One‐Stone‐for‐Multiple‐Birds Additive Strategy for Highly Efficient and Stable Carbon‐Based Hole‐Transport‐Layer‐Free CsPbI₂Br Solar Cells

During fabrication and operation of perovskite solar cells (PSCs), defects commonly arise within the crystals as well as at grain boundaries. However, conventional additive strategies typically only serve to mitigate the occurrence of a single defect and fail to significantly enhance device performance. Herein, carbon-based hole-transport-layer-free CsPbI₂Br devices are focused on, one kind of important PSCs with more stable structure and an appropriate bandgap for a semitransparent solar cell or a top cell in a tandem configuration, and present a highly efficient one-stone-for-multiple-birds additive strategy based on lanthanide trifluoromethanesulfonates (Ln(OTF)₃, Ln: neodymium (Nd), europium (Eu), dysprosium (Dy), thulium (Tm)). Density functional theory calculations reveal that the Ln³⁺ ions with a smaller radius can elevate defect formation energy for Pb and I vacancies within the crystals, while the presence of OTF^- can effectively passivating uncoordinated Pb²⁺ at grain boundaries. In addition, Ln(OTF)₃ addition increases the grain size and meanwhile reduces the surface roughness of the CsPbI₂Br layers. All these positive contributions lead to a significant enhancement in power conversion efficiency (PCE) to 15.13% which is among the top PCEs reported for the corresponding solar cells, from 11.80% of the pristine device without Tm(OTF)₃ addition, while notably boosting long-term stability and reducing current-voltage hysteresis.