Ethylene thiourea additive engineering for high-performance and stable lead-free Cu2AgBiI6 photodetectors

Lead-free bismuth-based halides, such as Cu2AgBiI6 (CABI), are promising candidates for environmentally friendly photodetectors, yet solution-processed films typically exhibit poor crystallinity, high defect density, and limited stability. Here, we demonstrate that ethylene thiourea (ETU) serves as a multifunctional additive to overcome these limitations. The sulfur moiety in ETU coordinates with metal cations, while its amino groups form hydrogen bonds with iodide ions, synergistically improving crystallinity and optimizing energy level alignment. The resulting CABI photodetector achieves a responsivity of 0.68 A W−1 and a specific detectivity of 1.19 × 1013 Jones at 365 nm under zero bias, with an ultralow dark current of merely 0.4 nA and response times (rise/fall) of 8.6/13.8 ms. The device retains 80% of its initial photocurrent after 14 days in ambient conditions (25 °C and 30% relative humidity) and exhibits stable operation over 1500 s of continuous cycling. Encrypted optical communication is demonstrated as a proof-of-concept application, leveraging the broadband response of the device. These results establish thiourea-based additive engineering as an effective strategy for high-performance, stable, and lead-free photodetectors.