Exploring the Effects of Fluorination at the Central Unit of Y6-Type Nonfullerene Acceptors on Photovoltaic Properties: A Computational Investigation.

Abstract

The strategy of modification of central units of Y6-type nonfullerene acceptors (NFAs) with halogenation has become popular for designing new photovoltaic materials and has shown dramatic effects in improving photovoltaic properties. However, the underlying mechanism of how halogenation of central units of these NFAs influences photoelectric properties remains rather elusive. In this paper, focusing on two reported promising NFAs Qx-1 and Qx-2, with varying degrees of ring fusion at central units, we designed 4 new NFAs and modeled 10 NFAs systematically through fluorination at the central units. Using density functional theory (DFT) and time-dependent DFT calculations, we explore the impact of an altered fluorinated location at the central units of Qx-1 and Qx-2 on the photoelectric properties. The molecular planarity, dipole moment, electrostatic potential (ESP) and its fluctuation, exciton binding energy (E_b), singlet-triplet energy gap, and absorption spectrum are obtained with combinations of traditional hybrid or long-range corrected density functionals and Pople basis sets. We also developed a new numerical method to analyze the fluctuation of ESP quantitatively because recent reports discussed its importance. The computed data suggest that newly designed Qx2-bf and Qx2-cf are promising NFAs because they exhibit enhanced planarity, lower E_b (by at least 0.002 eV), and higher averaged ESP (by at least 0.247 kcal/mol) compared to Qx-2. We also find that fluorination of the core units reduces E_b noticeably, increases the ESP standard deviation, and raises the average ESP except for ortho (outside) substitutions. These findings offer valuable physical insights into the effects of core fluorination, which can serve as a guide for the rational design of high-performance QX-based NFAs.