Formulation engineering for optimizing ternary electron acceptors exemplified by isomeric PC71BM in planar perovskite solar cells

As the most prevalently used fullerene-based electron acceptor in organic–inorganic solar cells, [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) was isolated into three typical isomers of α-, β1- and β2-PC71BM with comparable molecular orbital energy levels for exemplifying a formulation engineering based on blending the three isomers to improve photovoltaic performance. The power conversion efficiency (PCE), photocurrent hysteresis and stability of planar heterojunction perovskite (CH3NH3PbI3) solar cells have been optimized by formulation engineering with mixed PC71BM (α:β1:β2 = 17:1:2), the specific mixture which represents the best electron acceptor superior to either each of the purified isomers or any other ternary isomers of PC71BM. Microscopic analyses support that molecular aggregation of the isomeric PC71BM was critical to influence the surface morphology and, in turn, the PCE in the range of 0.38–17.56% of the perovskite solar cells involved. This finding about isomer-dependent photovoltaic performance launches a heretofore unknown strategy of formulation engineering for making efficient electron acceptors by mixing various fullerene derivatives having isomeric structures or beyond.