Phenalene—A New Ring-Locked Vinyl Bridge for Nonfullerene Acceptors With Enhanced Chemical and Photochemical Stabilities

Hongtao Liu,Zhong’An Li,Chu-Chen Chueh,Cheng-Tien Hsieh,Yaxin He

doi:10.3389/femat.2022.851294

Abstract

Currently, the two exocyclic vinyl bridges in the acceptor–donor–acceptor (A–D–A)-type nonfullerene acceptors (NFAs) have been widely recognized as one of the most vulnerable sites under external stresses. Embedding the exocyclic vinyl bridges into an aromatic ring could be a feasible solution to stabilize them. Herein, we successfully develop a phenalene-locked vinyl bridge via a titanium tetrachloride—pyridine catalytic Knoevenagel condensation, to synthesize two new A–D–A-type unfused NFAs, EH-FPCN and O-CPCN, wherein malononitrile is used as the electron-deficient terminal group while fluorene and carbazole rings are used as the electron-rich cores, respectively. These two NFAs possess wide bandgaps associated with deep energy levels, and significantly enhanced chemical and photochemical stabilities compared to the analogue molecule O-CzCN with normal exocyclic vinyl bridges. When pairing with a narrow bandgap polymer donor PTB7-Th, the fabricated EH-FPCN- and O-CPCN-based organic solar cells achieved power conversion efficiencies of 0.91 and 1.62%, respectively. The higher efficiencies for O-CPCN is attributed to its better film morphology and higher electron mobility in the blend film. Overall, this work provides a new design strategy to stabilize the vulnerable vinyl bridges of A–D–A-type NFAs.

Highlights

As one of the third-generation photovoltaic techniques, organic solar cells (OSCs) have demonstrated outstanding potentials in the fields of flexible devices, semi-transparent devices, indoor photovoltaics, and so on (Liu et al, 2021c; Dauzon et al, 2021; Kini et al, 2021; Xie et al, 2021)
OTf-PCN reacted with fluorene and carbazole borate esters such as EH-Fl-Bpin and O-Cz-Bpin to produce EH-FPCN and O-CPCN via a Pd-catalyzed Suzuki coupling, respectively
The nonplanar molecular conformations were observed for both nonfullerene acceptors (NFAs) by density functional theory (DFT) calculations, which could negatively affect the ICT process

Summary

Introduction

As one of the third-generation photovoltaic techniques, organic solar cells (OSCs) have demonstrated outstanding potentials in the fields of flexible devices, semi-transparent devices, indoor photovoltaics, and so on (Liu et al, 2021c; Dauzon et al, 2021; Kini et al, 2021; Xie et al, 2021). Based on our ring-locked strategy, we successfully develop a feasible synthetic route to embed the exocyclic vinyl bridges into the phenalene unit as a conjugation bridge of A–D–A-type NFAs (Figure 1A).

Results

Conclusion