Low-Temperature Synthesis of High-Power-Factor Mg2(Si,Ge,Sn) Layers for Flexible Thermoelectrics

Abstract Mg–IV compound semiconductors are promising candidates for environmentally compatible thermoelectric materials because their alloy systems allow the tuning of carrier transport and phonon scattering using abundant and low-toxicity elements. In this study, we systematically investigated low-temperature-synthesized Mg2(Si1–xGex)1–ySny thin films on glass and flexible plastic substrates. Binary Mg1–xSnx films were first examined to clarify the formation behavior of Mg2Sn-based thin films, revealing that a Sn fraction close to the stoichiometric composition of Mg2Sn is important for obtaining a uniform semiconducting phase while suppressing metallic Sn precipitation. By extending this composition design to multicomponent Mg2(Si1–xGex)1–ySny films, we found that the Sn fraction y primarily governs the trade-off between electrical conductivity and Seebeck coefficient, while the Si/Ge ratio has a relatively minor influence. As a result, a high power factor exceeding 800 μW m–1 K–2 was achieved near room temperature for Mg2Si0.02Ge0.05Sn0.93 after crystallization at annealing temperatures as low as 200 °C. Furthermore, Mg2Si0.02Ge0.05Sn0.93 films fabricated on a flexible polyimide substrate exhibited a power factor of approximately 700 μW m–1 K–2 while retaining mechanical flexibility. These findings demonstrate that composition and low-temperature crystallization control provide an effective strategy for developing environmentally compatible flexible thermoelectric thin films based on Mg–IV compound semiconductors.