Synergistically optimizing thermoelectric power factor and stability in GeI2-doped CsSnI3 thin film

All-inorganic CsSnI3 is a promising thermoelectric (TE) material featuring “phonon glass-electron crystal” (PGEC) behavior, yet its solution-processed thin films suffer from low electrical conductivity (σ) and poor environmental stability. In this work, we systematically study, by incorporating GeI2 into CsSnI3 thin films, the mechanism of synergistically enhancing the TE performance and stability. Tetragonal CsSn1−xGexI3 (x = 0–4 mol. %, x is the nominal component) films are fabricated via spin-coating and a two-step annealing process. Importantly, Ge incorporation synergistically improves the carrier mobility (μH) and hole concentration by passivating Sn vacancies (reducing scattering) and providing hole compensation via preferential oxidation of Ge. As a result, a maximum power factor (PF) of 112.9 μW m−1 K−2 is achieved at 1 mol. % GeI2 doping, which is 70% higher than that of the undoped sample. Meanwhile, the incorporated Ge ions regulate micro-morphology and act as sacrificial agents via their preferential oxidation, during which a more dense and stable surface forms, protecting the oxidation of Sn2+ and inhabiting H2O invasion. Furthermore, Ge doping increases the tolerance factor (t). These combined effects stabilize the black perovskite phase against its detrimental transition to the yellow non-perovskite phase. This work offers a straightforward and effective route to synergistically boost both the stability and TE PF of all-inorganic Sn-based perovskite films.