posted on 2021-11-18, 15:04authored byWonkyung Choi, Seo Hyun Nam, Hyeon-Kyeong So, Sang-Eon Lee, Myung-Hwa Jung, Joon I. Jang
Two-dimensional (2D) semiconductors
have emerged as an excellent
platform for studying various excitonic matter under strong quantum
and dielectric confinements. However, such effects can be seriously
overestimated for Coulomb binding of two excitons to form a biexciton
by a naive interpretation of the corresponding photoluminescence (PL)
spectrum. By using 2D halide perovskite single crystals of [CH3(CH2)3NH3]2Pb1–xMnxBr4 (x = 0–0.09) as a model system, we
investigated both population and relaxation kinetics of biexcitons
as a function of excitation density, temperature, polarization, and
Mn doping. We show that the biexciton is formed by binding of two
dark excitons, which are partially bright, but they radiatively recombine
to yield a bright exciton in the final state. This renders the spectral
distance between the exciton peak and the biexciton peak as very different
from the actual biexciton binding energy (ϕ) because of large
bright–dark splitting. We show that Mn doping introduces paramagnetism
to our 2D system and improves the biexciton stability as evidenced
by increase in ϕ from 18.8 ± 0.7 to 20.0 ± 0.7 meV
and the increase of the exciton–exciton capture coefficient C from 2.4 × 10–11 to 4.3 ×
10–11cm2/ns within our doping range.
The precisely determined ϕ values are significantly smaller
than the previously reported ones, but they are consistent with the
instability of the biexciton against thermal dissociation at room
temperature. Our results demonstrate that electron–hole exchange
interaction must be considered for precisely locating the biexciton
level; therefore, the ϕ values should be reassessed for other
2D halide perovskites that even do not exhibit any dark exciton PL.