Thermal stability of low-bandgap copolymers PTB7 and PTB7-Th and their bulk heterojunction composites

Abstract

The impact of PC61BM and PC71BM on the thermal stability of two low-bandgap conjugated polymers widely used in organic photovoltaic applications, namely PTB7 and PTB7-Th (also known as PBDTTT-EFT), was studied under nitrogen and air atmospheres. The thermal stability effect observed is mainly additive, i.e., the stability of the composites is similar to the weighted average of the stabilities of the individual components. The thiophene-substituted polymer (PTB7-Th) displays higher thermal stability, both in nitrogen and air, than its ether-substituted analogue (PTB7). The corresponding bulk heterojunction composites display the same order of relative thermal stabilities, i.e., PTB7-Th:PC71BM (1:1.5) > PTB7:PC71BM (1:1.5), which interestingly is also the order of lifetime stabilities reported in the literature for the corresponding OPV devices. These results suggest that simple and rapid thermogravimetry tests might be used as a valid rapid screening test to infer on the relative lifetime stability of OPV devices.