The abundance of low-temperature waste heat necessitates the development of reliable and scalable thermal-to-electric energy conversion technology. The thermoelectric device is one viable option. Commercially available Bi₂Te₃-based materials are optimized for near room temperature cooling applications. Currently there are no mass-produced materials available for 400 K to 650 K thermoelectric power generation. We report the successful realization of high performance n-type Bi₂(Te_1−xSe_x)₃-based materials for the temperature range of interest, using a commercial zone-melting technique. The introduction of Se effectively increases the band gap, which significantly suppresses the “turn-over” in Seebeck coefficient and the appearance of a pronounced bipolar effect, shifting the corresponding temperature of the optimum thermoelectric figure of merit ZT towards a higher temperature range. Furthermore, we demonstrate that the electron concentration of Bi₂(Te_0.5Se_0.5)₃ can be effectively adjusted by iodine doping. The samples with electron concentrations between 3 × 10¹⁹ and 4.5 × 10¹⁹ cm⁻³ display optimal thermoelectric performances. The highest ZT value reaches 0.86 at 600 K for the sample with the electron concentration of 4.0 × 10¹⁹ cm⁻³, whose average ZT between 400 K and 640 K is 0.8, making this scalable zone-melted low-Te content Bi₂(Te_0.5Se_0.5)₃ compound a promising candidate for low-temperature power generation.