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 1C. 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 1C, the width of
the symmetric CQC stretching Raman mode and the CQC/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 (43 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.
