In conventional solar cell semiconductor materials (predominantly Si) photons with energy higher than the band gap initially generate hot electrons and holes, which subsequently cool down to the band edge by phonon emission. Due to the latter process, the energy of the charge carriers in excess of the band gap is lost as heat and does not contribute to the
Photon recycling,
the iterative process of re-absorption and re-emission
of photons in an absorbing medium, can play an important role in the
power-conversion efficiency of photovoltaic cells. To date, several
studies have proposed that this process may occur in bulk or thin
films of inorganic lead-halide perovskites, but conclusive proof of
the occurrence and magnitude of this effect is missing. Here, we provide
clear evidence and quantitative estimation of photon recycling in
CsPbBr
3
nanocrystal suspensions by combining measurements
of steady-state and time-resolved photoluminescence (PL) and PL quantum
yield with simulations of photon diffusion through the suspension.
The steady-state PL shows clear spectral modifications including red
shifts and quantum yield decrease, while the time-resolved measurements
show prolonged PL decay and rise times. These effects grow as the
nanocrystal concentration and distance traveled through the suspension
increase. Monte Carlo simulations of photons diffusing through the
medium and exhibiting absorption and re-emission account quantitatively
for the observed trends and show that up to five re-emission cycles
are involved. We thus identify 4 quantifiable measures, PL red shift,
PL QY, PL decay time, and PL rise time that together all point toward
repeated, energy-directed radiative transfer between nanocrystals.
These results highlight the importance of photon recycling for both
optical properties and photovoltaic applications of inorganic perovskite
nanocrystals.
Assembled perovskite
nanocrystals (NCs), known as supercrystals
(SCs), can have many exotic optical and electronic properties different
from the individual NCs due to energy transfer and electronic coupling
in the dense superstructures. We investigate the optical properties
and ultrafast carrier dynamics of highly ordered SCs and the dispersed
NCs by absorption, photoluminescence (PL), and femtosecond transient
absorption (TA) spectroscopy to determine the influence of the assembly
on the excitonic properties. Next to a red shift of absorption and
PL peak with respect to the individual NCs, we identify signatures
of the collective band-like states in the SCs. A smaller Stokes shift,
decreased biexciton binding energy, and increased carrier cooling
rates support the formation of delocalized states as a result of the
coupling between the individual NC states. These results open perspectives
for assembled perovskite NCs for application in optoelectronic devices,
with design opportunities exceeding the level of NCs and bulk materials.
Silicon nanoparticles
(Si-NPs) represent one of many types of nanomaterials,
where the origin of emission is difficult to assess due to a complex
interplay between the core and surface chemistry. Band-gap tunability
in Si-NPs is predicted to span from the infrared to the ultraviolet
spectral range, which is rarely observed in practice. In this work,
we directly assess the size dependence of the optical band gap using
a single-dot correlative microscopy tool, where the size of the individual
NPs is measured using atomic force microscopy (AFM) and the optical
band gap is evaluated from single-dot photoluminescence measured on
the very same NPs. We analyze 2–8 nm alkyl-capped Si-NPs prepared
by a sol–gel method, followed by annealing at 1300 °C.
Surprisingly, we find that the optical band gap is given by the amorphous
shell, as evidenced by the convergence of the optical band gap size
dependence toward the amorphous Si band gap of ∼1.56 eV. We
propose that the structural disorder might be the reason behind the
often reported limited emission tunability from various Si-NPs in
the literature. We believe that our message points toward a pressing
need for development and broader use of such direct correlative single-dot
microscopy methods to avoid possible misinterpretations that could
arise from attempts to recover size–band gap relation from
ensemble methods, as practiced nowadays.
We demonstrated the synthesis of ultra small and stable Cs3BiBr6 nanocrystals, ∼1.5–3 nm, via a room-temperature antisolvent method. Red-shift of bandgap was observed in low temperature PL measurements with an activation energy of ∼41 meV.
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