Morphological stability and performance of polymer-fullerene solar cells under thermal stress: The impact of photoinduced PC₆₀BM oligomerization

We report a general light processing strategy for organic solar cells (OSC) that exploits the propensity of the fullerene derivative PC₆₀BM to photo-oligomerize, which is capable of both stabilizing the polymer:PC ₆₀BM active layer morphology and enhancing the device stability under thermal annealing. The observations hold for blends of PC₆₀BM with an array of benchmark donor polymer systems, including P3HT, DPP-TT-T, PTB7, and PCDTBT. The morphology and kinetics of the thermally induced PC₆₀BM crystallization within the blend films are investigated as a function of substrate and temperature. PC₆₀BM nucleation rates on SiOx substrates exhibit a pronounced peak profile with temperature, whose maximum is polymer and blend-composition dependent. Modest illumination (<10 mW/cm²) significantly suppresses nucleation, which is quantified as function of dose, but does not affect crystalline shape or growth, in the micrometer range. On PEDOT:PSS substrates, thermally induced PC₆₀BM aggregation is observed on smaller (≈100 nm) length scales, depending upon donor polymer, and also suppressed by light exposure. The concurrent thermal dissociation process of PC₆₀BM oligomers in blend films is also investigated and the activation energy of the fullerene-fullerene bond is estimated to be 0.96 ± 0.04 eV. Following light processing, the thermal stability, and thus lifetime, of PCDTBT:PC₆₀BM devices increases for annealing times up to 150 h. In contrast, PCDTBT:PC₇₀BM OSCs are found to be largely light insensitive. The results are rationalized in terms of the suppression of PC₆₀BM micro-and nanoscopic crystallization processes upon thermal annealing caused by photoinduced PC₆₀BM oligomerization.