Influences of the terminal groups on the performances of asymmetric small molecule acceptors-based polymer solar cells

Abstract

By altering the end-capping groups from 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile to 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile, three asymmetric non-fullerene electron-acceptors (T-TT, T-TT-4F, and T-TT-4Cl) containing thiophene–phenylene–thieno[3,2-b]thiophene fused-ring as the electron-donating core were synthesized. Although the three asymmetric molecules all have bent geometries, the di-fluorinated and di-chlorinated acceptors show bathochromically-shifted spectra, enhanced molar extinct coefficients, deepened molecular energy levels, and reduced dipole moments. When blending with a wide-bandgap polymer PM6 in inverted solar cells, the T-TT-based device exhibits the highest open-circuit voltage (VOC) of 0.966 V, while the T-TT-4Cl-based device has the biggest short-circuit current density (JSC) of 19.00 mA cm−2, and the T-TT-4F-based device possesses the best fill factor (FF) of 66.1%, a moderate VOC (0.859 V), and a medium JSC (18.48 mA cm−2). As a result of the above properties and parameters, the T-TT-4F- and T-TT-4Cl-based solar cells successfully achieved high power conversion efficiencies of 10.49% and 10.16%, in comparison with that of 9.70% for the T-TT-based device, illustrating the importance of the influences of the terminal groups on the performances of asymmetric small molecular acceptors-based polymer solar cells.