Record-high power factors in low-temperature polycrystalline Ge for flexible thermoelectric generators

Flexible thermoelectric devices offer unique advantages, including mechanical conformability and suitability for wearable and Internet of Things energy harvesting. However, their integration with low-cost polymer substrates requires the low-temperature synthesis of high-performance thermoelectric materials. In this study, impurity-doped polycrystalline Ge thin films were fabricated via solid-phase crystallization at low temperatures (<500 °C), and their microstructure and transport properties were systematically optimized by controlling the dopant concentration and deposition temperature. As a result, both P-doped n-type and Ga-doped p-type Ge films achieved record-high power factors of 3180 μW⸱m -1 ⸱K -2 and 1210 μW⸱m -1 ⸱K -2 , respectively, establishing the highest performance reported to date among polycrystalline, environmentally benign thermoelectric materials. The flexible devices demonstrated stable power generation, achieving maximum power densities of 0.70 mW⸱cm -2 in the cross-plane configuration, which represent the highest output characteristics among eco-friendly flexible thermoelectric systems. These results establish low-temperature solid-phase crystallization of doped Ge thin films as a promising route to next-generation flexible thermoelectric devices. • Low-temperature solid-phase crystallization enables high-quality polycrystalline Ge films. • Carrier transport was optimized by simultaneous control of dopant concentration and deposition temperature. • Record-high power factors were achieved among eco-friendly, low-temperature–processed thin films. • Flexible thermoelectric generators on flexible plastic substrates delivered high-power density. • Eco-friendly, group-IV–based Ge films offer a promising route for next-generation flexible TEGs.