Selective Charge Carrier Transport and Bipolar Conduction in an Inorganic/Organic Bulk-Phase Composite: Optimization for Low-Temperature Thermoelectric Performance

Abundant conducting polymers are promising organic substances for low-temperature thermoelectric applications due to their inherently low thermal conductivities. By introducing a conducting polymer filler (PEDOT:PSS─poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid)) into a representative inorganic thermoelectric matrix (Bi2Te3), a bulk-phase composite (i.e., inorganic matrix/organic filler) for low-temperature thermoelectric applications is proposed. This composite hosts an interfacial energy barrier between the inorganic and organic components, facilitating controlled carrier transport based on its energy level, known as the energy filtering effect, and thus the composite exhibits a highly improved Seebeck coefficient compared to pristine Bi2Te3. The composite also displays a completely different temperature dependence on the Seebeck coefficient from Bi2Te3 due to its distinct bipolar conduction tendency. By regulation of the energy filtering effect and bipolar conduction tendency, the composite undergoes noticeable variations in the Seebeck coefficient, resulting in a significantly enhanced power factor. Furthermore, the composite shows a substantially reduced thermal conductivity compared to Bi2Te3 because it has lower carrier/lattice thermal contributions, possibly attributed to its high carrier/phonon scattering probabilities. Owing to the superior power factor and reduced thermal conductivity, the composite exhibits markedly enhanced thermoelectric performance, achieving a maximum figure of merit of approximately 1.26 at 380 K and an average figure of merit of approximately 1.23 in the temperature range of 323–423 K. The performance of the composite is competitive with previously reported n-type Bi2Te3 binary or ternary analogues. Therefore, the composite is highly expected to be a promising n-type counterpart of p-type Bi2Te3-based alloys for various low-temperature thermoelectric applications.