Non‐Fullerene Acceptors with an Extended π‐Conjugated Core: Third Components in Ternary Blends for High‐Efficiency, Post‐Treatment‐Free Organic Solar Cells

Abstract

The synthesis of four non-fullerene acceptors (NFAs) with a "Aπ-D-π-A" structure, in which the electron-donating core is extended, was achieved. The molecules differed by the nature of the solubilizing groups on the π-spacer and/or the presence of fluorine atoms on the peripheral electron-accepting units. The optoelectronic properties of the molecules were characterized in solution, in thin film, and in photovoltaic devices. The nature of the solubilizing groups had a minor influence on the optoelectronic properties but affected the organization in the solid state. On the other hand, the fluorine atoms influenced the optoelectronics properties and increased the photo-stability of the molecules in thin films. Compared to reference ITIC, the extended molecules showed a wider absorption across the visible range and higher lowest unoccupied molecular orbital energy levels. The photovoltaic performances of the four NFAs were assessed in binary blends using PM6 (PBDB-T-2F) as the donating polymer and in ternary blends with ITIC-4F. Solar cells (active area 0.27 cm 2 ) showed power conversion efficiencies of up to 11.1 % when ternary blends were processed from nonhalogenated solvents, without any thermal post-treatment or use of halogenated additives, making this process compatible with industrial requirements.

Effects of Halogenation on the Benzotriazole Unit of Non-Fullerene Acceptors in Organic Solar Cells with High Voltages

“…In addition, compared with the C–C bond, due to the higher dipole moment between the halogen atom and the C atom, it can enhance the intramolecular charge transfer and induce more red-shifted absorption and ordered molecular packing. − For the molecules with A–D–A and A–DA′D–A structures, the halogenation on the terminal IC group has achieved remarkable results. For example, since Zhan et al reported a non-fullerene small molecule acceptor ITIC ,, in 2015, the halogenated molecules based on the A–D–A structure have made great progress. − In 2018, Hou et al introduced different numbers of chlorine substituents on the end group and obtained the molecules IT-2Cl and IT-4Cl. The large dipole moment between the C–Cl bond and the strong electron-withdrawing ability of Cl atoms led to the red-shifted absorption and decreased LUMO levels.…”

Star-shape non-fullerene acceptor featuring an aza-triangulene core for organic solar cells

“…60 AzaTk contains fluorinated end-groups as previously observed on other NFA molecules BITIC-C 8 F 4 and BITIC-PhC 6 F 4 , and the presence of fluorine substituents seems to have a positive effect on the intrinsic stability of the NFAs. 8…”

“…4 Since then, considerable efforts have inspired new research directions in semiconductors synthesis, 5 device engineering, 6,7 and tandem or ternary approach. 8,9 Swapping fullerenes for other molecules in bulk-heterojunction solar cells has been a challenge. In recent years, the rise of non-fullerene acceptors (NFAs) resulted in quick improvements in device efficiency and opened new perspectives for the launch of this technology in building integrated photovoltaics.…”

Review: materials and modelling for organic photovoltaic devices

“…For example, tertiary blends of fluorinated ITIC (ITIC-4F) and new extended NFAs with acceptor-(π-conjugated)-donor-(π-conjugated)-acceptor structures with PM6 have delivered 11.1% efficiencies. 94 Another recently advanced molecule is Y6, also shown in Fig. 16, which when associated with PM6 brought about an efficiency greater than 16% in 2019.…”

“…Other work has also shown that these systems are adaptable to nonchlorinated solvents making them more accessible for industrial processing. For example, tertiary blends of fluorinated ITIC (ITIC‐4F) and new extended NFAs with acceptor–( π ‐conjugated)–donor–( π ‐conjugated)–acceptor structures with PM6 have delivered 11.1% efficiencies 94 …”