Designing small molecules as ternary energy-cascade additives for polymer:fullerene solar cell blends

Ternary blends comprising an ‘energy-cascade former’ in addition to the donor and the acceptor materials increasingly attract attention in the organic solar cell area as they seem to provide a tool to positively manipulate the open-circuit voltage of bulk-heterojunction devices. By comparing two additives that have similar HOMO/LUMO levels and that can be expected to lead to an energy cascade in ternaries with the prototypical P3HT:PC60BM system, we demonstrate here that the compatibility of the additive with, in this specific case, the fullerene, that can be tailored by peripheral chemical functionalization, plays a critical role in energy cascade formation. A compromise needs to be found between good mixing (favoring energy cascade formation) and phase separation (supporting charge extraction) that affect the open-circuit voltage in an as important fashion as their electronic features, providing critical insights for future materials design activities.