How the Energetic Landscape in the Mixed Phase of Organic Bulk Heterojunction Solar Cells Evolves with Fullerene Content

Energy levels in the mixed polymer:fullerene phase of three-phase bulk heterojunction solar cells are significantly shifted from their values in the pure materials. These shifts provide an important driving force for separating charge carriers. Through cyclic voltammetry, we measure a gradual shift in the polymer HOMO and fullerene LUMO as a function of blend composition in a model bulk heterojunction (BHJ) system that only contains amorphous polymer. The effective band gap of the polymer:fullerene blend varies by up to 300 meV with varying blend composition. The shifts in polymer HOMO and fullerene LUMO can be quantitatively accounted for by the electrostatic potential generated by induced dipoles at the polymer:fullerene interfaces. Remarkably, however, the measured charge transfer state energy and open-circuit voltage shift far less.