Inkjet Printed Large‐Area Flexible Few‐Layer Graphene Thermoelectrics

Graphene‐based organic nanocomposites have ascended as promising candidates for thermoelectric energy conversion. In order to adopt existing scalable printing methods for developing thermostable graphene‐based thermoelectric devices, optimization of both the material ink and the thermoelectric properties of the resulting films are required. Here, inkjet‐printed large‐area flexible graphene thin films with outstanding thermoelectric properties are reported. The thermal and electronic transport properties of the films reveal the so‐called phonon‐glass electron‐crystal character (i.e., electrical transport behavior akin to that of few‐layer graphene flakes with quenched thermal transport arising from the disordered nanoporous structure). As a result, the all‐graphene films show a room‐temperature thermoelectric power factor of 18.7 µW m^?1 K^?2, representing over a threefold improvement to previous solution‐processed all‐graphene structures. The demonstration of inkjet‐printed thermoelectric devices underscores the potential for future flexible, scalable, and low‐cost thermoelectric applications, such as harvesting energy from body heat in wearable applications.     

Flexible Thermoelectric Devices of Ultrahigh Power Factor by Scalable Printing and Interface Engineering

Printing is a versatile method to transform semiconducting nanoparticle inks into functional and flexible devices. In particular, thermoelectric nanoparticles are attractive building blocks to fabricate flexible devices for energy harvesting and cooling applications. However, the performance of printed devices are plagued by poor interfacial connections between nanoparticles and resulting low carrier mobility. While many rigid bulk materials have shown a thermoelectric figure of merit ZT greater than unity, it is an exacting challenge to develop flexible materials with ZT near unity. Here, a scalable screen‐printing method to fabricate high‐performance and flexible thermoelectric devices is reported. A tellurium‐based nanosolder approach is employed to bridge the interfaces between the BiSbTe particles during the postprinting sintering process. The printed BiSbTe flexible films demonstrate an ultrahigh room‐temperature power factor of 3 mW m^?1 K^?2 and ZT about 1, significantly higher than the best reported values for flexible films. A fully printed thermoelectric generator produces a high power density of 18.8 mW cm^?2 achievable with a small temperature gradient of 80 °C. This screen‐printing method, which directly transforms thermoelectric nanoparticles into high‐performance and flexible devices, presents a significant leap to make thermoelectrics a commercially viable technology for a broad range of energy harvesting and cooling applications.     

3D Conformal Printing and Photonic Sintering of High‐Performance Flexible Thermoelectric Films Using 2D Nanoplates

Flexible thermoelectric (TE) devices hold great promise for energy harvesting and cooling applications, with increasing significance to serve as perpetual power sources for flexible electronics and wearable devices. Despite unique and superior TE properties widely reported in nanocrystals, transforming these nanocrystals into flexible and functional forms remains a major challenge. Herein, demonstrated is a transformative 3D conformal aerosol jet printing and rapid photonic sintering process to print and sinter solution‐processed Bi₂Te_2.7Se_0.3 nanoplate inks onto virtually any flexible substrates. Within seconds of photonic sintering, the electrical conductivity of the printed film is dramatically improved from nonconductive to 2.7 × 10⁴ S m^?1. The films demonstrate a room temperature power factor of 730 µW m^?1 K^?2, which is among the highest values reported in flexible TE films. Additionally, the film shows negligible performance changes after 500 bending cycles. The highly scalable and low‐cost fabrication process paves the way for large‐scale manufacturing of flexible devices using a variety of high‐performing nanoparticle inks.     

All-inkjet-printed dissolved oxygen sensors on flexible plastic substrates

Inkjet printing is a promising alternative manufacturing method to conventional standard microfabrication techniques for the development of flexible and low-cost devices. Although the use of inkjet printing for the deposition of selected materials for the development of sensor devices has been reported many times in literature, it is still a challenge and a potential route towards commercialization to completely manufacture sensor devices with inkjet technology. In this work is demonstrated the fabrication of a functional low-cost dissolved oxygen (DO) amperometric sensor with feature sizes in the micrometer range using inkjet printing. All the required technological steps for the fabrication of a complete electrochemical three electrodes system are discussed in detail. The working and counter electrodes have been printed using a gold nanoparticle ink, whereas a silver nanoparticle ink was used to print a pseudo-reference electrode. Both inks are commercially available and can be sintered at low temperatures, starting already at 120 °C, which allows the use of plastic substrates. In addition, a printable SU8 ink formulation cured by UV is applied as passivation layer in the sensor device. Finally, as the performance of analytical methods strongly depends on the working electrode material, is demonstrated the electrochemical feasibility of this printed DO sensor, which shows a linear response in the range between 0 and 8 mg L⁻¹ of DO, and affords a detection limit of 0.11 mg L⁻¹, and a sensitivity of 0.03 μA L mg⁻¹. The use of flexible plastic substrates and biocompatible inks, and the rapid prototyping and low-cost of the fabricated sensors, makes that the proposed manufacturing approach opens new opportunities in the field of biological and medical sensor applications.     

Hierarchical Architectural Structures Induce High Performance in n-Type GeTe-Based Thermoelectrics

Compatible p- and n-type materials are necessary for high-performance GeTe thermoelectric modules, where the n-type counterparts are in urgent need. Here, it is reported that the p-type GeTe can be tuned into n-type by decreasing the formation energy of Te vacancies via AgBiTe₂ alloying. AgBiTe₂ alloying induces Ag₂Te precipitates and tunes the carrier concentration close to the optimal level, leading to a high-power factor of 6.2 µW cm⁻¹ K⁻² at 423 K. Particularly, the observed hierarchical architectural structures, including phase boundaries, nano-precipitates, and point defects, contribute an ultralow lattice thermal conductivity of 0.39 W m⁻¹ K⁻¹ at 423 K. Correspondingly, an increased ZT of 0.5 at 423 K is observed in n-type (GeTe)_0.45(AgBiTe₂)_0.55. Furthermore, a single-leg module demonstrates a maximum η of 6.6% at the temperature range from 300 to 500 K. This study indicates that AgBiTe₂ alloying can successfully turn GeTe into n-type with simultaneously optimized thermoelectric performance.     

High-brightness perovskite quantum dot light-emitting devices using inkjet printing

The performance of perovskite quantum dot light-emitting diodes (PeQLEDs) has been rapidly enhanced recently, but the devices are still stuck in the stage of using small-scale solution processes, such as spin-coating. In this work, we report the realization of high performance PeQLEDs by using inkjet printing technique. We demonstrate the preparation of a printable perovskite quantum dot ink by using a hybrid solvent consisting of high boiling solvent dodecane and low boiling solvent n-octane. A universal strategy for eliminating coffee rings during inkjet printing of perovskite inks is developed based on the modulation of ink formulation, and the stacking model of perovskite quantum dot in a pixel pit structure is proposed. The inkjet-printed PeLEDs exhibit a low turn-on voltage of 2.7 V, a brightness of 10992 cd/m² at 6.6 V and a maximum current efficiency of 8.67 cd/A, which is by far the highest value reported for inkjet-printed PeLEDs. The results pave a way for future realization of high performance pixelated PeLED displays with inkjet printing technique.     

Inkjet Printing of Perovskites for Breaking Performance–Temperature Tradeoffs in Fabric-Based Thermistors

A novel low-temperature route is developed for inkjet printing of the perovskite Cs₂SnI₆, to create wearable negative-temperature-coefficient thermistors with unprecedented performance on thermally sensitive fabrics. A low processing temperature of 120 °C is achieved by creating a stable and printable ink using binary metal iodide salts, which is thermally transformed into dense Cs₂SnI₆ crystals after printing. The optimally printed Cs₂SnI₆ shows a temperature measurement range up to 120 °C, high sensitivity (4400 K), and temperature coefficient of resistivity (0.05 °C⁻¹), and stability under ambient environmental conditions and bending. The approach breaks a critical tradeoff that has hindered wearable fabric-based thermistors by enabling damage-free fabrication of devices with commercially comparable performance, evincing significant applications in multifunctional textiles and beyond.     

All inkjet-printed piezoelectric polymer actuators: Characterization and applications for micropumps in lab-on-a-chip systems

Digital printing technologies are promising as future manufacturing approaches due to their capabilities of highly flexible and additive material deposition on various substrates. In this contribution, all inkjet-printed piezoelectric polymer actuators are presented based on polyvinylidene fluoride trifluoroethylene (P(VDF-TrFE)) and electrodes printed from silver nanoparticle dispersions. The target application for the actuators described here are membrane pumps for microfluidic lab-on-a-chip (LOC) systems. For the first time, all-inkjet-printed P(VDF-TrFE) actuators are reported and the corresponding piezoelectric d₃₁ coefficient is measured. For manufacturing the actuators, a low-cost procedure is employed that consists of only three inkjet printing and post-processing steps where moderate thermal treatments (T_max = 130 °C) are combined with plasma sintering. The processing is therefore compatible with a wide range of temperature sensitive polymer substrates, completely additive and highly flexible. A sandwich-like structure of a piezoelectric P(VDF-TrFE) layer between two silver electrodes is inkjet-printed onto a polyethylene terephthalate (PET) substrate. When a voltage is applied across the piezoelectric layer, the reverse piezoelectric effect will lead to a bending deflection of this unimorph structure. The piezoelectric d₃₁ coefficients are found to be approximately 7 to 9 pm V⁻¹, which allows the generation of significant actuator deflections. For the application in a micropump, flow rates of several 100 μL min⁻¹ are anticipated, which is promising for LOC applications. Most current micropumps are based on actuator elements that are fabricated separately and mounted on a passive membrane. By using all inkjet-printed actuators, as presented here, the joining step is avoided and the benefits of low-cost printed devices are added to the well-developed processing approaches for microfluidic chips.     

Dichlorinated Dithienylethene-Based Copolymers for Air-Stable n-Type Conductivity and Thermoelectricity

Two donor–acceptor (D–A) polymers are obtained by coupling difluoro- and dichloro-substituted forms of the electron-deficient unit BDOPV and the relatively weak donor moiety dichlorodithienylethene (ClTVT). The conductivity and power factors of doped devices are different for the chlorinated and fluorinated BDOPV polymers. A high electron conductivity of 38.3 and 16.1 S cm⁻¹ are obtained from the chlorinated and fluorinated polymers with N-DMBI, respectively, and 12.4 and 2.4 S cm⁻¹ are obtained from the chlorinated and fluorinated polymers with CoCp₂, respectively, from drop-cast devices. The corresponding power factors are 22.7, 7.6, 39.5, and 8.0  µ W m⁻¹ K⁻², respectively. Doping of PClClTVT with N-DMBI results in excellent air stability; the electron conductivity of devices with 50 mol% N-DMBI as dopant remained up to 4.9 S m⁻¹ after 222 days in the air, the longest for an n-doped polymer stored in air, with a thermoelectric power factor of 9.3  µ W m⁻¹ K⁻². However, the conductivity of PFClTVT-based devices can hardly be measured after 103 days. These observations are consistent with morphologies determined by grazing incidence wide angle X-ray scattering and atomic force microscopy.     

High Entropy Semiconductor AgMnGeSbTe4 with Desirable Thermoelectric Performance

A new p-type high entropy semiconductor AgMnGeSbTe₄ with a band gap of ≈0.28 eV is reported as a promising thermoelectric material. AgMnGeSbTe₄ crystallizes in the rock-salt NaCl structure with cations Ag, Mn, Ge, and Sb randomly disordered over the Na site. Thus, a strong lattice distortion forms from the large difference in the atomic radii of Ag, Mn, Ge, and Sb, resulting in a low lattice thermal conductivity of 0.54 W m⁻¹ K⁻¹ at 600 K. In addition, the AgMnGeSbTe₄ exhibits a degenerate semiconductor behavior and a large average power factor of 10.36 µW cm⁻¹ K⁻² in the temperature range of 400–773 K. As a consequence, the AgMnGeSbTe₄ has a peak figure of merit (ZT) of 1.05 at 773 K and a desirable average ZT value of 0.84 in the temperature range of 400–773 K. Moreover, the thermoelectric performance of AgMnGeSbTe₄ can be further enhanced by precipitating of Ag₈GeTe₆, which acts as extra scatting centers for holes with low energy and phonons with medium wavelength. The simultaneous optimization in power factor and lattice thermal conductivity yields a peak ZT of 1.27 at 773 K and an average ZT of 0.92 (400–773 K) in AgMnGeSbTe₄-1 mol% Ag₈GeTe₆.     

Inkjet Printed Disposable High-Rate On-Paper Microsupercapacitors

On-paper microsupercapacitors (MSCs) are a key energy storage component for disposable electronics that are anticipated to essentially address the increasing global concern of electronic waste. However, nearly none of the present on-paper MSCs combine eco-friendliness with high electrochemical performance (especially the rate capacity). In this work, highly reliable conductive inks based on the ternary composite of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), graphene quantum dots and graphene are developed for scalable inkjet printing of compact (footprint area ≈ 20 mm²) disposable MSCs on commercial paper substrates. Without any post treatment, the printed patterns attain a sheet resistance as low as 4 Ω ?^?1. The metal-free all-solid-state MSCs exhibit a maximum areal capacitance > 2 mF cm^?2 at a high scan rate of 1000 mV s^?1, long cycle life (>95% capacitance retention after 10 000 cycles), excellent flexibility, and long service time. Remarkably, the “totally metal-free” MSC arrays are fully inkjet printed on paper substrates and also exhibit high rate performance. The life cycle assessment indicates that these printed devices have much lower eco-toxicity and global warming potential than other on-paper MSCs.     

Inkjet printed silver nanowire percolation networks as electrodes for highly efficient semitransparent organic solar cells

In this work, we demonstrate inkjet printing of silver nanowires (AgNW) with an average length of 10's of μm using industrial printheads with nozzle diameters in the same size range. The printed silver nanowire mesh reveals uniform distribution and a good balance between conductivity and transmittance, which is comparable to layers fabricated by conventional methods like slot-die or spray coating. Employing a novel AgNW ink formulation based on a high boiling alcohol allows printing directly on PEDOT:PSS and prevents nozzle clogging. Using silver nanowire meshes as bottom and top electrodes, a fully inkjet printed semitransparent organic solar cell with a power conversion efficiency of 4.3% for 1 cm² area is demonstrated, which is the highest value reported so far for fully inkjet printed organic photovoltaic cells.     

Thermoelectric Properties of TlCu3Te2 and TlCu2Te2

We performed thermoelectric characterizations on TlCu₃Te₂: (Tl¹⁺)(Cu¹⁺)₃ (Te²⁻)₂ and TlCu₂Te₂: (Tl¹⁺)(Tl³⁺)(Cu¹⁺)₄(Te²⁻)₄, in order to understand the relationship between the thermoelectric properties (especially the lattice thermal conductivity κ _lat) and the valence states of Tl. The thermal conductivity of TlCu₂Te₂ is high (about 8 W m⁻¹ K⁻¹), while that of TlCu₃Te₂ is extremely low (around 0.5 W m⁻¹ K⁻¹) like other thallium tellurides. This high κ of TlCu₂Te₂ was caused not only by its large electronic contribution but also by its intrinsically high κ _lat. The present study implies that the valence states of Tl would play some important roles in determining the magnitude of κ _lat.     

Modulation Doping Leads to Optimized Thermoelectric Properties in n-Type Bi6Cu2Se4O6 due to Interface Effects

Heterogeneous composites consisting of Bi₆Cu₂Se_3.6Cl_0.4O₆ and Bi₂O₂Se are prepared according to the concept of modulation doping. With prominently increased carrier mobility and almost unchanged effective mass, the electrical transport properties are considerably optimized resulting in a peak power factor ≈1.8 µW cm⁻¹ K⁻² at 873 K, although the carrier concentration is slightly deteriorated. Meanwhile, the lattice thermal conductivity is lowered to ≈0.62 W m⁻¹ K⁻¹ due to the introduction of the second phase. The modified Self-consistent Effective Medium Theory is utilized to explain the deeper mechanism of modulation doping. The enhancement of apparent carrier mobility is derived from the highly active phase interfaces as fast carrier transport channels, while the reduced apparent thermal conductivity is ascribed to the existence of thermal resistance at the phase interfaces. Ultimately, an optimized ZT ≈0.23 is obtained at 873 K in Bi₆Cu₂Se_3.6Cl_0.4O₆ + 13% Bi₂O₂Se. This research demonstrates the effectiveness of modulation doping for optimizing thermoelectric properties once again, and provides the direct microstructure observation and consistent theoretical model calculation to emphasize the role of interface effects in modulation doping, which should be probably applicable to other thermoelectrics.     

Tailoring Interfacial Charge Transfer for Optimizing Thermoelectric Performances of MnTe-Sb2Te3 Superlattice-Like Films

Interfacial charge transfer has a vital role in tailoring the thermoelectric performance of superlattices (SLs), which, however, is rarely clarified by experiments. Herein, based on epitaxially grown p-type (MnTe)_x(Sb₂Te₃)_y superlattice-like films, synergistically optimized thermoelectric parameters of carrier density, carrier mobility, and Seebeck coefficient are achieved by introducing interfacial charge transfer, in which effects of hole injection, modulation doping, and energy filtering are involved. Carrier transport measurements and angle-resolved photoemission spectroscopy (ARPES) characterizations reveal a strong hole injection from the MnTe layer to the Sb₂Te₃ layer in the SLs, originating from the work function difference between MnTe and Sb₂Te₃. By reducing the thickness of MnTe less than one monolayer, all electronic transport parameters are synergistically optimized in the quantum-dots (MnTe)_x(Sb₂Te₃)₁₂ superlattice-like films, leading to much improved thermoelectric power factors (PFs). The (MnTe)_0.1(Sb₂Te₃)₁₂ obtains the highest room-temperature PF of 2.50 mWm⁻¹K⁻², while the (MnTe)_0.25(Sb₂Te₃)₁₂ possesses the highest PF of 2.79 mWm⁻¹K⁻² at 381 K, remarkably superior to the values acquired in binary MnTe and Sb₂Te₃ films. This research provides valuable guidance on understanding and rationally tailoring the interfacial charge transfer of thermoelectric SLs to further enhance thermoelectric performances.     

Inkjet printed thin and uniform dielectrics for capacitors and organic thin film transistors enabled by the coffee ring effect

The deposition of a thin and uniform dielectric layer is required for high performance printed capacitors and thin film transistors (TFTs), however this is difficult to achieve with printing methods. We have demonstrated inkjet-printed dielectrics with a uniform thickness from 70 nm to 200 nm by taking advantage of the coffee ring effect. A high capacitance per unit area of 230 pF/mm² is achieved from capacitors with linear morphologies fully printed onto flexible substrates. We also demonstrate organic TFTs with an average mobility of 0.86 cm²/Vs and a source drain current of 57 μA obtained with a supply voltage of 15 V. This performance was shown to be consistent, with a standard deviation of 15% obtained from hundreds of printed organic TFTs on PET substrates. This consistency was further validated by the production of functional NAND, NOR, AND and OR logic gates. Our results demonstrate that the coffee ring effect, which is usually viewed as undesirable, can enable higher performance in printed electronic devices.     

Flexible organic phototransistors based on a combination of printing methods

Highly photosensitive organic phototransistors (OPTs) are successfully demonstrated on a flexible substrate using all-solution process as well as a combination of printing methods which consist of roll-to-plate reverse offset printing (ROP), inkjet printing and bar coating. Excellent electrical switching characteristics are obtained from heterogeneous interfacial properties of the reverse-offset-printed silver nanoparticle electrode and the inkjet-printed p-channel polymeric semiconductor. In particular, the OPTs exhibit remarkably photosensitivity with a photo-to-dark current ratio exceeding 5 orders. This optoelectronic properties of the combinational printed OPTs are theoretically and experimentally studied, and found the comparable tendency. In addition, excellent mechanical stability is observed with up to 0.5% of strain applied to the OPTs. Hence, by manufactured with a combination of various graphic art printing methods such as roll-to-plate ROP, inkjet printing, and bar coating, these devices are very promising candidates for large-area and low-cost printed and flexible optoelectronics applications.     

Inkjet-printed platinum counter electrodes for dye-sensitized solar cells

In this study, inkjet printing method was successfully demonstrated to produce catalytic platinum layers for dye-sensitized solar cells. Our work includes meticulous optical, morphological, and electrocatalytical analyses of precisely inkjet-patterned counter electrodes as well as traditionally drop-cast samples. Similar catalytic performance was obtained with both methods (Rʹ_CT = 1.2 Ω cm² for drop-cast and Rʹ_CT = 1.6 Ω cm² for inkjet-printed) at same Pt loading (ca. 2.5 μg/cm²), and correspondingly almost same cell efficiencies (η_drop-cast = 6.5% and η_printed = 6.7%). All the cells exhibited high stability by keeping their efficiencies after being subjected to a 1000 h aging test under 1 Sun and 35 °C at the open circuit condition. These results highlight the potential of inkjet printing to realize precisely patterned and no-material-wasting counter electrodes by controlled dispensing of the functional solution.     

Multi-Functional and Stretchable Thermoelectric Bi2Te3 Fabric for Strain,Pressure, and Temperature-Sensing

Fiber-based electronics are essential components for human-friendly wearable devices due to their flexibility, stretchability, and wearing comfort. Many thermoelectric (TE) fabrics are investigated with diverse materials and manufacturing methods to meet these potential demands. Despite such advancements, applying inorganic TE materials to stretchable platforms remains challenging, constraining their broad adoption in wearable electronics. Herein, a multi-functional and stretchable bismuth telluride (Bi₂Te₃) TE fabric is fabricated by in situ reduction to optimize the formation of Bi₂Te₃ nanoparticles (NPs) inside and outside of cotton fabric. Due to the high durability of Bi₂Te₃ NP networks, the Bi₂Te₃ TE fabric exhibits excellent electrical reliability under 10,000 cycles of both stretching and compression. Interestingly, intrinsic negative piezoresistance of Bi₂Te₃ NPs under lateral strain is found, which is caused by the band gap change. Furthermore, the TE unit achieves a power factor of 25.77 µWm⁻¹K⁻² with electrical conductivity of 36.7 Scm⁻¹ and a Seebeck coefficient of −83.79 µVK⁻¹ at room temperature. The Bi₂Te₃ TE fabric is applied to a system that can detect both normal pressure and temperature difference. Balance weight and a finger put on top of the 3 × 3 Bi₂Te₃ fabric assembly are differentiated through the sensing system in real time.