Performance of asymmetric non-fullerene acceptors containing the 4,4,9,9-tetramethyl-4,9-dihydroselenopheno[2',3':5,6]-s-indaceno[1,2-b]thiophene core

Abstract

Through changing the terminated groups, three asymmetric non-fullerene acceptors containing a 4,4,9,9-tetramethyl-4,9-dihydroselenopheno[2',3':5,6]-s-indaceno[1,2-b]thiophene fused-ring core were designed and synthesized. The resultant acceptor-donor-acceptor (A-D-A) type of small molecules all show broad absorption spectra with tunable molecular energy levels and distinct molar extinction coefficients in solutions. The 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile terminated molecule (T-Se), has the smallest molar extinction coefficient of 1.83 × 105 M−1cm−1 with a bandgap of 1.62 eV, and a high-lying lowest unoccupied molecular orbital (LUMO) energy level (−3.83 eV). For the fluorinated molecule T-Se-4F, its molar absorption coefficient is enhanced to 2.05 × 105 M−1 cm−1, and its bandgap is narrowed to 1.58 eV, while its LUMO energy level is down-shifted to −3.93 eV. As for the molecule T-Se-Th having thiophene functionalized end-group, although the acquired LUMO energy level (−3.88 eV) is in between that of T-Se and T-Se-4F, it possesses the largest molar extinction coefficient (2.40 × 105 M−1cm−1), and a moderate bandgap of 1.60 eV. In solar cells with PM6 as electron-donor, the T-Se-4F- and T-Se-Th-based devices have superior exciton dissociation and charge collection properties, more weakly bimolecular recombination probabilities, and better electron transportability. As the results of the preceding characteristics, the T-Se-4F- and T-Se-Th-based devices achieve a higher current of 18.09 and 16.90 mA cm−2, better fill factors of 66.6 and 66.8%, relative to the 12.80 mA cm−2 and 62.1% of the T-Se-based device. Benefitting from the medium open-circuit voltage (VOC, 0.912 V), the T-Se-Th-based device successfully achieved the best power conversion efficiency of 10.29% among the three molecules. In contrast, the T-Se- and T-Se-4F-based devices owned the highest VOC of 0.935 V and the lowest VOC of 0.781 V, leading to the low efficiency of 7.44% and moderate efficiency of 9.41%, respectively, revealing the significant impact of the terminal groups on the properties of 4,4,9,9-tetramethyl-4,9-dihydroselenopheno[2',3':5,6]-s-indaceno[1,2-b]thiophene based asymmetric non-fullerene acceptors.