Charge Carrier Recombination Dynamics Research Articles

Positional Isomerism of Aromatic Heterocyclic Spacer Cations in Two-Dimensional Dion-Jacobson Hybrid Perovskites.

Ligand engineering of aromatic heterocyclic cations in two-dimensional (2D) Dion-Jacobson (DJ) perovskites has been widely explored in recent years. In this study, how the positional isomers of aromatic heterocyclic cations tune the lattice of 2D perovskites, thereby influencing the transport and recombination dynamics of charge carriers, has been investigated through nonadiabatic molecular dynamics simulations. We demonstrate that the meta-substituted 3-(aminomethyl)pyridinium (3AMPY) cations greatly reduce the strength of electron-vibration coupling since the strong hydrogen-bonding network introduced by the changes in the arrangement of spacer cations significantly suppresses the structural thermal fluctuations. Compared to the para-substituted 4-(aminomethyl)pyridinium (4AMPY) cation, using the asymmetric 3AMPY as a spacer cation can achieve improved in-plane transport performance, enhanced thermal stability, and suppressed charge carrier recombination through weakening electron-vibration interactions. Our results explain the observed lifetime difference between the two types of DJ-phase perovskites in experiments and provide new guidance for optimizing the performance of perovskite devices.

Achieving Inkjet‐Printed 2D Tin Iodide Perovskites: Excitonic and Electro‐Optical Properties

AbstractCurrently, there is a great demand for non‐toxic lead‐free halide perovskites as active materials for solar cells, light‐emitting diodes and other optoelectronic devices. Although an essential progress has been made using tin(II) halide perovskites, still greater efforts are needed to improve their stability and manufacture films and devices under a scalable technology. The first goal of the work is to achieve suitable physical properties of 2D Sn(II) polycrystalline perovskite films obtained by the industrially scalable inkjet printing deposition technique. In the present work, inks of 2D tin(II) halide perovskite 2‐thiopheneethylammonium tin(II) iodide, TEA2SnI4, have been successfully formulated in DMF (toxic) and DMSO (non‐toxic) solutions and using appropriate additives (SnF2 and reducing agents) for improving the stability of the inks and the resulting films. Room‐ and low‐temperature excitonic photoluminescence (PL), charge carrier recombination dynamics and µ‐PL is used to explain the observed two excitonic bands, which are associated to the bulk and edges of perovskite grains nanoplatelet‐like composing the polycrystalline films. Promising electro‐optical properties are also obtained in the TEA2SnI4 films inkjet‐printed from DMSO formulations onto ITO‐interdigitated electrodes, such as low dark currents, ≈10 – 20 nA at 10 V of bias voltage, and high responsivities ≈1–20 A/W.

Improving the photocatalytic activity of graphitic carbon nitride through ionic liquid-mediated thermal annealing

Graphitic carbon nitride (g-C3N4) has shown a great promise in photocatalytic hydrogen evolution. However, the catalytic activity of g-C3N4 derived from nitrogen-rich compounds is still limited by fast charge carrier recombination and slow surface reaction dynamics. Herein, a series of primary g-C3N4 materials were prepared by thermal polymerization of melamine and/or urea, and then thermally treated with the involvement of different ionic liquids under altering heat conditions. Imidazole-containing ionic liquids, such as 1-propyl-3-methylimidazolium chloride ([PMIm]Cl), acted as an effective additive to mediate the thermal annealing behavior of primary g-C3N4, leading to the dissociation of aggregates into porous nanosheets with varying element compositions and with increased surface area. Regardless of little change in optical absorption and electronic structure, the resulting porous g-C3N4 nanosheets promoted the separation and transfer of photogenerated charges and modified surface hydrogen desorption. Hydrogen evolution tests confirmed that the catalytic activity of g-C3N4 from the co-polymerization of melamine and urea was enhanced by ∼3.4 and ∼7.4 times upon [PMIm]Cl-free and -mediated thermal annealing, respectively. This work suggests the potential effect of the additive involvement in the thermal annealing on the photocatalytic activity of g-C3N4.

Optical Absorption, Photocarrier Recombination Dynamics and Terahertz Dielectric Properties of Electron-Irradiated GaSe Crystals

Optical absorption spectra of 9 MeV electron-irradiated GaSe crystals were studied. Two absorption bands with the low-photon-energy threshold at 1.35 and 1.73 eV (T = 300 K) appeared in the transparency region of GaSe after the high-energy-electron irradiation. The observed absorption bands were attributed to the defect states induced by Ga vacancies in two charge states, having the energy positions at 0.23 and 0.61 eV above the valence band maximum at T = 300 K. The optical pump-terahertz probe technique (OPTP) was employed to study the dark and photoexcited terahertz conductivity and charge carrier recombination dynamics at two-photon excitation of as-grown and 9 MeV electron-irradiated GaSe crystals. The measured values of the differential terahertz transmission at a specified photoexcitation condition were used to extract the terahertz charge carrier mobilities. The determined terahertz charge carrier mobility values were ~46 cm2/V·s and ~14 cm2/V·s for as-grown and heavily electron-irradiated GaSe crystals, respectively. These are quite close to the values determined from the Lorentz–Drude–Smith fitting of the measured dielectric constant spectra. The photo-injection-level-dependent charge carrier lifetimes were determined from the measured OPTP data, bearing in mind the model injection-level dependencies of the recombination rates governed by interband and trap-assisted Auger recombination, bulk and surface Shockley–Read–Hall (SRH) recombination and interband radiative transitions in the limit of a high injection level. It was found that GaSe possesses a long charge carrier lifetime (a~1.9 × 10−6 ps−1, b~2.7 × 10−21 cm3ps−1 and c~1.3 × 10−37 cm6ps−1), i.e., τ~0.53 μs in the limit of a relatively low injection, when the contribution from SRH recombination is dominant. The electron irradiation of as-grown GaSe crystals reduced the charge carrier lifetime at a high injection level due to Auger recombination through radiation-induced defects. It was found that the terahertz spectra of the dielectric constants of as-grown and electron-irradiated GaSe crystals can be fitted with acceptable accuracy using the Lorentz model with the Drude–Smith term accounting for the free-carrier conductivity.

(Invited) Charge Carrier Relaxation, Recombination and Extraction Dynamics in Metal Halide Perovskite Nanocrystals

Charge Carrier Relaxation, Recombination and Extraction Dynamics in Metal Halide Perovskite Nanocrystals Anunay Samanta School of Chemistry, University of Hyderabad, Hyderabad 500046 (anunay@uohyd.ac.in)The lead halide-based all-inorganic and hybrid perovskites are under intense investigation due to their immense potential in solar photovoltaic and light emitting applications.1,2 As realization of the full potential of these materials in various applications requires a clear understanding of the relaxation pathways and dynamics of the photo-generated charge carriers, we have been studying these key photo-processes in a wide range of perovskite nanocrystals.3-10 In this talk, we would like to present some of our recent results highlighting how a combination of femtosecond pump-probe and single-particle fluorescence techniques can be used to obtain valuable information on the nature and energy states of the photogenerated species, pathways and dynamics of relaxation of the charge carriers, location and nature of the trap states and their role in fluctuation of the photoluminescence of different caesium lead halide (CsPbX3) perovskite nanocrystals. References Dey, A. et. al. ACS Nano 2021, 15, 10775.Green, M.A.; Ho-Baillie, A.; Snaith, H.J. Nature Photonics 2014, 8, 506.Mondal, N; De, A.; Das, S.; Paul, S.; Samanta, A. Nanoscale 2019, 4, 9796.Mondal, N; De, A.; Samanta, A. ACS Energy Letts. 2019, 4, 32.Seth, S.; Ahmed, T.; Samanta, A. J. Phys. Chem. Lett. 2018, 9, 7007.Mondal, N; De, A.; Samanta, A. J. Phys. Chem. Lett. 2018, 9, 3673.Ahmed, T; Seth. S.; Samanta, A. ACS Nano 2019, 13, 13537.De, A.; Das, A.; Samanta, A. ACS Energy Letts. 2020, 5, 2246.Ahmed, T.; Paul, S.; Samanta, A. J. Phys. Chem. C 2022, 126, 9109.Paul, S.; Samanta, A. J. Phys. Chem. Lett. 2022, 13, 5742.

Electrical Relaxation and Transport Properties of ZnGeP2 and 4H-SiC Crystals Measured with Terahertz Spectroscopy

Terahertz photoconductivity and charge carrier recombination dynamics at two-photon (ZnGeP2) and three-photon (4H-SiC) excitation were studied. Thermally annealed, high-energy electron-irradiated and Sc-doped ZnGeP2 crystals were tested. The terahertz charge carrier mobilities were extracted from both the differential terahertz transmission at a specified photoexcitation condition and the Drude–Smith fitting of the photoconductivity spectra. The determined terahertz charge carrier mobility values are ~453 cm2/V·s for 4H-SiC and ~37 cm2/V·s for ZnGeP2 crystals. The charge carrier lifetimes and the contributions from various recombination mechanisms were determined at different injection levels using the model, which takes into account the influence of bulk and surface Shockley–Read–Hall (SRH) recombination, interband radiative transitions and interband and trap-assisted Auger recombination. It was found that ZnGeP2 possesses short charge carrier lifetimes (a~0.01 ps−1, b~6 × 10−19 cm3·ps−1 and c~7 × 10−40 cm6·ps−1) compared with 4H-SiC (a~0.001 ps−1, b~3 × 10−18 cm3·ps−1 and c~2 × 10−36 cm6·ps−1), i.e., τ~100 ps and τ~1 ns at the limit of relatively low injection, when the contribution from Auger and interband radiative recombination is small. The thermal annealing of as-grown ZnGeP2 crystals and the electron irradiation reduced the charge carrier lifetime, while their doping with 0.01 mass % of Sc increased the charger carrier lifetime and reduced mobility. It was found that the dark terahertz complex conductivity of the measured crystals is not fitted by the Drude–Smith model with reasonable parameters, while their terahertz photoconductivity can be fitted with acceptable accuracy.

The effect of extended alkyl length on molecular orientation of nonfullerene acceptor: A case study in as-cast polymer solar cells

In this work, the effective electronic coupling on the indacenodithiophene (IDT)-based nonfullerene core was investigated by changing the length of the linear alkyl side chain. Extended linear alkyl side chain reduces the extinction coefficient in thin films and promotes the tendency of nonfullerene molecules to edge-on orientation. The theoretical calculation illustrates that the enhanced electronic coupling stem from short alkyl side chain could be beneficial for face-on orientation and charge transport. For the as-cast solar cells, the devices based on IDIC with the short alkyl side chain possessed the best efficiency. Our result manifests that the linear alkyl chain on the core of nonfullerene acceptor dominates molecular packing mode, light absorption, bulk heterojunction size, charge carrier transport and recombination dynamics.

Interplay between charge separation and hole back transfer determines the efficiency of non-fullerene organic solar cells with low energy level offset

Organic bulk heterojunction solar cells with electron acceptors based on small donor-acceptor type molecules show record efficiencies mainly due to their long wavelength absorption, which enables efficient harvesting of solar light and, thus, causes high current density. Meanwhile, relative positions of HOMO and LUMO levels of donor and acceptor materials determine the open circuit voltage. Here, we apply ultrafast transient absorption and transient luminescence techniques together with specially-designed modelling technique to address charge carrier generation and recombination dynamics in detail. We demonstrate the importance of careful adjustment of the HOMO and LUMO levels, as their positions determine formation and recombination rates of interfacial charge transfer (CT) states. An insufficient donor and acceptor LUMO level offset, lower than ∼300 meV, leads to slow and inefficient CT state formation, while an offset of the HOMO level below ∼100 meV leads to fast CT state recombination, which we attribute to the back transfer of a hole from the donor to the acceptor. • CT state formation was investigated by ultrafast transient techniques and modelling. • ∼300 meV LUMO offset is required for efficient CT state generation. • > 100 meV HOMO offset is sufficient for ultrafast hole transfer. • Low HOMO offset opens a loss channel via the back transfer of an electron.

Lifetime of Photogenerated Positive Charges in Hybrid Cerium Oxide-Based Materials from Space and Mirror Charge Effects in Time-Resolved Photoemission Spectroscopy

Space and mirror charge effects in time-resolved photoemission spectroscopy can be modeled to obtain relevant information on the recombination dynamics of charge carriers. We successfully extracted from these phenomena the reneutralization characteristic time of positive charges generated by photoexcitation in CeO2-based films. For the above-band-gap excitation, a large fraction of positive carriers with a lifetime that exceeds 100 ps are generated. Otherwise, the sub-band-gap excitation induces the formation of a significantly smaller fraction of charges with lifetimes of tens of picoseconds, ascribed to the excitation of defect sites or to multiphoton absorption. When the oxide is combined with Ag nanoparticles, the sub-band-gap excitation of localized surface plasmon resonances leads to reneutralization times longer than 300 ps. This was interpreted by considering the electronic unbalance at the surface of the nanoparticles generated by the injection of electrons, via localized surface plasmon resonance (LSPR) decay, into CeO2. This study represents an example of how to exploit the space charge effect in gaining access to the surface carrier dynamics in CeO2 within the picosecond range of time, which is fundamental to describe the photocatalytic processes.

Toward Continuous-Wave Pumped Metal Halide Perovskite Lasers: Strategies and Challenges.

Reliable and efficient continuous-wave (CW) lasers have been intensively pursued in the field of optoelectronic integrated circuits. Metal perovskites have emerged as promising gain materials for solution-processed laser diodes. Recently, the performance of CW perovskite lasers has been improved with the optimization of material and device levels. Nevertheless, the realization of CW pumped perovskite lasers is still hampered by thermal runaway, unwanted parasitic species, and poor long-term stability. This review starts with the charge carrier recombination dynamics and fundamentals of CW lasing in perovskites. We examine the potential strategies that can be used to improve the performance of perovskite CW lasers from the materials to device levels. We also propose the open challenges and future opportunities in developing high-performance and stable CW pumped perovskite lasers.

Efficiency Limits in Wide‐Bandgap Ge‐Containing Donor Polymer:Nonfullerene Acceptor Bulk Heterojunction Solar Cells

A precise picture of the photophysics that determine the efficiency of nonfullerene acceptor (NFA) bulk heterojunction (BHJ) organic solar cells is still missing, yet needed to mitigate the remaining loss channels. Herein, charge carrier generation and recombination dynamics in blends of a novel wide‐bandgap germanium‐containing donor polymer, namely, PEHGeNDT‐BT, paired with either O‐IDTBR or O‐IDTBCN as NFA in BHJ solar cells are investigated by (ultrafast) transient spectroscopy and time‐delayed collection field (TDCF) experiments. Photovoltaic devices yield moderate power conversion efficiencies (PCEs) of 5.3% when using O‐IDTBCN as the acceptor, and only about 2% when using O‐IDTBR as the acceptor, the latter severely limited by its low photocurrent and moderate fill factor (FF) of ≈43%. Time‐resolved photoluminescence experiments reveal limited exciton quenching as one loss channel in O‐IDTBR‐based blends, accompanied by significant geminate recombination further reducing the photocurrent, while field‐dependent charge generation is identified as the origin of the low FF. Geminate recombination is less in the O‐IDTBCN blend and charge generation is field‐independent, leading to improved photocurrent and FF. Carrier drift‐diffusion simulations of the devices’ current–voltage (J–V) characteristics confirm that the experimentally determined kinetic parameters and process yields can reproduce the measured J–V curves under steady‐state solar illumination.

Comparative Study of Recombination Dynamics in Optimized Composition of Sn- Versus Pb-Based Perovskite Solar Cells.

The energy band gaps of Pb halide perovskites are higher than the optimal band gap required for single-junction solar cells, governed by the Shockley-Queisser radiative limit. The pure Sn and Pb-Sn mixed-based perovskites have drawn significant attention due to their ability to lead to lower band gaps and open a new door for all perovskite tandem applications. There has been continuous progress toward the rapid improvement in the power conversion efficiency of Sn and Pb-Sn mixed-based perovskite solar cells (PSCs). Along with efforts for efficiency, it is worth analyzing the in-depth recombination dynamics for further development of Sn-based PSCs. The lower bimolecular recombination rate constant (k) is often attributed to the high performance of PSCs. Herein, we study the role of "B" cations in charge carrier recombination dynamics (CCRD) of ABX3 (A = MA+, FA+, and Cs+; B = Pb2+, Sn2+, and X = I-)-based PSCs. We fabricated p-i-n configuration-based FA0.95Cs0.05PbI3 (pure Pb), MA0.20FA0.75Cs0.05SnI3 (pure Sn), and (MAPbI3)0.4(FASnI3)0.6 (Pb-Sn mixed) PSCs and compared the CCRD of all the three PSCs. We optimized the Sn-based perovskite thin film (pure Sn) in terms of moisture and thermal stability in order to minimize the error due to perovskite degradation. We note that despite having lower open-circuit voltage (VOC), a pure Sn-based PSC shows lower k than that of Pb-Sn mixed and pure Pb-based PSCs, which is a contradictory result. This slow relaxation lifetime of the charge carrier in Sn-based PSCs can be correlated with recombination through the defect states without introducing the quasi-Fermi-level splitting. Furthermore, our results suggest that the rate law of charge carrier decay has nonlinear dependence of k on n in Sn-based PSCs, whereas it is linear in the other two cases.

Facile lattice tensile strain compensation in mixed-cation halide perovskite solar cells

Despite the rapid development of power conversion efficiency (PCE) for halide perovskite solar cells (PSCs), the lattice strain engineering in perovskite thin films has been rarely probed in recent years. Herein, a strain compensation by homogeneous crystallization in perovskite films is achieved with the aid of precursor aging in the mixed-cation perovskite of Cs0.05(FA0.83MA0.17)Pb(I0.90Br0.10)3 with near 20% PCE in inverted devices. The homogeneous crystallization releases the residual tensile stress and induces more compressive stress at the edges of perovskite films, thus elongating the carrier lifetime and reducing the trap-assisted carrier recombination. The high dependence on the perovskite components in strain engineering strategy was systematically revealed, wherein MAPbI3 and Cs0.05(FA0.83MA0.17)PbI3 film showed an increased compressive strain and FAPbI3 film showed adverse tensile strain after aging. The density functional theory (DFT) calculations are further performed to reveal the change of electronic features. The precursor aging-induced strain modulation was correlated with a systematic characterization of the charge carrier transport and recombination dynamics in the mixed-cation perovskite films. We believe that this facile approach provides a novel strain engineering strategy for PSCs technology.

(Invited) Ultrafast Pump-Probe and Single-Particle Photoluminescence Studies of Charge Carrier Recombination and Extraction Dynamics of the Caesium Lead Halide Perovskite Nanocrystals

The lead halide perovskites have emerged in recent years as the most promising materials in photovoltaic and photonic applications.1,2 As utilization of the full potential of these materials in these applications requires a thorough understanding of the pathways through which the photo-generated charge carriers relax and the rate of their relaxation and capture by other substances prior to recombination, we have been investigating the dynamics of charge carrier recombination and extraction in a variety of caesium lead halide perovskite (CsPbX3) nanocrystals employing femtosecond pump-probe, fluorescence upconversion and single-particle fluorescence techniques. In this talk, some of our recent studies, which throw important insight on these topics, will be presented.3-8

Interfacial defect mediated charge carrier trapping and recombination dynamics in TiO2-based nanoheterojunctions

The construction of heterojunctions is an effective strategy to prevent the charge carrier recombination. Owing to the lattice mismatch, it is unavoidable that many defects would be introduced at the heterojunction interface, thus developing a quantitative description on effect of interfacial defects on carrier dynamics become very imminent. In this paper, we employ ultrafast transient absorption (TA) and time resolved photoluminescence (TRPL) spectroscopy to investigate the interfacial defects related carrier trapping, separation, and recombination dynamics in WS2/TiO2 heterojunction. The results reveal the interfacial defects act as the carrier trapping center, and the trapping of electrons possesses active effect, while the co-trapping of both electrons and holes exhibits passive effect on charge carriers transition and separation. Accordingly, both the population and lifetime of carriers are accelerated substantially when only electrons are trapped, whereas the lifetime of charge carriers cannot be prolonged effectively when both electrons and holes are trapped. This work provides a detailed understanding of the interfacial defects mediated charge carrier trapping and recombination dynamics, providing a significant guidance for the future design of high-performance photoactive nanoheterojunctions.

Microwave‐assisted hydrothermal synthesis of Cu‐doped TiO2 nanoparticles for efficient dye‐sensitized solar cell with improved open‐circuit voltage

SummaryAn important reason for the underlying power conversion efficiency (PCE) of the dye sensitized solar cells (DSSCs) is the low open‐circuit voltage (Voc). Considerable efforts have been made to increase the Voc of DSSCs by suppressing charge carrier recombination dynamics, co‐sensitization, and other strategies. Herein, we report an enhancement of the Voc by the incorporation of Cu into TiO2 microstructure via microwave‐assisted hydrothermal synthesis. The prepared nanoparticles and the photoanodes were tested for their efficacy with various structural, optical, and electrical characterization tools. It was found that the incorporation of Cu2+ into TiO2 lattice raises its conduction band (CB) edge and shifted the Fermi level upwards, which contribute towards enhanced Voc. A significant enhancement in Voc from 0.714 to 0.781 V was observed by the Cu incorporation. An optimized 1 wt% Cu modified TiO2 DSSC has shown an efficiency of 6.94% with a high Voc of 0.762 V.

Spin-Dependent Charge-Carrier Recombination Processes in Tris (8-Hydroxyquinolinato) Aluminum

We have studied the nature and dynamics of spin-dependent charge carrier recombination in Tris(8-hydroxyquinolinato) aluminum (Alq$_3$) films in light emitting diodes at room temperature using continuous wave and pulsed electrically detected magnetic resonance (EDMR) spectroscopy. We found that the EDMR signal is dominated by an electron-hole recombination process, and another, weaker EDMR signal whose fundamental nature was investigated. From the pulsed EDMR measurements we obtained a carrier spin relaxation time, $T_2 = 45\pm 25$ ns which is much shorter than $T_2$ in conjugated polymers, but relatively long for a molecule containing elements with high atomic number. Using multi-frequency continuous wave EDMR spectroscopy, we obtained the local hyperfine field distributions for electrons and holes, as well as their respective spin-orbit coupling induced g-factor and g-strain values.

Direct Spectroscopic Observation of the Hole Polaron in Lead Halide Perovskites

Intrinsic photophysical origin of lead halide perovskites (LHPs) succeeding in optolectronic applications still remains hotly debated. Here, by using the ultrafast X-ray transient absorption spectroscopy, we successfully tracked the fate of photogenerated charge carriers at room temperature within the thin films of two classic LHPs, namely MAPbBr3 (MA: CH3NH3) and FAPbBr3 (FA: CH(NH2)2). We clearly observed in both thin films that the hole polaron is formed by localizing the photogenerated hole at Br 4p orbital and concurrently distorting the local structure surrounding Br atom after the photoexcitation. Furthermore, the bigger cation FA in the cavity of [PbBr6]4- octahedral framework induces larger hole polaron effect due to its p orbital hybridization into valence and conduction bands, correlating with the slower charge carrier recombination dynamics. Our direct experimental observation of the localized hole polaron in perovskites should advance the fundamental comprehension of charge carrier behavior within LHPs and their related devices.

Synthesis and Localized Photoluminescence Blinking of Lead-Free 2D Nanostructures of Cs3 Bi2 I6 Cl3 Perovskite.

Two-dimensional (2D) lead-free halide perovskites have generated enormous perception in the field of optoelectronics due to their fascinating optical properties. However, an in-depth understanding on their shape-controlled charge-carrier recombination dynamics is still lacking, which could be resolved by exploring the photoluminescence (PL) blinking behaviour at the single-particle level. Herein, we demonstrate, for the first time, the synthesis of nanocrystals (NCs) and 2D nanosheets (NSs) of layered mixed halide, Cs3 Bi2 I6 Cl3 , by solution-based method. We applied fluorescence microscopy and super-resolution optical imaging at single-particle level to investigate their morphology-dependent PL properties. Narrow emission line widths and passivation of non-radiative defects were evidenced for 2D layered nanostructures, whereas the activation of shallow trap states was recognized at 77 K. Interestingly, individual NCs were found to display temporal intermittency (blinking) in PL emission. On the other hand, NS showed temporal PL intensity fluctuations within localized domains of the crystal. In addition, super-resolution optical image of the NS from localization-based method showed spatial inhomogeneity of the PL intensity within perovskite crystal.

(Invited) Charge Carrier Dynamics and Single-Particle Photoluminescence of the Caesium Lead Halide Perovskite Nanocrystals

Because of their potential in photovoltaic and photonic applications, great attention is being paid to both hybrid and all-inorganic lead halide perovskite nanocrystals.1,2 However, despite growing interests on these substances, our understanding of different pathways of relaxation of the photo-generated charge carriers and their dynamics, which is key to utilization of the full potential of these materials in various applications, is still quite limited. In this talk, we plan to discuss some of our recent works on charge carrier recombination dynamics of caesium lead halide perovskite (CsPbX3) nanocrystals highlighting how femtosecond pump-probe and single-particle fluorescence based techniques can be effectively employed to obtain information on the identity and energy states of the photogenerated species, pathways and dynamics of relaxation of the charge carriers (including their recombination), location and nature of the trap states and their role in fluctuation of the photoluminescence of different CsPbX3 perovskite nanocrystals.3-7