Correcting hybrid density functionals to model Y6 and other non-fullerene acceptors

Abstract

Recently developed fused-ring organic electron-acceptors such as Y6 have strong oscillator strength, good charge-carrier transport and low bandgaps. They therefore have enormous current technical application to optoelectronic devices, such as solar cells. Due to the large number of atoms involved in representative aggregates of these materials, we need an efficient electronic structure method to model them. Standard density functional theory poorly describe charge-transfer states, and were developed for vacuum calculations of individual molecules. In this work we tune a range-separated hybrid functional for Y6. We characterise representative dimers of the solid-state and show that Y6 dimers show the extensive solvatochromic effects are due, in part, to oscillator strength borrowing. We provide an explanation for the short optimally-tuned range-separation parameter, based in the Penn model for the frequency dependent dielectric of a semiconductor. We caution that standard 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 functionals, without an involved tuning process.

Figures (21)

• Figure 1: Jablonski diagram of the D2 dimer (example of J aggregate), D4 dimer (example of H aggregate) and monomer. Triplet states are in orange and the Singlet states are in blue.
• Figure 2: Comparison of the energies ($E$) and extent of charge transfer ($\omega_{CT}$) given by OT-SRSH, B3LYP, PBE0 and CAM-B3LYP-tuned (CAM-B3LYP with a changed range-separated parameter).
• Figure S1: The singlet and triplet excited states of the D2 dimer using CAMB3LYP functional vary with basis set.
• Figure S2: The properties of the singlet and triplet states of the D2 dimer depend on the dielectric constant $\epsilon_r$ used in the tuning of the OT-SRSH and in the PCM.
• Figure S3: The energies ($E$) and the extent of charge transfer ($\omega_{CT}$) for the different singlet and triplet excite states of the D1 dimer changes with functional in PCM with $\epsilon_r=6$. The black dotted line shows the results of the GW/BSE/MM calculations from the paper by Giannini et alakram2025analyzing.