Convenient modulation of bandgap for the mixed halide perovskites (MHPs) (e.g., CsPbBr x I1–x ) through varying the halide composition (i.e., the ratio of bromide to iodide) allows for optimizing the light-harvesting properties in perovskite solar cells (PSCs) and emission color in perovskite light-emitting diodes (PeLEDs). Such MHPs, yet, severely suffered from the instability under light irradiation and electrical bias as a result of an intrinsic soft, ionic lattice and a high halide ion mobility. Understanding the halide ion migration (mediated through halide vacancies) and suppressing the halide ion segregation, thus, remain a significant challenge both in the field of PSCs and PeLEDs since it is directly linked to the long-term stability and performances of the corresponding devices. In this Mini-Review, we discuss the intrinsic instability of the MHPs arising from the ionic nature of perovskites. The liquid crystalline properties with the low formation energy of halide ion defects facilitate the defect-mediated halide ion migration. Several different mechanistic models are provided to explain the fundamental origin of the photo- or electric field-driven halide ion segregation based upon thermodynamics and kinetics. These reflect that lattice strains (internal or polaron-induced) and bandgap energy differences between parent mixed halide and iodide-rich domain serve as the thermodynamic driving forces for halide segregation. On the basis of the deeper understanding of the underpinning segregation mechanism mediated through hole trapping and accumulation at the iodide-rich sites, we further discuss the strategies to mitigate the detrimental halide segregation through composition-, defect-, dimension-, and interface-engineering. Finally, we provide a fundamental insight into designing perovskite-based photovoltaic and optoelectronic devices for the long-term operational stability.

Mn-doped lead halide perovskite nanocrystals provide considerable opportunities to improve the photoluminescence quantum yield and stability and to modulate the optoelectronic and magnetic properties of the nanocrystals through doping. However, excited-state charge carrier recombination within host lattices and competing exciton-to-dopant energy transfer indeed require a deeper understanding of the complicated excited-state dynamics. Here, we have thus investigated such competing exciton recombination versus energy transfer dynamics seen in Mn-doped CsPb(Cl 1−y Br y ) 3 nanocrystals as a function of precisely controlling the Mn concentration and Br/Cl composition. The concentration of the dopant across the host lattice of the nanocrystals and the halide composition with a tunable band gap indeed determine the rate of forward (exciton-to-Mn) and backward energy transfer (Mn-to-exciton). Two different Mn concentration regimes (lightly vs heavily doped) are found with different excited-state behaviors while modulating the halide composition. Understanding such competing radiative, nonradiative, and forward and backward energy transfers observed in Mn states that are strongly dependent on the concentration of Mn and the band gap of the host nanocrystals (halide composition) can provide significant insights into full utilization of the dual-emissive features in the transition metal-doped lead halide perovskite nanocrystals.

Lead halide perovskite nanocrystals with inclusion of a transition-metal dopant of Mn 2+ offer a substantial degree of freedom to modulate the optoelectronic and magnetic properties owing to the introduced dopant in the host lattices. However, complexity as a result of the excited interactions between the exciton and dopant, involving dynamics of exciton recombination, competing forward and backward energy transfer (and vice versa), and Mn recombination, makes it difficult to understand and predict the Mn sensitization. Here, we have created machine learningdirected models using different nonlinear algorithms with initial 86 samples to decipher the complex energy transfer by navigating the reaction design space of various concentrations of Mn along with different halide compositions (band gap) in Mn-doped CsPb(Cl 1−y Br y ) 3 nanocrystals. Knearest neighbor-based predictive models coupled with time-correlated single photon counting measurements allow for fully elucidating the complex and competing energy transfer kinetics occurring in two different Mn concentration regimes. Importantly, forward exciton-to-Mn energy transfer is more governed by the Mn concentration, while the backward Mn-to-exciton energy transfer is strongly dependent on the energy gap difference between the exciton and Mn energy state. This machine learning-guided approach and modeling can not only provide an efficient means for navigating the vast reaction design space but also provide significant insight into understanding and elucidating the complex physical phenomena throughout analyzing and predicting the dataset trend.

Lead halide perovskites nanocrystals have emerged as a leading candidate in perovskite solar cells and light-emitting diodes. Given their favorable, tunable optoelectronic properties through modifying the size of nanocrystals, it is imperative to understand and control the growth of lead halide perovskite nanocrystals. However, during the nanocrystal growth into bulk films, the effect of halide bonding on growth kinetics remains elusive. To understand how a chemical bonding of PbÀ X (covalency and ionicity) impact on growth of nanocrystals, we have examined two different halide perovskite nanocrystals of CsPbCl 3 (more ionic) and CsPbI 3 (more covalent) derived from the same parent CsPbBr 3 nanocrystals. Tracking the growth of nanocrystals by monitoring the spectral features of bulk peaks (at 445 nm for Cl and at 650 nm for I) enables us to determine the growth activation energy to be 92 kJ/mol (for CsPbCl 3 ) versus 71 kJ/mol (for CsPbI 3 ). The electronegativity of halides in PbÀ X bonds governs the bond strength (150-240 kJ/mol), characteristics of bonding (ionic versus covalent), and growth kinetics and resulting activation energies. A fundamental understanding of PbÀ X bonding provides a significant insight into controlling the size of the perovskite nanocrystals with more desired optoelectronic properties.

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