Thermoelectric modules can create voltage from a temperature difference because of the electronic properties of the semiconductor materials (thermoelectric elements).
A module has a hot side (contacting heat source) and a cold side (usually atmosphere).
The N and P type semiconductor materials work together to drive electron flow resulting from the thermal gradient between the cold and hot sides.
The thermoelectric couples are arranged thermally in parallel and electrically in series, terminating in a pair of leads.
The voltage across the leads depends on the module design. It is proportional to the Seebeck coefficient (S) and the temperature difference (ΔT). V = S (ΔT)