Efficient organic photovoltaic cells based on nanocrystalline mixtures of boron subphthalocyanine chloride and C ₆₀

The electrical and structural behavior of uniformly mixed films of boron subphthalocyanine chloride (SubPc) and C ₆₀ and their performance in organic photovoltaic cells is explored. Device performance shows a strong dependence on active-layer donor-acceptor composition, and peak efficiency is realized at 80 wt.% C ₆₀. The origin of this C ₆₀-rich optimum composition is elucidated in terms of morphological changes in the active layer upon diluting SubPc with C ₆₀. While neat SubPc is found to be amorphous, mixed films containing 80 wt.% C ₆₀ show clear nanocrystalline domains of SubPc. Supporting electrical characterization indicates that this change in morphology coincides with an increase in the hole mobility of the SubPc:C ₆₀ mixture, with peak mobility observed at a composition of 80 wt.% C ₆₀. Organic photovoltaic cells constructed using this optimum SubPc:C ₆₀ ratio realize a power conversion efficiency of (3.7 ± 0.1)% under 100 mW cm ^-2 simulated AM1.5G solar illumination.