Terminal alkyl substitution in an A-D-A-type nonfullerene acceptor: simultaneous improvements in the open-circuit voltage and short-circuit current for efficient indoor power generation

Two types of small molecule nonfullerene acceptors (IDICO1 and IDICO2) based on 2,2′-((2 Z ,2′ Z )-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2- b :5,6- b ′]dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1 H -indene-2,1-diylidene))dimalononitrile (IDIC) are synthesized by attaching octyl side-chains onto terminal end groups. The alkyl substitution increases the lowest unoccupied molecular orbitals (−3.81 to −3.86 eV) of the two acceptors, compared to that of IDIC (−3.94 eV). Interestingly, the IDICO1 and IDICO2 films have higher integrated absorption coefficients (1.49 × 10 7 cm −1 ) than the IDIC (1.29 × 10 7 cm −1 ) film. Also, the electron mobilities of IDICO1 and IDICO2 are approximately twice as high as that of IDIC. The terminal octyl substitution also improves the miscibility with a donor polymer (PBDB-T) to form well-intermixed blends with a decreased π-π stacking distance. As a result, their photovoltaic devices exhibit significant improvements in both the open-circuit voltage and short-circuit current density, compared to those of the reference PBDB-T:IDIC device, exhibiting maximum power conversion efficiencies of up to 9.64%, 20.4%, and 1.68% under 1-sun, 1000-lx LED, and halogen lamp illumination, respectively, which are significantly higher than those of PBDB-T:IDIC (7.2%, 11.7%, and 1.2%, respectively). It is worth noting that a maximum power density of 141.4 μW cm −2 is achieved for the PBDB-T:IDICO2-based device under a halogen lamp, which is the highest value reported to date among those achieved under indoor lighting conditions. Terminal alkyl substituents in IDIC nonfullerene acceptors improve light absorption, crystalline packing and charge mobility in films.