Suppressing Photooxidation and Iodine Migration in HTL‐Free Carbon‐Based Perovskite Solar Cells via a Tailor‐Made Redox Guard

ABSTRACT

Hole transport layer‐free carbon‐based perovskite solar cells (HTL‐free C‐PSCs) suffer rapid performance decay under operational stressors, driven by iodine migration and photogenerated superoxide radicals (O ₂ ^•− ). To break this photo‐oxidation‐iodine migration vicious cycle, we design a multifunctional molecular stabilizer, tetrakis(4‐carboxyphenyl)tetrathiafulvalene (4B‐TTF). This tailor‐made “molecular guard” works via two synergistic mechanisms: its reversible TTF core acts as a charge‐transfer mediator, reducing I ₂ to I ⁻ and scavenging O ₂ ^•− , while four peripheral benzoic acid groups anchor to undercoordinated Pb ²⁺ sites, passivating grain‐boundary defects. Incorporating 4B‐TTF into MAPbI ₃ ‐based devices yields a champion power‐conversion efficiency (PCE) of 19.96%, vs. 17.58% for the control, by suppressing non‐radiative recombination. Unencapsulated devices retain 96.4% of their initial efficiency after 1200 h under ambient storage and 93.5% after 120 h under harsh UV‐oxygen exposure; encapsulated devices retain 90.32% after 700 h of continuous maximum power point (MPP) tracking under 1‐sun illumination at 55°C in nitrogen. The strategy is further validated in Cs _0.17 FA _0.83 PbI ₃ systems, delivering a state‐of‐the‐art PCE of 20.70%. This work provides a unified molecular‐level strategy to concurrently address ionic, radical, and defect‐related degradation, bridging the performance gap between low‐cost HTL‐free devices and conventional high‐cost architectures.