Effects of Heteroatom Substitution on the Photovoltaic Performance of Donor Materials in Organic Solar Cells

Abstract

ConspectusOver the past few years, the innovation of narrow bandgap acceptor combined with wide bandgap donor materials significantly promotes the power conversion efficiencies (PCEs) of organic solar cells (OSCs) to exceed 18%. To build a state-of-the-art OSC, absorption spectra, frontier molecular orbital energy levels, molecular packing and crystallinity, and charge carrier mobilities of the photovoltaic materials should be considered in their molecular design. The donor and acceptor materials are the key components determining the photovoltaic performance of the OSCs. The side chain engineering on the conjugated backbone is a critical strategy to optimize the photovoltaic properties of the donor materials. In this Account, we focus on the topic of heteroatom substitution on the molecular backbone of the donor materials for improving their photovoltaic performance, aiming to provide in-depth understanding of the molecular structure optimization for the design of state-of-the-art photovoltaic materials.First, we highlight the halogen (fluorine and chlorine) atom substitution strategies applied on the conjugated molecular backbone of the organic photovoltaic materials. The strong electronegativity of halogen atoms can downshift the highest occupied molecular orbital (HOMO) energy levels of the donor materials, which could increase open-circuit voltages of the resulting OSCs. In addition, the hydrogen bonding aroused by the halogen atoms is beneficial to improve their charge transport property and crystallinity of the organic semiconductors. On the other hand, flexible side chains are critical components for improving the solubility of the photovoltaic materials. Using the flexible side chains of alkylthio and alkylsilyl is an easy and effective approach to tune the electronic energy levels and absorption spectra of the photovoltaic materials. Due to the formation of pπ(C)–dπ(S) orbital overlap, the empty 3d-orbitals of the sulfur atom in the alkylthio substituents can accept the π-electron of conjugated skeleton to modulate the optical and electrical properties of the photovoltaic materials. Similarly, the silicon atom in the alkylsilyl side chains can stabilize the lowest unoccupied molecular orbital (LUMO) level and downshift the HOMO level of the organic semiconductor, which leads to the improved photovoltaic performance of the organic donor materials. Finally, we briefly discussed the challenges for the photovoltaic materials toward performance optimization and practical application of the OSCs.