Improved charge transport based on donor-acceptor type solid additive with large dipole moment for efficient organic solar cells

Applying solid additive into the active layer has been demonstrated to be one of the most efficient approaches in improving the photovoltaic performance of organic solar cells (OSCs). Through the combination of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile and benzodithiophene units, this work presents a novel donor-acceptor type solid additive IC-BDTBr. The IC-BDTBr additive possesses a large dipole moment with an efficient intramolecular charge transfer, attributed to the push-pull effect and the S⋯O noncovalently conformational lock. It is able to form strong electrostatic interaction with the active layer materials, which helps to tune the miscibility. Thus, the charge transport of OSCs is significantly promoted by the incorporation of IC-BDTBr. The IC-BDTBr additive exhibits suitable energy levels and optical absorption, which enhances the photovoltaic performance of the devices. As a result, incorporating the IC-BDTBr additive into the active layer improves both the photocurrent and fill factor of the OSCs. Thus, the average power conversion efficiency (PCE) is increased from 15.70 % of the PM6:Y6 control device to 16.51 % (PCEmax = 16.60 %) of the device with 2 wt% IC-BDTBr. This research demonstrates a feasible design strategy for the solid additive, by applying a donor-acceptor molecular structure with large dipole moment.