Data driven high quantum yield halide perovskite phosphors design and fabrication

Effect of static local distortions vs. dynamic motions on the stability and band gaps of cubic oxide and halide perovskites

Because perovskite crystals exhibit unique magnetic, conductive, and optical properties, they have been the subject of many fundamental investigations in various research fields. However, investigations related to their use as optoelectronic device materials are still in their early days. Regarding oxide perovskites, which have been investigated for a long time, the efficiency of photoluminescence (PL) induced by band-to-band transitions is extremely low because of the localized nature of the carriers in these materials. On the other hand, halide perovskites exhibit a highly efficient band-edge PL attributable to the recombination of delocalized photocarriers. Therefore, it is expected that this class of high-quality materials will be advantageous for optoelectronic devices such as solar cells and light-emitting diodes. In this Minireview, we discuss various aspects of the PL properties and carrier dynamics of SrTiO3 and CH3 NH3 PbX3 (X=I, Br), which are representative oxide and halide perovskites.

Precise control of the thickness of large-area two-dimensional (2D) organometal halide perovskite layers is extremely challenging owing to the inherent instability of the organic component. Herein, a novel, highly reproducible, and facile solvothermal route is reported to synthesize and tailor the thickness and optical band gap of the organic-inorganic halide perovskite nanosheets (NSs). Our study reveals that self-assembly of randomly oriented perovskite nanorods leads to the growth of multilayered perovskite NSs at ∼100 °C, while at higher temperature, large-area few-layer to bilayer 2D NSs (CH3NH3PbBr3) are obtained through lattice expansion and layer separation depending precisely on the temperature. Interestingly, the thickness of the 2D NSs shows a linear dependence on the reaction temperature and thus enables precise tuning of the thickness from 14 layers to 2 layers, giving rise to a systematic increase in the band gap and appearance of excitonic absorption bands. Quantitative analysis of the change in the band gap with thickness revealed a strong quantum confinement effect in the 2D layers. The perovskite 2D NSs exhibit tunable color and a high photoluminescence (PL) quantum yield (QY) up to 84%. Through a careful analysis of the steady-state and time-resolved PL spectra, the origin of the lower PL QY in thinner NSs is traced to surface defects in the 2D layers, for the first time. A white light converter was fabricated using the composition-tuned 2D CH3NH3PbBrI2 NS on a blue light-emitting diode chip. The 2D perovskite photodetector exhibits a stable and very fast rise/fall time (24 μs/103 μs) along with high responsivity and detectivity of ∼1.93 A/W and 1.04 × 1012 Jones, respectively. Storage, operational, and temperature-dependent stability studies reveal high stability of the 2D perovskite NSs under the ambient condition with high humidity. The reported method is highly promising for the development of large-area stable 2D perovskite layers for various cutting-edge optoelectronic applications.

Easy-to-perform organic-solvent-free synthesis of carbon dots with strong green photoluminescence

The white emissions based on lead‐free perovskites with broadband emission due to self‐trapped excitons (STEs) are considered as an ideal candidate for next‐generation white light‐emitting diodes (WLEDs). Recently, copper‐based halide perovskites Cs3Cu2I5@CsCu2I3 composites films with a high color‐rendering index (CRI) have been developed for WLEDs. However, the Cs3Cu2I5@CsCu2I3 composites generally exhibit a relatively low photoluminescence quantum yield (PLQY). Herein, a high PLQY up to 86.75% copper‐based perovskite Gua3Cu2I5 (Gua = guanidine) films with a broadband strong yellow STEs emission centered at 544 nm is synthesized. By combining with efficient blue STEs emission Cs3Cu2I5 component via composition engineering of Cs3Cu2I5@Gua3Cu2I5, a high PLQY of up to 96.1% is achieved on the white emission composites films. Moreover, a high CRI of 93 WLEDs is achieved by integrating Cs3Cu2I5@Gua3Cu2I5 composites film onto a UV LED chip. These findings provide a simple and effective strategy for designing highly efficient white light‐emitting diodes.

Near-unity photoluminescence quantum yield Mn-doped two-dimensional halide perovskite platelets via hydrobromic acid-assisted synthesis

Lead‐free halide perovskites (LHP) have attracted extensive attention in the field of next‐generation solid‐state lighting. However, single perovskite phosphor with tunable white‐light emission and ultra‐high photoluminescence quantum yield (PLQY) are rare. Here, the tunable dual‐color emission LHP phosphor is developed with a near‐unity PLQY of 99.5%. The LHP phosphor comprises Cs2NaInCl6: Sb3+/Bi3+ and Cs2InCl5·H2O: Sb3+/Bi3+ crystals, with the transition between these two perovskite structures being adjustable by the concentration of Na+. The addition of NaCl inhibits the hydrolysis of the [Sb(H2O)Cl5]2− octahedron, resulting in 3D connected [SbCl6]3− octahedron with less Jahn‐Teller distortion, and the PLQY of LHP is greater than 90%. Finally, the white‐light diode device is fabricated by assembling the LHP with a ultraviolet light‐emitting diode chip and the white light‐emitting diode device achieved Commission Internationale de l'Eclairage color coordinates of (0.34, 0.33), correlated color temperature of 4936 K, and a high color rendering index of 92.9. The LHP perovskite exhibited multimodal luminescence when excited by 365, 335, and 310 nm UV light, showing the multi‐mode anti‐counterfeiting capabilities of the LHP crystals.

Ultrastable PVB films-protected CsPbBr3/Cs4PbBr6 perovskites with high color purity for nearing Rec. 2020 standard

Three aryl-hydrazone O,N,O tridentate ligands with a different electron-withdrawing substituent were prepared. The introduction of a flexible charged chain in the ligands guaranteed solubility in many organic solvents and in water. The increasing withdrawing aptitude of the substituents red-shifted the emission in the correspondent metallopolymers. The metallated polymers were obtained by grafting ligand-zinc (II) coordination fragments onto commercial poly-(4-vinylpyridine). Metallopolymers thin films exhibited red, green and blue emission colors defined by Commission Internationale d’Eclairage (CIE) coordinates and medium to excellent photoluminescence (PL) quantum yields (PLQYs) comparable with other highly-performing active materials for Light-Emitting Diodes (LEDs). By grafting a suitable mix of the three different coordination pendants, an efficient single-component white emissive metallopolymer with CIE (0.30, 0.31) was prepared. Thanks to the charged moiety, the polymers resulted miscible with an ionic liquid. The addition produced homogeneous polymeric layers with unaltered PL performances, potentially employable in Light-emitting Electrochemical Cells (LECs).

An innovative strategic route to the green synthesis of CsPbBr3 films on rigid and flexible substrates

Doping is able to create novel optoelectronic properties of halide perovskites, and the involved mechanism of efficient emission is still a challenge. Herein Mn2+ substitution into 2D layered perovskites (C8H20N2)PbBr4 was investigated, demonstrating broad-band orange-red emission originating from the 4T1 → 6A1 transition of Mn2+ dopant. The photoluminescence quantum yield (PLQY) of Mn2+ emission is up to 60.8% related to the energy transfer in coupled states. We verify that an actual Mn2+ dopant as low as 0.476% reaches a high PLQY, whereas the nominal adding amount is 0.8 as the Mn2+/Pb2+ ratio. The small activation energy (∼6.72 meV) between the Mn2+ d state and the trap state accounts for this highly efficient energy transfer and photoluminescence. The proposed luminescence mechanism in Mn2+-doped 2D halide perovskites would provide unique insights into the doping design toward high-performance luminescence materials.

Inkjet printing has emerged as a promising technique for the fabrication of halide perovskite (HP) thin films, as it enables precise and controlled deposition of the perovskite ink on a variety of substrates. One main advantage of inkjet printing for the fabrication of HP thin films is its ability to produce uniform films with controlled thickness and high coverage, which is critical for achieving high‐performance devices. Additionally, inkjet printing allows for the deposition of patterned thin films, enabling the fabrication of complex device architectures such as light‐emitting diodes (LEDs). In this work, flexible LEDs based on inkjet printed Pb‐free HP thiophene‐ethylammonium tin iodide (TEA2SnI4) are produced that has gained attention as a potential alternative to Pb‐based HPs in optoelectronic devices due to its lower toxicity, environmental impact, and high performance. The role of ink solutions is compared using pure solvents: toxic dimethyl formamide (DMF) and more eco‐friendly dimethyl sulfoxide (DMSO). Red‐emitting devices (λmax = 633 nm) exhibit, in ambient conditions, a maximum external quantum efficiency (EQEmax) of 0.5% with a related brightness of 21 cd m−2 at 54 mA cm−2 for DMSO‐based LEDs. The environmental impacts of films prepared with DMSO‐based solvents ensure only 40% of the impact caused by DMF.

Recently, lead halide perovskite nanocrystals (NCs) have gained tremendous attention in optoelectronic devices due to their excellent optical properties. However, the toxicity of lead limits their practical applications. Here, the synthesis of Zn2+-alloyed CsZnxPb1-xX3 (up to 15%) NCs is reported to achieve lead-reduced white light-emitting diodes (WLEDs). The incorporation of Zn2+ into CsPbX3 host NCs results in a lattice contraction, without altering the structure and morphology, which has a direct effect on the optical properties. The blue-shifts in the photoluminescence emission and increase in bandgap is observed while retaining high photoluminescence quantum yield. Then by engineering the different compositions of halides for 15% Zn2+-alloyed CsZnxPb1-xX3 NCs, tunable emission (411–636 nm) is obtained. Notably, the WLEDs are experimentally demonstrated employing the lead-reduced NCs (blue, green, yellow, and red). By varying the ratios of the amount of NCs, white lights with a tunable correlated-color temperature (2218–8335 K), an exemplary color-rendering index (up to 93) and high luminous efficacy of radiation (268–318 lm·W−1) are obtained. Best of our knowledge, these are superior to other reported WLEDs based on CsPbX3 NCs doped with transition metal ions. This work places the halide perovskite NCs one-step closer in designing the environmentally benign and energy-efficient WLEDs.

ConspectusThe past decade has witnessed the great success and rapid development of halide perovskite materials in photovoltaics and light-emitting devices, which is due to the inherently high photoluminescence quantum yield, long carrier diffusion length, high light absorption coefficient, great defect tolerance, and remarkable tunability. For example, the record efficiency of perovskite solar cells has ramped up to 25.5%, comparable to single-crystal silicon solar cells. In combination with the facile solution processing, perovskite solar cells have gained tremendous attention and interest from both academia and the photovoltaic market. Moreover, as a newly emerging class of semiconductors with excellent properties and low manufacturing cost, it is also promising and meaningful to explore new applications beyond optoelectronic devices, such as transistors, lasers, and memory devices. To this end, the integration of different halide perovskites into epitaxial heterostructures provides an ideal option. On the other hand, from the perspective of fundamental studies, halide perovskite heterostructures also provide a platform to directly visualize ion migration and to study the charge transfer behavior and anomalous exciton phenomena, etc.The aim of our Account is to summarize the synthetic strategies to achieve halide perovskite epitaxial heterostructures and to highlight the effective routes to stabilize halide perovskites and heterostructures. We focus this Account on two-dimensional (2D) halide perovskites and emphasize the important roles the organic ligands play. Specifically, we discuss our recent findings on the incorporation of conjugated organic ligands into 2D halide perovskites leading to enhanced thermal and environmental stability and suppressed halide interdiffusion in the corresponding heterostructures. Molecular dynamics simulation and low-dose aberration corrected transmission electron microscopy were used as powerful tools to unveil and elucidate the working mechanism of stabilization and anion interdiffusion suppression. Following this strategy, diverse halide perovskite heterostructures between different halides, metals, or organic cations and more complex heterostructures and superlattices can be realized. Due to the reduced ion motion and improved stability, the interface of the heterostructures can be kept quite sharp. In the outlook of this Account, we discuss the challenges and a few promising directions with a special focus on the vertical van der Waals (vdW) heterostructures, which are expected to offer more flexibility in heterostructure integration and more opportunities in studying the charge transfer and carrier interaction along the out-of-plane direction. Moreover, the existence of semiconducting organic ligands provides new insights to tune the carrier and exciton behaviors along both in-plane and out-of-plane directions.

Laser-driven liquid crystal displays (LCDs) comprising metal halide perovskites (MHPs) as the blue-to-green/red color converters are at the forefront of ongoing intense research on the development and improvement of display devices. However, the inferior high photoluminescence quantum yield (PLQY) of MHPs under the excitation of high-power blue light and photoluminescence deterioration at high temperatures remain major concerns. Herein, we design a kind of octylamine-modified MHP via binding energy engineering, and the synthesized materials show PLQY of 97.6% under the excitation of a blue laser at 450 nm. Meanwhile, this design endows a structural self-healing ability to achieve a high PLQY and luminescence stability under high temperature (90 °C) and high flux excitation (386 mW cm-2). The blue light-excitable MHPs with a near unity PLQY, strong stability, and low PLQY deterioration are further encapsulated into a laser-driven LCD device. This prototype demonstrates excellent color gamut (132% NTSC, 98% Rec. 2020), illuminance intensity (>10 000 lux), and energy consumption (47.5% of commercial consumption), and hence is expected to be beneficial for the reduction of energy consumption in backlight display devices, particularly in large-screen outdoor displays.