Metal-halide perovskites are emerging as an intriguing class of semiconductors with significant potential for photovoltaics. Here, several perovskites are discussed that have been assessed via various experimental techniques to determine the effects of their composition, dimensionality, and structural stability on hot carriers and polaron formation. It will be shown that polarons formed in these systems are strongly affected by the binding energy and nature of the excitons in the materials. Notably, the hot carrier dynamics in perovskites is strongly affected by their low thermal conductivity, which inhibits the dissipation of heat in the material.
A new type of organic light-emitting diode (OLED) has emerged that shows enhanced operational stability and large internal quantum efficiency approaching 100%, which is based on exciplexes in donor-acceptor (D-A) blends having thermally activated delayed fluorescence (TADF) when doped with fluorescent emitters. We have investigated magnetoelectroluminescence (MEL) and magneto-conductivity in such TADF-based OLEDs, as well as magnetophotoluminescence (MPL) in thin films based on the OLEDs active layers, with various fluorescence emitters. We found that both MEL and MPL responses are thermally activated with substantially lower activation energy compared to that in the pristine undoped D-A exciplex host blend. In addition, both MPL and MEL steeply decrease with the emitters’ concentration. This indicates the existence of a loss mechanism, whereby the triplet charge-transfer state in the D-A exciplex host blend may directly decay to the lowest, non-emissive triplet state of the additive fluorescent emitter molecules.
We used steady state and picosecond transient photoinduced absorption (PA),
excitation dependence (EXPA(ω)) spectrum of the triplet exciton PA band, and its
magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot-excitons into
two separated triplet excitons in luminescent π-conjugated polymers. From the high
energy step in the triplet EXPA(ω) spectrum of poly(dioctyloxy)-phenylenevinylene
(DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold
energy at E≈2ET (=2.8 eV, where ET is the energy of the lowest lying triplet exciton),
which is about 0.8 eV above the lowest singlet exciton energy. The picosecond
transient PA with 3.1 eV pump excitation shows that in DOO-PPV film a triplet
exciton is generated at time, t<500 ps. However the ultrafast triplet generation is
missing in DOO-PPV solution, indicating that the HE-SF is predominantly interchain
in nature. The HE-SF process was confirmed by the triplet MPA(B) response for
excitation at E>2ET, which shows a typical SF response. Our work shows that the SF
process in π-conjugated polymers is a much more general process than thought
previously.
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