Efficient Organic Solar Cells Enabled by Structurally Modified Quinoxaline-Based Small Molecule Acceptors with Brominated End Groups

Abstract

Scientific interest in organic solar cells (OSCs) has increased significantly in recent years. This surge is largely due to advances in A-DA′D-A-type small molecule acceptors (SMAs), which have played a key role in improving the power conversion efficiency (PCE) of OSC devices. Nevertheless, there is a prevailing need to continue exploring avenues that would further elevate the performance of OSCs, particularly improving their open-circuit voltage (V_OC). The structural modification of the fused-ring electron-withdrawing A′ unit with the quinoxaline unit is an approach that holds considerable promise for enhancing the V_OC and PCE of A-DA′D-A type molecules. Furthermore, it has been demonstrated that the incorporation of bromine atoms into SMAs can result in the synthesis of highly prospective SMAs. This is attributable to the fact that bromine atoms possess lower electronegativity, larger atomic dimensions, and a comparatively more straightforward and cost-effective synthetic procedure compared to the commonly used fluorine and chlorine atoms. To develop promising brominated SMAs to enhance the V_OC of OSCs, two alkoxypheny-substituted quinoxaline-based A-DA′D-A molecules (BQ-2FBr and BQ-2Cl-FBr) were synthesized, with the former sealed with (5-bromo-4-fluoro-3-oxy-2,3-dihydro-1H-indole-1-ylidene)malonitrile (FBr-INCN) unit and the latter sealed with 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malonitrile (2Cl-INCN) and FBr-INCN units simultaneously. The symmetrical molecule BQ-2FBr possesses an elevated LUMO energy level, while the asymmetrical molecule BQ-2Cl-FBr displays a broadened absorption spectrum with a high extinction coefficient and better molecular stacking property. When combined with PM6, the BQ-2FBr device achieves a very good V_OC of 0.944 V and a moderate PCE of 10.11%. Despite a decline in the V_OC of the PM6:BQ-2Cl-FBr device to 0.928 V, simultaneous enhancement in the short circuit current density (J_SC) and fill factor (FF) was observed. This resulted in an augmented PCE of 11.54%, a development primarily attributed to the improved charge collection and exciton dissociation properties, suppression of charge recombination, enhancement of molecular stacking property and better morphology of the blend film, and acceleration of exciton diffusion time. This work describes the important influence of distinct brominated terminal groups on the photovoltaic performance of alkoxyphenyl-substituted quinoxalinyl A-DA′D-A SMAs, which may offer a useful structural guideline for the development of promising SMAs for high-efficiency and large V_OC OSCs.