Spiro‐Phenothiazine Hole‐Transporting Materials: Unlocking Stability and Scalability in Perovskite Solar Cells

Abstract Improving both the efficiency and long‐term stability of perovskite solar cells (PSCs) is critical for their commercial deployment. Despite the widespread use of spiro‐OMeTAD as a hole‐transporting material (HTM), its inhomogeneous doping behavior and susceptibility to moisture and heat have hindered its large‐scale industrial implementation. Here, a family of spiro‐phenothiazine‐based HTMs (PTZ) is reported to address these drawbacks. Among them, the fluorene derivative (PTZ‐Fl) shows a larger Li + affinity and forms a compact interphase by intercalation in the perovskite passivating layer that prevents Li + migration. PSCs incorporating PTZ‐Fl exhibit power conversion efficiencies (PCEs) up to 25.8% (certified 25.2% under reverse scan), retaining 80% of their initial performance after 1000 h under ISOS‐L‐3 protocol. Furthermore, a 5 × 5 cm mini‐module reaches a PCE of 22.1%, surpassing spiro‐OMeTAD‐based PSCs and retaining over 85% of its efficiency after 1100 h under ISOS‐D‐1 protocol. These results demonstrate that PTZ‐Fl not only enables high PCEs but also substantially improves operational stability, offering a promising pathway toward the large‐scale deployment of next‐generation PSCs.