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, BiI6 3–, 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 BiI6 3–. The findings give insight into the excited-state relaxation dynamics and the photochemical reaction mechanisms in halide complexes of heavy ns2 metal ions.

Exploring Bismuth Coordination Complexes as Visible-Light Absorbers: Synthesis, Characterization, and Photophysical Properties

“…Moreover, comparing the initial transient absorption studies of the complexes in DMF with the Bi­(III)­I 6 3– ions reported in the literature, the results show that only one triplet excited-state species is present in the complexes in DMF. The observation of the single species, especially in 2 and 3 , is in contrast to the reported studies, which show that halobismuthate­(III) complexes undergo photodissociation and excited-state reactions to give rise to long-lived multiple species. ,, These initial studies further confirm the stability of the complexes in the excited state and thus provide the opportunity to explore the excited-state dynamics of the bismuth-based coordination complexes for optoelectronics applications.…”

“…In the case of the bismuth salts in DMF, a highly intense absorption band was observed between 260 and 275 nm, and a lower-energy band was observed for BiBr 3 and BiI 3 at 375 nm along with a weak absorbing tail for BiCl 3 (350–410 nm) ( Figure S24 ). As reported, 60 − 62 the halide complexes of bismuth are known to show two types of electronic bands, metal-centered (MC) sp bands and halogen to metal-centered LMCT bands. Generally, the LMCT transitions are observed to be higher in energy; however, upon changing the halides coordinated with a bismuth atom from Cl and Br to I, the contribution of a higher energy LMCT transition shifts toward the lower-energy side and its contribution increases in sp absorption bands ( Figure S24 ).…”

“…Comparing these excited-state results with the ground-state properties in ACN for 3 suggests that ACN strongly influences the ground- and excited-state properties of the complexes. Taking the idea from the previous literature published on the photochemistry of the Bi­(III)­I 6 3– ions , and halide photochemistry, we assume that the lower-energy absorption band (450–550 nm in DMF and 440–525 nm in ACN) is purely the halide LMCT in nature. Additionally, when the complexes were excited at 440–525 nm in ACN, no emission was observed, which further confirms that these bands lead to the population of LMCT states, which are generally known to be nonemissive.…”

“…The observation of the new band and its extension to 550 nm in BiI 3 is caused by the mixture of an interconfigurational transition from 6s 2 to 6s 1 6p 1 along with the LMCT transition from the σ orbital of iodine to the p orbital of bismuth. 60,63 Therefore, the observation of the lowerenergy band at 450−500 nm of 3 in DMF can be clearly assigned to transitions taking place in the inorganic part of the complex. Whereas in the case of 1 and 2, no lower-energy band extending up to ∼550 nm is observed due to the comparatively weaker electron-donating tendency of the chlorine-and bromine-based complexes, hence weaker electronic transitions from halogens are involved in the 1 and 2.…”

Unveiling the Influence of Organic Chain Length on the Physical Properties of Two-Dimensional Cobalt-Based Hybrid Perovskites

“…The TRPL decay is best-fitted with two exponential decay functions . We note that the first and second components (τ 1 and τ 2 ) are associated with surface and bulk states, respectively, ,, as LMCT and defect emission are, respectively, too fast (∼tens of ps) and too slow (>1 μs) than both processes. It is widely accepted that the fast average decay time of a hybrid perovskite material is a useful figure of merit for timing applications. , Notably, we note that the weight contribution of C 1 and C 2 exhibits an odd–even trend, as shown in Table .…”