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

Abstract

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.