Photonic Curing Enables Ultrarapid Processing of Highly Conducting β-Cu2-δSe Printed Thermoelectric Films in Less Than 10 ms.

It has been a challenge to obtain high electrical conductivity in inorganic printed thermoelectric (TE) films due to their high interfacial resistance. In this work, we report a facile synthesis process of Cu-Se-based printable ink for screen printing. A highly conducting TE β-Cu2-δSe phase forms in the screen-printed Cu-Se-based film through ≤10 ms sintering using photonic-curing technology, minimizing the interfacial resistance. This enables overcoming the major challenges associated with printed thermoelectrics: (a) to obtain the desired phase, (b) to attain high electrical conductivity, and (c) to obtain flexibility. Furthermore, the photonic-curing process reduces the synthesis time of the TE β-Cu2-δSe film from several days to a few milliseconds. The sintered film exhibits a remarkably high electrical conductivity of ∼3710 S cm-1 with a TE power factor of ∼100 μW m-1 K-2. The fast processing and high conductivity of the film could also be potentially useful for different printed electronics applications.

Introduction

In general, different chemical and physical synthesis methods such as arc melting, solid-state reaction melt and growth, mechanical alloying, and hydrothermal synthesis are employed to prepare various thermoelectric (TE) materials.[1] Depending on the type of material, a suitable synthesis method is implemented. The synthesis processes are extremely important; they should be appropriate to get an expected phase and high TE performance. The melt and growth method is widely used to synthesize low-melting-temperature alloys, such as telluride/selenide chalcogenides and Zintl phases. In this process, metal elements with a stoichiometric ratio are kept in a vacuum-sealed quartz tube followed by heating at elevated temperatures for several hours to days so that all of the precursor elements interdiffuse to form the desired phases.[2] The melting points of the metal elements and annealing temperatures are generally determined from the phase diagram of the targeted alloys. The resulting consolidated ingots are ground into a powder and made into pellets using hot-pressing or spark plasma sintering (SPS) to prepare TE components. The mechanical alloying method has also been adopted to synthesize similar TE materials.[3−5] Here, powder elements are mixed in a stoichiometric ratio and ground with a high-energy ball mill for several hours. The milling time and rotation speed of the jar containing balls and the ingredients depend on the formation energies of the alloys to be prepared. The resulting homogeneous milled powder samples are then consolidated into pellets using hot press or SPS. Sometimes, further heat treatment at high temperatures is required to acquire the desirable phase. The Cu2Se chalcogenide phase is one of the most interesting TE phases for its unique transport properties. The monoclinic α-Cu2Se is the common phase at room temperature (RT) and transforms into the cubic disordered superionic β-Cu2Se phase at elevated temperatures. In this phase, two separate sublattices, “Cu” and “Se”, build the structure of Cu2Se. A rigid crystalline symmetric framework is formed by the large Se atoms that help transfer the charge carriers, and the disordered Cu ions diffuse around the Se sublattices, resulting in a strong phonon scattering, facilitating a low lattice thermal conductivity. Therefore, the β-Cu2Se phase exhibits a liquid-like behavior of the phonons and a crystal-like behavior of the charge carriers, a “phonon-liquid electron-crystal” behavior.[6] Hence, β-Cu2Se exhibits simultaneously high electrical conductivity and low thermal conductivity. Both the “melt and growth” and mechanical alloying synthesis processes are employed to prepare the bulk Cu2Se phase, followed by grinding, pressing, and dicing to design Cu2Se-based bulk TE components. The complete synthesis processes are reported to take up to 10 days.[7−11] Chemical synthesis methods have also been reported to prepare Cu2Se, which also involves multiple steps.[12] Apart from a complex and long synthesis process, the Cu2Se-based bulk TE components reported so far are neither shape-conformal nor flexible. A flexible TE component would potentially be advantageous over its bulk counterparts as this allows wearable and complex geometry applications.[13,14] Hence, efforts have been put forward to synthesize flexible Cu2Se-based TE films using bulk or chemically prepared Cu2Se particles.[15,16] Recently, PEDOT:PSS/Cu2Se-based composite TE films prepared by the vacuum filtration and pressing technique have been reported to exhibit good performance.[17,18] However, unlike printing technology, the synthesis processes of these TE components are not low-cost and involve a set of complex processes.

Recently, we have reported Sb–Bi–Te/β-Cu2−δSe TE films prepared by sintering in a vacuum oven. Nevertheless, there is a trade-off between high performance and good flexibility in the films.[19−21] We have also reported high-performance Ag2Se-based printed n-type TE films prepared by a similar sintering process.[13,22,23] In this work, we have employed a fast photonic-curing technology on printed Cu–Se-based films to address the drawbacks associated with the traditional procedure to fabricate flexible β-Cu2−δSe-based TE films. We have used screen printing to fabricate TE films with high spatial resolution, and a very fast photonic-curing process is implemented to sinter the films. We have discovered that the TE β-Cu2−δSe phase is formed in less than 10 ms in the film, facilitating an ultrahigh electrical conductivity and mechanical flexibility without damaging the low-temperature substrates. The sintering time to form the Cu2−δSe phase reduces by 8 orders of magnitude from ∼106 s in bulk to ≤10–2 s in the printed film.

Experimental Methods

Materials

Copper powder (spheroidal) (10–25 μm, 98%, Sigma-Aldrich), Se powder (100 mesh, ≥99.5% trace metal basis, Sigma-Aldrich), N-methyl-2-pyrrolidone (NMP) (anhydrous, 99.5%, Sigma-Aldrich), poly(vinylpyrrolidone) (PVP) (average Mw ∼40 000, Sigma-Aldrich), and polyethylene naphthalate (PEN) (75 μm, DuPont).

Synthesis of TE Inks and Films

The Cu–Se-based printable TE ink was prepared using a ball mill, Fritsch Planetary Mill PULVERISETTE 5 premium line. The Cu and Se elemental powders were mixed in a 2:1 atomic ratio in a PVP–NMP (6:94) solution. The blend was then kept in a N2 purged zirconia jar containing 10 mm zirconia balls followed by wet milling for 30 min at 200 rpm. The weight ratio of the balls to the mixture was 10:1. We printed the obtained Cu–Se-based ink on glass and flexible substrates using a semiautomated screen printer ROKUPRINT machine with a screen specification of 600 × 300 90–40 y/22° Hitex. The printed films of the order of 10 μm were dried on a hotplate after printing. After drying at 343 K, the printed films were flash-sintered with 35 μPulses for 8–10 ms, varying the external input flashlamp voltage (VP) in the range of 300–400 V using a PulseForge 1200 (Novacentrix) photonic-curing machine. The temperatures of the top surface of the printed film, at the interface, and at the bottom of the substrate have been simulated using the interactive numerical model SimPulse for PulseForge photonic-curing tools based on the heat transfer model. It can be seen that the top and interface temperatures reach 973 for VP = 380 V where the substrate temperature remains near RT (cf. Figure S1). The film temperature decreases below 473 K within 50 ms.

Characterization Techniques

The phase and crystallographic analyses of the photonic-sintered printed films were conducted using X-ray diffraction (XRD) on a Bruker D8 diffractometer with a Lynxeye XE detector. The temperature-dependent transport parameters of the films were studied using a Hall measurement setup (Linseis HCS 10). The electrical conductivity is determined by the van der paw method with four contacts. A magnetic field is applied perpendicular to the film to produce a Hall voltage (VH). The Hall coefficient (RH) is determined from the VH, which is essential to determine the carrier concentration and mobility. The RT Seebeck coefficient α of the sintered film with VP = 380 was measured using a custom-built setup. The working principle of the custom-built setup is given in Supporting Information Note 2 of the previous report.[14] The thicknesses and surface morphologies of the printed films were studied by a Bruker 3D microscope based on white-light interferometry (WLI). Microstructural and elemental analyses of the sintered films were conducted using an FEI Quanta 650 environmental scanning electron microscope (ESEM) equipped with a Schottky field emitter and an SSD detector operating with 5 and 15 kV.

Results and Discussion

The crystallographic and phase analyses of the nonsintered and sintered films were extensively studied using the X-ray diffraction (XRD) technique (cf. Figure ). XRD patterns of the sintered films for different VP from 300 to 400 V were collected while keeping the sintering time constant at 10 ms. The XRD pattern of the nonsintered printed film corresponds to the two unreacted Cu and Se. The β-Cu2−δSe phase starts to grow when VP reaches 320 V and is found to be fully developed for VP ≥ 380 V (cf. Figure a). To get insight into the crystallographic structure of the β-Cu2−δSe phase, Rietveld refinement of the XRD pattern of the sintered film for VP = 380 V was performed (cf. Figure b). The results indicate that the XRD pattern corresponds to a main cubic β-Cu2−δSe phase with space group Fm3̅m and a fraction of unreacted Cu. The nonstoichiometry “δ” is estimated to be up to 0.1. Most probably, the δ is responsible for the RT β-Cu2−δSe phase as it is also reported that δ = 0.03 can enable a phase transformation of the conventional α-Cu2−δSe phase to the cubic β-Cu2Se phase. The lattice parameter of the unit cell of the β-Cu2−δSe phase is estimated to be a = 5.7(6) Å. The Se atoms in the β-Cu2−δSe crystal form a rigid face-centered cubic cell occupying the Wyckoff position 4a (0, 0, 0), and the Cu atoms occupy two different Wyckoff positions 8c (1/4, 1/4, 1/4) and 32f (x, x, x).

Figure 1

VP-dependent XRD patterns of the Cu–Se-based printed films at RT (a). Rietveld refinement results of the film for VP = 380 V (b). β-Cu2−δSe is identified as the main phase with a small fraction of unreacted Cu. Unit cell and lattice structure of β-Cu2−δSe (c, d).

The detailed microstructural and morphological analyses of the printed sintered and nonsintered films are included in Figure . The screen printability of the Cu–Se-based film was checked using the ROKUPRINT screen-printing machine (cf. Figure a). Macroscopic morphologies of the printed Cu2Se films for sintering and nonsintering conditions were studied using three-dimensional (3D) microscopy; see Figure b–d. The morphology of the nonsintered film shows segregated island-like structures of the Cu–Se material. SEM images of the nonsintered and sintered films also show that the film was compacted after sintering with VP = 380 V and becomes denser (cf. Figure e,g). The elemental mapping of the printed films indicates that the Cu and Se elements were segregated in the nonsintered film and they almost fully reacted to form the β-Cu2Se phase in the sintered film. The b/w regions in the elemental mappings in Figure h correspond to the potholes created due to the expulsion of the organic constituents from the film during the sintering process. A sufficient number of data from potholes did not reach the energy-dispersive X-ray analysis (EDX) detector.

Figure 2

Screen-printed sintered Cu2Se film on the poly(ethylene terephthalate) (PET) substrate (a). Surface morphologies of the nonsintered film (b), sintered film with VP = 340 V (c), and sintered film with VP = 380 V (d) captured by a WLI 3D microscope. The microstructures and elemental maps of the nonsintered film (e, f) and the sintered film with VP ≥ 340 V (g, h).

The remnant of organic binder PVP could be present in the films; however, it constitutes only 1.6 wt % (<10 vol %) of the films. After sintering, it decomposes releasing <3 vol % of carbon in the film, which is far below the percolation threshold to influence the transport properties.[24] The mechanical flexibility of a 15 mm sintered film for VP = 380 V has been checked by a semicircular bending test multiple times. The normalized resistance of the film was found to be 1.5 after 10 bending cycles (cf. Figure S2).

Thermotransport Properties of Photonic-Cured Films

The temperature-dependent transport properties were studied for VP ≤ 380 V (cf. Figure ). The mechanical flexibility and the electrical conductivities σ of the printed film deteriorate for VP > 380 V. The nonsintered film is found to be insulating and becomes highly conductive after sintering. All of the films show p-type conduction, and the σ increases with increasing VP, altering the transport properties and the nature of the films for VP > 340 V (cf. Figure a). The negative value of (RH) at T > 323 K for the film with VP = 320 V indicates alteration of the conduction nature, from p-type to n-type, as the β-Cu2Se phase is not formed. Although the Hall carrier concentration (pH) decreases with increasing VP, the σ increases because of significant enhancement of the carrier mobility (μH) (cf. Figure b,d). The films for VP ≥ 360 V show a metallic-like nature, consistent with the previously reported transport phenomena of the pristine β-Cu2Se-based film.[20]

Figure 3

Variation of electrical conductivity σ (a), Hall carrier concentration pH (b), Hall coefficient RH (c), and Hall carrier mobility μH (d) of the Cu–Se-based printed film with temperature for different VP, 320 ≤ x ≤ 380.

A surprisingly high σ ∼3710 S cm–1 is achieved at RT in the flexible printed film for VP = 380 V. The σ value is much higher than those of most reported Cu2Se-based bulk materials[6−9] because of leftover unreacted Cu elements in the film. The electrical conductivity σ and Hall mobility μH of the printed film increase with increasing the flashlamp voltage VP, which is directly proportional to the sintering temperature of the film. The insulating organic ingredients in the printed film start to be removed with increasing VP. The elemental Cu and Se then react to form β-Cu2Se with further increase of VP, enhancing the σ of the sintered film (Figure ). At first, the charge carrier concentration pH of the film increases with increasing VP for ≤ 340 V due to the removal of the insulating organic constituents, leaving partially unreacted Cu and Se in the printed film, as shown in Figure a.

Figure 4

Variation of the transport parameters σ, pH, and μH with the input flashlamp voltage VP (a). Electric conductivity is modeled, and the line through the data points is a fit (b). Temperature-dependent α and power factor of the photonic-cured film for VP = 380 V (c). Semicircular bending test of a 15 mm sintered film for VP = 380 V to check its mechanical flexibility (d).

The amount of the unreacted elements decreases with increasing VP for VP > 340 V resulting in the decrease of pH in the film. As the pH of the Cu element (∼8.55 × 1022 cm–3) is one order of magnitude higher than that for the β-Cu2Se phase (<5 × 1021 cm–3), with decreasing unreacted Cu, the overall σ decreases for VP > 340 V. The temperature-dependent electrical transport of the film for VP = 380 V is modeled using a metal-semiconducting transport equation.[25] The film is found to be metallic in nature (cf. Figure b). The RT σ of the sintered film stored in the air atmosphere for VP = 380 V was measured after many weeks and it is found to be decreased slightly. Storing in an N2 or inert gas environment could prevent decreasing the conductivity. Due to the presence of unreacted Cu elements, the Seebeck coefficient (α) is also found to be lower. A positive α ∼16 μV K–1 and a TE power factor of ∼95 μW m–1 K–2 are achieved in the film at RT (cf. Figure c). The power factor value is twice as high as the previously reported vacuum-sintered β-Cu2Se film.[20] Unfortunately, we could not determine the thermal conductivity of the film due to the nature of the synthesis process and film specification. However, the thermal conductivity value could be higher compared to the β-Cu2Se film, resulting in lower ZT due to its high conductivity. In addition to TE applications, the highly conductive p-type β-Cu2Se film could be potentially employed for printed sensors and optoelectronic applications.[26−28]

Conclusions

A long sintering process at elevated temperatures is one of the most important steps to synthesis a TE phase. However, in printed thermoelectrics, low-temperature substrates with printed precursor materials do not withstand the high-temperature sintering process to form a TE phase. This work employs photonic-curing technology to sinter the printed Cu–Se-based material without damaging the flexible low-temperature substrates. The TE β-Cu2Se phase is formed within 10 ms through the photonic curing in the printed film. The obtained sintered flexible film exhibits a remarkably high electrical conductivity of 3710 S cm–1 with a TE power factor of 95 μW m–1 K–2 at RT.