Correcting Hybrid Density Functionals to Model Y6 and Other Nonfullerene Acceptors

Recently developed fused-ring electron acceptors such as Y6 (BTP-4F) have strong oscillator strength, good charge-carrier transport, and a small band gap. They therefore have enormous current technical applications to organic optoelectronics, such as solar cells. To design new materials, it would be useful to predict the electronic structure accurately. Because of the large number of atoms involved in representative aggregates of these materials, we need an efficient electronic structure method. Standard density functional theory poorly describes charge-transfer states and was typically parametrized for vacuum calculations of individual molecules. In this work, we tune a range-separated hybrid functional for Y6 and characterize representative dimers extracted from the solid state. We demonstrate that the extensive solvatochromic effects of Y6 are due, in part, to oscillator strength borrowing between the charge transfer and Frenkel excitons. We provide an explanation for the short, optimally tuned range-separation parameter, based on the Penn model for the frequency-dependent dielectric of a semiconductor. We caution that nontuned range-separated hybrids are less accurate than global hybrids for these, and similar, materials. We show how reducing the range-separation length improves the accuracy of standard range-separation functionals without an involved tuning process.