30 Single-Molecule Level Study and Control …
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NCs, etc. To examine these phenomena on model systems of controllable size, we use
the method of single-molecule/particle spectroscopy to study well-defined aggregates
of NCs of the perovskite CsPbBr 3 . We compare PL and electroluminescence (EL)
from the same aggregates in a microscopic EL device to get insight into the processes
of charge and energy transport between individual NCs within the aggregate [3].
To compare PL and EL on the same single aggregates of CsPbBr 3,
we prepared EL devices directly on microscope coverslips for use on top
of a high N.A. fluorescence microscope. The device has the structure of
ITO/PEDOT:PSS/PVK:CsPbBr 3 :PBD/TPBi/LiF/Al and uses as the emissive layer a
spin-coated film of PVK and PBD into which CsPbBr 3 NCs are doped. The CsPbBr 3
NCs have cube-like shape and average edge size of 16 nm, and their surface is
passivated with oleic acid and oleylamine ligands. The self-assembled aggregates
have sizes on the order of 100 nm, and each aggregate contains tens to hundreds of
individual perovskite NCs.
Results of the simultaneous PL and EL microscopic imaging are shown in
Fig. 30.1. The same aggregates appear in both the PL (Fig. 30.1a) and EL (Fig. 30.1b)
images but most of them with different intensity. We have shown recently that the
excitation rates in both PL and EL emission processes are comparable. The differences in the emission intensities in Fig. 30.1 thus originate from differences in PL and
EL QY, due to different fractions of charged NCs in each aggregate. An important
finding is that the individual aggregates show the dependence on their emission intensities on time (time traces) in PL and EL. In PL, the emission intensity is constant,
as shown in Fig. 30.1c. In contrast, the EL intensity time trace shows strong fluctuations (blinking) in the form of intensity spikes starting from the background level
(Fig. 30.1d). Generally, individual NCs do exhibit blinking in PL under ambient
conditions. Therefore, the fact that we do not observe any blinking for the aggregates in PL implies that all NCs within the aggregate behave as isolated units and
absorb and emit independently, as shown schematically in Fig. 30.1e. The blinking of
the individual NCs is averaged out over the aggregate, and only constant PL intensity
is observed.
The strong blinking in EL, on the other hand, indicates that only a small number of
NCs are actively emitting EL. This has been confirmed by super-resolution analysis
of the blinking images. An example of such analysis is shown in Fig. 30.1f, where
three distinct emitting spots are evident. On average 3–7 emission spots are EL
active in each aggregate, and these spots emit consequentially, i.e., at each time the
emission is coming only from one single NC. This fact helps to explain the strong
blinking observed in the EL. Further, there is a size distribution between individual
NCs within each aggregate, as illustrated in the TEM image in Fig. 30.1g. Due to the
quantum confinement effect, the largest NCs within the aggregate have the smallest
bandgap and work as traps for charges captured originally at the surrounding NCs.
The conductive environment present between the NCs enables efficient migration
of the charges to the traps (the largest NCs) from where the EL takes place, as
shown schematically in Fig. 30.1h. This phenomenon of charge funneling is thus
an important aspect of EL devices composed of densely packed perovskite NCs,
especially in the presence of size distribution.
517
NCs, etc. To examine these phenomena on model systems of controllable size, we use
the method of single-molecule/particle spectroscopy to study well-defined aggregates
of NCs of the perovskite CsPbBr 3 . We compare PL and electroluminescence (EL)
from the same aggregates in a microscopic EL device to get insight into the processes
of charge and energy transport between individual NCs within the aggregate [3].
To compare PL and EL on the same single aggregates of CsPbBr 3,
we prepared EL devices directly on microscope coverslips for use on top
of a high N.A. fluorescence microscope. The device has the structure of
ITO/PEDOT:PSS/PVK:CsPbBr 3 :PBD/TPBi/LiF/Al and uses as the emissive layer a
spin-coated film of PVK and PBD into which CsPbBr 3 NCs are doped. The CsPbBr 3
NCs have cube-like shape and average edge size of 16 nm, and their surface is
passivated with oleic acid and oleylamine ligands. The self-assembled aggregates
have sizes on the order of 100 nm, and each aggregate contains tens to hundreds of
individual perovskite NCs.
Results of the simultaneous PL and EL microscopic imaging are shown in
Fig. 30.1. The same aggregates appear in both the PL (Fig. 30.1a) and EL (Fig. 30.1b)
images but most of them with different intensity. We have shown recently that the
excitation rates in both PL and EL emission processes are comparable. The differences in the emission intensities in Fig. 30.1 thus originate from differences in PL and
EL QY, due to different fractions of charged NCs in each aggregate. An important
finding is that the individual aggregates show the dependence on their emission intensities on time (time traces) in PL and EL. In PL, the emission intensity is constant,
as shown in Fig. 30.1c. In contrast, the EL intensity time trace shows strong fluctuations (blinking) in the form of intensity spikes starting from the background level
(Fig. 30.1d). Generally, individual NCs do exhibit blinking in PL under ambient
conditions. Therefore, the fact that we do not observe any blinking for the aggregates in PL implies that all NCs within the aggregate behave as isolated units and
absorb and emit independently, as shown schematically in Fig. 30.1e. The blinking of
the individual NCs is averaged out over the aggregate, and only constant PL intensity
is observed.
The strong blinking in EL, on the other hand, indicates that only a small number of
NCs are actively emitting EL. This has been confirmed by super-resolution analysis
of the blinking images. An example of such analysis is shown in Fig. 30.1f, where
three distinct emitting spots are evident. On average 3–7 emission spots are EL
active in each aggregate, and these spots emit consequentially, i.e., at each time the
emission is coming only from one single NC. This fact helps to explain the strong
blinking observed in the EL. Further, there is a size distribution between individual
NCs within each aggregate, as illustrated in the TEM image in Fig. 30.1g. Due to the
quantum confinement effect, the largest NCs within the aggregate have the smallest
bandgap and work as traps for charges captured originally at the surrounding NCs.
The conductive environment present between the NCs enables efficient migration
of the charges to the traps (the largest NCs) from where the EL takes place, as
shown schematically in Fig. 30.1h. This phenomenon of charge funneling is thus
an important aspect of EL devices composed of densely packed perovskite NCs,
especially in the presence of size distribution.