Optoelectronic properties and aggregation effects on the performance of planar versus contorted pyrene-cored perylenediimide dimers for organic solar cells

In this work, we present a new strategy to develop small molecules based on perylenediimde (PDI) with fused and contorted conjugated backbones as electron acceptors for fullerene-free organic solar cells. The two new electron acceptors, 27-Py-PDI and 49-Py-PDI, containing binary PDI units fused with pyrene at different positions were structurally isomers, which were investigated systematically. Theoretical calculations indicated that the two positional isomers exhibit distinct molecular geometries (planar for 27-Py-PDI vs contorted for 49-Py-PDI), which lead to huge differences on their synthetic methods, aggregation effects and their optoelectronic properties. The effects of structural isomerism on the molecular geometry, optical spectra, energy levels, charge carrier mobility and the morphology discrepancies as well as the corresponding photovoltaic performance were fully investigated. Temperature-dependent H-1 NMR and the film UV-vis spectroscopy were used to study the molecular aggregation behaviors. Calculations of nuclear independent chemical shifts (NICS) indicate significant difference of the aromaticity between the isomers. Blended with donor materials of PTB7-Th to fabricate the inverted solar cells, an encouraging PCE of 4.53% along with an impressive open-circuit voltage (V-OC) of 1.0 V (higher than the other acceptors based on PDIs and the PC71BM) were achieved by using 49-Py-PDI, which were superior to those of its isomer 27-Py-PDI (2.51%). The work suggests that introducing rigid and contorted features into fully fused acceptors based on PDI motifs can enhance the interface energy gap (Delta E-DA) extend the pi-delocalization and decrease the conformational disorder resulting into improved V-OC without sacrificing J(SC) in OPV devices. This design strategy by introducing rigid and steric hindrance to increase intermolecular strain to construct fully ring fused contorted acceptors is an effective approach for the development of novel NFAs.