Tetracyanoquinodimethane and Its Derivatives as Promising Sustainable Materials for Clean Energy Storage and Conversion Technologies: A Review

Abstract

7,7′,8,8′-tetracyanoquinodimethane (TCNQ) is one of the most widely studied redox-active molecules and effective π-acceptors, possessing excellent electrical properties. TCNQ derivatives are considered exciting materials due to their multitude of uses. TCNQ-based electrode and dopant materials offer enormous potential in revolutionary energy storage and conversion devices because of their safety, long-term stability, environmental friendliness, charge mobility, and low electrode potential. This review presents the most important advances in applications of TCNQ and its derivatives as promising sustainable organic materials for the optimization of electrical properties in energy storage and conversion devices compared to inorganic materials. This paper shows that materials based on TCNQ can be used as promising candidates for designing efficient and stable photovoltaics, sustainable batteries, and triboelectric nanogenerators, which are very important to the development of energy harvesting. The results presented in this review are sufficiently convincing for us to conclude that applying the TCNQ doping strategy to improve the performance of the discussed devices is very promising. The presented data could serve as inspiration for the development and subsequent design of new, effective, high-performance micro- and nano-systems based on TCNQ for energy storage and conversion.

1. Introduction

Society’s increasing demand for energy is one of the greatest challenges for humanity. Therefore, the most important goal for today’s world is to find renewable, sustainable and ecological sources of energy. Current research in the field of energy covers topics such as the acquisition of energy, sustainable energy sources for micro- and nano-systems, the storage of energy, energy conversion, renewable energy, and green energy. An important aspect of the development of energy harvesting is also achieving it on a smaller scale for electrodes, sensitive chemical and biological sensors, electromechanical systems, nanorobots, and portable personal electronics. The development of processing technologies and advanced electrode materials to satisfy requirements relating to reliability and cost-effectiveness is important as well.

Since the 1960s, the production and application of 7,7′,8,8′-tetracyanoquinodimethane and its derivatives have been actively developing [1,2,3,4]. TCNQ has been one of the most commonly studied redox-active molecules since its first synthesis in 1962 [5]. The body of research on the use of this molecule is constantly growing [1,2,3,4,5,6,7,8,9]. TCNQ has a central hexagonal ring, which is aromatized when taking an electron, and four low-lying vacant C≡N π*-orbitals. This can help to accommodate and delocalize the incoming electron [10]. Moreover, TCNQ, as an active acceptor molecule and redox ligand, can easily transform into an open-shell TCNQ^•− radical anion via electrochemical methods or chemical reduction when it is placed in contact with different electron donors (Figure 1) [2,4,11,12]. The morphology of TCNQ depends on its form and its interaction with the surface of other materials. As a result, different structures are formed [2,5,6]. This variability is a result of different synthesis methods and conditions. The neutral form of TCNQ is often grown as a thin film on substrates, while TCNQ salts and complexes can be grown as microcrystals or nanostructures. However, it should be taken into account that the morphology of TCNQ structures is dependent on specific reactions, such as vapor transport, and the composition of the system. This is related to additional requirements when constructing devices.

The four widely separated cyan groups (–CN) in the TCNQ structure may also act as coordination sites, making TCNQ a potential polydentate ligand capable of binding to four metal centers [1,6,13]. After reacting with a metal ion, including the charge transfer from a metal ion to the TCNQ molecule, TCNQ forms a charge transfer complex (CTC) which participates in the formation of a close π–π bond. This π stacked column structure becomes the basis for the enviable electric and magnetic properties of TCNQ-based compounds [2,4,14]. Charge transfer in TCNQ involves the transfer of an electron from a donor molecule to an electron-accepting TCNQ molecule, forming highly conductive salts and CTCs. This interaction between the donor and acceptor is crucial for creating materials with unique electronic properties. Depending on the donor and reaction conditions, TCNQ-based materials can exhibit incomplete or complete charge transfer. The tilt angle between the donor and strong TCNQ acceptor also changes the degree of charge transfer (q) and electrical conductivity. If 0.5 < q < 0.74, the complex usually exhibits metallic behavior because of its segregated structure. Therefore, combinations of different donors with a strong TCNQ acceptor are already successfully used in the electronics industry [2,3,4,14,15,16,17,18,19,20,21,22,23,24,25,26], including spintronic and non-linear optics [2,3,15], biosensing [4,20,21,22], and electrochemical analysis [4,23,24,25,26]. So far, a large number of TCNQ derivatives have been synthesized, exhibiting various unique physicochemical properties [2,3,4,27,28,29,30]. The deposited active layers doped with a TCNQ thin film work as charge transfer electron-accepting layers. According to dimensionality, these layers can be categorized as 0D to 3D [2,3,30].

Converting dispersed mechanical energy into electrical energy can effectively mitigate the global energy shortage problem. Not all current battery technologies meet the needs of the modern world [31,32,33]. The popularity of organic electrode materials based on organic compounds with redox activity as an alternative to conventional inorganic electrodes is increasing due to their safety, stability, and environmental friendliness [34,35,36,37,38]. In particular, since the beginning of the 21st century, TCNQ and its derivatives have demonstrated extraordinary potential for use in different electronics because of their suitability in energy devices [2,4,39,40,41,42]. For example, the small TCNQ molecule can be used as a cathode material for aqueous zinc batteries due to its very low water solubility and energy gap of 2.58 eV [2,4,43,44,45]. This significantly smaller energy gap than most organic electrodes indicates better internal electrical conductivity [46,47]. Therefore, TCNQ could be a promising alternative to standard conductive polymeric materials such as polythiophene (PTh), polypyrrole, and polyaniline (PANI) used in electronic technologies [48,49]. TCNQ-based electrode materials have also been applied in non-aqueous, redox flow, multivalent-metal, dual-ion, and all-solid-state batteries due to their low cost and high sustainability, among other factors [50,51,52,53,54,55]. Thus, TCNQ can clearly be seen as a future alternative for sustainable battery technologies [56]. Scheme 1 summarizes the physicochemical properties of the TCNQ molecule, indicating that TCNQ is a potential candidate for battery construction.

Thin-film solar cells are another important source of renewable energy that is worth mentioning. The most efficient organic solar cells (OSCs) and perovskite solar cells (PSCs) are made from organic materials, including TCNQ thin-film dopants [57,58,59]. These thin films have the potential to reduce the cost of photovoltaics by eliminating the use of silicon wafers, which account for over 50% of the total production cost of solar cell devices. TCNQ doping significantly facilitates the recombination of majority carriers at the OSC interface. Recombination centers are hidden in the case of excitons and minority carriers. This reduces energy losses during electron transfer processes, which leads to higher device efficiency [60]. Because of their solubility in organic solvents, like acetonitrile or chlorobenzene [60,61], TCNQ dopants can also be integrated into fully solution-processed devices. On the other hand, TCNQ-based PSCs stand out for the robustness of their absorber material. TCNQ’s high chemical stability and the wide variety of successful TCNQ preparation methods available make TCNQ suitable for large-scale PSC production. Thus, TCNQ can be used as a “morphology modulator” in organic solar cells. By introducing and then vaporizing TCNQ during thermal annealing, a better-optimized film morphology can be achieved. Recently, organic perovskite photovoltaic cells have shown great promise in energy storage technologies due to their simple and cheap production process and an unprecedentedly fast increase in efficiency [62,63]. TCNQ is a suitable candidate for PSC applications because of its various desirable properties, including its ability to melt without decomposition, variable solid-state morphology, and high conductivity.

In addition to the development of many “green” energy sectors such as wind, water, and solar energy, and the research on methods for storing the energy obtained in these ways, work is also being carried out in parallel on recovering energy generated during everyday activities such as walking or writing. Known for their wide availability and exceptional biocompatibility, TCNQ and some of its derivatives are good candidates for the development of environmentally friendly triboelectric nanogenerators (TENGs) [64]. TCNQ doping helps to reduce mechanical input and surface wear. Energy from such nanogenerators can help to power nano- and micro-devices.

To summarize, thanks to their unique properties, organic TCNQ-based molecular dopants of high-performing and stable solar cells and batteries offer an interesting alternative to inorganic structures. The widespread implementation of new TCNQ dopants not only increases photovoltaic performance but also decreases the optical band gap, enables diffusion-controlled redox processes, and improves manufacturing compatibility (Figure 2).

Furthermore, metal–organic TCNQ-based compounds have also been placed in the spotlight as catalysts, phosphorescent materials, hydrogen storage materials, etc. [65,66]. Some of these materials exhibit metal-like conductivity and superconductivity at low temperatures. Thus, TCNQ and some of its derivatives can be considered alternative sustainable dopants. In this introductory work, for the first time, attention is focused on the recent progress made in three energy technologies based on the use of TCNQ:

• Modern sustainable TCNQ organic molecular solids for efficient stable photovoltaics;
• Cathode TCNQ-based materials in sustainable high-capacity and long-lifetime batteries;
• TCNQ-based triboelectric nanogenerators as an efficient and high-performance wearable electronic power source.

This pioneering review has been prepared with the hope that the data contained herein will help scientists to exploit the full potential of TCNQ-based materials and encourage researchers to continue research in these application areas.

2. Using TCNQ and Its Derivatives for High-Efficiency Stable Photovoltaics

Solar cells are photovoltaic devices that directly convert sunlight into electrical energy [33,67,68,69,70,71]. The history of photovoltaics began with experiments conducted by the French scientist Edmund Becquerel. In 1839, at the age of 19, he discovered the photovoltaic phenomenon—the generation of an electric current under the influence of light. He created the first prototype of a photovoltaic cell, built from two electrodes composed of platinum and silver, immersed in nitric acid. In 1887, German physicist Heinrich Hertz discovered the photoelectric effect, which showed that light could liberate electrons from metal surfaces. This phenomenon became the basis for the further development of photovoltaics, now essential to modern society. The history of photovoltaics, from the scientific discoveries in the 19th century to contemporary innovations, including inventions in the field of TCNQ, is illustrated in Figure 3 [72,73,74,75]. In recent years, rapid developments in the field of photovoltaics have been achieved, both in terms of efficiency and the diversity of technologies [76,77]. Products such as solar roof tiles and transparent panels have started to appear on the market, enabling the increasingly broader use of photovoltaics in urban architecture. Meanwhile, third-generation photovoltaics (PVs), including organic solar cells [23,39,78,79,80], dye-sensitized solar cells (DSSCs) [81,82,83,84,85], solar thermal selective coatings (STSCs) [86], and perovskite solar cells [58,61,62,63,87,88] are also making inroads, opening the door to novel, unconventional PV applications. DSSCs give great hope due to their characteristics such as inexpensive synthesis, simple construction, elasticity, tunable transparency, high efficiency, and eco-friendliness [85,89,90,91]. Meanwhile, PSCs exhibit high potential due to their good optical absorption coefficient, band gap, photovoltaic parameters (open circuit voltage greater than 1.0 V), carrier length, mobility, and conversion efficiency of more than 20%, as compared to silicon and gallium arsenide solar cells [92,93,94,95].

In recent times, organic batteries have become very popular [96,97]. In the research on OSCs [72,75,98,99], modern organic TCNQ-based molecular solids also offer an interesting alternative to inorganic structures. These sustainable TCNQ systems are excellent photovoltaic materials for the production of energy devices, with the ability to obtain a fill factor (FF) and high open-circuit voltage (V_oc). In 2012, organic solar cells based on TCNQ/copper phthalocyanine (CuPc) and TCNQ/zinc phthalocyanine (ZnPc) were fabricated [100]. The relationship between their microstructure and photovoltaic properties has been investigated. In the case of a TCNQ/ZnPc heterojunction solar cell, the open-circuit voltage, short circuit current (J_sc), fill factor, and power conversion efficiency (PCE) were found to be 0.58 V, 0.27 × 10⁻⁶ A cm⁻², 0.18, and 1.6 × 10⁻⁵%, respectively. Regarding a TCNQ/CuPc solar cell, the V_oc, J_sc, FF, and PCE were obtained to be 0.48 V, 0.37 × 10⁻⁶ A cm⁻², 0.16, and 2.8 × 10⁻⁵%, respectively. According to Suzuki et al., the low performance in both cases is caused by the low crystallinity of the TCNQ layers, which are responsible for the charge transfer from the ZnPc electron-donor layer to the TCNQ acceptor layer [100]. The authors suggest that in both cases of the heterojunction TCNQ/CuPc and TCNQ/ZnPc solar cells, the TCNQ thin film acts as a strong electron-accepting layer and an n-type semiconductor. Ref. [100] is one of the few highly valuable works which propose guidelines for the optimization of photovoltaic properties in heterojunction solar cells.

Large-area graphene is a promising candidate for applications in elastic optoelectronic devices. In 2012, a TCNQ doping process on graphene for the purposes of forming sandwiched graphene/TCNQ/graphene stacked films for solar cell anodes was reported [101]. In this case, TCNQ p-dopants were securely embedded between two graphene layers. The anode based on sandwiched graphene/TCNQ/graphene stacked films showed optimum PCE (~2.58%). This is an encouraging result, because power conversion efficiency using a pristine chemical vapor deposition (CVD) graphene anode is still not appealing due to its much lower conductivity [102]. The TCNQ-based anode film proposed in [101] is propitious for next-generation extensible conductivity devices.

Organic PVs have attracted attention in research [77,97,98,103] due to their high bendability and compatibility with electronic fabrication processes. For example, a very strong electron acceptor tetrafluoro-tetracyanoquinodimethane (F₄TCNQ) has been proven to be a perfect candidate for manufacturing effective OSCs. Liu et al. [103] incorporated F₄TCNQ into a poly(3-hexylthiophene) (P3HT), leading to the realization of devices with an improved power conversion efficiency of 5.83%. Figure 4 shows an electron transferring from P3HT to F₄TCNQ. This TCNQ derivative, with an exceptionally high potential value of 5.2 eV, possesses an even stronger electron-accepting character than TCNQ [104]. Therefore, the excited electrons in P3HT have a high possibility of transferring to F₄TCNQ, thus reducing the recombination of electrons and holes. F₄TCNQⁿ⁻ (n = 0, 1 or 2) also forms strong intermolecular hydrogen bonds, favoring strong electronic interactions [105,106]. In addition, a fluoride can increase the redox potential of the organic molecule and improve the average operating voltage. As a result, the higher charge carrier mobility and photoconductivity of F₄TCNQ/P3HT OSCs with an increased J_sc from 8.19 to 9.85 mA cm⁻² and an increased FF from 63.2% to 68.0% is observed. The electron mobility of the F₄TCNQ-doped device increased from 4.92 × 10⁻⁴ to 6.91 × 10⁻⁴ cm² V⁻¹ s⁻¹, being higher than that of the control device. The incorporation of F₄TCNQ can effectively increase the charge carrier density, which is beneficial for improving the FF of solar cells. Thus, doping increases the quantity of charge carriers, leading to a higher conductivity, as also described in [107,108,109,110]. Moreover, organic p-doping is promising for creating sustainable solar cells due to the high absorption and electrical properties of these cells, which in many respects are comparable to those of crystalline semiconductors. For this reason, p-type doping in organic layers with TCNQ is a good way of improving the performance of OSCs.

The TCNQ molecule can exist in a neutral, anion-radical, or dianion form. The negative charge of the molecule arises from the strengthening of bonds between the cyan groups and the surface atoms by means of the transfer of electrons from the surface into the molecule. As a consequence, the degree of electron density (degree of charge transfer) in the TCNQ molecule increases. The bond lengths of –C=C– and –C≡N are reduced, resulting in the formation of a stable TCNQ^•− radical anion with an active center in its structure. These active sites help to maintain strong contacts (bonds) with other active molecules. Thanks to this, 7,7′,8,8′-tetracyanoquinodimethane also experiences a nucleophilic substitution reaction when it reacts with primary or secondary amines (Scheme 2) to form a number of diaminodicyanoquinodimethane (DADQs) compounds [60,111,112,113,114,115,116,117] with attractive and unpredictable properties. The ease of formation of these compounds results from TCNQ being one of the strongest acceptors and the amino group (–NH₂) being a strong electron donor group. Consequently, TCNQ-based DADQ compounds have been used as conductive materials with promising optoelectronic effects.

For example, in 2023, Mohitkar and his team [60] described an amine-TCNQ/TiO₂ solar cell that displays an improved V_oc of 3 V, a J_sc from 1.25 to 9.12 mA cm⁻², an FF from 0.21 to 0.59, and an increased PCE from 2.26% to 11.75%. The optical band gap among DADQ molecules is about 2.0 eV, which also suggests enhanced intramolecular charge transfer. The authors also concluded that these DADQ compounds have potential applications in OSCs.

TCNQ doping also enhances the performance of dye-sensitized solar cells [118]. Dye sensitizer is the most important part of DSSCs affecting photovoltaic processes [119,120,121]. The TCNQ-based dye structure with TiO₂ increases the driving force of electrons without undesirable charge recombination between the oxidized dye and the semiconductor in comparison with standard DSSCs [118,122]. The affinity of the dye for electron transfer to the semiconductor increases due to its lower electrophilicity, which then improves the final efficiency. For example, in [118], Pakravesh et al. described (2-(4-(3,3-dicyano-1-(4-(dimethylamino)-phenyl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile)/TCNQ/TiO₂ (AQ) and (2-(4-(3,3-dicyano-1-(4-(diphenylamino)-phenyl)allylidene)-cyclohexa-2,5-dien-1-ylidene)malononitrile)/TCNQ/TiO₂ (TQ) dye solar cells. These TCNQ-adduct dyes in DSSCs have a higher electron transfer rate constant, longer excited-state lifetime, and higher polarization. In the case of AQ dye solar cells, the measured parameters V_oc, J_sc, and FF were found to be 3.25 V, 10.63 mA cm⁻², and 95%, respectively. In the case of TQ dye solar cells, the V_oc, J_sc, and FF were 2.72 V, 11.86 mA cm⁻², and 94%, respectively. The results obtained allowed the authors to conclude that these AQ and TQ TCNQ-based dyes could be seen as favorable candidates for use in sustainable DSSCs.

In recent years [58,88,92,93,123,124,125,126], there has been an increasing demand for portable energy systems based on embedded perovskite with an excellent power-to-weight ratio, superior efficiency, facile features, and low cost. In single-junction PSCs, a PCE over 25.8% has been realized [127]. Currently, the widespread use of silicon solar cells is limited due to the expensive production process of these devices [128,129]. The manufacturing of sustainable organic–inorganic halide perovskite (OIHP) solar cells [130,131,132,133,134,135] with a high PCE (reaching 26.7%) has emerged as an alternative technology and surprised the community. TCNQ passivation plays a key role in improving the photoelectric properties of perovskite devices. For example, Abicho and co-authors [57] proposed a strategy to introduce a TCNQ interface passivator between OIHP and various organic molecular sheets. The authors showed that TCNQ is an excellent sustainable candidate for minimizing interface defects in OIHP solar cells. Earlier, in 2014 [136], it was also found that large current leakages caused losses in the V_oc and FF, which had depended on the quality of the interfaces of the OIHP layers and charge transport. The TCNQ-treated glass/ITO/PEDOT:PSC/OIHP thin films (ITO—indium tin oxide; PEDOT—poly(3,4-ethylenedioxythiophene)) reported by Wang et al. exhibited lower current leakages, which led to increases in V_oc by ~1.03 V, J_sc by ~20.1 mA cm^− 2, and PCE by 16.5% [136]. The authors therefore concluded that the use of TCNQ interface passivation is a good option to improve the yield of perovskite devices.

One of the most important approaches to improving the efficiency and stability of perovskite solar cells is rational hole transport layer (HTL) material design. Liu and co-workers [137] described the effect of 0.30 wt. % F₄TCNQ doping on the optical and electrical properties of these devices. The highest power conversion efficiency was notably improved from 13.30% (undoped) to 17.22% (doped) due to efficient carrier transport from the perovskite absorber sheet to the HTL. The obtained data indicated that the new F₄TCNQ-doped film is an efficient HTL, providing high-performance perovskite solar cells. For example, according to [138], the measured FF and PCE parameters for F₄TCNQ are 69.9% and 14.3%, respectively. Meanwhile, according to [139], a F₄TCNQ interface layer was used to increase efficiency by passivating the halogen bond and doping the interface. In this solar cell, the parameters FF and PCE were measured to be 75.4%, and 16.4, respectively. In contrast, F₆TCNQ [140] successfully doped a 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) HTL device with a high FF of 79.8% and a higher PCE of 18%.

Zhao and co-workers [141] described a F₄TCNQ doping strategy of nickel oxide (NiO_x) as a highly efficient hole transporting material for inverted PSCs. Nickel oxide [142,143] is a promising material for hole transport in inverted perovskite solar cells due to its low cost and high photostability. A new high-temperature-resistant F₄TCNQ/NiO_x film demonstrated reduced surface roughness and a higher carrier transport capacity compared to pure NiO_x. The F₄TCNQ/NiO_x-based hole transport layer exhibits an open-circuit voltage of 1.02 V, a short circuit current density of 20.07 mA cm⁻², a fill factor of 74.5%, and a PCE of 15.7%, which is 16.2% higher than that of the undoped HTL. In addition, F₄TCNQ-doped NiO_x substrates facilitated hole migration to the HTL and therefore were characterized by increased transmittance, improving the final photoelectric properties. These results prove that F₄TCNQ/NiO_x can be used as an excellent sustainable and controllable hole extraction layer for realizing inverted sustainable planar PSCs in the future.

In contrast, Hu and co-authors [144] presented highly efficient perovskite solar cells with a poly(triarylamine) (PTAA) hole transport layer enabled by polymethyl methacrylate (PMMA) doping F₄TCNQ. These PSCs exhibited a charger open-circuit voltage of 1.06 V, an FF of 76.3%, and a PCE of 17.9% for a concentration of the F₄TCNQ solution of 0.025 mg/mL. In the same year, Liu and colleagues [59] described a way of doping the HTL with F₄TCNQ and improving the initial stability of mesoporous spiro-OMeTAD PSCs. A highly stable mesoporous triple-cation PSC based on F₄TCNQ with a very long T₈₀ lifetime of more than 1 year (~380 days) for devices stored in air (RH ~ 40%) has been developed. This device exhibits a V_oc of 1.05 V, a J_sc of 19.5 mA cm⁻², an FF of 63.4%, and a PCE of 13.3%, which are higher than those for the undoped HTL (V_oc, J_sc, FF, and PCE of 0.79 V, 18.9 mA cm⁻², 43.4%, and 6.2%, respectively). Similar requests were observed in [145,146,147,148]: the conductivity in the spiro-OMeTAD layer is significantly influenced by the F₄TCNQ doping level. Thus, the spiro-OMeTAD layer doped with F₄TCNQ is primary responsible for the increased stability of the perovskite solar cells when compared to conventional hygroscopic dopants [149,150,151,152,153,154]. p-type doping of F₄TCNQ hole-transporting materials enables the protection of the lower perovskite layer due to their hydrophobic properties (Figure 5).

Furthermore, new solar cells doped with TCNQ or its derivatives have an advantage in the construction of sustainable devices due to the stability of their photovoltaic performance under an air atmosphere. Thus, the optimization and engineering of TCNQ-doped perovskite solar cells could lead to highly efficient devices with long-term stability in the future.

TCNQ-based charge transfer materials also support carrier generation. As a result, an increase in the short-circuit current density related to conversion efficiency (η, %) is observed [155,156]. In 2023 [157], the fabrication and characterization of methylammonium perovskite solar cells using a TCNQ-doped decaphenylpentacyclosilane (DPPS)/CuPc(NH₂)₄ complex as a hole-transporting material were performed. According to Suzuki and co-authors, an increase in J_sc, FF, and V_oc related to η was observed. This was caused by the charge transfer due to molecular interaction with p-orbital overlap between CuPc(NH₂)₄ and TCNQ molecules. This provided measured V_oc, J_sc, FF, and η values of 0.766 V, 23.1 mA cm⁻², 61.84%, and 10.9%, respectively. These DPPS/CuPc(NH₂)₄/TCNQ photovoltaic perovskite cells are characterized by stable photovoltaic performance with a good conversion efficiency even after 180 days compared to the above-described spiro-OMeTAD.

In summary, the use of TCNQ and its F₄TCNQ derivative [4,20,21,22,23,24,25,57,58,59,60,100,114,115,116,137,141,144,145,156,157] is a promising doping technique as it allows for a non-hygroscopic additive. As a result, promising V_oc, J_sc, FF, and PCE values for solar cells are being observed (

Table 1. Photovoltaic characteristics of solar cells doped with TCNQ or its F₄TCNQ derivative.

TCNQ-Based Organic Solar Cell	Voc (V)	Jsc (mA/cm)	FF (%)	PCE (%)	Ref.
ZnPc/TCNQ CuPc/TCNQ	0.58 0.48	0.27·10−3 0.37·10−3	18.0 16.0	1.6·10−5 2.8·10−5	[100]
Graphene/TCNQ/graphene	0.60	8.90	48.0	2.58	[101]
Spiro-MeOTAD/DMC/F4TCNQ	0.98	24.00	62.4	14.40	[145]
P3HT/F4TCNQ	0.87	9.85	63.2	5.83	[103]
DADQs/TCNQ/TiO2	3.00	9.12	59.0	11.75	[60]
Spiro-MeOTAD/F4TCNQ	1.04	19.40	69.9	14.30	[139]
Spiro-MeOTAD/F4TCNQ	0.95	18.72	56.8	10.59	[154]
CH3NH3PbI3/F4TCNQ	1.06	20.30	75.4	18.10	[140]
PBDTTT-EFT/F4TCNQ	0.80	17.39	61.8	8.60	[164]
PEDOT:PSS/F4TCNQ	1.02	21.93	77.0	17.22	[137]
PTAA/F4TCNQ/Bphen/Al	1.12	23.38	77.1	20.16	[150]
FTAZ:IT-M/F4TCNQ	0.95	18.70	71.0	12.40	[165]
2mF-X59/spiro-OMeTAD/TCNQ/CsPbI2Br	1.20	14.78	75.85	13.42	[166]
F4TCNQ HTL/NiOx	1.02	20.07	74.5	15.70	[141]
AQ/TCNQ/TiO2 TQ/TCNQ/TiO2	3.25 2.72	10.63 11.86	95.0 94.0	18.92 18.90	[118]
PMMA/F4TCNQ	1.06	21.81	76.3	17.90	[144]
CuPc(NH2)4/TCNQ/CH3NH3PbI3	0.77	23.1	61.84	10.90	[157]

Abbreviations: bphen—7-diphenyl-1,10-phenanthroline; DMC—decamethylcobaltocene; FTAZ:IT-M—a high-performance polymer/small-molecule blend; 2mF-X59—N′,N′,N″,N″-tetrakis(4-methoxyphenyl)spiro[fluorene-9,9′-xanthene]-2,7-diamine; PBDTTT-EFT—poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-3b]thiophene-)-2-carboxylate-2-6-diyl)]; PTAA—poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine.

). TCNQ-based OSCs, including perovskites, have extraordinary features such as a simple design, stability, flexibility, and a light weight compared to polymer solar cells. Through extensive work over the past several decades [64,75,77,79,86,87,88,92,93,94,95,131,132,133,134,135,138,139,140,146,147,148,149,150,151,152,153,154,155,158,159,160,161,162,163], the efficiency of solar cells, including PSCs doped with TCNQ, now rivals that of commercial photovoltaic materials.

3. TCNQ-Based Materials in Sustainable Green Energy Devices

Materials based on TCNQ are also suitable for use in portable and flexible green energy devices [167,168,169,170,171]. Starting from 2022 [172], multifunctional solar cells have seen significant improvements and now have an efficiency of 27.1% for flexible PSCs and of 33.9% for monolithic perovskite/Si solar cells [173]. The continuous research and development in this field has led to greater comprehension of the synthesis of new organic photovoltaic materials, resulting in more effective doping techniques. Modern strategies offer promising opportunities for achieving a higher PCE and advancing the field of TCNQ-derivative-based OSCs with enhanced efficiency and stability in the future. Thus, TCNQ and some TCNQ-based materials could contribute to the advancement of organic green energy PVs for the cost-effective realization of solar energy conversion.

4. Cathode TCNQ-Based Materials in Sustainable Batteries

Electrode materials [4,174,175,176,177,178] which use redox-active organic compounds as active components have attracted attention as promising alternatives to inorganic electrodes because they offer safety, stability, and environmental friendliness. Conductive TCNQ, as a small organic molecule with four cyan groups, is promoted today as a positive electrode material. The use of less than 20 wt.% conductive TCNQ can lead to high specific energy values [40,179,180,181].

Aqueous zinc ion batteries (AZIB) have been one of the most favorable power sources for portable and wearable electronic devices in recent years due to their low toxicity, excellent stability, and, most importantly, the appreciable specific energy (1086 Wh kg⁻¹) and high theoretical capacity (820 mAh g⁻¹) of zinc. As a result, metallic zinc anodes are regarded as one of the safest electrodes in aqueous batteries, while also having good compatibility with water and low redox potential (0.76 V vs. SHE) [182,183]. It is known that cyan groups can easily be reduced and regenerated via further oxidation [2,3,184,185]. For this reason, small-size organic electrode materials based on TCNQ [186] have also become a prominent choice for AZIBs. The stability of the cathode increases by confining TCNQ material inside the aqueous zinc batteries. Kundu et al. [187] reported similar small-size organic molecules as cathode materials for sustainable AZIBs. Chola and Nagarale [188] suggested the usefulness of TCNQ-based cathode materials for rechargeable aqueous zinc batteries with zinc plates as an anode. The cyclic voltammograms presented in [188] showed an oxidation peak at 1.4 V, whereas a reduction peak was observed at 0.65 V because of the coordination/decoordination of Zn²⁺ ions with TCNQ molecules. According to Chola and Nagarale, the stability and efficiency of a TCNQ-based cathode can be significantly increased by encapsulating the TCNQ inside a specially designed organic porous polymer, cyanuric chloride and pyrene (CCP). In contrast, TCNQ cathodes provide a higher capacity than a graphite positive electrode (ca. 60 mAh g⁻¹).

Recently, TCNQ-based cathodes have also been used in AZIBs at room temperature. For example, Wang and his team [189] investigated highly purified TCNQ (p-TCNQ) as a cathode in AZIBs in a quite large temperature range of 25 °C to −40 °C. This p-TCNQ cathode delivered a high capacity of 160.7 mAh g⁻¹ at 0 °C (a current density of 3 A g⁻¹) and capacities of 172.9 and 96.6 mAh g⁻¹ (a current density of 100 mA g⁻¹) at −20 °C and −40 °C, respectively. This suggests better surface control with p-TCNQ and significantly better cycling stability than with TCNQ at various temperatures. In another work [190], Wang et al. described the charge transfer of TTF-TCNQ, TTF, and TCNQ organic sustainable electrode materials in AZIBs with an initial capacity of 153.4, 65.7, and 142.8 mAh g⁻¹, respectively, at a current density of 1 A g⁻¹.

A very strong TTF π-electron donor and its derivatives have recently emerged as key materials for new applications in electronics and molecular sensing due to their unusual electrical properties and smaller charge transfer resistance [45,98,100,190,191]. For example, according to Wang et al. [190], the TTF-TCNQ cathode material showed a far superior rate, cycling performance, and ionic diffusivity in ionic storage than each single-moiety counterpart, especially at low temperatures. It showed a specific capacity of 123.3 mAh g⁻¹ at a current density of 3 A g⁻¹ at −20 °C. Thus, Wang provides a new idea for the design of new organic electrode materials for sustainable high-performance batteries in the future.

In 2025, Ma and co-workers [192] demonstrated that because of TCNQ’s cyan groups, a high output voltage (1.15 V vs. Zn²⁺/Zn) with the incorporation of Na⁺ ions and an I⁻/I³⁻ redox mediator in a ZnSO₄ aqueous electrolyte is possible. The TCNQ cathode demonstrated a significant improvement in rate capability and perfect cycle performance (2000 cycles with a decay rate of 0.0035% per cycle). The authors pointed to a higher reduction voltage with Zn²⁺ ions inserted into Zn(TCNQ)₂. These results help us to understand the higher conductivity and ion storage capacity of TCNQ in the presence of Na⁺ and Zn²⁺ ions. Ref. [192] offered a promising approach for advancing sustainable energy storage systems. Also in 2025, Lee et al. [193] described the four-electron redox activity of an organic charge transfer complex (OCTC) TCNQ/PNZ (PNZ—phenazine) with superior cycle stability. It retained 88% of the maximum capacity of 122 mAh g⁻¹ at a rate of 2 A g⁻¹ over 100 cycles, using Zn-aqueous electrolytes (1 M Zn(TFSI)₂ in H₂O). Surprisingly, the full redox reaction achieves an unprecedentedly high electrode-level energy density, delivering ~10 mAh cm⁻² of areal capacity (580 μm thick electrodes) in Zn batteries. Compared with electrodes of individual molecules, the TCNQ/PNZ OCTC could exhibit a significant enhancement in rate capability and cycle stability, which contributed to the electronic conductivity increasing by ~105 and ~20 times compared to PNZ and TCNQ molecules, respectively, and the suppressed dissolution in the electrolyte. When cycled within a broad voltage range of 1–3.8 V (vs. Li⁺/Li) encompassing redox activities of both entities (e.g., PNZ at 1.3, 1.9 V, and TCNQ at 2.6, 3.2 V (vs. Li⁺/Li), respectively), the electrochemical performance deteriorated rapidly, jeopardizing the expected benefits of the OCTC electrode in this trade-off. The OCT TCNQ/PNZ electrode exhibited extended redox capabilities to a four-electron transfer reaction with discharge voltages at 2.86, 2.92, 3.13, and 3.29 V (vs. Li⁺/Li) or 0.56, 0.62, 0.83, and 0.99 V (vs. Zn²⁺/Zn), respectively. The authors elucidate the complex interplay between organic electrodes and electrolytes in the charge storage mechanism. The same problem was explained in [194]. The results described in [193,194] highlight the importance of electrolyte design in developing sustainable organic electrode materials.

In the 1990s, lithium-ion batteries (LIBs), with their high energy efficiency of over 90%, were introduced to the battery market [195,196]. However, the main challenges related to their application in energy storage are their theoretical specific energy (400 Wh kg⁻¹) and long charging time [197,198]. It is known that the ionic conductivity of non-aqueous electrolytes is two orders of magnitude lower than that of aqueous electrolytes [37,45,187,194,199,200,201]. In this regard, TCNQ, as one of the strongest π-acceptors [4,50,51,202,203,204,205], has also been proposed for the storage of Li ions. Fujihara and co-authors [206] developed a charge transfer TTF-TCNQ mixture as a cathode material without any additive. An overshooting potential was observed, especially at 0–10 mAh g^–1 and 0.1–0.2 V. Similar behavior is often observed in cathode materials for LIBs [207,208].

It is known that the cycle stability of all organic conventional batteries is mainly governed by the aging and degradation of the electrolyte and electrodes due to repeated electrochemical reactions [209]. Copper-tetracyanoquinodimethane (CuTCNQ) is a perfect model of a sustainable electrode material [50,52,53,204] that can reversibly store Li⁺ ions under lower working voltages, leading to an increase in the life cycle length. CuTCNQ salts can exist in two polymorphic forms: the kinetically stable phase I, which has a needle morphology, and the low-conductivity thermodynamically stable phase II, which has a platelet morphology [210]. Both the Cu⁺ cation and the TCNQ- anion are electrochemically active, allowing CuTCNQ to participate in electron transfer redox reactions. For example, a self-supported CuTCNQ electrode for LIBs has been fabricated by Meng and co-authors [50]. The charge specific capacity of this electrode is 219.4 mAh g⁻¹, reaching 280.9 mAh g⁻¹ at a current density of 100 mA g⁻¹ after 500 cycles, which was also obtained by Song et al. [211]. The reversible insertion and exiting of Li⁺ ions into the benzene rings of TCNQ ligands were observed. Moreover, the conductivity of the CuTCNQ film is 0.305 S cm⁻¹ [50,212]. Thus, this CuTCNQ film can be used directly as a perfect candidate to design Li-based batteries in the future. For example, in [213], Yin and co-authors described a high-conductivity thin layer (TL) lithophilic large-surface-area CuTCNQ metal–organic framework (MOF). Recently, MOFs [214,215,216] emerged as a promising class of materials for battery applications due to the hybrid structure of their organic and inorganic parts. The CuTCNQ MOF TL has polar surfaces with a porous structure, providing it with a strong ability to adsorb Li⁺ ions from the electrolyte. This Li/CuTCNQ-MOF electrode has a very stable potential of around 13.1 mV, and therefore it could be a promising candidate in designing sustainable metal-based batteries.

In recent times, there has been a demand for advancements in the development of new positive electrode materials for reversible anion storage in dual-ion battery (DIB) cells. DIB technology using the appropriate electrolyte could be considered an environmentally friendly option for grid energy storage [217,218,219]. For example, Wang et al. [54] described CuTCNQ/lithium/sodium/graphite-based dual-ion batteries with ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate as an electrolyte. The results of [54] demonstrate a higher working voltage, an excellent rate capacity of 76 mAh g⁻¹ with a current density of 0.1 A g⁻¹, and good reversibility of CuTCNQ DIB. Thus, TCNQ represents a promising type of sustainable material to overcome the limits of typical positive DIB electrodes. The CuTCNQ MOF positive electrode in DIB cells has also been investigated by Dühnen et al. [220]. Many known cathode materials for LIBs exhibit a conversion reaction with Li⁺ ions at high potentials. In contrast, the complete discharge of CuTCNQ (2.4 V vs. Li/Li⁺) results in a total specific capacity of 157 mAh g⁻¹ in the first cycle [221,222,223]. According to Dühnen et al., a new CuTCNQ MOF represents an unexplored class of sustainable materials for anion storage in dual-ion cells.

In 2022, Geng et al. [55] presented some surprising results for F₄TCNQ. A new solid-state high-voltage (3.4 V) Li/F₄TCNQ battery was found to have a relatively high redox potential of 3.0 V vs. Li⁺/Li, which is higher than the redox potential of most organic cathode materials [224,225,226,227,228]!

Canals-Riclot and co-workers [229] designed an organic TCNQ all-solid-state lithium metal battery using a polymer covalent organic framework (COF)/polyethylene oxide (PEO) composite as a solid electrolyte. According to [229], the Li(TCNQ)-PEO-COF@LiI/PEO/Li cell was cycled at 70 °C and C/20 rate between 2.7 and 3.15 V vs. Li⁺/Li. A much better reversibility was observed, with 0.67 Li⁺ inserted per TCNQ unit at the second discharge (~88 mAh g⁻¹) and 0.44 Li⁺ at the tenth discharge (~58 mAh g⁻¹). The observed potentials for these two reversible charge/discharge processes are in good agreement with potentials reported by other authors [230] for the first and the second lithiation processes of TCNQ, respectively: TCNQ + Li⁺ + e⁻ ⟶ LiTCNQ and LiTCNQ + Li⁺ + e⁻ ⟶ Li₂TCNQ (Figure 6). Therefore, very weak polarization was observed for these two processes (0.04 V and 0.1 V), representing a very good result for an all-solid-state cell. This improvement of the electrochemical behavior of the battery may be explained by the increased ionic conductivity of the composite polymer electrolyte containing COF@LiI.

Generally, highly porous π-electronic COFs can be used as materials for functional designs and as stable electrode materials at high potentials [230,231]. The electrochemical properties of COF-based materials have been significantly improved by introducing redox groups into their covalent organic framework. This favors the more widespread use of COFs in electrochemical sensors and other electronic devices [230,232,233,234]. TCNQ COFs also show a high electrical conductivity of 3.4 S m⁻¹, which is a new record for 3D COF materials [235]. This presents a strong pretext for constructing new efficient energy storage devices based on TCNQ in the future.

Nowadays, because of lithium’s scarcity, batteries based on a sodium-ion (NIBs) [38,215,236,237,238] are also very promising from a practical point of view. NIB cells have huge potential in energy storage applications due to the abundance of the resources required for their creation, their low price, and their environmental friendliness. Organic compounds composed of C, H, and O are also cost-effective. Recently, CuTCNQ [202,204,205,239], as a promising high-capacity sustainable cathode material for both NIBs and potassium-ion batteries (KIBs), was investigated. In each charge/discharge process (voltage 2.0–4.1 V) a three-electron redox reaction occurs, which provides a very high reversible discharge capacity of 255 mAh g⁻¹ [202]. As a result, cathode materials based on CuTCNQ have an extremely large capacity of 252 mAh g⁻¹ and a specific energy density of more than 900 Wh kg⁻¹. After oxidation, Na[Cu(TCNQ)²⁻] (Cu^II ⟶ Cu^I) forms a more conductive structure (phase I) of CuTCNQ with a record redox potential of 3.8 V vs. Na⁺/Na. According to [202], metal–organic CuTCNQ compounds present a new opportunity to achieve high energy density for NIBs and could provide a new way to design potential sustainable cathode materials for energy storage.

Recently, potassium ion batteries have drawn attention as a promising technology due to the relative abundance of potassium. A significant advantage of KIBs is the possibility of using graphite anodes in them, in contrast to NIBs. However, both KIBs and NIBs are easy to implement [240]. Potassium and sodium, which do not alloy with aluminum, can be used as anode current collectors, replacing copper in LIBs [241,242]. The abundant potassium resources (i.e., 1.5 wt. % in the Earth’s crust and 380.0 mg L⁻¹ in seawater) mean that potassium-ion batteries can successfully be used in electric vehicles and in power grids for energy storage [243,244]. However, standard KIBs have low capacity even at voltages higher than 4.0 V. Therefore, modifying these devices by coating the electrode surfaces with TCNQ-based materials may enhance their performance. For example, the CuTCNQ organic material mentioned above shows a specific capacity of 244 mAh g⁻¹, an average discharge potential of 2.8 V (vs. K⁺/K), and high performance of 70% of its initial capacity after 50 cycles [204].

Conductive TCNQ is a positive electrode material for rechargeable aluminum batteries as well. According to Guo et al. [245], TCNQ delivered a specific capacity of 180 mAh g⁻¹ and a high discharging potential of 1.6 V (vs. Al³⁺/Al) for devices. The reversible capacity was still 115 mAh g⁻¹ even after 2000 cycles at 500 mA g⁻¹. There is thus an excellent opportunity to develop effective cyan-organic electrodes for sustainable aluminum batteries in the future. In contrast, Zhong et al. [43] found that the insertion/extraction of Al³⁺ into and out of TCNQ is highly reversible and leads to the improvement of the life cycle and specific capacity. The energy binding in the connection of TCNQ⁻ with Al³⁺ is −2.34 eV for the TCNQ⁰/TCNQ⁻ redox process. A TCNQ/PPy cathode exhibited a reversible capacity of 245.8 mAh g⁻¹ at 0.5 A g⁻¹ and a life cycle of 1000 rounds at 3 A g⁻¹. This work demonstrates that adding highly valent cations in the presence of TCNQ is an effective strategy to enhance the electrochemical performance of organic electrodes in aqueous rechargeable batteries. Thus, aluminum TCNQ-based batteries [43,246,247,248] have also been regarded as the most effective for energy storage due to their high specific capacity. Inorganic positive electrode materials cannot properly meet the requirements of high-energy-density aluminum batteries [249,250,251]. TCNQ cyan groups [209,252,253] have also been used as the substituents for nucleophilic reactions, which allowed TCNQ-based organic molecules to coordinate with aluminum-chlorine coordination ions. TCNQ exhibits a great electron affinity, specific capacity (262 mAh g⁻¹), and high reduction potential (1.96 V vs. Al³⁺/Al) [174,246]. Thus, TCNQ is the most appropriate cyan-organic material for sustainable aluminum batteries.

To summarize, organic TCNQ cathode materials are more attractive in view of their large surface area, good safety, and perfect physicochemical properties, including their low electrode potential, high redox capability, and high cycle stability (Scheme 3).

The features of TCNQ cathode materials for sustainable batteries are illustrated in

Table 2. Features of sustainable TCNQ-based cathode materials for rechargeable batteries.

Cathode Material	Type of Battery	Average Voltage (V)	Cycling Stability (%)/Cycles	Specific Capacity (mAh g−1)	Ref.
CuTCNQ	Li+	0.01–3.20	75/500	280.9	[50]
CuTCNQ	Li/Na DIBs	4.26	75/200	195.4	[54]
CuTCNQ-MOF	Li+ DIBs	3.75	70/50	157.0	[226]
F4TCNQ/ LLZTO-PEO	Li+	3.40	77/50	80.00	[55]
CuTCNQ	Na+	2.00–4.10	84/50	255.0	[202]
TCNQ	Na+	2.20–3.00	70/50	233.0	[239]
CuTCNQ	K+	2.80	75/50	244.0	[204]
TCNQ/CCP	Zn2+	1.10	78.54/1000	171.0	[186]
TCNQ/PPy	Zn2+	0.45	75.40/1000	245.8	[50]
p-TCNQ	Zn2+	1.10	55.20/50	250.0	[189]
TCNQ	Zn2+	1.02	98.90/50	244.4	[190]
FTCNQ	Zn2+	1.20	93.40/50	168.5	[190]
F4TCNQ	Zn2+	1.40	75.30/50	135.1	[190]
TCNQ/PNZ	Zn2+	1.00–3.80	88/100	150.0	[193]
TCNQ	Al3+	1.60	75/2000	180.0	[245]
TCNQ/PPy	Al3+	0.45–0.85	76/100	245.8	[43]

.

As illustrated in the table above, TCNQ is successfully used as a positive electrode material in batteries. Nonetheless, it has high theoretic specific capacity and its performance can sometimes decline because of material dissolution [43].

5. TCNQ-Based Triboelectric Nanogenerators as Sustainable Wearable Power Sources for Energy Conversion

The increasing intelligence of wearable electronics has led to growing demand for higher energy levels but smaller volume or weight at the same time. In recent years [254,255,256,257,258,259,260], there has been a demand for triboelectric nanogenerators that can convert various types of mechanical energy into electrical energy, including body motions, water waves, or other vibrations. Through the contact electrification effect and the electrostatic induction effect, TENGs generate electrostatic charges and an external electric current, respectively [257,258,261,262]. Organic conductive materials can be used as electrification materials for the production of TENGs based on the tribovoltaic phenomenon [262,263,264]. Recently, several biodegradable and natural materials have been used to fabricate TENGs: cellulose, marine biomaterials, silk fibroin, petal roses, lotus, rice paper, etc. [31,265,266,267,268,269]. A wide range of carbon materials have also been tested for their suitability in energy harvesting devices. TCNQ conductive materials have been designed as sustainable electrode materials for TENGs due to their excellent electrical and physicochemical properties [270,271,272,273]. For example, Wang et al. [270] designed a TCNQ/PVA-based positive tribomaterial (PVA-polyvinyl alcohol) composed of Ni/Ag conductive tape as the top and bottom electrodes. This TCNQ-based TENG shows relatively high output power and can generate a V_oc of 520 V and a J_sc of 218 mA m⁻², regardless of the concentration of TCNQ. This demonstrates the versatile applications of TCNQ TENGs as sustainable capacitors for electronic devices and as wireless power transmitters. In another work, Zhang and co-workers [271] fabricated a new PVK:TCNQ(THF)/Ag (PVK—poly(9-vinylcarbazole) hybrid device with ideal 100% productivity. The TCNQ cyan groups coordinated very efficiently with Ag through the charge transfer between Ag and TCNQ: Ag_x + TCNQ_x + [Ag⁺TCNQ⁻]_n−x ↔ [Ag⁺TCNQ⁻]_n (0 < x ≤ n). According to [271], a new TCNQ-based hybrid device was found to have low threshold voltages of ~0.21 V and −0.3 V and a low operation power of 1.52 × 10⁻⁵ W, representing the lowest levels amongst organic devices. According to Zhang et al. [271], their device will inspire novel strategies for rationally designing multifunctional memory devices in the future.

A very strong F₄TCNQ electron acceptor was used for the construction of a PEAI CABI/Spiro-PTAA-F₄TCNQ-based TENG (PEAI—phenylethylammonium iodide; CABI—n-type lead-free Cu₂AgBiI₆ perovskite; PTAA—poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) by Choi et al. [274]. This nanogenerator shows a current density of 11.23 μA cm⁻², a power density of 1.25 W m⁻², and an upper voltage of 0.81 V. A p-type F₄TCNQ dopant increases the charge mobility of the film. Jiao et al. [64] came to the same conclusion. This new F₄TCNQ-based device is also temperature- and humidity-sensitive. The output voltage of the PEAI CABI/Spiro-PTAA-F₄TCNQ-based TENG decreases with humidity. Thus, this TCNQ-based TENG can be used as a sustainable temperature and humidity sensor in the future.

All known examples of TCNQ-based nanogenerators meet the high sensitivity, safety, and biocompatibility requirements. Therefore, it is necessary to further develop highly efficient power management circuits and continue research that could lead to significant advances in the development and application of TCNQ-based TENGs. The future prospects of TCNQ-based TENGs are illustrated in Figure 7.

6. Conclusions and Perspectives

Since the beginning of the 21st century, due to their excellent physicochemical and electrical properties, TCNQ-based compounds have been exceptional candidates for various electronic applications. Over the last few decades, a wide range of TCNQ derivatives have been tested in this respect. This review presented the evolution of stable TCNQ-based materials for energy storage and conversion and the most important applications of TCNQ organic compounds as prospective sustainable cathode materials in solar cells, sustainable batteries, and TENGs.

This paper demonstrates that TCNQ organic compounds offer an interesting alternative to inorganic structures. TCNQ doping plays the dual role in improving the photoelectric properties and stability of perovskite devices due to the unusual redox behavior of TCNQ. CTCs based on TCNQ support charge transfer, yielding increases in short-circuit current density, which is related to conversion efficiency. These charge transfer processes are fundamental to creating advanced materials for application in electronics.

TCNQ-based electrode materials can be applied in a wide variety of energy storage devices such as non-aqueous Li⁺, Na⁺, K⁺-ion, dual-ion, multivalent-metal, and aqueous batteries. These electrodes can be fabricated with more than 90 wt.% of active material, enhancing the electrode-level specific energy. CuTCNQ is one of the best examples of a good electrode material, being able to reversibly store Li⁺/Na⁺/K⁺ ions under a lower working voltage. High-conductivity TCNQ positive electrode materials can also be used in rechargeable aluminum batteries. TCNQ-based batteries are regarded as the perfect sustainable candidates for low-cost and large-scale energy storage.

F₄TCNQ, as a TCNQ derivative, is often used as an effective acceptor on electrode surfaces and a p-type dopant in sustainable energy devices due to its exceptionally high potential value of 5.2 eV.

Additionally, this review discusses the fundamental challenges involved in elevating TENGs to the next level, using TCNQ and some of its derivatives as sustainable materials. All TCNQ-based nanogenerators meet the high sensitivity, safety, and biocompatibility requirements.

In summary, using TCNQ-based materials with these unique properties expands the list of candidates for sustainable high-energy organic materials. This can be considered as a future alternative for high-performance solar cells, sustainable batteries, and high-sensitivity triboelectric nanogenerators. The presented data can be used as a guide for future research and should help to drive the development of new sustainable energy storage and conversion devices for a cleaner environment. This paper expands on a number of research papers discussing recent advances in energy storage and conversion. Hopefully, the information featured in this work will provide readers with a better understanding of TCNQ chemistry and illustrate the reasons why 7,7′,8,8′-tetracyanoquinodimethane can be considered as a sustainable material for sustainable energy device design.