The key processing parameters in screen printing include mesh count, squeegee pressure, and squeegee angle, which strongly influence the properties of the printed films, including thickness, uniformity, and pattern fidelity^[60-62]. The selection of these parameters is primarily guided by the characteristics of the ink, the type of substrate, and the required level of pattern detail^[61,63].

Ink properties play a central role in determining suitable printing conditions. High-viscosity inks, such as plastisol, typically require lower mesh counts (110-156), moderate squeegee pressures (20-25 N), and relatively low angles (around 45°) to ensure adequate ink transfer and coverage^[60-62]. In contrast, low-viscosity inks, such as water-based formulations, benefit from higher mesh counts (≥ 230), higher squeegee pressures (25-35 N), and steeper angles (around 75°) to control ink flow and prevent excessive penetration into the substrate^[62,64,65]. Metallic inks, which contain larger particles, generally require even lower mesh counts (38-86), along with reduced pressure (15-20 N) and lower angles to accommodate particle size and maintain print quality^[62,64,65].

In addition to ink properties, substrate characteristics are a critical factor in parameter selection. Screen-printing substrates can be broadly categorized into textiles, smooth surfaces, and rough surfaces^[62,65]. Textiles, such as fabrics, typically require lower mesh counts (110-156) to compensate for ink absorption and surface porosity. In contrast, smooth substrates, including paper and plastics, allow for higher mesh counts (230-305), enabling finer feature definition and improved pattern resolution^[62].

Beyond conventional printing quality, these parameters are particularly critical for thermoelectric inks because they directly influence electrical percolation, porosity evolution, and interparticle transport pathways. Thermoelectric inks typically require relatively high particle loading (commonly > 60-80 wt.% for inorganic systems) to establish continuous conductive networks and preserve thermoelectric transport, whereas excessive binder content, although beneficial for printability and adhesion, may introduce insulating interfaces that reduce σ. Accordingly, most reported systems maintain the combined binder and solvent content below ~ 20 wt.%^{[36,47,48,54,66]}. Similarly, commonly reported viscosity windows for thermoelectric inks range from 1000 to 5000 mPa·s, balancing mesh transfer and pattern fidelity with particle dispersion, densification, and microstructure evolution during post-treatment^[66]. Insufficient densification can lead to high porosity and poor electrical contact between thermoelectric particles, ultimately increasing internal resistance and degrading device-level performance. Additionally, solvent selection and drying conditions play an important role in determining film formation and thermoelectric performance. Solvents with different volatilities and boiling points can significantly influence evaporation kinetics during drying, thereby affecting particle redistribution, film uniformity, and microstructure evolution^[31,64]. Uncontrolled solvent evaporation may induce thickness non-uniformity and coffee-ring effects, leading to spatial variations in σ and S across the printed film^[64]. Therefore, appropriate solvent selection and drying control are essential for achieving homogeneous thermoelectric films with reproducible transport properties and device performance. Therefore, thermoelectric-specific screen-printing windows must balance printability with electrical continuity, microstructure preservation, and interfacial transport.

Inorganic thermoelectric materials generally exhibit superior thermoelectric performance; however, their development has traditionally proceeded along two largely independent fabrication routes. Bulk thermoelectric materials are typically produced via powder-based approaches, including solvothermal synthesis^[18,20,67], melt processing^[8,9], and ball milling^[68,69]. In contrast, thermoelectric thin films are commonly fabricated using vacuum-based techniques, such as magnetron sputtering^[70,71], co-evaporation^[72], and other physical vapor deposition (PVD) methods^[73,74]. This inherent separation between bulk and thin-film processing pathways not only limits process compatibility but also complicates device integration, thereby posing substantial challenges for scalable and cost-effective manufacturing.

Screen printing provides a distinct advantage by integrating these traditionally separated material forms within a unified, powder-based processing framework^{[20,29-34,75]}. Powders synthesized via conventional bulk-material routes can be directly utilized as printable feedstocks, enabling the fabrication of both bulk-like thick films and thin-film thermoelectric components through careful control of particle morphology and size distribution^[34,66]. Importantly, this strategy preserves the microstructural features responsible for high thermoelectric performance, while simultaneously introducing enhanced design flexibility at the device level^[41,47]. For instance, CuAgSe powders with dendritic, topology-driven micro-nano architectures synthesized through solution-based methods inherently generate hierarchical structures and complex grain boundary networks during post-printing densification, resulting in suppressed lattice thermal conductivity (κ_l)^[20]. These microstructural characteristics can be effectively retained and incorporated into TED architectures via screen printing^[20]. Similarly, solution-synthesized n-type Bi₂Te₃ nanorods and nanoplates, as well as p-type Bi_0.4Sb_1.6Te₃ nanoparticles prepared by ball milling, have been successfully processed into high-performance thermoelectric films using screen-printing techniques^[3,30]. Collectively, these examples demonstrate that the compatibility of screen printing with diverse inorganic thermoelectric powders is not material-dependent but fundamentally enabled by the adaptability of the printing process itself. However, most inorganic thermoelectric films require post-printing annealing or sintering at relatively high temperatures, commonly ranging from 200 to 500 °C, as summarized in Table 1. Consequently, substrate selection for inorganic screen-printed thermoelectric is often limited to thermally stable materials such as polyimide, ceramics, Al₂O₃, glass, and SiO₂. Although screen printing provides a material-agnostic processing pathway, this high-temperature requirement restricts compatibility with low-cost polymer substrates and roll-to-roll manufacturing processes, thereby limiting scalability and increasing manufacturing complexity. Addressing the trade-off between high-temperature densification and substrate compatibility therefore remains a critical challenge for translating laboratory-scale inorganic thermoelectric materials into industrial manufacturing.

By bridging bulk-derived powders with film-based device architectures within a unified manufacturing framework, screen printing significantly reduces fabrication complexity, development cycles, and associated production costs. These advantages align closely with industrial demands, reinforcing the role of screen printing as an effective manufacturing interface for inorganic thermoelectric systems. Nevertheless, the continued reliance on particle-based inks and high-temperature densification processes constrains precise microstructural control and may limit large-scale manufacturing efficiency. This highlights the necessity for advanced strategies that can preserve bulk-level carrier transport properties while enabling scalable fabrication of screen-printed thermoelectric films.

Compared with inorganic thermoelectric materials, organic thermoelectrics offer intrinsic advantages in cost-effectiveness, mechanical flexibility, and biocompatibility^[76-79]. Owing to their polymeric nature, organic thermoelectric materials are inherently well suited to screen-printing processes, allowing precise tuning of ink rheology, low-temperature film formation, and the fabrication of mechanically compliant thick-film architectures^[59,80]. In contrast, inorganic thermoelectric inks typically consist of particulate suspensions that may suffer from dispersion instability and generally require high-temperature sintering, thereby restricting substrate selection and complicating device integration^[41,49,81].

The low-temperature processing characteristics of organic thermoelectrics substantially broaden the accessible manufacturing window for screen-printed TEDs. For example, n-type poly(benzodifurandione) (PBFDO) can be fabricated into uniform thermoelectric films via screen printing, followed by mild thermal annealing at approximately 100-120 °C^[30]. The resulting films and devices demonstrate excellent environmental stability across a wide temperature range (-8 °C to 200 °C) and under high-humidity conditions (up to 90% relative humidity), underscoring the structural robustness and practical viability of screen-printed organic thermoelectric architectures^[30].

Beyond purely organic systems, the low thermal budget associated with screen printing further enables the integration of organic thermoelectric materials with bio-derived and flexible components, giving rise to hybrid thermoelectric systems that are inaccessible through conventional high-temperature processing routes. Such hybridization facilitates direct printing onto a wide range of flexible substrates (including polymer films, textiles, and paper), thereby enabling conformal contact with human skin and highlighting strong potential for wearable thermoelectric applications^[27,31,76]. For example, hybrid thermoelectric films composed of multiwall carbon nanotubes (MWCNTs) and chitin nanocrystals (ChNCs) have been successfully fabricated on cellulose paper substrates via screen printing^[31]. As naturally derived nanomaterials extracted from shrimp and crab shells, ChNCs impart intrinsic biocompatibility to the printed devices while preserving functional thermoelectric performance^[31]. Collectively, these examples demonstrate that the compatibility of screen printing with organic and hybrid thermoelectric materials is not incidental, but fundamentally enabled by its low-temperature, rheology-governed processing characteristics.

By extending the screen-printing processing window beyond rigid inorganic systems to encompass flexible, biocompatible, and hybrid materials, screen printing establishes a versatile manufacturing interface for emerging thermoelectric applications, particularly in wearable and human-centric energy harvesting. Future progress will require concurrent enhancement of σ and thermal stability while preserving low-temperature processability, thereby enabling the development of integrated hybrid organic-inorganic printing platforms for scalable TED fabrication.

The material compatibility of screen printing across diverse thermoelectric systems can be rationalized within a unified processing window defined by three interrelated parameters: ink rheology, particle attributes, and interfacial chemistry^{[20,29,32,34]}. Rather than being material-specific, printability arises from the dynamic balance among viscous, capillary, and interfacial forces that govern ink transfer, mesh filling, and pattern fidelity during the printing process^[82]. This balance can be quantitatively described using dimensionless relationships that couple surface tension (γ), viscosity (η), and printing speed (U). In particular, the capillary number (Ca) provides a generalized framework for predicting printable regimes across different material systems, thereby establishing a process-based criterion for screen-printing compatibility^[82]. Here,

$$ C a= \frac{\eta U}{\gamma} $$

Particle size and morphology play a dual role within this processing framework. They influence not only the ability of the ink to infiltrate the screen mesh and form continuous, thickness-controlled films, but also the microstructural evolution during post-printing densification, which ultimately governs thermoelectric transport properties^[47,83]. Carefully engineered particle size distributions facilitate uniform film formation while preserving critical microstructural features, such as grain boundaries and hierarchical architectures, that contribute to reduced κ_l and optimized carrier transport^[20]. For example, Sb₂Te₃ inks formulated with smaller particle sizes yield films with superior thickness uniformity and enhanced thermoelectric performance compared with those derived from coarser powders, underscoring the importance of particle-size-controlled film formation^[33].

Interfacial modification further broadens the printable processing window by mitigating the intrinsic porosity and weak interparticle contacts commonly observed in screen-printed films^[34,41]. The incorporation of organic binders and conductive polymers can enhance film continuity and reduce porosity; for example, introducing acid-doped polyaniline (PANI) into Cu₂Se inks significantly decreases film porosity while simultaneously improving the power factor (S²σ)^[83]. In parallel, inorganic nanosolders and secondary phases can facilitate densification and strengthen interfacial coupling without compromising electronic transport^[34,47]. For instance, the addition of Te into Bi-Sb-Te systems suppresses defect formation and promotes sintering, enabling screen-printed films to achieve thermoelectric performance comparable to, or even surpassing, that of conventionally processed bulk counterparts^[34,47,48]. Importantly, these strategies extend beyond simple improvements in film integrity; they enable the retention or even enhancement of thermoelectric performance through deliberate tuning of carrier filtering effects, contact resistance, and phonon scattering mechanisms.

These considerations underscore a critical distinction for thermoelectric systems: printability alone is insufficient. A truly material-agnostic processing window must not only enable pattern formation, but also preserve the microstructural features that govern thermoelectric transport. However, the establishment of quantitative printability metrics that explicitly correlate rheological parameters with transport performance remains an open challenge for predictive thermoelectric manufacturing. Defining microstructure-preserving printability would provide a rigorous and predictive framework for translating diverse thermoelectric materials into screen-printed architectures, thereby laying the foundation for the device-level design strategies discussed in the following section.

Film-based TEDs represent the planar limit of the screen-printing design space and are typically fabricated on flexible substrates to enable conformal integration with curved or soft surfaces. Their lateral geometry facilitates large-area coverage and efficient operation under small ΔTs, making them particularly well-suited for body-heat harvesting and distributed thermal sensing applications. For example, wearable CuAgSe-based arrays printed on textile substrates demonstrate the feasibility of screen-printed thermoelectrics for body-heat harvesting, highlighting the compatibility of planar architectures with flexible and fabric-based platforms (nanofiber membranes, PI, and glass fiber fabrics), as shown in Figure 3A^[20]. Beyond energy harvesting, the high pattern fidelity enabled by mesh-controlled printing allows thermoelectric elements to be integrated into circuit-like layouts. Pattern-defined thermoelectric tracks fabricated from printable composite inks composed of ChNCs and MWCNTs illustrate how thermoelectric functionality can be embedded within electronic architectures, thereby enabling multifunctional energy-harvesting and sensing platforms, as shown in Figure 3B^[31]. At higher pattern resolutions, micro-thermoelectric generators with radially defined p-n geometries (Sb₂Te₃/Bi₂Te_2.7Se_0.3), shown in Figure 3C, further demonstrate the capability of screen printing to produce compact, device-level architectures while retaining the advantages of planar heat spreading^[85].

Collectively, these examples reveal a resolution-dependent functional regime: low-resolution, large-area patterns tailored for wearable and distributed applications; intermediate-resolution layouts enabling circuit-level integration; and high-resolution micro-architectures designed for compact energy harvesting. This pattern-enabled design flexibility represents a defining feature of screen-printed film-based thermoelectrics, offering a direct pathway to match device geometry with application-specific thermal conditions. Nevertheless, scaling planar architectures toward higher power densities (ω) while preserving mechanical compliance remains a central challenge for wearable and distributed thermoelectric systems.

Bulk-like TEDs represent the thickness-dominated limit of the screen-printing design space, where cross-plane heat transport and multilayer stacking enable higher ω and operation under larger ΔTs. In this regime, device performance is determined not only by intrinsic material properties but also by structural parameters such as the number of printed layers, leg thickness, and interfacial engineering. As illustrated in Figure 3D, thermoelectric legs fabricated via screen printing can be integrated with sputtered electrodes and conventional bonding processes on ceramic substrates, demonstrating compatibility with established microfabrication workflows and enabling densely packed p-n arrays for compact power-generation modules^[37]. Beyond rigid platforms, vertically integrated multilayer architectures can also be realized on flexible or paper-based substrates, producing semi-flexible devices that retain the advantages of thickness-controlled thermoelectric legs. Figure 3E shows flexible TEDs comprising thermoelectric arrays (p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.7Se_0.3) fabricated by screen printing on printed copy paper substrates^[66]. Such architectures underscore the capability of screen printing to translate bulk-like device concepts into mechanically compliant formats. Importantly, multilayer designs also offer an effective strategy to mitigate one of the principal limitations of printed thermoelectrics (namely, high contact resistance) by incorporating engineered interfacial layers and conductive interconnects. For example, the introduction of carbon-based interlayers reduces contact resistance and enables milliwatt-level power output from stacked screen-printed devices, as shown in Figure 3F^[21].

These examples delineate a thickness-dependent functional regime in which increasing the number of printed layers and leg thickness enhances cross-plane thermal gradients and ω, while interfacial engineering critically determines electrical continuity and overall device efficiency. Together with the planar architectures discussed above, bulk-like configurations complete the continuous geometric design space enabled by screen printing, offering a scalable pathway from flexible, low-temperature-gradient systems to compact, high-output thermoelectric modules. Despite these advances, contact resistance remains a central challenge, underscoring the urgent need to develop low-resistance, thermally stable interlayers to realize high-efficiency multilayer thermoelectric modules.