Efficient Organic Photovoltaic Cells Based on Nanocrystalline Mixtures of Boron Subphthalocyanine Chloride and C60

The electrical and structural behavior of uniformly mixed films of boron subphthalocyanine chloride (SubPc) and C60 and their performance in organic photovoltaic cells is explored. Device performance shows a strong dependence on active-layer donoracceptor composition, and peak efficiency is realized at 80 wt.% C60. The origin of this C60-rich optimum composition is elucidated in terms of morphological changes in the active layer upon diluting SubPc with C60. While neat SubPc is found to be amorphous, mixed films containing 80 wt.% C60 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:C60 mixture, with peak mobility observed at a composition of 80 wt.% C60. Organic photovoltaic cells constructed using this optimum SubPc:C60 ratio realize a power conversion efficiency of (3.7 +/- 0.1)% under 100 mW cm-2 simulated AM1.5G solar illumination.