Lead halide perovskite materials have shown strong promise in energy harvesting and generation over the past five years. However, their poor ambient stability and lead toxicity issues hinder optoelectronic applications. In the quest for alternatives, metal halide perovskites with lower toxicity and more stable metals have recently emerged. The divalent Pb2+ could be replaced with isoelectronic Sn2+, but Sn2+ tends to oxidize rapidly in presence of air to Sn4+, forming a defect in the structure. However, Sn2+‐based perovskites have been stabilized in 2D structures. Recently Sn4+ based halide perovskites nanocrystals with have been reported with poor luminescence. The replacement of Pb2+ with isoelectronic trivalent elements (Sb3+, Bi3+) results in A3B2X9 type defect‐order perovskite structure, which shows promises for optoelectronic applications. The perovskite nanocrystals of Sb, Bi have been reported in the form of dimers and layered structures. In addition to these, double perovskites, where two divalent Pb2+ are replaced with a monovalent and a trivalent cation have been reported very recently. In this Focus Review, we give a brief summary of different non‐lead perovskite nanocrystals starting from synthesis, characterization, stability, properties to applications, along with potential future directions.

A deep understanding of hot carrier (HC) dynamics is important to improve the performance of optoelectronic devices by reducing the thermalization losses. Here, we investigate the hot hole cooling and transfer dynamics of CsPbBr 3 nanocrystals (NCs) using 5,10,15,20-tetra(4pyridyl) porphyrin (TpyP) molecules. Density functional theory (DFT) is used to elucidate the mechanism underlying charge extraction as well as the HC transfer process in the CsPbBr 3 −TpyP system. It is noted that the hot hole states are localized around the top surface of CsPbBr 3 , while the hot electron states are delocalized away from its top surface, indicating easy extraction of hot holes from the CsPbBr 3 by TpyP molecules, as compared to the hot electrons. The significant drop of initial hot carrier temperature from 1140 to 638 K at 400 nm excitation confirms the hot hole transfer from CsPbBr 3 NCs to TpyP molecules, which is dependent on the excitation energy, and the maximum transfer efficiency is found to be 42% (for 0.85 eV above band edge photoexcitation). In addition, the hot hole transfer rate is almost 11 times faster than the band edge hole transfer rate. Our findings are relevant for the development of next-generation perovskite-based optoelectronic devices.

Thermalization of photogenerated HCs occurs by dissipating their excess energy as heat energy through phonons which is the major intrinsic loss channel for solar cell devices. [3] Harnessing the excess energy of photoexcited HCs will allow us to achieve maximum power conversion efficiency (PCE) up to 67% for a single-junction solar cell under one sun illumination, [4] breaking the socalled Shockley-Queisser (SQ) limit of 34%. [5] Photoexcited HCs are used in photo-catalysis, photodetection, and highpower optoelectronic devices to improve efficiency. [6,7] However, rapid energy loss mechanisms of HCs in most of the conventional semiconductor nanomaterials (e.g., GaAs, PbSe, InN, and CdSe) through carrier-phonon scattering processes in sub-picosecond timescale severely restrict the utilization of non-thermalized excess energy of photo-excited HCs. [8][9][10][11] Therefore, it is essential to develop a solar absorber with retarded HC cooling rate. [12,13] Due to their extraordinary performance, metal halide perovskite nanocrystals (NCs) have recently emerged as front-runner materials in low-cost, high-performance solar cells. [14][15][16] A lot of interest has been shown to understand the HC cooling dynamics of lead halide perovskite (LHP)to find out potential applications. [17][18][19][20][21] Slow HC relaxation mechanisms in perovskite materials are reported due to the hot-phonon bottleneck effect, [22] Auger-heating effect, [23] band-filling effects, [24] dielectric screening, [25] and significant polaron screening effects. [26] However, in most cases, high pump fluence with photo-excited carrier densities of 10 18 -10 19 cm −3 was used, which is hard to accomplish in practical conditions. [22,27] It is worth noting that HC relaxation of LHP still occurs very rapidly (within hundreds of femtoseconds) under weak carrier densities (comparable to sun illumination level, ≈10 17 cm −3 ). [28,29] Thus, slowing down the HC relaxation rate of halide-based perovskite materials under low excitation power densities is a challenge for hotcarrier-based optoelectronic applications. Recently, delayed HC cooling rate has been reported in CsPbBr 3 based asymmetric multiple quantum wells (MQWs) due to sequential hot-electron transfer between CsPbBr 3 layers. [30] Tuning the HC cooling dynamics of metal halide perovskite is mainly limited to reduction of dimensionality, [31] changing cation/ halide ions, [32][33][34] and doping impurity ions. [35] Therefore, significant efforts are Metal halide perovskite nanocrystals have recently emerged as a front-runner material for high-performance solar cells. However, slowing down the hot carrier (HC) cooling of perovskites at carrier densities comparable to the sun-illumination level (≈10 17 cm −3 ) is still a thriving challenge. A new strategy is presented to retard the HC cooling via charge localization at the CsPbBr 3 / PbSe heterostructure interface. Ultrafast transient absorption study reveals two times slower HC relaxation time (from 770 fs to 1.4 ps) and much higher initial HC temper...

Manipulation of intrinsic carrier relaxation is crucial for designing efficient lead halide perovskite nanocrystal (NC) based optoelectronic devices. The influence of heterovalent Bi 3+ doping on the ultrafast carrier dynamics and hot carrier (HC) cooling relaxation of CsPbBr 3 NCs has been studied using femtosecond transient absorption spectroscopy and first-principles calculations. The initial HC temperature and LO phonon decay time point to a faster HC relaxation rate in the Bi 3+ -doped CsPbBr 3 NCs. The first-principles calculations disclose the acceleration of carrier relaxation in Bi 3+ -doped CsPbBr 3 NCs due to the appearance of localized bands (antitrap states) within the conduction band. The higher Born effective charges (Z*) and higher soft energetic optical phonon density of states cause higher electron−phonon scattering rates in the Bi-doped CsPbBr 3 system, which is responsible for the faster HC cooling rate in doped systems.

Two-dimensional (2D) material-based composites are considered to be an important class of materials for light-harvesting applications because of their efficient charge separation. In this article, we have designed composites of 2D CdSe nanoplatelets (NPLs) and CsPbX 3 (X = mixture of Br and I or I) perovskite nanocrystals and investigated their ultrafast carrier dynamics using ultrafast spectroscopy. A time-resolved fluorescence upconversion study reveals that the electron transfer from CdSe NPLs to CsPbX 3 varies with changing the composition of perovskite from CsPbBr 1.5 I 1.5 to CsPbI 3 . From the transient absorption spectroscopic study, the shortening of the faster component of bleach recovery kinetics of CdSe NPLs along with the enhancement of growth time of CsPbX 3 NCs in composites indicates the ultrafast electron transfer from CdSe NPLs to CsPbX 3 NCs. The ultrafast electron transfer from 2D CdSe NPLs to CsPbX 3 NCs enhances in the following order: CsPbI 3 > CsPbBrI 2 > CsPbBr 1.5 I 1.5 . The dark current and photocurrent are 0.04 and 62.4 μA in the CdSe−CsPbI 3 composite. The dramatically improved photocurrent response in the CdSe−CsPbI 3 composite confirms the enhancement of their efficient charge separation because of the ultrafast electron transfer from CdSe NPLs to pervoskite NCs. Our finding reveals that the integration of 2D CdSe NPLs with perovskite NCs offers a great opportunity for the improvement of the efficiency of perovskite solar cells by engineering the interfacial chargetransfer dynamics.

Two-dimensional (2D) cesium lead halide perovskite nanoplatelets (NPLs) have received tremendous attention due to their unique properties for designing solar cell applications. Here, we investigated the crystal structure of 2D CsPbBr 3 nanoplatelets (NPLs) and their ultrafast carrier relaxation dynamics with varying the monolayer (ML) thickness using femtosecond transient absorption spectroscopy (fs-TAS). Rietveld analysis suggested that the basal planes of the NPLs are composed of ( 101) and ( 101) planes while the remaining four facets (thickness) are composed of ( 101), ( 101), (010), and (010) planes of the orthorhombic phase. The formation of the orthorhombic CsPbBr 3 NPLs by stacking the structural motifs of PbBr 6 octahedra in the crystallographic directions is evident from the atomic modeling. The change of monolayer thickness leads to a red-shift of the excitonic absorption band and PL band and enhancement of decay time. The cooling dynamics of the hot carrier to the band-edge state by phonon emission varies from 140 to 210 fs with thickness by modification of quantum confinement and dielectric screening. We observed both energy and charge transfer between 2D CsPbBr 3 NPLs with an organic chromophore, N,N′-bis(hexadecyl)perylene-3,4,9,10tetracarboxylic acid diimide (PDI), which is thickness-dependent. A deep understanding of the photoinduced carrier dynamics of the 2D CsPbBr 3 NPLs will pave the way to designing 2D perovskite-based photovoltaic devices.

Red-emitting carbon dots (C-dots) have tremendous potential for bioimaging and optoelectronic applications. Here, we investigated the structural modification of red-emitting C-dots due to boron doping and their ultrafast relaxation dynamics. It is evident from the X-ray photoelectron spectroscopy study that the relative percentage of pyrridinic nitrogen is increased at the expense of amino nitrogen and graphitic nitrogen in B-doped C-dots. Transient absorption spectroscopy and global target analysis reveal the formation of an additional excited-state level that takes away a significant amount of the excited-state population after boron doping. This new excited state slows the initial relaxation process toward the emissive state from 317 to 750 fs and increases the overall lifetime from 1.03 to 1.45 ns in B-doped C-dots.

Oligothiophenes and their aggregates play a dominant role in optoelectronic and light-harvesting applications. Here, we controlled the degree of aggregation of 2,2′:5′,2″:5′′,2‴quaterthiophene (QTH) to shed light on the impact of the aggregation on the excited state dynamics. QTH aggregation realized the control over the Intersystem Crossing (ISC) rates and, in turn, the formation of triplet excited states via the simple addition of water to QTH solutions in THF. From global target analysis, the time scale was 345.5 ps for ISC for QTHs in THF, but it was 2.33 ns in the case of QTH solutions featuring 70% water. Notably, the excitonic coupling between closely packed QTHs occurred predominantly in the aggregates formed in the presence of large water concentrations. Relaxation dynamics of the resulting QTH-aggregates differed substantially from QTH solutions at lower water content. For example, QTH-aggregates lacked any triplet excited states, and the unusual emission occurs from lower excitonic states from these predominantly H-aggregates.