11 The Confinement and Migration of Charge-Carriers in Lead …
199
exciton with high E b is called Frenkel exciton whose R B is much smaller than the a
of the material (Fig. 11.1b). In halide perovskites, E b lies in between those of MottWannier and Frenkel excitons and is either less than or comparable to the thermal
energy. Hence, free charge-carriers dominate in the films and bulk single crystals
of these materials. Studies have shown that the free charge-carriers are the consequence of dissociation of excitons when the associated binding energy is overcome
by the thermal energy [8–12]. The free charge-carrier property of halide perovskites
is associated with long carrier diffusion lengths and lifetimes, as observed in the case
of bulk films and single crystals [2, 3]. Nevertheless, the carrier lifetime varies in the
range of sub-nanoseconds to microseconds in different types of halide perovskites,
depending upon their size, dimensionality, and composition [2, 3, 8–15].
Exciton binding energies in perovskite nanocrystals (PNCs) are higher than the
corresponding bulk crystal. For example, E b of 320 meV is reported for MAPbBr 3
(MA = methylammonium, CH 3 NH 3
+ ) nanocrystals, which is more than three times
higher (84 meV) than that for the corresponding bulk crystal [16]. Therefore, quantum
confinement effects are observed in PNCs as well, when their physical size becomes
smaller than the corresponding R B values [13, 15]. Additionally, the quantum confinement effects in PNCs are depended upon not only the size but also the dimensionality.
For instance, blue-shift in absorption and PL spectra, which is associated with the
increase in optical band-gap, is observed in the case of perovskite nanoplatelets
when the layer thickness decreases below R B [15]. Despite the quantum confinement effects in PNCs, when compared to the conventional chalcogenide QDs, slow
cooling and efficient extraction of hot carriers are reported in their colloidal solutions [17]. The intrinsic phonon bottleneck and Auger heating effects at low and high
carrier densities, respectively, in PNCs account for such observations. Apart from
this, the excellent optoelectronic properties of PNCs, which are comparable to the
conventional QDs, are maintained even at the weak quantum confinement regime.
While R B in lead halide perovskites lies in the range of 2–7 nm [13, 15, 16, 18, 19],
most of the studies demonstrating the light-emitting [4, 6] and photovoltaic [17, 18]
applications of PNCs show the nanocrystal size either only slightly smaller or larger
than the corresponding R B value. This suggests that, compared to the chalcogenide
QDs, the application of PNCs in light-harvesting and light-emitting device is diverse
and immense.
11.2 Photoluminescence Blinking
In quantum-confined semiconductor nanocrystals or QDs, the photogenerated electrons and holes are strongly correlated. Carrier dynamics in such strongly confined
QDs is governed by the discrete energy states showing sharp excitonic band in the
absorption spectrum, fast radiative recombination of charge-carriers, carrier multiplication or multi-exciton generation, and increased Auger recombination [20]. Moreover, the PL properties of these semiconductor nanocrystals at their single particle
level are often intricated by the stochastic emission behavior with a sequence of bright
199
exciton with high E b is called Frenkel exciton whose R B is much smaller than the a
of the material (Fig. 11.1b). In halide perovskites, E b lies in between those of MottWannier and Frenkel excitons and is either less than or comparable to the thermal
energy. Hence, free charge-carriers dominate in the films and bulk single crystals
of these materials. Studies have shown that the free charge-carriers are the consequence of dissociation of excitons when the associated binding energy is overcome
by the thermal energy [8–12]. The free charge-carrier property of halide perovskites
is associated with long carrier diffusion lengths and lifetimes, as observed in the case
of bulk films and single crystals [2, 3]. Nevertheless, the carrier lifetime varies in the
range of sub-nanoseconds to microseconds in different types of halide perovskites,
depending upon their size, dimensionality, and composition [2, 3, 8–15].
Exciton binding energies in perovskite nanocrystals (PNCs) are higher than the
corresponding bulk crystal. For example, E b of 320 meV is reported for MAPbBr 3
(MA = methylammonium, CH 3 NH 3
+ ) nanocrystals, which is more than three times
higher (84 meV) than that for the corresponding bulk crystal [16]. Therefore, quantum
confinement effects are observed in PNCs as well, when their physical size becomes
smaller than the corresponding R B values [13, 15]. Additionally, the quantum confinement effects in PNCs are depended upon not only the size but also the dimensionality.
For instance, blue-shift in absorption and PL spectra, which is associated with the
increase in optical band-gap, is observed in the case of perovskite nanoplatelets
when the layer thickness decreases below R B [15]. Despite the quantum confinement effects in PNCs, when compared to the conventional chalcogenide QDs, slow
cooling and efficient extraction of hot carriers are reported in their colloidal solutions [17]. The intrinsic phonon bottleneck and Auger heating effects at low and high
carrier densities, respectively, in PNCs account for such observations. Apart from
this, the excellent optoelectronic properties of PNCs, which are comparable to the
conventional QDs, are maintained even at the weak quantum confinement regime.
While R B in lead halide perovskites lies in the range of 2–7 nm [13, 15, 16, 18, 19],
most of the studies demonstrating the light-emitting [4, 6] and photovoltaic [17, 18]
applications of PNCs show the nanocrystal size either only slightly smaller or larger
than the corresponding R B value. This suggests that, compared to the chalcogenide
QDs, the application of PNCs in light-harvesting and light-emitting device is diverse
and immense.
11.2 Photoluminescence Blinking
In quantum-confined semiconductor nanocrystals or QDs, the photogenerated electrons and holes are strongly correlated. Carrier dynamics in such strongly confined
QDs is governed by the discrete energy states showing sharp excitonic band in the
absorption spectrum, fast radiative recombination of charge-carriers, carrier multiplication or multi-exciton generation, and increased Auger recombination [20]. Moreover, the PL properties of these semiconductor nanocrystals at their single particle
level are often intricated by the stochastic emission behavior with a sequence of bright