Hot Carrier Relaxation in CsPbBr3-Based Perovskites: A Polaron Perspective

Time-Resolved Dynamics of Metal Halide Perovskite under High Pressure: Recent Progress and Challenges

Metal halide perovskites have garnered significant attention in the scientific community for their promising applications in optoelectronic devices. The application of pressure engineering, a viable technique, has played a crucial role in substantially improving the optoelectronic characteristics of perovskites. Despite notable progress in understanding ground-state structural changes under high pressure, a comprehensive exploration of excited-state dynamics influencing luminescence remains incomplete. This Perspective delves into recent advances in time-resolved dynamics studies of photoexcited metal halide perovskites under high pressure. With a focus on the intricate interplay between structural alterations and electronic properties, we investigate electron-phonon interactions, carrier transport mechanisms, and the influential roles of self-trapped excitons (STEs) and coherent phonons in luminescence. However, significant challenges persist, notably the need for more advanced measurement techniques and a deeper understanding of the phenomena induced by high pressure in perovskites.

Controlling Charge Carrier Dynamics in Porphyrin Nanorings by Optically Active Templates

Understanding the dynamics of photogenerated charge carriers is essential for enhancing the performance of solar and optoelectronic devices. Using atomistic quantum dynamics simulations, we demonstrate that a short π-conjugated optically active template can be used to control hot carrier relaxation, charge carrier separation, and carrier recombination in light-harvesting porphyrin nanorings. Relaxation of hot holes is slowed by 60% with an optically active template compared to that with an analogous optically inactive template. Both systems exhibit subpicosecond electron transfer from the photoactive core to the templates. Notably, charge recombination is suppressed 6-fold by the optically active template. The atomistic time-domain simulations rationalize these effects by the extent of electron and hole localization, modification of the density of states, participation of distinct vibrational motions, and changes in quantum coherence. Extension of the hot carrier lifetime and reduction of charge carrier recombination, without hampering charge separation, demonstrate a strategy for enhancing efficiencies of energy materials with optically active templates.

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Probing the charge transfer mechanisms in type-II Cs2AgBiBr6-CdSe composite system: ultrafast insights

Lead-free halide-based double perovskites (DPs) have established themselves as the emerging nontoxic alternatives for photovoltaic (PV) applications thus substituting the long-standing lead halide perovskites. Among the prospective lead-free DPs, Cs₂AgBiBr₆has gained immense popularity owing to the fascinating properties demonstrated by them including low carrier effective mass and microsecond lifetime for electron-hole recombination. Nevertheless, the large, indirect bandgap remains the prime hurdle that restrains commercialization of the Cs₂AgBiBr₆DPs based PV devices. A rational solution could be designing its heterostructure with another suitable material that could mitigate the inadequacies of Cs₂AgBiBr₆DPs. With this line of thought, herein we synthesized a composite of Cs₂AgBiBr₆DPs with CdSe NCs and then performed transient absorption (TA) spectroscopic measurements to introspect its photophysical aspects. Executing excitation energy-dependent studies clearly reveal the carrier transfer efficiency to be strongly pump-dependent. Upon exciting with 350 nm pump, in compliance with the energy band alignment and tendency of both the constituents to be photoexcited across their bandgap, there is a bidirectional transfer of hot electrons anticipated in the composite system. Nevertheless, the TA outcomes indicate the transfer of hot electrons from CdSe to Cs₂AgBiBr₆to be more favorable out of the bidirectional pathways. Employing further lower pump energies (480 nm) when only CdSe NCs are capable of being excited, the transfer efficiency of the electrons from CdSe to Cs₂AgBiBr₆is noticed to be fairly low. Besides this, when the pump wavelength is tuned to 530 nm i.e. quite close to the CdSe band edge, no electron transfer is noticeable despite the anticipation from thermodynamic feasibility. Thus, as reflected by the TA kinetics, electron transfer is discerned to be more efficient from the hot states rather than the band edges. Most advantageously, charge separation is successfully achieved in this never explored composite architecture which eases the carrier extraction and minimizes the otherwise prevalent fast recombination processes.

Elucidating the Unique Hot Carrier Cooling in Two-Dimensional Inorganic Halide Perovskites: The Role of Out-of-Plane Carrier-Phonon Coupling

Two-dimensional (2D) halide perovskites represent the natural semiconductor quantum wells (QWs), which hold great promise for optoelectronics. However, due to the hybrid structure of Ruddlesden-Popper 2D perovskites, the intrinsic nature of hot-carrier kinetics remains shielded within. Herein, we adopt CsPbBr₃ nanoplates as a model system to reveal the intrinsic carrier dynamics in inorganic perovskite QWs. Interestingly, we revealed an ultrafast and hot-phonon-bottleneck (HPB)-free carrier cooling in monodisperse CsPbBr₃ QWs, which is in sharp contrast to the bulk and nanocrystalline perovskites. The absence of HPB was attributed to the efficient out-of-plane triplet-exciton-LO-phonon coupling in 2D perovskites because of the structural anisotropy. Accordingly, the HPB can be activated by shutting down the out-of-plane energy loss route through forming the layer-stacked perovskite superlattice. The controllable on and off of HPB may provide new possibilities in optoelectronic devices and these findings deepen the understanding of a hot-carrier cooling mechanism in 2D perovskites.

Nanoscale-Resolved Surface-to-Bulk Electron Transport in CsPbBr3 Perovskite

Describing the nanoscale charge carrier transport at surfaces and interfaces is fundamental for designing high-performance optoelectronic devices. To achieve this, we employ time- and angle-resolved photoelectron spectroscopy with ultraviolet pump and extreme ultraviolet probe pulses. The resulting high surface sensitivity reveals an ultrafast carrier population decay associated with surface-to-bulk transport, which was tracked with a sub-nanometer spatial resolution normal to the surface, and on a femtosecond time scale, in the case of the inorganic CsPbBr₃ lead halide perovskite. The decay time exhibits a pronounced carrier density dependence, which is attributed via modeling to enhanced diffusive transport and concurrent recombination. The transport is found to approach an ordinary diffusive regime, limited by electron-hole scattering, at the highest excitation fluences. This approach constitutes an important milestone in our capability to probe hot-carrier transport at solid interfaces with sub-nanometer resolution in a theoretically and experimentally challenging, yet technologically relevant, high-carrier-density regime.

Defect-Interceded Cascading Energy Transfer and Underlying Charge Transfer in Europium-Doped CsPbCl3 Nanocrystals

Rare-earth ion (RE³⁺) doping in cesium lead chloride (CsPbCl₃) has unlocked novel prospects to explore changes in optical, magnetic, and charge carrier transport properties. This leads to a huge advancement in optoelectronic applications, yet deep understanding of the photophysics governing the energy transfer processes is lacking and demands vital attention. Herein, we probe into the mechanistic transfer processes from the band edge of the host (CsPbCl₃) to the dopant europium ion (Eu³⁺) with the aid of femtosecond fluorescence upconversion and transient absorption (TA) spectroscopy. The upconversion measurement portrays a defect-mediated cascading energy transfer from CsPbCl₃ to Eu³⁺ and further cross-relaxation among Eu³⁺ states. Moreover, TA studies reveal that there is charge transfer from the band edge of CsPbCl₃ to doping-induced shallow defect states. Furthermore, two-photon absorption study establishes no compromise in the transfer mechanism even upon bandgap excitation. This work validates that Eu-CsPbCl₃ is an apt entrant for optoelectronic applications.

Scalable Ehrenfest Molecular Dynamics Exploiting the Locality of Density-Functional Tight-Binding Hamiltonian

To explore the science behind excited-state dynamics in high-complexity chemical systems, a scalable nonadiabatic molecular dynamics (MD) technique is indispensable. In this study, by treating the electronic degrees of freedom at the density-functional tight-binding level, we developed and implemented a reduced scaling and multinode-parallelizable Ehrenfest MD method. To achieve this goal, we introduced a concept called patchwork approximation (PA), where the effective Hamiltonian for real-time propagation of the electronic density matrix is partitioned into a set of local parts. Numerical results for giant icosahedral fullerenes, which comprise up to 6000 atoms, suggest that the scaling of the present PA-based method is less than quadratic, which yields a significant advantage over the conventional cubic scaling method in terms of computational time. The acceleration by the parallelization on multiple nodes was also assessed. Furthermore, the electronic and structural dynamics resulting from the perturbation by the external electric field were accurately reproduced with the PA, even when the electronic excitation was spatially delocalized.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Concurrent Energy- and Electron-Transfer Dynamics in Photoexcited Mn-Doped CsPbBr3 Perovskite Nanoplatelet Architecture

Mn-doped perovskites have already been widely explored in the context of interesting optical, electronic, and magnetic properties. Such fascinating traits showcased by them explain the huge augmentation in the device efficiency, directing their widespread application in the field of solar cells, energy- harvesting sectors, and light-emitting diodes. However, the underlying photophysics governing the overall charge carrier dynamics in Mn-doped CsPbBr₃ nanoplatelets (NPLs) has never been discussed and therefore demands an in-depth investigation. Herein, fluorescence up-conversion and femtosecond transient absorption (TA) spectroscopy are employed for gaining a comprehensive understanding of the excited-state dynamics and the fundamental energy/charge-transfer processes for two-dimensional CsPbBr₃ nanoplatelets (NPLs) and their Mn-doped counterparts. The up-conversion measurement clearly suggests the possibility of energy-transfer pathways in the Mn-doped CsPbBr₃ NPLs. Interestingly, strong indication of charge transfer (CT) in Mn-doped CsPbBr₃ NPLs was unambiguously established by an ultrafast TA approach. Our investigation clearly suggests that both the probable processes viz. the ultrafast energy and electron transfers noticeable in the Mn²⁺-doped CsPbBr₃ NPLs are utterly competitive and rapid owing to the highly confined nature of the two-dimensional NPLs. This extensive probing of concurrent charge/energy-transfer processes may pave help clarify unresolved anomalies in Mn-doped perovskites, which may prove advantageous for a wide range of practical applicability.