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Strong quantum confinement in Mn-doped semiconductor nanocrystals enhances dopant–carrier exchange interactions. Here, we report the synthesis and optical properties of strongly quantum confined, quasi two-dimensional, Mn-doped CsPbCl3 nanoplatelets. A room-temperature synthesis was employed to prepare the platelets with thickness 2.2 nm (4 monolayers), which is significantly smaller than the Bohr excitonic diameter of CsPbCl3 (5 nm). Efficient transfer of excitonic energy from the host to the Mn2+ dopant ions leads to a spin-forbidden 4T1–6A1 Mn d-electron emission with the highest quantum yield of ∼20% and exhibits a long lifetime of 1.6 ms. Subsequent anion exchange reactions at room temperature lead to the formation of Mn-doped CsPbBr3 nanoplatelets, with weak Mn emission. These newly developed Mn-doped cesium lead halide nanoplatelets are suitable candidates for exploring the effects of quantum confinement on dopant–carrier exchange interaction and exhibiting interesting magneto-optic properties.

Lead bromide (PbBr 2 ,99.999%), lead iodide (PbI 2 , 99.999%), lead chloride (PbCl 2 , 98%), manganese (II) bromide (MnBr 2 , 98%), ytterbium (III) nitrate pentahydrate {Yb(NO 3) 3 .5H 2 O, 99.99%}, caesium carbonate (Cs 2 CO 3 , 99.9%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), oleylamine (OLA, 98%), trioctylphosphine (TOP, 90%), methyl acetate (MeOAc, 99.5%), N,N-dimethylformamide (DMF, 99.8%), hydrogen bromide (HBr, 43 wt%, 99.99%) were purchased from Aldrich and were used without further purification. Acetone (99.0%, SDFCL), Hexane (99.99%, Rankem), Toluene (99.5%, Rankem) solvents are used as obtained.

Lanthanides have been widely explored as optically active dopants in inorganic crystal lattices, which are often insulating in nature. Doping trivalent lanthanide (Ln 3+) into traditional semiconductor nanocrystals, such as CdSe, is challenging because of their tetrahedral coordination. Interestingly, CsPbX 3 (X = Cl, Br, I) perovskite nanocrystals provide the octahedral coordination suitable for Ln 3+ doping. Over the last two years, tremendous success has been achieved in doping Ln 3+ into CsPbX 3 nanocrystals, combining the excellent optoelectronic properties of the host with the f-f electronic transitions of the dopants. For example, the efficient quantum cutting phenomenon in Yb 3+-doped CsPb(Cl,Br) 3 nanocrystals yields a photoluminescence quantum yield close to 200%. Other approaches of Ln 3+ doping and codoping have enabled promising proof-of-principle demonstration of solid-state lighting and solar photovoltaics. In this perspective article, we highlight the salient features of the material design (including doping in Pb-free perovskites), optical properties and potential optoelectronic applications of lanthanide-doped metal halide perovskite nanocrystals. While review articles on doping different metal ions into perovskite nanocrystals are present, the present review-type article is solely dedicated to lanthanide-doped metal halide perovskite nanocrystals.

Cs2AgInCl6 double perovskite (DP) is a direct band gap, stable, and environmentally benign material with a three-dimensional perovskite structure. However, the band gap (∼3.3 eV) is too wide for optoelectronic applications in the near-infrared (NIR) region. To achieve optical functionality in the NIR window, we have doped Yb3+ in both microcrystals and colloidal nanocrystals of Cs2AgInCl6 DPs. Our Yb-doped Cs2AgInCl6 DP microcrystals and nanocrystals show NIR emission at ∼994 nm (1.24 eV), whereas the band gap of the hosts is in the UV region. Therefore, the Yb-emission does not suffer from self-absorption. The microcrystals exhibit a single-exponential decay with 2.7 ms long lifetime corresponding to 2F5/2 → 2F7/2 transition of Yb3+ f electrons. The nanocrystals show biexponential decay (3 ms and 749 μs), probably because of the proximity of Yb to the surface of nanocrystals. Interestingly, NIR emission and perovskite structure of samples are stable for >3 months in ambient conditions for future optoelectronic applications.

Lead halide compounds, including lead halide perovskite nanocrystals (NCs), have attracted the interest of researchers in optoelectronics and photonics because of their high photoluminescence quantum yields (PLQYs) coupled with relatively short PL lifetimes (on the order of a few nanoseconds). However, lead-free metal halides of high PLQY, including double perovskites and their doped NCs, typically possess long PL lifetimes (up to microseconds) that limit their application space. Here, we introduce CsMnBr3 NCs, which are lead-free and red-emitting, that combine a high PLQY with an exceptionally short radiative lifetime (on the order of picoseconds). We find that the octahedral coordination of Mn2+ in CsMnBr3 induces a red emission centered at ∼643 nm with a PLQY of ∼54% and a fast radiative decay rate. Femtosecond transient absorption and transient PL spectroscopies reveal the existence of a low-lying excited state of Mn2+ that relaxes to the ground state within around 605 ps by emitting light at around 643 nm. At greater excitation energies, higher excited states of Mn2+ relax in the sub-nanosecond time scale to this low-lying excited state. A similarly positioned PL peak with a short picosecond scale PL lifetime and a PLQY of ∼6.7% was also detected in bulk CsMnBr3 single crystals reported in this studya relatively high quantum yield for a bulk material. Our experimental results and density functional theory modelling show that the crystal structure and the strong coupling among Mn2+ ions govern those luminescence properties of CsMnBr3 NCs and single crystals. These findings pave the way for new lead-free materials that combine high PLQY and ultrafast luminescence.

Defect tolerant perovskite nanocrystals of the general formula Cs-Pb-X3 (where X= Cl, Br and I) have shown exceptional potential in fundamental physics as well as in novel optoelectronic applications as the next generation solar cells. Although exciton many-body interactions such as biexciton Stark shift, state filling, and Auger recombination are studied extensively, other important correlated effects like bandgap renormalization (BGR) and hot phonon bottleneck are not explored in these nanocrystals. Here we experimentally demonstrate the carrier density dependence of the BGR and an effective hot phonon bottleneck in CsPb(Cl0.20Br0.80)3 mixed halide nanocrystals. The results are compared with two other halide compositions, namely, CsPbBr3 and CsPb(Br0.55I0.45)3 nanocrystals with varying bandgaps. The optical response of the nanocrystals changes dramatically across the spectral range of many hundreds of meV at high carrier density due to large BGR. We have calculated the BGR constant ≈ (6.0±0.3)×10-8 eVcm for CsPb(Cl0.20Br0.80)3 nanocrystals that provides the amount of bandgap shift as a function of carrier density. In these nanocrystals, an efficient hot phonon bottleneck is observed at a carrier density of 3.1×10 17 cm-3 that slows down the thermalization by one order of magnitude. Our findings reveal that the complex kinetic profile of the exciton dynamics can be analyzed by the global target analysis using the sequential model with increasing lifetimes.

Metal chalcogenide perovskites were proposed as potential solar cell material in 2015. The theoretical maximum solar cell efficiencies of some chalcogenide perovskites are ∼30%, similar to CH 3 NH 3 PbI 3 perovskites. The foreseen advantages of chalcogenide perovskites are high thermal and aqueous stability along with nontoxic elemental composition. To date, a reasonable amount of computational and experimental work has been reported on the synthesis, electronic and optical properties of chalcogenide perovskites. Major experimentally studied compounds are AZrS 3 (A = Ba and Sr), Ba 3 Zr 2 S 7 , and LaYS 3 , which have direct band gaps in the range 1.3 to 2 eV, along with strong light absorption coefficients and small effective masses of charge carriers. There are a few more compositions with similar properties that have been suggested by computational screening. In this perspective, we summarize both the computational and experimental progresses made in designing chalcogenide perovskites for optoelectronic properties. Then we discuss the material design challenges that need to be addressed in the coming years for successful solar cell application.