The SSE is the conversion of heat (or a temperature gradient) into electricity like the charge-based thermoelectric effect, but it utilizes electron ‘spin’ instead of charge. The additional spin degree-of-freedom not only provides a larger parameter space for optimizing the device performance, but also enables new device design which could overcome the shortcomings of the existing thermoelectric devices. While a SSE device possesses the same advantages (no moving parts, long life time, high power density, etc.) with those conventional thermoelectric devices have, the use of electron spin and the resulting device geometry make it possible to separately tune the thermal conduction and electrical conduction in the device, which has been one of the biggest challenges in conventional thermoelectric devices. Also, the fact that the temperature gradient and the electrical output are perpendicular to each other makes it possible to use only one type of material, whereas conventional thermoelectric devices require two different types of materials (e.g. n- and p-type semiconductors) (Fig. 2).