Learning from Polymeric -Backbone to Film Sequences Unravels that a Coexistence of Bilayered and Mixed Morphologies Optimally Strengthens Thermoelectrics

pi-Conjugated polymers can be manipulated at the molecular and thin-film levels to strengthen thermoelectric performance. Herein, two distinctive polymers-PBDP-T and PBDB-T with respective outstanding Seebeck coefficient (S) and electrical conductivity (sigma) are selected and their functional units are integrated to synthesize random terpolymers via the ' backbone sequence ' strategy. The resulting disordered pi-backbones of terpolymers however lead to an unsuccessful acquisition of high power factor (PF). Alternately, the ' film sequence ' strategy is utilized and representative high-sigma P2F and high-S PTB7-Th polymers are blended. Note that 3D conductive paths are generated in the blend films when the PTB7-Th content falls between 35-80 wt%, which favors charge transportation and yields high sigma. At an optimal 65 wt% PTB7-Th, a maximum PF of 145.5 mu W m-1 K-2 is obtained, far surpassing that of each parent polymer, which marks a breakthrough of the ' two-phase combination ' rule. The peak PF achieved in medium PTB7-Th content is attributed to the coexistence of bilayered and mixed morphologies, the former of which delivers slowly declining sigma while the latter results in greatly elevated S with increasing PTB7-Th. Given the notably lower thermal conductivity acquired in blend than neat polymers, the room-temperature thermoelectric figure-of-merit as high as 0.15 is achieved. A coexistence of bilayered and mixed morphologies is unveiled to optimally strengthen polymeric thermoelectrics thanks to a concurrence of the slowly declining S and the significantly elevating sigma with the increase of high-sigma component in polymer blend films via the ' film sequence ' strategy.image