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

TL;DR

This work tunes a range-separated hybrid functional for Y6, a non-fullerene acceptor, to improve electronic structure predictions relevant for organic optoelectronics, highlighting the importance of proper parameter tuning.

Contribution

The paper introduces a method to tune a range-separated hybrid functional specifically for Y6, enhancing the accuracy of electronic structure calculations for non-fullerene acceptors.

Findings

Tuned a range-separated hybrid functional for Y6.

Extensive solvatochromic effects are partly due to oscillator strength borrowing.

Reducing the range-separation length improves accuracy without complex tuning.

Abstract

Recently developed fused-ring electron-acceptors such as Y6 (BTP-4F) have strong oscillator strength, good charge-carrier transport and a small bandgap. They therefore have enormous current technical application to organic optoelectronics, such as solar cells. To design new materials, it would be useful to predict the electronic structure accurately. Due to 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 were typically parameterised for vacuum calculations of individual molecules. In this work we tune a range-separated hybrid functional for Y6, and characterise 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 non-tuned 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.