Lead-halide　perovskites　exhibit　outstanding　performance　in　X-ray　detection　due　to　their　intrinsic　features　such　as　high　charge　carrier　mobility,　large　atomic　number,　and　long　carrier　lifetime,　but　the　toxicity　of　lead　is　regarded　as　the　major　factor　hindering　their　development.　Here,　we　introduce　organic　molecule　(R)-(-)-2-methylpiperazine　(R-MPz)　into　the　bismuth-based　structure　to　synthesize　lead-free　(R)-(H2MPz)BiI5　(R-MBI).　The　high-quality　centimeter-sized　single　crystals　have　been　obtained,　which　show　a　low　dark　current　and　superior　environmental　stability.　Particularly,　the　single-crystal　device　of　R-MBI　exhibits　a　high　µτ　product　up　to　1.88　×　10−4　cm2/V　and　a　low　trap　density　of　1.21　×　1010　cm−3.　Further,　the　detector　displays　excellent　detection　sensitivity　of　263.58　µC　Gyair−1　cm−2　and　a　favorable　low　detection　limit　of　4.35　µGyair/s,　both　of　which　meet　the　requirement　for　medical　diagnostics.　These　findings　shed　light　on　the　exploration　of　innovative　bismuth-based　hybrid　perovskites　for　high-performance　X-ray　detection.　©　2024