Steric modulation of tetradentate platinum(II) bis-carbene complex enables over 18.4 % efficiency of layer-by-layer all-polymer solar cells

Triplet photovoltaic materials are increasingly recognized for their potential to enhance the efficiency of organic solar cells due to the strong emission and extended exciton lifetime compared to singlet materials. The layer-by-layer (LbL) structure provides a good opportunity to produce the highly efficient triplet photovoltaic cells, which partly overcome the challenge of material miscibility between donor and acceptor or the third component through two-step deposition. This work developed two new tetradentate platinum(II) bis-carbene complexes, Pt-CF3 and Pt-AD, as triplet additive for the LbL all-PSCs fabrication. Pt-AD, featuring a bulky adamantyl group, proves to be a more effective additive than Pt-CF3, as the latter largely damages the molecular packing of the host materials due to its strong aggregation behavior. Incorporating 0.2 wt% of Pt-AD into the PBQx-TCl/PY-DT blend elevates the fill factor (FF) to 75.8 % and the PCE to 18.45 %, outperforming the control device with PCE of 17.46 %. The effective energy transfers from Pt-AD to both the donor and acceptor can be demonstrated, leading to an increased exciton yield and lifetime in the active layers. This study indicates that steric modulation of platinum(II) complex is an effective strategy of molecular design for their application in highly efficient LbL all-PSCs.