Exploring Novel Flat-Band Polymorphs of Single-Layered Germanium Sulfide for High-Efficiency Thermoelectric Applications

Improving the performance of thermoelectric materials for clean energy production via structural manipulation is considered as an effective route to advanced thermoelectrics. Here, we report the improved thermoelectric response of single-layered GeS (alpha-GeS) by developing its two new polymorphs, namely, gamma-GeS and epsilon-GeS. The two new polymorphs exhibited flat-band gap edges, which resulted in large Seebeck coefficients (thermopowers) due to the large density of states around the Fermi level. The favorable combination of large thermopowers and considerable electrical conductivities resulted in large values of thermoelectric power factors (PFs) and thermoelectric figure of merit (ZT). The room-temperature ZT values of gamma-GeS and epsilon-GeS exceeded a benchmark value of unity and approached as large as 1.67 (for gamma-GeS at 500 K) and 1.07 (for epsilon-GeS at 1000 K). These monolayers demonstrated large PFs corresponding to band gap edges that can be enhanced by low-level p-type doping. Moreover, their optimal PFs go through further enhancement with increase in temperature. The investigations of energetic stability revealed high thermodynamic stability of these monolayers, which suggest the feasibility of their experimental growth. These investigations were performed through the density functional theory and Boltzmann transport theory-based computational approaches. Our study suggests structural manipulation as an effective tool for improvement of the thermoelectric response of IV monochalcogenides for high-efficiency thermoelectric applications.