Thermally stable direct arylation derived terthiophene-isoindigo copolymer for organic solar cell application

Abstract

The lifetime of organic solar cells (OSCs) remains a significant challenge for their commercial viability. In this study, we successfully synthesized a low band gap copolymer, P3T-IID, with an energy gap (E_g) of 1.61 eV, and HOMO and LUMO levels of −5.43 eV and −3.78 eV, respectively, using direct heteroarylation polycondensation (DAP) copolymerization of a terthiophene donor moiety with an isoindigo acceptor moiety. We investigate the stability of pristine P3T-IID copolymer films under conditions of 75% relative humidity, UV light illumination, and an elevated temperature of 85 °C for over 50 h. The results reveal significant absorbance loss due to UV-irradiation-induced processes, such as chain scission and chemical reactions involving free radicals, while the polymer integrity remains largely unaffected by high humidity and elevated temperatures. Interestingly, the copolymer demonstrates excellent thermal stability with a high decomposition temperature of 390 °C. Thermal stability studies on inverted OSCs fabricated with a 1 : 4 ratio of P3T-IID : PC₇₀BM show that the devices retained over 40% of their initial power conversion efficiency (PCE) after 24 h of annealing at 85 °C. Further investigation into the degradation mechanism reveals that the PCE loss is primarily due to the deteriorated morphology of the P3T-IID:PC₇₀BM active layer and the formation of an unwanted interlayer between the active layer and the electrodes.