Flexible thermoelectrics provide a different solution for developing portable and sustainable flexible power supplies. The discovery of silver sulfide–based ductile semiconductors has driven a shift in the potential for flexible thermoelectrics, but the lack of good p-type ductile thermoelectric materials has restricted the reality of fabricating conventional cross-plane π-shaped flexible devices. We report a series of high-performance p-type ductile thermoelectric materials based on the composition-performance phase diagram in AgCu(Se,S,Te) pseudoternary solid solutions, with high figure-of-merit values (0.45 at 300 kelvin and 0.68 at 340 kelvin) compared with other flexible thermoelectric materials. We further demonstrate thin and flexible π-shaped devices with a maximum normalized power density that reaches 30 μW cm ⁻² K ⁻² . This output is promising for the use of flexible thermoelectrics in wearable electronics.

Deformable semiconductors: Semiconductors are usually brittle and do not deform easily. Wei et al. found that bulk single crystals of indium selenide instead have excellent flexibility (see the Perspective by Han). The deformability comes from the compliant intralayer bonding between indium and selenium. The authors used these observations along with a previously discovered silver sulfide to determine a deformability factor for materials that may help find other deformable semiconductors. Science , this issue p. 542 ; see also p. 509

Flexible thermoelectrics is a synergy of flexible electronics and thermoelectric energy conversion. To date, state-of-the-art thermoelectrics is based on inorganic semiconductors that afford high electron mobility but lack in mechanical flexibility. By contrast, organic materials are amply flexible but low in electrical mobility and power output; the inorganic–organic hybrid design is a viable material-level option but has critical device-level issues for practical application. Here, we reported high intrinsic flexibility and state-of-the-art figures of merit (up to 0.44 at 300 K and 0.63 at 450 K) in Ag₂S-based inorganic materials, opening a new avenue of flexible thermoelectrics. In the flexible full-inorganic devices made of such Ag₂S-based materials, high electrical mobility yielded a normalized maximum power density up to 0.08 W m⁻¹ under a temperature difference of 20 K near room temperature, orders of magnitude higher than organic devices and organic–inorganic hybrid devices. These results promised an emerging paradigm and market of wearable thermoelectrics.

We present that earth-abundant and environmentally friendly SnS is a promising thermoelectric material due to its high ZT of 0.6 despite its relatively wide band gap of 1.2 eV. Ag doping significantly improved electrical conductivity but maintained the Seebeck coefficient above 400 μV K⁻¹ and the thermal conductivity below 0.45 W m⁻¹ K⁻¹ at 873 K.

This review summarizes the recent advances and future challenges for Cu₂Se-based thermoelectric materials.