Lignin-Based Thin Films in Emerging Organic Transistor Devices: Challenges, Strategies, and Applications

Abstract

Lignocellulosic biomass, a rich and underutilized source of lignin, presents considerable potential for advancing sustainable electronic materials. This review explores the lignin’s role in organic transistor-based devices, considering its integration into various components. It highlights lignin’s structural and chemical characteristics that influence its performance in such devices, along with key factors affecting its processability, interfacial behavior, and compatibility with existing organic electronic materials. By outlining current research directions and emerging applications, this work aims to provide a foundation for further exploration of lignin-based thin films in next-generation, green organic electronics.

1. Introduction

Lignocellulosic biomass, the most abundant raw material on Earth, is a natural source of lignin, a complex aromatic polymer that is an integral constituent of plant cell walls. Lignin acts as a binder for hemicellulose and cellulose, conferring structural rigidity and hydrophobicity to plants [1]. Its structure is characterized by three monolignol subunits, i.e., synapyl alcohol, coniferyl alcohol, and p-coumaryl alcohol, which form syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) groups, respectively [1,2].

Despite its abundance, approximately 98% of the lignin extracted annually from the pulp and paper industries is burned as a low-value energy source, while only a small fraction is utilized for high-value applications [3,4]. This underutilization stems from lignin structural complexity and heterogeneity, which vary with the botanical source and extraction method involved. However, lignin’s unique properties, such as high carbon content, thermal stability, antioxidant properties, biodegradability, and biocompatibility, make it a promising candidate for eco-friendly advanced materials [5,6,7,8,9,10,11,12].

The increasing demand for sustainable alternatives in the electronic industry has driven interest in organic electronics, enabling flexible, lightweight, and low-energy-consuming technologies. Although outstanding advancements in this field have been achieved, a critical need for even greener, renewable materials remains [13]. In fact, sustainability has become a key focus in organic electronics research, with papers addressing various aspects of sustainability constituting more than 28% of total publications on the topic in 2024 (SciFinder^®® database). With its abundance, renewable nature, biodegradability, and biocompatibility, lignin represents a unique opportunity in this context. It has primarily been investigated as an alternative to conventional synthetic dielectrics (e.g., poly(methyl methacrylate)—PMMA, polyvinylphenol—PVP, SiO₂) and plastic-based substrates (polyethylene terephthalate—PET, polyethylene naphthalate—PEN). While traditional dielectrics like SiO₂ exhibit superior dielectric strength and stability, lignin-based materials can achieve adequate permittivity and low processing temperatures. When incorporated into flexible substrates, these hybrid systems aim to replace plastic-based supports such as PET and PEN, further contributing to the sustainability of organic electronics. Lignin has also been explored as an electrolyte, surface modifier, conductive ink precursor (after carbonization), semiconductive polymer dopant for photoanodes in photovoltaic cells, and an active material in sensing layers—either as a structural matrix or as part of composites or blend films [5,8,11,14].

However, integrating lignin into electronic devices remains technically challenging, due to differences in its physical–chemical properties compared to other electronic components. Lignin’s complex structure and the presence of a variety of functional groups—primarily phenolic hydroxyls, along with aliphatic hydroxyls, carboxyl, and methoxy groups—can lead to issues such as solubility and processability [15,16], adhesion properties at interfaces, and the introduction of electronic trap states at semiconductor interfaces that impacts charge transport [17]. Specifically, the irreproducibility of results due to variations in lignin source, extraction, and processing conditions remains a critical hurdle. In addition, the functional groups can engage in specific interactions with other materials during the formation of composites, which might affect the material properties unpredictably [14,18].

While reviews covering the use of lignin in organic (opto)electronics exist, a comprehensive analysis of the challenges and strategies for integrating lignin thin films into organic transistor-based devices—an emerging yet underexplored niche in sustainable electronics—is lacking.

This review addresses this gap by providing a focused analysis of the potential of lignin in thin films for organic transistor-based devices, including, but not limited to, organic field-effect transistors (OFETs), electrolyte-gated transistors (EGTs), and sensors. Specifically, it discusses: (i) functionalization and processing strategies to enhance the compatibility of lignin with transistor components, (ii) key integration challenges such as solubility, adhesion, and interfacial properties, and (iii) the role of lignin-based thin-film composites in advancing sustainable devices, as reported in the literature. By addressing these aspects, this work aims to highlight both the opportunities and issues associated with leveraging lignin in next-generation, greener transistor technologies.

While the studies discussed in this review rely on commercially available lignin or do not explicitly detail its origin, it is nonetheless important to provide context and begin with outlining the main approaches to lignin extraction and purification. These processes directly affect the structure, functional groups, and consistency of lignin, all of which are critical parameters in thin-film electronic applications.

2. Extraction and Purification of Lignin from Lignocellulosic Biomass

2.1. Extraction Processes

Extraction of lignin from lignocellulosic biomass can be broadly classified into chemical, physical/physicochemical, and emerging methods. The choice of extraction technique is critical as it significantly influences the chemical structure, molecular weight distribution, and purity of the isolated lignin. Lignin extracted from softwood differs considerably from that extracted from hardwood or agricultural residues, leading to variations in functional group content (e.g., hydroxyl, carboxyl, phenolic groups) and consequently affecting its solubility, film-forming ability, and electrical properties.

2.1.1. Chemical Methods

Kraft Process

The Kraft process stands out as one of the most prevalent industrial methods for lignin extraction, particularly in the pulp and paper sector, where it is utilized to separate lignin from wood chips [19,20,21]. This method involves cooking wood chips in a solution of sodium hydroxide and sodium sulfide, known as white liquor, at elevated temperatures ranging from 150 to 180 °C and a highly alkaline pH of 13–14 for approximately two hours. During the process, the alkaline conditions facilitate the cleavage of ether bonds within the lignin structure through an episulfide intermediate. A color transition of the liquor, which changes from white to dark brown or black, can be observed. The resulting Kraft lignin is typically characterized by its dark color and a sulfur content of 1%–2% due to the use of sodium sulfide. It may also contain some carbohydrate impurities. The Kraft process offers high lignin removal efficiency, and the extracted lignin exhibits solubility in alkali solutions and various polar organic solvents. An advanced variation of the Kraft process, known as the LignoBoost process [22], employs a two-step precipitation procedure using carbon dioxide followed by sulfuric acid to achieve higher lignin purity and improved filtration performance, enhancing its potential for high-value applications.

Sulfite Process

The sulfite process employs solutions of sulfite and bisulfite ions, utilizing bases such as sodium, calcium, magnesium, potassium, or ammonium to treat wood chips in large pressure vessels called digesters. The pulping is carried out at pH values ranging from 1.5 to 5 and temperatures between 130 and 160 °C for a duration of 4 to 14 h. During this process, the lignin is converted into water-soluble lignosulfonates, which can then be readily separated from the cellulose fibers. Sulfite lignin, or lignosulfonates, is distinguished by its water solubility and a sulfur content of up to 5% in the form of sulfonate groups. It may also contain carbohydrate impurities. Compared to the Kraft process, the sulfite process generally results in a lower degree of lignin degradation, making lignosulfonates valuable byproducts [19,20,21,22].

Alkaline Extraction (Soda Pulping)

Alkaline extraction, also known as soda pulping, utilizes alkaline solutions such as sodium hydroxide, sodium carbonate, potassium hydroxide, or ammonium hydroxide to dissolve lignin and hemicellulose from lignocellulosic biomass. This method can be conducted under mild conditions, sometimes even at ambient temperature and atmospheric pressure. The alkaline environment facilitates the saponification of intermolecular bonds and hydrolysis reactions within the biomass. Soda lignin, the product of this process, is sulfur-free, which is a significant advantage for certain applications. However, it may exhibit a more condensed structure and have moderate purity, with residual ash content ranging from 2 to 5%. Alkaline extraction is particularly suitable for non-woody biomasses like straw and bagasse, and it has the potential to selectively remove lignin without extensively altering the carbohydrate structure, while also increasing the porosity, crystallinity, and surface area of the remaining cellulose. However, the reaction times for alkaline extraction can be relatively long [19,20,21,22,23,24,25].

Organosolv Process

The Organosolv process employs organic solvents such as ethanol, methanol, acetone, acetic acid, ethyl acetate, dioxane, glycerol, formic acid, butanol, gamma-valerolactone, tetrahydrofuran, and 1,4-butylene glycol, often in combination with water and an acid catalyst (H₂SO₄, HCl, or organic acids), at elevated temperatures to dissolve lignin from biomass. These solvents can be readily recovered through distillation. One of the key advantages of the Organosolv process is the production of lignin with lower levels of sulfur and other impurities compared to Kraft lignin. Organosolv lignin is typically sulfur-free and has a low ash content, generally less than 1%. Under mild conditions (temperatures usually below 150 °C), the extracted lignin can retain a more uniform structure, exhibit higher purity, have a lower molecular weight (typically ranging from 1000 to 6000 g/mol), and show better preservation of the valuable β-O-4 linkages. The efficiency of the process and the properties of the resulting lignin are significantly influenced by factors such as the type and concentration of the organic solvent, the operating temperature and pressure, and the presence and type of catalyst used. Acidified Organosolv treatment has shown promise for recovering not only lignin but also water-soluble hemicellulose and cellulose pulp with enhanced accessibility to hydrolytic enzymes [19,20,22,26].

Acid Hydrolysis

Acid hydrolysis involves the use of diluted or concentrated acids, such as sulfuric acid or hydrochloric acid, to hydrolyze the carbohydrate components (hemicellulose and cellulose) of lignocellulosic biomass, leaving lignin as a solid residue. While this method can achieve high lignin yields, especially under severe conditions (e.g., high acid concentrations and temperatures), it often results in lignin with lower purity and significant structural changes. The harsh acidic environment can lead to the condensation of lignin, forming C-C linkages and causing the cleavage of β-aryl ether units. Acid hydrolysis lignin typically has a molecular weight range of 2000–5000 g/mol and moderate to low purity. Despite these drawbacks, acid hydrolysis remains a method for obtaining a lignin-rich fraction from biomass, particularly when the primary objective is the production of sugars from the carbohydrate components [20,27].

2.1.2. Physical and Physicochemical Methods. Steam Explosion

Steam explosion is a physicochemical pretreatment method that involves subjecting lignocellulosic biomass to high-pressure steam at temperatures ranging from 160 to 260 °C and pressures of 5 to 45 bar for a short period, followed by rapid decompression. This process, also known as autohydrolysis, effectively disrupts the intricate structure of the biomass by increasing its surface area and porosity. Steam explosion can lead to partial delignification by cleaving lignin inter-units and lignocellulosic complexes, and it can also facilitate the separation of lignin, hemicellulose, and cellulose fractions. The efficiency of the process and the properties of the resulting lignin are influenced by factors such as temperature, pressure, and residence time. While steam explosion is considered an environmentally benign method due to its reduced chemical usage, the characteristics of the recovered lignin are contingent on the severity of the treatment conditions [20,28].

2.1.3. Emerging Extraction Methods

Ionic Liquids (ILs)

Ionic liquids represent a class of organic salts that are liquid at temperatures below 100 °C. ILs possess unique properties such as tunability, high solvation ability for a wide range of molecules, comparatively low flammability, high ionic conductivity, and recyclability. Certain ionic liquids, such as 1-ethyl-3-methylimidazolium acetate (EmimAc), can completely dissolve lignocellulosic biomass or selectively dissolve its individual components, including lignin, by disrupting the intricate structural network of lignin, cellulose, and hemicellulose [29]. Protic ionic liquids (PILs) have also demonstrated high efficiency in the selective extraction of lignin [30]. Lignin extracted using ILs can serve as a high-value raw material for various applications. Ongoing research continues to focus on developing cost-effective and environmentally benign ILs to facilitate their large-scale industrial application in lignin extraction [31,32,33].

Deep Eutectic Solvents (DES)

Deep eutectic solvents are formed by mixing two or more pure compounds, typically a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), which results in a mixture with a significantly lower melting point than that of the individual components. Often regarded as “green solvents”, DES offer advantages such as low cost, ease of preparation, biodegradability, and tunability. Ternary DES have also emerged as highly tunable solvents with enhanced extraction performance. These solvents have proven effective in lignin extraction by disrupting lignin–carbohydrate complexes and cleaving the ether linkages within lignin. Notably, lactic acid- and choline chloride-based DES have been applied for extracting lignin from pine wood with high purity (>90%) and a high content of hydroxyl groups without detectable sulfur. Natural-origin DES (NADES) are also being explored for sustainable lignin fractionation. While DES present a promising alternative for lignin extraction, their high viscosity can be a significant challenge in their application [34,35,36,37,38,39].

2.2. Lignin Purification Techniques

The purification of lignin is a critical step to obtaining high-quality lignin suitable for various industrial applications, specifically electronics. Technical lignins extracted from biomass often contain impurities such as carbohydrates, proteins, inorganic substances (ash), and sulfur, which can negatively impact their properties and performance. Several techniques are employed to purify lignin, including solvent fractionation, precipitation, membrane filtration, and chromatographic methods [40].

2.2.1. Solvent Fractionation

Solvent fractionation is a versatile technique that involves dissolving lignin in a suitable solvent and then separating it into different fractions based on their solubility in the same or different solvents. This method allows for the separation of lignin fractions with different molecular weights and properties. Solvent fractionation can be performed using a single solvent, a series of solvents sequentially, or mixtures of solvents. The choice of solvent, its polarity, and its solubility parameters play a crucial role in determining the solubility of lignin and the characteristics of the resulting fractions. For instance, solvents like isopropanol, ethanol, gamma-valerolactone, acetone, methanol, ethyl acetate, tetrahydrofuran, and 1-methoxy-2-propanol are commonly used. Solvent fractionation can effectively reduce the ash content of lignin and increase its calorific value and carbon content. This technique is straightforward and scalable, providing a means to obtain lignin fractions with narrower molecular weight distributions and tailored properties for enhanced applicability in various fields, specifically the electronic industry [20,40,41].

2.2.2. Precipitation Methods

Precipitation is a widely used technique for lignin purification that involves reducing the solubility of lignin in a solution, causing it to be separated as a solid. Acid precipitation, often coupled with heat coagulation, is a common method for recovering Kraft lignin from black liquor. This typically involves acidification of the black liquor using mineral acids like sulfuric or hydrochloric acid, or through the use of CO₂ in the LignoBoost process [22]. The precipitation yield is significantly influenced by factors such as pH, temperature, and the ionic strength of the solution, with lower pH and temperature and higher ionic strength generally leading to increased yields. Organic acids, including acetic, citric, lactic, malic, and oxalic acids, are also being explored as greener alternatives for lignin precipitation, potentially resulting in higher-purity lignin. Precipitation is a cost-effective and scalable technique for isolating lignin from industrial streams, and by carefully controlling the conditions, some degree of fractionation and enhanced purity can be achieved [40,42,43,44].

2.2.3. Membrane Filtration

Membrane filtration techniques, including microfiltration, ultrafiltration, and nanofiltration, utilize semi-permeable membranes to separate lignin from impurities based on differences in size or molecular weight. Ultrafiltration, for instance, can be employed as a fractionation process to concentrate high-molecular-mass lignin. These methods are particularly effective in removing low-molecular-weight contaminants and offer the potential for fractionation based on the molecular weight cut-off (MWCO) of the membrane. However, a common challenge in membrane filtration is flux decline, often attributed to the wide molecular weight distribution of lignin molecules and the formation of a gel layer on the membrane surface. Membrane filtration can be integrated with other purification techniques, such as ultrafiltration following precipitation or alkaline extraction, to enhance the overall efficiency of lignin recovery and purification. This approach provides a chemical-free method for lignin purification and fractionation, allowing for precise control over the molecular weight of the resulting fractions [40,45,46].

2.2.4. Chromatographic Techniques

Advanced chromatographic techniques such as size-exclusion chromatography (SEC), high-performance liquid chromatography (HPLC), and counter-current chromatography (CCC) are utilized for the high-resolution purification of lignin based on molecular weight or specific chemical properties. SEC is a commonly employed method for determining the molecular weight of lignin. CCC, a liquid–liquid chromatography technique, has shown effectiveness in separating lignin monomers from oligomers and other compounds present in complex mixtures resulting from lignin depolymerization. While these techniques offer high resolution and the ability to isolate specific lignin components, they can be complex and expensive, often requiring specialized equipment and expertise. A significant challenge in lignin analysis using chromatography is the heterogeneity of lignin and the absence of universal standards for molecular weight calibration, which often necessitates the use of polystyrene standards [40,47].

3. Overcoming Integration Challenges

3.1. Enhancing Solubility and Processability

Lignin is typically insoluble in many of the organic solvents used for fabrication in organic transistor-based devices [15]. This poor solubility complicates the preparation of homogeneous thin films, which are essential for high-performance electronic applications. Moreover, there is an increasing interest in processing device components using green solvents as part of the push for more sustainable electronics [48,49,50,51,52,53]. Examples are anisole, o-methyl anisole, cyclopentyl methyl ether, 2-methyl terahydrofuran, t-amyl methyl ether, water, and ethanol [54].

These solvents may not effectively dissolve lignin or key organic components, including semiconductors, substrates, and dielectrics. As a result, the development of a more nuanced understanding of solubility and alternative deposition techniques are required. Therefore, strategies to increase lignin’s solubility in both conventional and green solvents are essential. These strategies may include the use of additives, solvent vapor annealing techniques, and nanoparticle ink techniques for assembling the device-active layer [53].

A promising approach to improving lignin solubility and processability is to leverage its inherent functionalities (vide supra) and chemically modify them through functionalization methods, such as alkylation, esterification, urethanation, and amidation [16,55,56,57].

Another useful strategy to enhance lignin solubility is applying the solubility parameter theory, which relies on cohesive energy density and solubility parameters. This theory helps identify compatible solvents and solvent mixtures based on molecular interactions, ultimately optimizing its dissolution behavior in organic solvents [16,58].

3.2. Addressing Interfacial Defects and Adhesion

The integration of lignin-based thin films into organic transistor devices is hindered by poor interfacial adhesion with organic electronic components. Differences in surface properties result in weak bonding, interfacial gaps, and charge transport disruptions that degrade device performance. To overcome these limitations, multiple strategies have been explored, including chemical modifications, interfacial engineering, optimized processing techniques, and advanced material design. A fundamental approach to improving adhesion is surface functionalization, which modifies lignin’s chemistry to enhance compatibility with adjacent layers [59].

Chemical grafting [60] with conductive polymers such as PEDOT:PSS or polyaniline (PANI) has been found to improve its electronic coupling with organic semiconductors [61,62]. Additionally, oxidation treatments introduce oxygen-containing functional groups that increase compatibility with polar materials, while oxygen and nitrogen plasma treatments enhance surface energy and generate reactive sites that strengthen adhesion [63].

Beyond chemical modification, interfacial engineering provides an effective means to bridge the incompatibilities between lignin and other electronic components (e.g., electrodes, semiconductors). The complex and sometimes unpredictable surface chemistry of lignin can lead to inconsistent interfacial adhesion and the formation of interfacial defects (e.g., traps, voids), which hinder device performance. Introducing interfacial layers between lignin-based materials and electrodes or other device components can help to mitigate defects and improve adhesion. These interfacial layers can act as buffer layers, reducing energy level mismatches and improving charge injection and transport [64]. For example, self-assembled monolayers (SAMs), such as silane-based APTES, may improve lignin–electrode adhesion by forming a chemically compatible interface [65,66,67].

Optimizing processing conditions refines interfacial stability and film quality. Solvent engineering (vide supra) enhances film homogeneity and adhesion by ensuring better dissolution and dispersion of lignin. Deposition techniques (e.g., spin-coating parameters) and post-deposition treatments (e.g., thermal annealing) can significantly impact film uniformity, thickness, and the presence of defects [68]. Slight variations in these parameters can lead to substantial differences in device performance, contributing to irreproducible results.

Finally, material engineering provides additional reinforcement by incorporating nanomaterials such as graphene oxide or carbon nanotubes, which enhance adhesion while improving electrical conductivity [69].

Thorough characterization and testing are crucial to evaluating these improvements. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) provide insights into surface morphology, roughness, and chemical composition, while mechanical tests assess adhesion strength and flexibility.

4. Applications of Lignin in Organic Transistor-Based Devices

4.1. Lignin in Transistor Components

Kraft lignin has demonstrated significant potential as a gate dielectric material in organic field-effect transistor (OFET) devices. In a recent investigation by D’Orsi et al. [15], the authors examined two types of kraft lignin—referred to as L1 and L2—sourced from softwood via the Kraft process, with variations in purification and isolation treatments during production (not provided). The lignins were subjected to extensive characterization, utilizing techniques including Fourier-transformed infrared spectroscopy (FTIR), gel permeation chromatography (GPC), and nuclear magnetic resonance spectroscopy (NMR) to analyze their chemical structures and molecular degradation levels. For example, L1 shows a pH of 6.8, while L2 is more alkaline with a pH of 10.0 in water suspensions. In addition, L1 has a lower ash content (3.1%) compared to L2 (25.3%), indicating more degradation in L2. Sample L1 has a higher average molecular weight (M_n of 2150 Da, M_w of 4830 Da) than L2 (M_n of 1260 Da and M_w of 3390 Da). L2 contains a greater proportion of enol ethers and carboxylic groups compared to L1.

To assess the feasibility of using L1 and L2 as gate dielectric materials for OFETs, the researchers performed dielectric spectroscopy on both lignins in a metal–insulator–metal (MIM) structure. In this setup, the lignins were placed between aluminum electrodes (1 mm wide and 80 nm thick) arranged in a cross-geometry. L1 and L2 lignin films were deposited by doctor blading from a 100 mg/mL solution in ethanol/10% aq. ammonia (1:1 v:v) solvent mixture, using a blade speed of 2.5 mm s⁻¹ on a 50 °C hot plate. Then, the films were dried on a hot plate for 1 h at 110 °C. The dielectric behavior revealed that L1 exhibited a more consistent dielectric response across the frequency range than L2. L1 showed a stable capacitance, with minimal dielectric loss (maximum loss of 0.25 at 1 MHz). Additionally, L1 relaxed within a frequency range that did not interfere with the rapid electrical measurements necessary for OFETs.

Both lignins were used in the fabrication of OFETs with a staggered bottom-gate, top-contact architecture, as detailed in the works of D’Orsi et al. [15]. Figure 1 displays the OFET layer structure and geometry.

The team tested two dielectric configurations: a hybrid alumina–lignin bilayer and a stand-alone lignin dielectric. Results showed that OFETs with L1 performed significantly better than those with L2, exhibiting higher field-effect mobility (

Table 1. Comparative OFET performance of L1- and L2-based dielectrics.

Semiconductor	Dielectric	Mobility (cm2 V−1 s−1)
C60	L1 on Al2O3	7 × 10−2
C60	L1	5 × 10−3
C60	L2 on Al2O3	8 × 10−3 cm2
C60	L2	1.5 × 10−2
Pentacene	L1 on Al2O3	2 × 10−2
Pentacene	L1	4 × 10−3
Pentacene	L2 on Al2O3	7 × 10−3
Pentacene	L2	3.2 × 10−4

) and lower hysteresis.

This enhanced performance was attributed to the higher purity of L1, better interface characteristics, and a lower inorganic ash content compared to L2. Notably, OFETs with L1 as a stand-alone dielectric were successful, especially with C60 as the semiconductor. However, only L1 provided acceptable results when paired with pentacene, likely due to its ability to promote better self-organization of the semiconductor, which enhanced charge carrier mobility. Overall, L1 demonstrated superior performance as a gate dielectric in OFETs, with lower hysteresis and higher charge carrier mobilities, positioning it as a promising candidate for the development of cost-effective, solution-processable electronic devices. These results directly reflect the influence of lignin purity and composition and highlight batch-to-batch variability as a key challenge to reproducibility.

In addition, lignin has been utilized in the fabrication of large-scale electrolyte-gated transistor (EGT) arrays. Tian et al. [70] constructed a 10 × 10 EGT array using a coplanar-gate design, with lignin serving as the electrolyte. Their work addressed a significant challenge in earlier EGT designs, as large-scale arrays with inorganic electrolytes existed, and small-scale arrays with organic electrolytes have been reported. However, traditional bottom- or top-gate fabrication techniques often led to damage from acids used in lithography processes. To overcome this, Tian et al. [70] adopted a coplanar gate architecture, originally developed by Zhu et al. [71], which positions the source, drain, and gate electrodes on the same side of the dielectric layer, thereby preventing direct contact between acids and the organic electrolyte. This EGT array, fabricated using a standard lithography process on glass substrate, and incorporating a coplanar-gate IGZO (indium–gallium–zinc–oxide) synaptic transistor with a lignin electrolyte, mimics biological synaptic behavior. In this configuration, the lateral gate electrode functions as the presynapse, receiving input signals, while the IGZO channel operates as the postsynaptic terminal, responsible for generating the excitatory postsynaptic current (EPSC). To fabricate the electrolyte lignin film, a 10 wt% lignin solution was prepared by dissolving 10 wt% lignin and 10 wt% acetone in 80 wt% deionized water. This solution was then dispensed onto a substrate and spin-coated at 500 rpm for 30 s, resulting in a lignin film with a thickness of approximately 380 nm. Finally, the film was air-dried for 10 min. The capacitance of the resulting lignin film was measured using an Al/Lignin/Mo (metal–insulator–metal, MIM) sandwich configuration. The result showed that when lignin was dissolved in the binary solvent system, it facilitated the formation of an electric double layer (EDL) at the Al/electrolyte and Mo/electrolyte interfaces, yielding an electrolyte capacitance of 41 nF cm⁻² at 20 Hz.

The developed EGT array holds significant promise for Artificial Neural Network (ANN) applications. The array’s ability to replicate key synaptic behaviors—such as EPSC, paired-pulse facilitation (PPF), and high-pass filtering—underscores its potential for simulating ANN processes. These synaptic functions are critical for modeling the interconnected units (neurons) within ANNs, enabling advanced computational tasks. Low-voltage operation below 3 V is observed for a single device in the array.

Similarly, Zhang et al. [72] fabricated a 10 × 10 EGT array on glass substrates, using a lithographic process. They employed hybrid polyvinyl alcohol (PVA)/lignin films with various mass ratios (6.67:1, 4:1, 2.5:1, and 2:1) as gate dielectrics. For the optimal 4:1 lignin/PVA composite film, 2.5 wt% PVA and 10 wt% lignin solutions were prepared by stirring in water and acetone (for lignin) separately. These solutions were mixed (1:1 ratio), stirred at 70 °C, and drop-cast onto the transistor array before drying. This hybrid system measured using a vertical sandwich structure of Mo/lignin and PVA/Al, producing a bilayer capacitance of 740 nF cm⁻² at 20 Hz.

The I_ON/I_OFF ratio of a single transistor reached 10⁶, and the maximum gate leakage current was 43.96 pA in the voltage range of −3 to 3 V. The array successfully demonstrated synaptic properties, including EPSC and PPF. Additionally, with a pulse duration of 100 ms, the energy consumption of the transistors was measured at 0.63 nJ, showcasing the array’s ability to simulate dynamic memory and forgetting functions. This performance highlights its potential for advanced applications in neuromorphic computing.

It is worth noting here that other biopolymers are also being explored for their potential in organic transistor-based devices, including cellulose, silk, and chitosan. Cellulose is valued for its biodegradability and environmental friendliness, but its use in electronic devices is limited by its hydrophilicity and low mechanical strength due to randomly distributed fibers. Various strategies, such as nanocellulose production, wet stretching, and cross-linking, have been explored to improve interfibril bonding and reduce voids. However, these methods often involve complex and costly pretreatments, reducing cellulose’s cost-effectiveness [73,74]. Chitosan offers good film-forming ability and biodegradability. However, it is also hydrophilic and mechanically weak, and its processing often requires acidic conditions, which may not be compatible with all electronic applications [75]. Silk is another biopolymer that offers excellent mechanical and optical properties, but high production costs and supply instability hinder its practical use [76]. In comparison, lignin offers several advantages. Its superior hydrophobicity, mechanical strength, and aromatic structure make lignin particularly suitable for applications requiring water resistance and structural stability. In addition, lignin is both abundant and inexpensive to produce through well-established processes (vide supra).

4.2. Lignin in Enabling Materials and Systems

Yuan et al. [77] reported on an artificial tactile perception system (ATPS) that integrates piezoelectric nanogenerators (PNGs) and a multi-gate synaptic transistor to detect and process spatio-temporal pressure information. A key innovation in this study is the development of a lignin/polyvinyl alcohol (PVA)/glycerol hybrid electrolyte, which improves the stability of electrolyte-gated transistors (EGTs) and enables efficient pressure sensing. This electrolyte film combines a 10 wt% lignin solution (in acetone/water) with a PVA/glycerol mixture (3:2 ratio) at varying proportions, where glycerol is used to provide long-term moisture. The hybrid electrolyte film, optimized at a 1:1 lignin-to-PVA/glycerol ratio, exhibits high specific capacitance (4.2 µF/cm² at 20 Hz), low impedance, and excellent electronic insulation (maximum leakage current < 60 μA under a scanning voltage of −3 to 3 V), making it an effective biocompatible dielectric material. The multi-gate synaptic transistor fabricated using an IGZO (indium–gallium–zinc–oxide) semiconductor, mimics biological synaptic behavior, allowing for the encoding and learning of pressure-related information. This ATPS demonstrates low-power operation (0.1 V), high spatial resolution, and pattern recognition capabilities, showcasing its potential for intelligent touch-based interfaces such as password recognition systems and human–machine interaction applications.

The study by Zhu et al. [78] demonstrated a method to fabricate transparent films by removing lignin from natural wood. Specifically, the process involves delignification using sodium hypochlorite that removes lignin—turning the wood from yellow to white—while preserving the natural alignment of cellulose nanofibers (CNFs). This is followed by mechanical pressing to densify the structure with ~5× reduction in thickness, resulting in a highly transparent film with highly aligned CNFs (Figure 2).

This process enhances both optical clarity (≈90% transmittance and up to 350% intensity ratio of transmitted light) and mechanical strength (tensile strength up to 350 MPa), making the material more suitable for applications in electronics. This approach is relevant for lignin-based thin films because controlling polymer alignment at the nanoscale is crucial for improving charge transport, mechanical durability, and optical properties in organic electronic devices such as transistors and sensors. In transistors, structured lignin-based films could serve as dielectric layers or functional interfaces, where fiber orientation influences electrical performance. Similarly, in sensors, the ability to fine-tune lignin’s structure could enhance sensitivity, stability, and response to environmental stimuli. It is worth noting that, while sodium hypochlorite provides an efficient and straightforward method for lignin removal and effectively preserves the alignment of cellulose nanofibers, it also raises significant environmental and safety concerns.

Expanding on this concept, Fu et al. [79] developed a fully wood-based flexible electronic circuit, where a strong, transparent cellulose film served as the substrate and a lignin-derived carbon nanofiber conductive ink was printed onto the surface (Figure 3). The transparent wood substrate (TWF) was obtained by delignifying balsa wood with sodium chlorite in acetate buffer, followed by alkaline treatment with sodium hydroxide. The resulting samples were pressed and air-dried under mild load to produce a material that combines mechanical strength and flexibility with a Young’s modulus of 49.9 GPa, and a tensile strength of 469.9 MPa. It can be rolled on a 0.15 mm radius wire with approximately 95% shape recovery. In addition, it shows high optical transmittance (80% at 550 nm). The lignin-derived conductive ink (LCF) was synthesized via carbonization, converting lignin into a nanostructured conductive material that enabled the fabrication of biodegradable, flexible circuits. The resulting devices exhibited good and reliable electrical conductivity (stable resistance after 500 folding cycles, <80% increase after 1000 cycles), mechanical robustness, and flexibility, demonstrating their potential for applications in wearable electronics, sensors, and sustainable electronic packaging.

Y. Hao and colleagues [80] have introduced a dual-catalytic approach to coat cellulose nanocrystals (CNCs) with lignosulfonate (Ls). This process leverages catechol redox reactions involving silver ions (Ag⁺) and ammonium persulfate (APS) at an ambient temperature to initiate the formation of CNC@Ls-Ag-PAM nanocomposite hydrogels. Initially, Ls chelates Ag⁺ ions, forming a redox-active Ls-Ag complex that activates APS, which in turn triggers free-radical polymerization of acrylamide monomers (AMs) in the presence of CNCs and N,N′-methylene bisacrylamide (MBA). The resulting hydrogel forms rapidly—within minutes—without requiring heat or UV, yielding a flexible film with embedded silver nanoparticles and a porous, layered microstructure (Figure 4).

Lignosulfonate plays a dual role: it reduces Ag⁺ to Ag nanoparticles via catechol-to-quinone oxidation and contributes to redox cycling that enables continuous adhesion. CNCs act as reinforcing nanofillers and cross-linkers, enhancing mechanical strength through hydrogen bonding and network entanglement. Together, the lignin-based and CNC components produce a stretchable, conductive, and self-healing hydrogel suitable for epidermal electronics and other flexible devices.

This innovative hydrogel combines impressive attributes, including high toughness (260.74 kJ/m³), excellent tensile strength (406 kPa), ultrahigh stretchability (1880%), exceptional sensitivity (gauge factor of 2.46), conductivity (9.5 mS cm⁻¹), strong adhesion, antibacterial properties (above 98%), and UV shielding (100%). The incorporation of CNCs and Ls into the hydrogel significantly boosts its tensile strength (406 kPa) and polymerization rate compared to earlier hydrogels that utilized catechol chemistry. CNCs, serving as reinforcing nanofillers, form multiple hydrogen bonds with Ls-Ag, which greatly enhances the hydrogel’s mechanical toughness. The CNC@Ls-Ag-PAM hydrogels demonstrate outstanding mechanical properties, strong adhesion, high strain sensitivity, rapid response times, UV protection, and antibacterial properties. These characteristics make them ideal for use in human skin applications, particularly in the development of epidermal sensors for monitoring physiological signals, presenting a novel approach to human–machine interface technologies.

Stapf et al. [81] presented an innovative application of lignin-based thin films in chemical sensing, specifically utilizing Kraft lignin hydrogels as a responsive material in a Suspended Gate Field Effect Transistor (SGFET)-based sensor (Figure 5).

This study highlights the ability of lignin hydrogels to serve as humidity-sensitive layers, where their swelling behavior in response to water vapor modulates the electrical output of the transistor. The sensor exhibits a reversible shift in output current of up to 9% when exposed to water vapor concentrations of 5000 ppm, demonstrating its potential for real-time humidity sensing.

The fabrication process involves a precision dispensing technique to deposit Kraft lignin hydrogels onto the SGFET membrane, ensuring controlled and uniform thin-film formation.

The device exhibited a reversible channel current change of up to 9% in response to 5000 ppm water vapor (UG = −10 V, UDS = 5 V), with linear sensitivity across 100–5000 ppm and effective operation at low relative humidity levels (0.4%–18%). Its strong selectivity toward water vapor over organic analytes such as toluene is attributed to the hydrophilic, responsive swelling behaviour of the lignin hydrogel, which modulates gate capacitance. The study further highlights the scalability and integration potential of this approach for miniaturized sensing platforms targeting gases and ions.

5. Conclusions

This review has highlighted the potential of lignin as a sustainable material for various components in organic transistor-based devices. Lignin has been explored for applications ranging from gate dielectric materials in OFETs to electrolytes in EGT arrays and as a component in tactile perception systems. The use of lignin-derived materials in flexible electronic circuits and conductive inks further demonstrates its versatility. Promising trends in lignin-based devices include the development of large-scale EGT arrays for neuromorphic computing, tactile perception systems with spatial–temporal recognition capabilities, and fully wood-based flexible electronic circuits. These applications showcase the ability of lignin to contribute to both the functionality and sustainability of electronic devices. However, challenges remain in the widespread adoption of lignin. To improve reproducibility, further efforts are needed to develop standardized protocols for lignin extraction and purification to obtain more consistent starting materials. In addition, implementing precise control over processing parameters and optimizing deposition techniques can lead to high-quality and reproducible films in transistor fabrication; chemical modification of lignin surfaces using, e.g., silanes or SAMs, can improve interfacial adhesion and reduce defect density, leading to more reliable device performance; combining lignin with other polymers or nanomaterials can help to tailor its properties and improve processability and reproducibility; Additionally, understanding the structure–property relationships of lignin from different sources and under various processing conditions is crucial for achieving reproducible and reliable performance in lignin-based electronic devices. Scalable processing techniques will be essential for future adoption. Current studies suggest that forming composite materials—such as blending lignin with polymers like PVA or incorporating it into hydrogels—seems to show more immediate promise for scalability. These methods often involve simpler mixing or solution processing techniques, which can be readily adapted for large-scale production. In contrast, chemical modification methods may require further optimization to be economically and environmentally viable for large-scale applications. The development of effective functionalization strategies, the exploration of green solvents (e.g., dimethylsulfoxide, deep eutectic solvents, and certain ionic liquids), and the implementation of advanced material design are crucial for overcoming these challenges. It is important to note that lignin-based (and more broadly organic) electronics are not intended to replace silicon in its dominant applications. Instead, they aim to enable new functionalities and address niche areas where flexibility, low temperature processing, and biodegradability offer distinct advantages. Looking ahead, key research priorities include developing scalable functionalization strategies, enabling integration into flexible and neuromorphic platforms, and expanding applications in biosensing, environmental monitoring, and transient electronics. With ongoing progress, lignin is poised to play a central role in shaping next-generation sustainable electronic technologies.