with high intrinsic conductivity.J. Am. Chem. Soc. 2019; 141: 11929-11937Crossref PubMed Scopus (164) Google Scholar,15.Nath B. et al.A new azodioxy-linked porphyrin-based semiconductive covalent organic framework with I2 doping-enhanced photoconductivity.CrystEngComm. 2016; 18: 4259-4263Crossref Google Scholar]. Once mobile charges are formed, extended conduction channels are needed for efficient charge transfer. Within COF materials, charge transfer occurs through bonds and through space (Box 2 and Figure 1C). For 2D COFs, the bond channel is constructed by in-plane conjugation within individual COF layers. Thus, enhancing the overall in-plane delocalization facilitates through-bond transfer [23.Jin E. et al.Two-dimensional sp2 carbon-conjugated covalent organic frameworks.Science. 2017; 357: 673-676Crossref PubMed Scopus (533) Google Scholar,24.Kim S. Choi H.C. Light-promoted synthesis of highly-conjugated crystalline covalent organic framework.Commun. Chem. 2019; 2: 60Crossref Scopus (49) Google Scholar]. An example of this is sp2c-COF (Figure 1G), which has a structure solely comprising sp2 hybridized carbons, showing a conductivity of 7.1 × 10−2 S m−1 after I2 doping [23.Jin E. et al.Two-dimensional sp2 carbon-conjugated covalent organic frameworks.Science. 2017; 357: 673-676Crossref PubMed Scopus (533) Google Scholar]. The space channel is constructed by π–π stacking between different layers (Figure 1H). Thus, enhancing interlayer π–π interactions by reducing the layer distance and/or avoiding a sliding configuration of layers facilitates through-space transfer [10.Jin S. et al.Two-dimensional tetrathiafulvalene covalent organic frameworks: towards latticed conductive organic salts.Chem. Eur. J. 2014; 20: 14608-14613Crossref PubMed Scopus (115) Google Scholar]. For instance, COFs with an eclipsed stacking mode shows higher conductivity than staggered stacking mode [25.Cai S. et al.Reversible interlayer sliding and conductivity changes in adaptive tetrathiafulvalene-based covalent organic frameworks.ACS Appl. Mater. Interfaces. 2020; 12: 19054-19061Crossref PubMed Scopus (24) Google Scholar]. Fortunately, COF materials usually have a self-sorting ability to align in the most π-overlapped configuration [26.Xu X. et al.Semiconductive porphyrin-based covalent organic frameworks for sensitive near-infrared detection.ACS Appl. Mater. Interfaces. 2020; 12: 37427-37434Crossref PubMed Scopus (35) Google Scholar]. It is worth mentioning that the bond channel and space channel usually do not contribute equally to the conductivity, resulting in anisotropic conductivities for 2D COFs [18.Lakshmi V. et al.A two-dimensional poly(azatriangulene) covalent organic framework with semiconducting and paramagnetic states.J. Am. Chem. Soc. 2020; 142: 2155-2160Crossref PubMed Scopus (40) Google Scholar,27.Wang L. et al.A highly soluble, crystalline covalent organic framework compatible with device implementation.Chem. Sci. 2019; 10: 1023-1028Crossref PubMed Google Scholar,28.Thomas S. et al.Design and synthesis of two-dimensional covalent organic frameworks with four-arm cores: prediction of remarkable ambipolar charge-transport properties.Mater. Horiz. 2019; 6: 1868-1876Crossref Google Scholar]. This is because the in-plane electron delocalization and interplane π-orbital overlap create different types of conductive channels, corresponding to through-bond and hopping transfer mechanisms, respectively. There are two major antagonists to efficient conduction channels. First is the lack of continuity. Pressing COF powders into pellets results in a material with numerous boundaries, cracks, and even gaps inside, which scatter the mobile charges and even impede the conduction channels. This can be solved by exploring technologies to make continuous COF films [21.Medina D.D. et al.Directional charge-carrier transport in oriented benzodithiophene covalent organic framework thin films.ACS Nano. 2017; 11: 2706-2713Crossref PubMed Scopus (87) Google Scholar,27.Wang L. et al.A highly soluble, crystalline covalent organic framework compatible with device implementation.Chem. Sci. 2019; 10: 1023-1028Crossref PubMed Google Scholar]. For instance, SBFdiyne-COF films fabricated in a continuous-flow system show extremely high film quality with the conductivity reaching 3.4 S m−1 after doping by TCNQ [29.Yang Y. et al.A highly conductive all-carbon linked 3D covalent organic framework film.Small. 2021; 17e2103152Crossref PubMed Scopus (2) Google Scholar]. Second are the amorphous or less-ordered regions in the framework [30.Ghosh R. Paesani F. Unraveling the effect of defects, domain size, and chemical doping on photophys