Comparative Study of Thermal Stability and Performance of All-Polymer, Fullerene−Polymer, and Ternary Blend Solar Cells Based on the Same Polymer Donor

We compared the thermal and morphological stability of all-polymer solar cells (all-PSCs) and fullerene-based PSCs (fullerene-PSCs) having the same polymer donor with comparable power conversion efficiencies (PCEs) of >6%. We observed that the performance of the fullerene-PSCs completely deteriorated after annealing for 5 h at 150 °C, whereas that of the all-PSCs remained stable even after annealing for 50 h at 150 °C. Pronounced phase separation was observed in the fullerene-PSCs at two different length scales. In stark contrast, almost no morphological changes in the all-PSCs were observed, likely due to the low diffusion kinetics of the polymer acceptors. Next, we evaluated the thermal stability of ternary blends composed of PBDTTTPD:polymer acceptor:fullerene acceptor with different fullerene contents to understand the role of polymer acceptor on the morphological changes. When fullerene contents <30 wt % of the acceptor mixture, fullerene was well-dispersed in the amorphous portion of the donor/acceptor polymer blend under thermal stress and led to thermally stable devices with a higher PCE (7.12%) than both all-PSCs and fullerene-PSCs.