N-Annulated perylene diimide dimers: acetylene linkers as a strategy for controlling structural conformation and the impact on physical, electronic, optical and photovoltaic properties

Abstract

The geometry of organic π-conjugated small molecules can impact the morphology of blended-thin films and subsequent performance in opto-electronic devices. In this report, we investigate the role of molecular conformation of perylene diimide (PDI) dimers designed to act as non-fullerene acceptors in organic solar cells. A series of three PDI dimers is presented in which the PDI chromophores are directly linked via the bay position (PDI₂, 3) or separated by one (PDI₂Ac, 4) or two (PDI₂Ac₂, 5) acetylene spacers. In all cases, the exo-position of the PDI dimers is N-annulated. New compounds 4 and 5 were synthesized via an optimized and facile synthetic pathway. Directly linked PDI dimers adopted a highly twisted conformation whereas adding two acetylene spacers rendered the PDI chromophores coplanar. ¹H NMR spectroscopic analysis of each dimer revealed a highly sensitive electronic structure that is strongly influenced by the acetylene spacers. It was found that compounds 4 and 5 with less twisted structures exhibited similar electron affinities but lower ionization potentials, lower organic solvent solubility, and red-shifted optical absorption spectra when compared to the highly twisted dimer 3. In addition, 4 and 5 showed a stronger tendency to aggregate in both solution and the solid state. This had a large impact on the performance of organic solar cells using these materials as electron acceptors. Bulk-heterojunction solar cells based upon a PTB7-Th:3 active layer could reach high power conversion efficiencies of 5.23%. In contrast, PTB7-Th:4 and PTB7-Th:5 based devices had ∼5 times lower performance owing to the formation of unfavourable active layer morphologies.