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Thermoelectric energy conversion is one of the most important and desirable functions of materials, because the ability to recycle a part of the energy wasted as heat back to useful electric energy significantly contributes to a sustainable society in future. For practical applications of thermoelectric materials, sufficiently high conversion efficiency is required over a wide range of temperature. It is also desirable that the materials are composed of non-toxic elements from an environmental perspective. In this paper, we report the successful control of the hole-type charge carrier density in GeTe-based materials by co-doping Bi and Cu, and the resultant improvement in the thermoelectric figure of merit over a wide range of temperature from room temperature to around 800 K, especially below 500 K, compared to those of previously reported analogous materials, thereby demonstrating the potential of GeTe-based materials for practical applications.