A chlorinated low-bandgap small-molecule acceptor for organic solar cells with 14.1% efficiency and low energy loss

A new acceptor-donor-acceptor (A-D-A) type small-molecule acceptor NCBDT-4Cl using chlorinated end groups is reported. This new-designed molecule demonstrates wide and efficient absorption ability in the range of 600–900 nm with a narrow optical bandgap of 1.40 eV. The device based on PBDB-T-SF:NCBDT-4Cl shows a power conversion efficiency (PCE) of 13.1% without any post-treatment, which represents the best result for all as-cast organic solar cells (OSCs) to date. After device optimizations, the PCE was further enhanced to over 14% with a high short-circuit current density (Jsc) of 22.35 mA cm−2 and a fill-factor (FF) of 74.3%. The improved performance was attributed to the more efficient photo-electron conversion process in the optimal device. To our knowledge, this outstanding efficiency of 14.1% with an energy loss as low as 0.55 eV is among the best results for all single-junction OSCs.