Synergistic Interfacial Engineering With Butylamine Hydrochloride (BACl)‐Doped PCBM Enables Four‐Orders‐of‐Magnitude Responsivity Enhancement in Planar CsPbI ₃ Photodetectors

Planar perovskite photodetectors are limited by severe interfacial recombination and inefficient charge extraction. Here, we enhance the photoresponsivity of CsPbI 3 photodetectors by four orders of magnitude via incorporating butylamine hydrochloride (BACl) into the PCBM electron transport layer. BACl improves film morphology and conductivity while passivating perovskite surface defects, suppressing non‐radiative recombination and optimizing energy alignment. Transient photovoltage and photocurrent measurements confirm the reduced recombination and accelerated charge extraction. The enhancement arises from suppressed recombination and a photogating effect, where trapped holes at the interface create an internal gate that boosts conductivity. Consequently, the champion device achieves a high responsivity of 3.4 A W − 1 , a specific detectivity of 4.85 × 10 12 Jones, and a fast response time of 40 µs. Furthermore, the modified device exhibits outstanding operational and storage stability: under continuous 660 nm illumination for 6 h, it shows only a 15.9% fluctuation in the light on‐off ratio, vastly outperforming the pristine device (99.5%). After 500 h of storage in a glove box, it retains 48% of its initial photoresponsivity, while the pristine device loses all photosensitivity. This work highlights multifunctional ETL doping as a simple route to high‐performance perovskite photodetectors.