Band convergence brings distinct extrema of an electronic band structure close enough in energy that several valleys contribute to charge carrier transport at the operating temperature. Increasing the effective valley degeneracy can improve the balance between Seebeck coefficient and electrical conductivity, rather than merely increasing the number of carriers in one valley. Composition and semiconductor doping can tune this energy separation. A primary example is the lead telluride-selenide alloy study that combined measured transport with a multiband model.[ref]
Convergence of electronic bands for high performance bulk thermoelectrics by Yanzhong Pei and others. Abstract and pp. 66–69: composition/doping control and convergence of valence-band valleys
The thermoelectric power factor is not determined by valley degeneracy alone. Additional inter-valley scattering can reduce charge carrier mobility, so a larger number of populated valleys need not improve transport. The spacing of the valleys in both energy and wave vector, and the relevant electron-phonon interactions, matter.[ref]
https://www.nature.com/articles/s41467-021-23839-w. When band convergence is not beneficial for thermoelectrics, abstract and discussion of inter-valley scattering
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