3 research outputs found
Modulation of Charge Recombination in CsPbBr<sub>3</sub> Perovskite Films with Electrochemical Bias
The
charging of a mesoscopic TiO<sub>2</sub> layer in a metal halide
perovskite solar cell can influence the overall power conversion efficiency.
By employing CsPbBr<sub>3</sub> films deposited on a mesoscopic TiO<sub>2</sub> film, we have succeeded in probing the influence of electrochemical
bias on the charge carrier recombination process. The transient absorption
spectroscopy experiments conducted at different applied potentials
indicate a decrease in the charge carrier lifetimes of CsPbBr<sub>3</sub> as we increase the potential from −0.6 to +0.6 V vs
Ag/AgCl. The charge carrier lifetime increased upon reversing the
applied bias, thus indicating the reversibility of the photoresponse
to charging effects. The ultrafast spectroelectrochemical experiments
described here offer a convenient approach to probe the charging effects
in perovskite solar cells
Temperature Dependent Carrier Dynamics in Ga-Alloyed CdSe/ZnS Core–Shell Quantum Dots
In this work, temperature dependent transient absorption
spectroscopy
measurements are presented on gallium-alloyed CdSe/ZnS core–shell
nanoparticles between 30 and 130 °C. To our knowledge, temperature
dependent measurements in these systems have been reported only in
a few papers, although all processes related to carrier recombination
are affected by temperature. For these experiments, gallium-alloyed
CdSe/ZnS QD samples were used with nominal doping percentages of 2.5%,
7.5%, 15%. The experimental results show that the transient absorption
decay is faster for the pristine CdSe/ZnS samples than in the gallium-alloyed
samples at all temperatures. It is assumed that Ga-alloying promotes
the formation of trions in the samples by introducing occupied impurity
levels within the bandgap of CdSe. The resulting Coulomb blockade
will, in turn, prolong the hot-electron relaxation process. By variation
of the temperature, the distribution of charge carriers in the different
recombination channels can be altered to accelerate recombination
in the Ga-alloyed samples at higher temperatures. These measurements
demonstrated their usefulness for observing the redistribution of
charge carriers among different relaxation pathways
Exciton Dynamics in MoS<sub>2</sub>‑Pentacene and WSe<sub>2</sub>‑Pentacene Heterojunctions
We measured the exciton dynamics in van der Waals heterojunctions
of transition metal dichalcogenides (TMDCs) and organic semiconductors
(OSs). TMDCs and OSs are semiconducting materials with rich and highly
diverse optical and electronic properties. Their heterostructures,
exhibiting van der Waals bonding at their interfaces, can be utilized
in the field of optoelectronics and photovoltaics. Two types of heterojunctions,
MoS2-pentacene and WSe2-pentacene, were prepared
by layer transfer of 20 nm pentacene thin films as well as MoS2 and WSe2 monolayer crystals onto Au surfaces.
The samples were studied by means of transient absorption spectroscopy
in the reflectance mode. We found that A-exciton decay by hole transfer
from MoS2 to pentacene occurs with a characteristic time
of 21 ± 3 ps. This is slow compared to previously reported hole
transfer times of 6.7 ps in MoS2-pentacene junctions formed
by vapor deposition of pentacene molecules onto MoS2 on
SiO2. The B-exciton decay in WSe2 shows faster
hole transfer rates for WSe2-pentacene heterojunctions,
with a characteristic time of 7 ± 1 ps. The A-exciton in WSe2 also decays faster due to the presence of a pentacene overlayer;
however, fitting the decay traces did not allow for the unambiguous
assignment of the associated decay time. Our work provides important
insights into excitonic dynamics in the growing field of TMDC-OS heterojunctions