The effects of dual doping on the thermoelectric properties of Ca3−xMxCo4−yCuyO9 (M = Na, La)

► The effects of dual doping on the thermoelectric properties of Ca3−xMxCo3.8Cu0.2O9 (M = Na, La) have been investigated. ► Dual doping by Cu and La results in large increase in electric conductivity and small decrease in Seebeck coefficient. ► Dual doping by Cu and Na increase Seebeck coefficient and decrease electric conductivity slightly. ► Both types of doping reduce thermal conductivity, with substantial reduction observed in dual doping of Cu and La. ► Thermoelectric figure of merit is increased by dual doping in general, with 78.07% enhancement in Ca2.8La0.2Co3.8Cu0.2O9.