Synthesis, self-assembly, and solar cell performance of N-annulated perylene diimide non-fullerene acceptors

Abstract

The synthesis, characterization, and photovoltaic performance of a series of N-annulated PDI materials is presented. Four novel N-annulated PDI compounds are reported, each of which can be synthesized in gram scale without the need for purification using column chromatography. N-Annulation of the PDI chromophore results in a decrease in electron affinity and lowering of the ionization potential, and renders the chromophore insoluble in organic solvents. Installation of an alkyl group improves the solubility. Single crystal X-ray analysis reveals a bowing of the aromatic backbone and compression of phenyl rings adjacent to the N atom. A brominated N-annulated PDI derivate represents a valuable synthon for creating novel multi-PDI chromophore materials. To demonstrate the utility of the new synthon for making electron transporting materials, a dimerization strategy was employed to create a dimeric PDI material. The PDI dimer has excellent solubility and film forming ability along with energetically deep HOMO and LUMO energy levels. X-ray crystal structure analysis reveals that, despite the isotropic nature of the molecule, only 1-D charge transport pathways are formed. Solar cells based on the new PDI dimer with the standard donor polymer PTB7 gave a high power conversion efficiency of 2.21% for this system. Through N-alkyl chain modification this PCE was increased to 3.13%. Further increases in PCE to 5.54% and 7.55% were achieved by using the more advanced donor polymers PTB7-Th and P3TEA, respectively. The simple yet high performance devices coupled with the highly modular and scalable "acceptor" synthesis make fullerene-free organic solar cells an attractive and cost-effective clean energy technology.