The state-of-the-art record of thermoelectric figure-of-merit (ZT) of 1.53 at 425 degrees C is achieved by chlorine doping in In4Se3-xCl0.03 bulk crystalline materials as used for n-type thermoelectric energy harvesting. The low-dimensional property imparted by chlorine doping significantly increases the electrical conductivity and reduces the thermal conductivity resulting in a high power factor over a wide temperature range.