While perovskite nanocrystals (NCs) have shown great promise as materials for efficient light-emitting diodes (LEDs), low photoluminescence quantum yield (PLQY) of the blue-emitting perovskites is an impediment to the development of white LEDs of which blue is an essential component. Herein, we report that room temperature postsynthetic treatment of weakly blue-violet-emitting (PLQY 3%) CsPbCl3 NCs with CdCl2 results in an instantaneous enhancement of the PLQY to near-unity without affecting the PL peak position (406 nm) and spectral width. The time-resolved PL and ultrafast transient absorption measurements confirm the removal of nonradiative defect states of the CsPbCl3 NCs in treated sample. The elemental composition and structural data of the treated sample reveal facile doping of Cd2+ into the crystal lattice without affecting the size and shape of the NCs. Extraordinary PLQY, high air stability and photostability and ease of preparation of this Cd-doped CsPbCl3 make it by far the most attractive blue-emitting perovskite for development of efficient blue and white LEDs.

Understanding the nature and dynamics of the photo-induced transients of all-inorganic perovskite nanocrystals (NCs) is key to their exploitation in potential applications. In order to determine the nature of charge carriers, their deactivation pathways and dynamics, the photo-induced transients of CsPbBr, CsPbBrI, CsPbBrI and CsPbI NCs are spectrally and temporally characterized employing a combination of femtosecond transient absorption (TA) and photoluminescence (PL) up-conversion techniques and global analysis of the data. The results provide distinct identities of the excitons and free charge carriers and distinguish the hot charge carriers from the cold ones. The carrier trapping is attributed to the electrons and their dynamics is unaffected in mixed halide perovskites. The excitation energy dependence of the TA dynamics suggests that the trap states are shallow in nature and mainly limited near the band-edge level. In mixed halide perovskites, an increase in the iodine content leads to hole trapping in a short time scale (<5 ps). The insights obtained from this study are likely to be helpful for tuning the photo-response of these substances and their better utilization in light-based applications.

Mn-Doped perovskite nanocrystals (NCs) are a new class of materials offering exciting opportunities to control over their optical and magnetic properties. Herein, we report a series of Mn-doped CsPbCl NCs exhibiting a tunable Mn photoluminescence (PL) band with a PL peak wavelength pushed up to 625 nm and tuned over a range of 40 nm, the largest achieved so far, by only varying the Mn content. The X-band EPR data and Mn PL decay behaviour of the NCs reveal that the exchange interaction between Mn ions is mainly responsible for a large shift of the Mn PL band. Ultrafast pump-probe measurements show that exciton-dopant energy transfer in these NCs is slower (∼50-100 ps) than trapping of the carriers (∼8-10 ps) in the host lattice. The large PL tuning reported here along with the insights into the mechanism of tuning and carrier dynamics are expected to boost the potential of Mn-doped CsPbCl NCs in light-powered devices.

Study of the emission behavior of all-inorganic perovskite nanocrystals CsPbBr3 and CsPbBr2I as a function of the excitation power employing fluorescence correlation spectroscopy and conventional techniques reveals fluorescence blinking in the microsecond time scale and photoinduced emission enhancement. The observation provides insight into the radiative and nonradiative deactivation pathways of these promising substances. Because both blinking and photoactivation processes are intimately linked to the charge separation efficiency and dynamics of the nanocrystals, these key findings are likely to be helpful in realizing the true potential of these substances in photovoltaic and optoelectronic applications.

Studies on ultrafast dynamics of various photo-induced processes in perovskite nanocrystals and their composites, and insights obtained from them are presented in this review.

Rotational dynamics of two dipolar solutes, 4-aminophthalimide (AP) and 6-propionyl-2-dimethylaminonaphthalene (PRODAN), and a nonpolar solute, anthracene, have been studied in N-alkyl-N-methylmorpholinium (alkyl = ethyl, butyl, hexyl, and octyl) bis(trifluoromethansulfonyl)imide (Tf2N) ionic liquids as a function of temperature and excitation wavelength to probe the microheterogeneous nature of these ionic liquids, which are recently reported to be more structured than the imidazolium ionic liquids (Khara and Samanta, J. Phys. Chem. B2012, 116, 13430-13438). Analysis of the measured rotational time constants of the solutes in terms of the Stokes-Einstein-Debye (SED) hydrodynamic theory reveals that with increase in the alkyl chain length attached to the cationic component of the ionic liquids, AP shows stick to superstick behavior, PRODAN rotation lies between stick and slip boundary conditions, whereas anthracene exhibits slip to sub slip behavior. The contrasting rotational dynamics of these probe molecules is a reflection of their location in distinct environments of the ionic liquids thus demonstrating the heterogeneity of these ionic liquids. The microheterogeneity of these media, in particular, those with the long alkyl chain, is further evidence from the excitation wavelength dependence study of the rotational diffusion of the dipolar probe molecules.

Among the lead halide perovskites, photoluminescence quantum yield (PLQY) of violet-emitting CsPbCl3 nanocrystals (NCs) is the lowest (<5%). This is an impediment to the development of perovskite-based materials for optical applications covering the entire visible region. While PLQY of the green- and red-emitting perovskites of this class has been raised to near-unity, achieving a similar level for violet- and blue-emitting NCs is still quite challenging. Herein, we report a novel method of simultaneously passivating the surface defects and crystal disorder of violet-emitting CsPbCl3 NCs to dramatically enhance (by a factor of ∼120) the PLQY and stability without affecting the peak wavelength (403 nm) and full-width at half-maximum (FWHM) of the photoluminescence (PL) band. We show that the addition of the correct quantity of CuCl2 during the hot-injection synthesis of CsPbCl3 NCs leads to doping of Cu+ into the NCs, which rectifies octahedral distortion of the crystal and the Cl– passivates the surface; the combined influence of the two results in huge PL enhancement. NCs emitting throughout the blue region (430-460 nm) with near-unity PLQY (92%–98%) can then be obtained by partial halide-exchange of the doped sample. Femtosecond transient absorption studies suggest suppression of the ultrafast carrier trapping process in doped NCs. The results help extending the utility of these materials in optical applications by covering the violet–blue region as well.

The interaction between colloidal CdTe quantum dots (QDs) and silver nanoparticles (Ag NPs), both in their negatively charged state, is studied to determine the importance of surface charge and epitaxial coupling of the two components to achieve the potential activity of the metal–semiconductor nanocomposites. An unexpected strong interaction between the two similarly charged species is evident from dramatic quenching of the excitonic emission of the QDs by the Ag NPs. Direct evidence of electron transfer from the photoexcited QDs to Ag NPs is obtained from an accelerated bleach recovery of the first exciton band of the QDs and a faster carrier cooling in the presence of Ag NPs in transient absorption measurements. The evidence of charge carrier trapping is demonstrated by the observation of a new broad positive transient absorption in the visible–near-infrared region, which was absent in their isolated counterparts. The electron transfer and charge carrier trapping processes are further substantiated by the results of similar measurements on core/shell (CdTe/ZnS) QDs. As ultrafast charge carrier (especially the hole) trapping slows the charge recombination process, the present findings open up new possibilities of harnessing solar energy and improving the photocatalytic activity by employing these colloidal mixtures.