Over 18.2% efficiency of layer–by–layer all–polymer solar cells enabled by homoleptic iridium(III) carbene complex as solid additive

The vertical phase distribution of active layers plays a vital role in balancing exciton dissociation and charge transport for achieving efficient polymer solar cells (PSCs). The layer–by–layer (LbL) PSCs are commonly prepared by using sequential spin–coating method from donor and acceptor solutions with distinct solvents and solvent additives. The enhanced exciton dissociation is expected in the LbL PSCs with efficient charge transport in the relatively neat donor or acceptor layers. In this work, a series of LbL all–polymer solar cells (APSCs) were fabricated with PM6 as donor and PY–DT as acceptor, and triplet material m–Ir(CPmPB)₃ is deliberately incorporated into PY–DT layer to prolong exciton lifetimes of active layers. The power conversion efficiency (PCE) of LbL APSCs is improved to 18.24% from 17.32% by incorporating 0.3 wt% m–Ir(CPmPB)₃ in PY–DT layer, benefiting from the simultaneously enhanced short–circuit current density (J_SC) of 25.17 mA cm⁻² and fill factor (FF) of 74.70%. The enhancement of PCE is attributed to the efficient energy transfer of m–Ir(CPmPB)₃ to PM6 and PY–DT, resulting in the prolonged exciton lifetime in the active layer and the increased exciton diffusion distance. The efficient energy transfer from m–Ir(CPmPB)₃ to PM6 and PY–DT layer can be confirmed by the increased photoluminescence (PL) intensity and the prolonged PL lifetime of PM6 and PY–DT in PM6 + m–Ir(CPmPB)₃ and PY–DT + m–Ir(CPmPB)₃ films. This study indicates that the triplet material as solid additive has great potential in fabricating efficient LbL APSCs by prolonging exciton lifetimes in active layers.