Coupling Electronic Structure Modulation with Self-Trapped Exciton Dynamics in Dual-Doped Cs3Bi2Br9 for Efficient Photocatalytic Hydrogen Evolution

Lead-free halide perovskites have attracted considerable attention as promising optoelectronic materials owing to their low toxicity and tunable electronic structures. However, poor stability and an insufficient understanding of excited-state regulation remain major challenges. A synergistic Rb+/Sb3+ codoping strategy is employed here to construct a series of highly stable Cs3Bi2Br9:x%Rb+,25%Sb3+ (x = 3, 5, 8, and 10) microcrystals. Structural characterization has shown that codoping forms a relatively stable perovskite structure. Spectroscopic experiments have identified that Sb3+ substitution enhances electron–phonon coupling and strengthens the formation of self-trapped excitons (STEs), and Rb+ modifies STE relaxation dynamics and suppresses radiative recombination to improve carrier utilization efficiency. Femtosecond transient absorption (fs-TA) indicates controlled excited-state evolution and enhanced charge-transfer characteristics. Therefore, Cs3Bi2Br9:x%Rb+,25%Sb3+ has achieved a hydrogen evolution rate of 144 μmol·g−1 h−1, which is 5.8 times higher than that of pristine Cs3Bi2Br9, and after adding Pt, this rate further increased to 393 μmol·g−1 h−1 due to accelerated charge separation and proton reduction. This paper proposes a structure-excited state-function relationship and shows how to optimize photocatalytic hydrogen evolution in lead-free perovskites by exciton engineering.