Less is more: Annealing-free crystallization to boost photo-induced charge separation in Cs3Bi2I9 perovskite

Thermal annealing is standard for perovskite fabrication; however, for lead-free Cs 3 Bi 2 I 9 , it triggers a fatal trade-off between crystallinity and surface stoichiometry. We reveal that thermal stress induces severe iodine volatilization, generating deep-level vacancy traps that pin the Fermi level and reverse surface band bending, thereby sabotaging electronic properties. To decouple crystallization from defect generation, we introduce an annealing-free, room-temperature crystallization paradigm, exploiting the material's unique 0D molecular nature to drive spontaneous self-assembly. Consequently, non-annealed films maintain superior stoichiometry, delivering a >300% enhancement in charge separation efficiency and significantly prolonged carrier lifetimes. Kinetic modeling confirms that annealing introduces a rapid (∼75 ns) surface decay pathway while leaving the bulk carrier lifetime (∼150 μs) largely intact, identifying the surface as the exclusive locus of defect-induced degradation. These findings establish a 'less-is-more' methodology for optimizing volatile optoelectronic materials, shifting the processing focus from grain size optimization to stoichiometric integrity.