Thermoelectrics can generate electrical power from waste heat and could make an important contribution to sustainable energy production if their efficiency is improved. Engineering efficient thermoelectrics, however, requires a sophisticated understanding of the fundamental interdependencies between electrical and thermal transport, for which improvements in our understanding of how charge carriers are coupled to lattice vibrations (phonons) is needed. Using advanced ultrafast electron-based techniques, the Siwick group probes the fundamental nature of electron–phonon coupling in SnSe, one of the highest performing thermoelectric materials, in order to unravel the origins of its thermoelectric efficiency. The Siwick group shows that the charge carriers in SnSe interact strongly with lattice waves to form moving lattice distortions called polarons, which helps to explain some of SnSe’s most unusual properties. This exciting new result was published in PNAS.