Customized Buried Interface for High Performance All‐Inorganic CsSnI 3 ‐Based NIR Perovskite Light‐Emitting Diodes via a Guanidinium Derivative

ABSTRACT CsSnI 3 ‐based perovskite light‐emitting diodes (PeLEDs) show great potential in near‐infrared (NIR) applications, yet device performance is limited by interfacial degradation at the acidic hole‐transport layer (HTL) based on Poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). Herein, a multifunctional guanidine derivative of N‐(3‐methoxy‐phenyl)‐guanidine (NMPG), is used to modify the buried interface between the CsSnI 3 perovskite and the HTL. NMPG deprotonates the polystyrene sulfonic acid (PSSH) of PSS and reduces the acidity of PEDOT:PSS, while its methoxy and C═N groups coordinate with Sn 2+ , effectively inhibiting Sn 2+ oxidation, passivating Sn (II) induced defects. This customized buried interface enables uniform and discrete growth of perovskite grain, improves charge injection balance, and enhances vertical compositional and optical homogeneity. As a result, the target PeLEDs achieve a competitive maximum external quantum efficiency (EQE) of 9.0% and a high average EQE of 8.4%, significantly outperforming the control device ( EQE max = 6.6%, EQE ave = 5.5%). The target PeLEDs also demonstrate superior operational stability, with a T 50 lifetime of 67.9 h, representing a 5‐fold improvement compared to the control PeLEDs. This work reveals the pivotal role of buried interface chemistry and presents a viable molecular design strategy for achieving high‐performance CsSnI 3 ‐based PeLEDs.