Ultrafast Solution-Phase Photophysical and Photochemical Dynamics of Hexaiodobismuthate(III), the Heart of Bismuth Halide Perovskite Solar Cells.

Abstract

The ultrafast relaxation pathways in a hexaiodide bismuth(III) complex, BiI63-, excited at 530 nm in acetonitrile solution are studied by means of femtosecond transient absorption spectroscopy supported by steady-state absorption/emission measurements and DFT computations. Radiationless relaxation out of the Franck-Condon, largely metal-centered (MC) triply degenerate 3T1u state (46 ± 19 fs), is driven by vibronic coupling due to the Jahn-Teller effect in the excited state. The relaxation populates two lower-energy states: a ligand-to-metal charge transfer (LMCT) excited state of 3π I(5pπ) → Bi(6p) nature and a luminescent "trap" 3A1u(3P0) MC state. Coherent population transfer from the initial 3T1u into the 3π LMCT state occurs in an oscillatory, stepwise manner at ∼190 and ∼550 fs with a population ratio of ∼4:1. The 3π LMCT state decays with a 2.9 ps lifetime, yielding two short-lived reaction intermediates of which the first one reforms the parent ground state with a 15 ps time constant, and the second one decays on a ∼5 ps timescale generating the triplet product species, which persists to the longest 2 ns delay times investigated. This product is identified as the η2 metal-ligated diiodide-bismuth adduct with the intramolecularly formed I-I bond, [(η2-I2)Bi(II)I4]3-, which is the species of interest for solar energy conversion and storage applications. The lifetime of the "trap" 3A1u state is estimated to be 13 ns from the photoluminescence quenching of BiI63-. The findings give insight into the excited-state relaxation dynamics and the photochemical reaction mechanisms in halide complexes of heavy ns2 metal ions.