Mixing of Non-Fullerene Acceptors Ionization Energies: An Additional Tool to Tune the Quantum Efficiency of Ternary Organic Solar Cells

Abstract

SummaryNon-fullerene acceptor (NFA)-based ternary bulk heterojunction solar cells (TSC) are the most efficient organic solar cells (OSCs) today due to their broader absorption, and quantum efficiencies (QE) often surpassing those of corresponding binary blends. We study how the energetics driving charge transfer at the electron donor:electron acceptor (D/A) interfaces impact the QE in blends of a donor (PBDB-T-2F) with several pairs of lower bandgap NFAs. Similar to binary blends, the ionization energy offset between donor and acceptor (ΔIE) controls the QE, and maximizes for ΔIE > 0.5 eV. However, ΔIE is not controlled by the individual NFAs IEs but by their average, weighted for their blending ratio. Using this property, we improved the QE of a PBDB-T-2F:IEICO binary blend, having a ΔIE insufficient for charge generation, by adding a deep IE third component: IT-4F. Combining two NFAs enables to optimize the D/A energy alignment and cells QE without molecular engineering.

Keywords: Organic Photovoltaics, ternary solar cells, quantum efficiency, driving energy, solar cell design rules, ionization energy offset, charge transfer, mixing of energy levels, non-fullerene acceptors, transient absorption spectroscopy.