Publication date: 21st July 2025
Bismuth halide semiconductors like Cs3Bi2X9 (X = Br or I), have emerged as environmentally stable alternatives to lead halide perovskites in photovoltaics and photocatalysis. However, their wide, indirect bandgaps and strong exciton binding energies (Eb) hinder efficient exciton dissociation and charge transport, limiting their potential in optoelectronic applications. Halide alloying in Cs3Bi2(Br1-yIy)9 offers a strategy to tune electronic properties such as bandgap and Eb, yet the excitonic behavior of these alloys remains insufficiently understood.
In this study, we systematically investigate the influence of halide composition on excitonic properties and dynamics in Cs3Bi2(Br1-yIy)9 film. Steady-state UV–vis absorption spectroscopy combined with Elliott fitting reveals a non-linear dependence of both bandgap and Eb: decreasing from ~3.6 eV and 780 meV for y = 0 to ~2.8 eV and 380 meV at y = 0.6 and subsequently increasing to ~3.0 eV and 460 meV at y = 0.9-1.0. This bandgap bowing behavior may be attributed to a structural phase transition, as evidenced by the emergency of two distinct excitonic peaks at y = 0.8, indicating phase coexistence. Transient absorption spectroscopy further probes exciton dynamics over nanosecond to microsecond timescales. The photobleaching signals align with steady-state excitonic features, and kinetic analysis reveals two decay components: a fast (~100 nanoseconds) process and a slower (up to several microseconds) one. Both lifetimes peak at y = 0.6, reaching 170 ns and 4.3 μs, respectively. These findings demonstrate that halide alloying enables effective tuning of bandgap and excitonic properties in bismuth halide semiconductors, offering a pathway to improved optoelectronic performance via compositional engineering.
H.Z. and E.M.H. are supported by the Advanced Research Center Chemical Building Blocks (ARC−CBBC).
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