Benzodithiophene bridged dimeric perylene diimide amphiphiles as efficient solution-processed non-fullerene small molecules

Abstract

Two amphiphilic and highly twisting perylene diimide (PDI) dimers, Bis-PDI-BDT-EG, were synthesized by using 4,8-bis(2-(2-ethylhexylthienyl) benzo[1,2-b:4,5-b′]dithiophene (BDT-T) and 4,8-bis(2-ethylhexyloxy) BDT (BDT-O) as covalent bridges at the 7,7′-positions, while at the 1,1′-positions, they were functionalized with weakly solvophobic 2-methoxylethoxyl (EG) units. The subtle structural differences between BDT-O and BDT-T lead to distinct aggregation abilities: with respect to the over-strong aggregation ability of the BDT-O bridged dimer 2, the BDT-T bridged dimer 1 shows largely reduced aggregation ability and is solution-processable in the commonly used organic solvent. The highly twisted conformation between the PDI–BDT–PDI planes produced steric-pairing effects, which directed ordered packing of dimer 1. When dimer 1 was blended with P3HT in a weight D/A ratio of 1 : 2.5, the electron mobility (μ_e) was 3.4 × 10⁻⁵ cm² V⁻¹ s⁻¹ and the best PCE was 1.72%. Slowing the solvent evaporation speed benefited the packing order of the PDI dimer, and the μ_e value was slightly increased to 6.0 × 10⁻⁵ cm² V⁻¹ s⁻¹. The best PCE was improved up to 1.87%. The μ_e was further increased up to 3.4 × 10⁻⁴ cm² V⁻¹ s⁻¹ when the D/A ratio was decreased down to 1 : 2.2 and the best PCE of 1.95% was achieved. Solid absorption spectra and XRD data of the blended films supported the improvement of the packing order of the PDI dimer by slowing the solvent annealing speed. AFM images supported the largely reduced aggregation ability of dimer 1 when blended with P3HT. The observed phase size of 35 nm is formed under the slow solvent annealing speed and a D/A ratio of 1 : 2.2. Our results revealed that the amphiphilic nature of the bridged aromatic unit reduces the aggregation ability and facilitates the ordered packing of the PDI units, contributing to the improvement of efficiency.