摘要：SummaryFor over half a century, the development of thermoelectric materials has based on the dimensionless figure of meritzT, assuming that the efficiency is mainly determined by this single parameter. Here, we show that the thermoelectric conversion efficiency is determined bythreeindependent parameters,Zgen,τ, andβ, which we call the three thermoelectric degrees of freedom (DoFs).Zgenis the well-definedmeanof the traditionalzTunder nonzero temperature differences. The two additional parametersτandβare gradients of material properties and crucial to evaluating the heat current altered by nonzero Thomson heat and asymmetric Joule heat escape. Each parameter is a figure of merit. Therefore, increasing one of the three DoFs leads to higher efficiency. Our finding explains why the single-parameter theory is inaccurate. Further, it suggests an alternative direction in material discovery and device design in thermoelectrics, such as highτandβ, beyondzT.Graphical abstractDisplay OmittedHighlights•General efficiency theory of thermoelectric conversion is derived forT-dependent properties•Three degrees of freedom Zgen, τ, and β determine thermoelectric efficiency•Efficiency can vary up to approximately 40% with τ and β•Each degree of freedom is a figure of meritThermal engineering; Electrical property; Thermal property; Electrical materials
