An asymmetrical A–DAD–A-type acceptor simultaneously enhances voltage and current for efficient organic solar cells

The asymmetrical molecule is a promising class of fused-ring electron acceptor (FREA) for use in organic solar cells (OSCs). Hence, two molecules were synthesized to compare the differences between A–D–A (acceptor–donor–acceptor)-type and A–DAD–A (acceptor–donor–acceptor–donor–acceptor)-type asymmetrical acceptors, named ITIC-γCl-2F and BTIC-γCl-2F, respectively. BTIC-γCl-2F exhibits larger dipole moments (μm) and a red-shifted absorption, which would be beneficial in obtaining a greater short-circuit current (Jsc). After they were fabricated as OSC devices, an extremely large increase in Jsc was achieved in BTIC-γCl-2F-based devices without a decrease in the open circuit voltage (Voc); one important reason for this phenomenon is that the introduction of nitrogen atoms in the core of BTIC-γCl-2F can effectively increase the HOMO energy level but with little change in the LUMO energy level. Consequently, devices based on BTIC-γCl-2F achieve a champion power conversion efficiency (PCE) of 15.43%, which is the highest value reported to date for asymmetrical acceptor-based OSCs. The more obvious fibrillar network and more homogeneous morphology of BTIC-γCl-2F leads to faster electron and hole mobilities, which is consistent with higher Jsc and fill factor (FF). This suggests that A–DAD–A-type symmetry breaking is a promising strategy that can be used for designing high-performance asymmetrical FREAs.