Tunable polaronic transport and thermal conductivity in K + doped 0D metal halide perovskite Cs3Bi2I9

In this study, we explore the polaronic charge transport dynamics in zero-dimensional Cs3Bi2I9 perovskites, emphasizing the systematic transition from overlapping large polaron tunneling to non-overlapping small polaron tunneling induced by K+ doping. Employing solid-state impedance spectroscopy and thermal conductivity analyses, we unveil that alkali metal substitution enhances electron–phonon coupling, fostering small polaron formation and shifting the conduction mechanism toward thermally activated hopping. This transition is driven by increased polaron binding energy, higher activation energy, and larger polaron effective mass. The transition from large to small polaron results in a systematic decrease in lattice thermal conductivity, due to localized polarons serving as effective phonon scattering centers. The findings elucidate the role of polaron in regulating thermal and electrical behavior. This work charts a path for polaron engineering as a control parameter to balance carrier mobility and thermal resistance in perovskite-based energy materials.