Ag₂S is a typical fast ionic conductor with a relative density of 7.317 g/cm³ and a melting point of 825 ° C. Due to their excellent physical and chemical properties, Ag₂S have attracted wide attention in the fields of catalysis, biology, optoelectronics and solar cells^{[52⇓⇓~55]}. As early as 1951, Hebb et al. Have done related research on the electrical properties of Ag₂S, but the mechanical properties of Ag₂S, such as flexibility, have been neglected^[56]. Until 2018, Shi et al. Made α-Ag₂S polycrystal by high temperature melting combined with plasma sintering technology, and found that the α-Ag₂S showed excellent ductility and high plasticity, and the deformation far exceeded that of known ceramics and semiconductor materials, which was comparable to the mechanical properties of some metals, which opened the research on Ag₂S-based flexible thermoelectric materials^[57].

Structure and Properties of 1)Ag₂S-Based Compounds

Ag₂S can undergo a structural phase transition at 455 K, and there are two different phase structures before and after the transition, which are room temperature α phase and intermediate temperature β phase, respectively^[58]. As shown in Fig. 4, the room temperature α-Ag₂S phase has a zigzag layered monoclinic structure (space group P2₁/c), each layer is composed of multiple 8-atom rings, each atom ring is composed of four S atoms and four Ag atoms, and the rings are connected by S atoms, so the folded layers are stacked along the a axis. There is a relatively low slip energy barrier between the atomic layers of this layered structure, which makes it easy to slip between the layers, but because there is a certain atomic interaction force between the sliding planes, the material can avoid cracking or dissociation in the process of sliding, which makes the α-Ag₂S show good flexibility^[57]. When the temperature exceeds 455 K, the α-Ag₂S will transform into the body-centered cubic structure of the β-Ag₂S, so that its layered structure is destroyed, and the β-Ag₂S is no longer flexible.

The Ag₂S,N type direct band gap semiconductor, which has very low intrinsic lattice thermal conductivity, is a potential thermoelectric material. At room temperature, the theoretical band gap of α-Ag₂S is about 1.0 eV, and the wide band gap makes its conductivity not high, only 0.1~0.5 S·m^-1. However, in the intermediate temperature β-Ag₂S, the carrier mobility and electrical properties of the β-Ag₂S are improved due to the free migration of Ag ions in the unit cell gap due to its liquid-like characteristics, and the band gap is reduced to 0. 42 eV, which is comparable to that of traditional thermoelectric materials such as PbTe and SiGe^[16][17]. The improvement of electrical properties makes the thermoelectric performance of β-Ag₂S much higher than that of α-Ag₂S. For example, Wang et al. Prepared single-phase Ag₂S by high temperature melting combined with annealing, and the carrier concentration, carrier mobility and Seebeck coefficient of the Ag₂S were greatly improved before and after the phase transition at 455 K, which increased the power factor from lower than 0.3μW·cm^-1·K^-2 to 5μW·cm^-1·K^-2[59]. At the same time, due to the lower thermal conductivity, the maximum ZT value of α-Ag₂S is lower than that of 0.02,β-Ag₂S, which reaches 0.55 at 580 K. It can be seen that although α-Ag₂S has good flexibility, its thermoelectric performance is not high, which limits its application in flexible thermoelectric wearable devices. How to optimize the mechanical properties and thermoelectric properties of room temperature phase α-Ag₂S has become the main research content of Ag₂S-based flexible thermoelectric materials.

Performance optimization of 2)Ag₂S-based flexible thermoelectric materials

In silver chalcogenide Ag₂X(X=S,Se,Te), the thermoelectric properties of Ag₂S are much lower than those of Ag₂Se and Ag₂Te due to their large band gap.But among the three, Ag₂Se and Ag₂Te present brittleness, and only Ag₂S has flexibility, as shown in Fig. 5A^[30,60,61]. Meanwhile, the existence of natural minerals such as Ag₄SSe and Ag₄STe indicates that Ag₂S can form solid solution with Ag₂Se and Ag₂Te^[62][63]. Therefore, the synergistic optimization of flexibility and thermoelectric properties of Ag₂S based compounds is expected to be achieved through solid solution and the regulation of solid solution ratio.

Shi et al. Made a detailed study on solid solution Ag₂S based flexible thermoelectric materials, prepared a series of Se and Te solid solution Ag₂S_1-xSe_x, Ag₂S_0.8Te_0.2 and ternary solid solution Ag₂S_0.5Se_0.45Te_0.05 by high temperature melting and plasma sintering technology, and studied the effect of solid solution on the performance of Ag₂S based flexible thermoelectric materials^[30,31,57]. The results show that the conductivity of Ag₂S_0.5Se_0.5 and Ag₂S_0.5Se_0.45Te_0.05 increases to 3.0×10⁴S·m^-1 and 2.7×10⁴S·m^-1, respectively, after solution treatment.5 orders of magnitude higher than that of the room temperature phase α-Ag₂S (about 0.1 S·m^-1); At the same time, the solid solution of Se and Te can introduce point defects to scatter phonons and reduce the lattice thermal conductivity. Finally, the ZT values of Ag₂S_0.5Se_0.5 and Ag₂S_0.5Se_0.45Te_0.05 reach 0.26 and 0.44 at room temperature, respectively, which are greatly improved by four orders of magnitude compared with α-Ag₂S (about 8×10^-5), as shown in Fig. 5B. While the thermoelectric properties were improved, they also studied the flexibility of the sample after solid solution, and found that the transition boundary between monoclinic structure and orthorhombic structure in Ag₂ {{{\mathrm{Se}}_1}_{-}}_x S_x solid solution was about X = 0.3, and when X ≤ 0.2, the transition boundary between monoclinic structure and orthorhombic structure was about X = 0.3.The Ag₂ {{{\mathrm{Se}}_1}_{-}}_x S_x samples crystallize in the orthorhombic structure, while they crystallize in the monoclinic structure for X ≥ 0.4,Only the monoclinic Ag₂ {{{\mathrm{Se}}_1}_{-}}_x S_x samples with X ≥ 0.4 have good flexibility. For example, when X = 0.5, the flexibility of the Ag₂S_0.5Se_0.5 sample is comparable to that of α-Ag₂S (Figure 5C), and the thermoelectric properties of the sample are not greatly affected after repeated bending, so the Ag₂S_0.5Se_0.5 sample is also used to fabricate flexible thermoelectric devices (Figure 5D). Similarly, the sample Ag₂S_0.8Te_0.2, in which Te is solubilized with Ag₂S, has similar properties. This indicates that an appropriate amount of solid solution can not only control the thermoelectric properties of Ag₂S-based compounds, but also keep them flexible. In addition, He et al. Prepared Ag₂Te_1-xS_x-based flexible thermoelectric materials by high temperature melting combined with planar hot pressing, and found that the Ag₂Te_0.6S_0.4 sample showed good flexibility and thermoelectric properties.The carrier concentration of the sample at room temperature is remarkably improved, and meanwhile, the microstructure shows that amorphous Ag₂Te_0.6S_0.4 with a disordered structure exists in the sample, so that the lattice thermal conductivity of the material is remarkably reduced, and finally, the ZT value of the material at room temperature reaches 0.22^[32]; Meanwhile, the mechanical property test results of different solid solutions show that the flexibility of the Ag₂Te_0.6S_0.4 sample is the best, which is close to α-Ag₂S. To sum up, for Ag₂S-based flexible thermoelectric materials, solid solution is an effective way to improve their thermoelectric performance and maintain good flexibility.

Inspired by the α-Ag₂S, Shi et al. focused the development of new semiconductor flexible thermoelectric materials on two-dimensional materials containing van der Waals force, and found that the InSe single crystal with layered structure not only has the excellent physical properties of traditional inorganic semiconductors, but also has extraordinary plasticity and deformability like metals (Fig. 6C)^[64]. This brings new opportunities for the development and application of new semiconductor flexible thermoelectric materials.

1) Structure and properties of InSe-based compounds

InSe mainly includes hexagonal β phase and rhombohedral γ phase, both of which show layered structures, as shown In Fig. 6a, B. Each layer In these two structures is composed of covalently bonded Se-In-In-Se, and the layers are stacked along the C axis. The stacking sequences of β-InSe and γ-InSe are ABAB and ABCABC, respectively, and the layers are connected by weak van der Waals forces^{[65][66⇓~68]}. Similar to α-Ag₂S, InSe single crystal also has good flexibility, and related experiments also verify the flexibility of β-InSe and γ-InSe. For example, in 2002, Mosca et al. measured the hardness and elastic modulus of γ-InSe single crystal by nanoindentation test, which were about 1.6 and 28 GPa, respectively, proving that γ-InSe has good flexibility^[69]. Shi et al. also studied the flexibility of two-dimensional β-InSe single crystal, and found that the material can be bent, twisted, and even deformed into various shapes without fracture in the bulk form, showing good flexibility macroscopically (Fig. 6d)^[64]; The related mechanical test results also show that the compressive engineering strain of the material can reach 80%, and the bending and tensile engineering strains in specific directions are also higher than 10%, which further verifies the good mechanical properties of the material.

InSe is an n-type semiconductor, and the theoretical band gaps of β-InSe and γ-InSe are 1.2 and 0.52 eV, respectively, both of which are direct gap semiconductors. Shi et al. Showed that InSe can obtain a low thermal conductivity of about 1.6 W·m^-1K^-1 at room temperature^[70]; But at the same time, the carrier concentration of InSe is also very low, only 3.2×10¹³cm^-3, which is several orders of magnitude lower than that of traditional thermoelectric materials with better performance (such as Bi₂Te₃ with a carrier concentration of about 1.1×10¹⁹cm^-3), which largely limits the improvement of InSe thermoelectric performance^[71]. Therefore, how to improve the carrier concentration of InSe to further optimize its electrical properties is the key to the research of InSe-based flexible thermoelectric materials.

2) Performance optimization of InSe-based flexible thermoelectric materials

At present, the research on InSe-based flexible thermoelectric materials is mainly focused on InSe single crystals. Zhang et al. made a detailed study on the mechanical properties and thermoelectric properties of γ-InSe single crystal flexible thermoelectric materials. They found that the bending deformation process of γ-InSe single crystal not only did not affect the thermoelectric properties of the material, but also induced more defects, enhanced phonon scattering, thereby reducing the lattice thermal conductivity, which was conducive to the improvement of thermoelectric properties^[72]. However, the conductivity of γ-InSe single crystal is only 00027 at 280 K due to the low carrier concentration, which is only 0.00027. This indicates that the thermoelectric properties of γ-InSe single crystal need to be further optimized and improved, although it has good flexibility. Zhang et al also pointed out that the electrical properties of γ-InSe single crystal can be further optimized by doping or solid solution, so as to obtain better InSe-based flexible thermoelectric materials.

The fabrication of traditional cross-plane π-type flexible thermoelectric devices often requires both P-type and N-type thermoelectric materials with good flexibility. Although the discovered Ag₂S-based flexible thermoelectric materials and InSe-based flexible thermoelectric materials have good flexibility,However, both of them are N-type semiconductors, and there is a lack of P-type semiconductor thermoelectric materials with both flexibility and good thermoelectric performance, which greatly limits the development of flexible thermoelectric devices. Based on this, in 2022, Shi et al. Discovered a new type of p-type AgCuSe-based flexible thermoelectric material, namely AgCu (Se, S, Te) pseudo-ternary solid solution^[73].

Similar to Ag₂S and InSe compounds, AgCuSe also has two different phase structures, which can undergo a structural phase transition at 504 K from the low temperature β phase of the orthorhombic structure to the high temperature α phase of the face-centered cubic structure. Shi et al. Found that AgCuSe was brittle and not flexible. They used the AgCu (Se, S, Te) pseudo-ternary solid solution composition-performance phase diagram (Fig. 7 a) to dissolve S and Te into AgCuSe, which not only converted AgCuSe from a brittle N-type semiconductor to a P-type semiconductor with better flexibility, but also improved its thermoelectric performance, in which AgCuSe_0.22S_0.08Te_0.7 had the best performance. In addition, the mechanical property experiment of AgCuSe_0.22S_0.08Te_0.7 by the research group shows that the material is easy to bend without fracture (Fig. 7 C), and its better flexibility mainly comes from the S element. Although the content of S element is very low, in the three-point bending experiment, the engineering strain of AgCuSe_0.3Te_0.7 without S is only 3%, while the AgCuSe_0.22S_0.08Te_0.7 is significantly improved to 18%, which is comparable to that of Ag₂S-based flexible thermoelectric materials. Meanwhile, the electrical properties of AgCuSe are improved due to the solid solution of Te, which makes the ZT value of AgCuSe_0.22S_0.08Te_0.7 reach 0.45 at room temperature (Fig. 7 B). Based on the good flexibility and thermoelectric performance of AgCuSe_0.22S_0.08Te_0.7, the research group fabricated a cross-plane π-type flexible thermoelectric device with N-type Ag₂₀S₇Te₃ flexible semiconductor (Fig. 7d).