The perovskite-inspired
Cu2AgBiI6(CABI)
absorber shows promise for low-toxicity indoor photovoltaics. However,
the carrier self-trapping in this material limits its photovoltaic
performance. Herein, we examine the self-trapping mechanism in CABI
by analyzing the excited-state dynamics of its absorption band at
425 nm, which is responsible for the self-trapped exciton emission,
using a combination of photoluminescence and ultrafast transient absorption
spectroscopies. Photoexcitation in CABI rapidly generates charge carriers
in the silver iodide lattice sites, which localize into the self-trapped
states and luminesce. Furthermore, a Cu–Ag–I-rich phase
that exhibits similar spectral responses as CABI is synthesized, and
a comprehensive structural and photophysical study of this phase provides
insights into the nature of the excited states of CABI. Overall, this
work explains the origin of self-trapping in CABI. This understanding
will play a crucial role in optimizing its optoelectronic properties.
It also encourages compositional engineering as the key to suppressing
self-trapping in CABI.