30 Single-Molecule Level Study and Control …
519
ones in Fig. 30.1c, d. These integrated intensities reflect the total number of photons
emitted during the time interval. Since the excitation rates are comparable in PL and
EL, the differences in the integrated intensities (number of emitted photons) directly
reflect differences between the intrinsic QY in PL and EL. A ratio of integrated EL to
integrated PL intensity shows a large distribution which spans a range between 0.12
and 0.66, with the average EL/PL ratio of 0.36. We have previously measured the
PLQY of CsPbBr 3 NCs dispersed in PVK on ensemble level and found a value of 0.3,
which leads to the ELQY value of 10.8%. Assuming further a typical outcoupling
efficiency of a LED of 20% gives the external ELQY of about 2%. We can thus
conclude that the blinking phenomenon is a crucial factor that contributes to the
lower efficiency in electroluminescence of perovskite LED devices.
30.2.2 Stark Effect and Environment Induced Modulation
of Emission in Single Halide Perovskite Nanocrystals
In order to get further insight into the blinking phenomenon that is prominently
present in the electroluminescence measurements, we systematically examined
different external factors that can influence the emission characteristics of CsPbBr 3
nanocrystals in working LED devices. In particular, we investigated the impact of
an environment and of externally applied electric field on PL characteristics of
single CsPbBr 3 NCs by performing steady-state and time-resolved single-particle
PL imaging and spectroscopy in inert and conductive environments [4]. We used a
hole-conducting polymer polyvinyl carbazole (PVK) as the conductive matrix and
polymethyl methacrylate (PMMA) as the insulating matrix, respectively, and applied
electric field in a capacitance-like device. Also, we studied field-dependent changes
in PL peak position and discovered a combination of linear and quadratic Stark effect,
which was attributed to polymer matrix-induced polarization.
Single-particle PL imaging and spectroscopy of CsPbBr 3 NCs were performed
in the polymer matrices of PMMA and PVK. In the time-averaged single-particle
images, individual CsPbBr 3 appears brighter in PMMA as compared to PVK
(Fig. 30.2a, b). Intensity time trajectories of individual particles were monitored
to understand this apparent decrease of PL efficiency in the conductive PVK as
compared to the inert PMMA. The intensity fluctuations in PMMA mostly show
discrete two-state blinking (Fig. 30.2c), i.e., transitions between ON states (highintensity) and OFF states (at the background level). The intensity traces in PVK,
on contrary, show frequent intermediate intensity levels, so-called GRAY states, in
addition to the ON and OFF states (Fig. 30.2d). The difference in average intensity (brightness) of individual CsPbBr 3 in the two matrices can be attributed to the
frequent population of the GRAY states in PVK as compared to PMMA.
The above observations were further complemented by excited-state lifetime
measurements on single-particle level. Excited-state lifetime decay curves of
519
ones in Fig. 30.1c, d. These integrated intensities reflect the total number of photons
emitted during the time interval. Since the excitation rates are comparable in PL and
EL, the differences in the integrated intensities (number of emitted photons) directly
reflect differences between the intrinsic QY in PL and EL. A ratio of integrated EL to
integrated PL intensity shows a large distribution which spans a range between 0.12
and 0.66, with the average EL/PL ratio of 0.36. We have previously measured the
PLQY of CsPbBr 3 NCs dispersed in PVK on ensemble level and found a value of 0.3,
which leads to the ELQY value of 10.8%. Assuming further a typical outcoupling
efficiency of a LED of 20% gives the external ELQY of about 2%. We can thus
conclude that the blinking phenomenon is a crucial factor that contributes to the
lower efficiency in electroluminescence of perovskite LED devices.
30.2.2 Stark Effect and Environment Induced Modulation
of Emission in Single Halide Perovskite Nanocrystals
In order to get further insight into the blinking phenomenon that is prominently
present in the electroluminescence measurements, we systematically examined
different external factors that can influence the emission characteristics of CsPbBr 3
nanocrystals in working LED devices. In particular, we investigated the impact of
an environment and of externally applied electric field on PL characteristics of
single CsPbBr 3 NCs by performing steady-state and time-resolved single-particle
PL imaging and spectroscopy in inert and conductive environments [4]. We used a
hole-conducting polymer polyvinyl carbazole (PVK) as the conductive matrix and
polymethyl methacrylate (PMMA) as the insulating matrix, respectively, and applied
electric field in a capacitance-like device. Also, we studied field-dependent changes
in PL peak position and discovered a combination of linear and quadratic Stark effect,
which was attributed to polymer matrix-induced polarization.
Single-particle PL imaging and spectroscopy of CsPbBr 3 NCs were performed
in the polymer matrices of PMMA and PVK. In the time-averaged single-particle
images, individual CsPbBr 3 appears brighter in PMMA as compared to PVK
(Fig. 30.2a, b). Intensity time trajectories of individual particles were monitored
to understand this apparent decrease of PL efficiency in the conductive PVK as
compared to the inert PMMA. The intensity fluctuations in PMMA mostly show
discrete two-state blinking (Fig. 30.2c), i.e., transitions between ON states (highintensity) and OFF states (at the background level). The intensity traces in PVK,
on contrary, show frequent intermediate intensity levels, so-called GRAY states, in
addition to the ON and OFF states (Fig. 30.2d). The difference in average intensity (brightness) of individual CsPbBr 3 in the two matrices can be attributed to the
frequent population of the GRAY states in PVK as compared to PMMA.
The above observations were further complemented by excited-state lifetime
measurements on single-particle level. Excited-state lifetime decay curves of