Layered Germanium–Selenium Compounds as Phonon–Glass Electron–Crystals: A Pathway to Enhance the Thermoelectric Performance

The early concept of a "phonon-glass electron-crystal" for enhancing the thermoelectric figure of merit (<i>ZT</i>) is explored theoretically in layered Ge-Se crystals, where phonon transport exhibits glass-like behavior. <i>Ab initio</i> lattice dynamics and the rigid electronic band method project an ultrahigh <i>ZT</i> = 4.04 at 1000 K along the <i>a</i> axis in the high-temperature GeSe₂ phase at an electron doping concentration of 10²⁰ cm^-3. Meanwhile, the low-temperature Ge₄Se₉ phase achieves a high <i>ZT</i> = 2.19 at 600 K along the <i>a</i> axis with an electron doping concentration of 6 × 10¹⁹ cm^-3. These maximal values reflect the ultralow lattice thermal conductivity, 0.168 W m^-1 K^-1 (GeSe₂, 1000 K) and 0.289 W m^-1 K^-1 (Ge₄Se₉, 600 K), and high power factor at optimized carrier concentrations along the <i>a</i> axis. Our calculations indicate a promising pathway for approaching the early concept of maximizing <i>ZT</i>, by tailoring carrier doping in layered crystals with glass-like phononic transport.