Why did early thermoelectric generators made with metallic conductors have very low efficiency?

Early thermoelectric generators were limited to an efficiency of approximately 0.5 percent because they relied exclusively on metallic conductors.1

The primary physical constraint is described by the Wiedemann-Franz law, which states that for most metals, the ratio of thermal conductivity to electrical conductivity is proportional to the absolute temperature. In a thermoelectric generator, high efficiency requires materials that possess both a high Seebeck coefficient and low thermal conductivity to maintain a temperature differential, while also maintaining high electrical conductivity to minimize ohmic heating losses.23

Because metals are inherently excellent thermal conductors, they dissipate heat too rapidly across the device, preventing the maintenance of the necessary temperature gradient. Furthermore, the Seebeck effect in junctions between different metals is relatively small compared to that achieved with semiconductor materials, which allow for a much more favorable balance of electrical and thermal properties.3