Unveiling the thermal stability of diketopyrrolopyrrole-based terpolymers: a key element for enhanced efficiency and stability of organic solar cells

Abstract

With the advent of novel polymers, organic solar cell (OSC) research has evolved significantly over the past decade. The molecular engineering of terpolymers has allowed for simple morphological control in binary devices over ternary blends, with the highest power conversion efficiencies (PCEs) exceeding 18%. However, research on the stability of OSCs is still lagging behind. In this regard, we examined the thermal stability of a series of terpolymers comprising one electron donor (thienyl-substituted benzodithiophene, BDTT) and two types of electron acceptors namely fluorobenzotriazole (FTAZ) and thienothiophene-capped diketopyrrolopyrrole (TTDPP) and their blends with PC₇₁BM. The terpolymers demonstrated broad absorbance ranging from below 350 nm to 900 nm. The thermal stability of the terpolymers was investigated as pristine thin films and as bulk heterojunction (BHJ) films of the terpolymers blended with PC₇₁BM by heating at 85 °C. We observed that thermal degradation had no sizeable effect on the properties of the pristine terpolymers while the blended films demonstrated significant changes in their morphology due to the inclusion and aggregation of PC₇₁BM. After thermal annealing at 85 °C, the width of the symmetric CC stretching Raman mode and the CC/C–C intensity ratio of pristine terpolymers and terpolymer:PC₇₁BM thin films revealed that incorporation of the FTAZ acceptor improves the thermal stability of the BHJ active layers. Furthermore, prolonged thermal annealing times (>3 hours) resulted in the development of PC₇₁BM aggregates and terpolymer decomposition with no evident changes in the molecular and chemical structure of the terpolymers. Our findings indicate that by gradually annealing the blended films using an appropriate annealing time, the diffusion of PC₇₁BM molecules to form aggregates can be carefully regulated, resulting in a nanostructure critical to the efficiency of organic solar cells.