Comparative Study of Thermal Stability, Morphology, 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 (PBDTTTPD), which provided comparable peak power conversion efficiencies (PCEs) of >6%. We observed a remarkable contrast in thermal stability dependent upon the acceptor composition in the active layer, with the performance of the fullerene-PSCs completely deteriorating 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 active layer of 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. To develop a comprehensive understanding of the role of polymer acceptor on the thermal stability of devices, the morphology of ternary blend active layers composed of PBDTTTPD:polymer acceptor:fullerene acceptor with different fullerene contents was examined while annealing at 150 °C. The ternary blends showed two extreme trends of all-PSC- and fullerene-PSC-like behavior in thermal stability depending on the PCBM content. When included in the active layer as <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 without fullerene (6.67%) and polymer–fullerene active layers without a polymeric acceptor (6.12%).