P- and N-dopable ambipolar bulk heterojunction thermoelectrics based on ladder-type conjugated polymers

Ambipolar bulk heterojunction polymer thermoelectrics were demonstrated for the first time using a classic n-type poly(benzimidazobenzophenanthrolinedione) ladder polymer (BBL) and a new p-type 6H-pyrrolo[3,2-b:4,5-b′]bis[1,4]benzothiazine ladder polymer (PBBTL). The blend films were solution-processed and can be p-/n-doped using a FeCl₃/benzyl viologen radical cation (BV˙⁺). Well-balanced p- and n-doped conductivities can be achieved at a 1 : 1 blend ratio, and a full thermoelectric device consisting of a pair of p- and n-channel legs from a single layer blend film was fabricated. The full device exhibited a power density of 0.27 μW cm⁻² at ΔT = 3 K, operating at a voltage of 0.93 mV and a current density of 0.29 mA cm⁻². The simplification of the device fabrication process using such ambipolar bulk heterojunction blend films may help the realization of polymer micro-thermoelectric devices in the future.