Industry outlook of perovskite quantum dots for display applications

All-inorganic perovskite nanocrystals (PNCs) have been anticipated to be used in efficient and stable perovskite optoelectronic devices because of their suitable bandgap, broad absorption spectrum, and high color purity. However, long-term stability remains a major obstacle for the commercial application of PNCs. In particular, for red-emissive perovskites, the reaction to the environment is more sensitive than that for the green counterpart, which makes a phase transformation easily occur at room temperature. On the basis of CsPbI3 PNCs, we analyze the main instability factors of red-emissive PNCs (620–720 nm), including their structures, optoelectronic properties, and instability mechanism. We then summarize some strategies to improve the stability and performance of the perovskites and light-emitting diodes (LEDs). Furthermore, we discuss the challenge of scaling up the production of PNCs. Finally, we propose possible perspectives for the development of perovskite materials. 

Recent Progress in the Stability of Red-Emissive Perovskite Nanocrystals for Light-Emitting Diodes

Transport of excitons and charge carriers in molecular systems can be enhanced by coherent coupling to photons, giving rise to the formation of hybrid excitations known as polaritons. Such enhancement has far-reaching technological implications; however, the enhancement mechanism and the transport nature of these hybrid excitations remain elusive. Here we map the ultrafast spatiotemporal dynamics of polaritons formed by mixing surface-bound optical waves with Frenkel excitons in a self-assembled molecular layer, resolving polariton dynamics in energy/momentum space. We find that the interplay between the molecular disorder and long-range correlations induced by coherent mixing with light leads to a mobility transition between diffusive and ballistic transport, which can be controlled by varying the light–matter composition of the polaritons. Furthermore, we show that coupling to light enhances the diffusion coefficient of molecular excitons by six orders of magnitude and even leads to ballistic flow at two-thirds the speed of light. The authors study ultrafast spatiotemporal dynamics of polaritons formed by mixing surface-bound optical waves with excitons observing a mobility transition from diffusive to ballistic transport flow at two-thirds the speed of light. 

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From enhanced diffusion to ultrafast ballistic motion of hybrid light–matter excitations

Due to their marvelous electrical and optical properties, perovskite nanocrystals have reached remarkable landmarks in solar cells, light-emitting diodes, and photodetectors. However, the intrinsic instability of ionic perovskites, which would undergo an undesirable phase transition and decompose rapidly in ambient humidity, limits their long-term practical deployment. To address this challenge, halogenated trimethoxysilane as the passivation additive is chosen, which utilizes simultaneous halide and silica passivation to enhance the stability of perovskite nanoparticles via a dual-passivation mechanism. The processable nanoparticles show high photoluminescence quantum yield, tunable fluorescence wavelength, and excellent resistance against air and water, highlighting great potential as green to deep-red bio-labels after further phospholipid encapsulation. This work demonstrates that the dual-passivation mechanism could be used to maintain the long-term stability of ionic crystals, which sheds light on the opportunity of halide perovskite nanoparticles for usage in a humid environment.

Encapsulation of Dual-Passivated Perovskite Quantum Dots for Bio-Imaging.

Recent Advances in Lead-Free Halide Perovskites for Photocatalysis

Halide perovskites are considered to be next-generation semiconductor materials with bright prospects to advance the technology of photonics and optoelectronics. Because of the intrinsic ionic feature, the interactions between perovskites and water induce serious stability issues, which has been one of the fundamental problems hindering the practical application of perovskites. The degradation of halide perovskites upon water exposure has been intensively studied, resulting in chemical insights into key processes, including hydration, phase transformation, decomposition, and dissolution. In this Perspective, we try to illustrate what happens when halide perovskites meet with water. We summarize the research progress regarding the understanding of these processes and discuss the principle of strategy design toward improved stability against water. In addition to the instability-related interactions, we also discuss the aqueous solution of perovskite precursors for fabricating perovskite-based functional materials. Hopefully, this Perspective can inspire more fundamental studies on the interactions between perovskites and water, such as spectroscopy and simulation, crystal structure and material characterizations, and solution chemistry and crystallization.

What Happens When Halide Perovskites Meet with Water?

Semiconductor excitations can hybridize with cavity photons to form exciton-polaritons (EPs) with remarkable properties, including light-like energy flow combined with matter-like interactions. To fully harness these properties, EPs must maintain long-range coherence despite exciton-mediated interactions with lattice phonons. To address this open question, we develop a nonlinear momentum-resolved optical approach that images EPs in real space on femtosecond scales. We directly visualize EP wavepacket propagation in two-dimensional halide perovskite microcavities, allowing precise quantification of EP interactions with lattice phonons and reservoir exciton states throughout their lifetimes. We show that EP-lattice interactions strongly renormalize EP group velocities at high exciton content without sacrificing long-range coherence, enabling ballistic propagation to be maintained for up to half-exciton EPs. These results concur with our quantum simulations of dynamic disorder shielding through light-matter hybridization. We extend our measurements to molecular polaritons and self-hybridized EPs in two-dimensional materials, testifying to the remarkable generalizability of our approach.

Ultrafast imaging of coherent polariton propagation and interactions

Abstract Semiconductor excitations can hybridize with cavity photons to form exciton-polaritons (EPs) with remarkable properties, including light-like energy flow combined with matter-like interactions. To fully harness these properties, EPs must retain ballistic, coherent transport despite matter-mediated interactions with lattice phonons. Here we develop a nonlinear momentum-resolved optical approach that directly images EPs in real space on femtosecond scales in a range of polaritonic architectures. We focus our analysis on EP propagation in layered halide perovskite microcavities. We reveal that EP–phonon interactions lead to a large renormalization of EP velocities at high excitonic fractions at room temperature. Despite these strong EP–phonon interactions, ballistic transport is maintained for up to half-exciton EPs, in agreement with quantum simulations of dynamic disorder shielding through light-matter hybridization. Above 50% excitonic character, rapid decoherence leads to diffusive transport. Our work provides a general framework to precisely balance EP coherence, velocity, and nonlinear interactions. 

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Ultrafast imaging of polariton propagation and interactions

We first report on the aqueous solution growth of centimeter-sized Na-doped CsPb2Br5 single crystals using a precursor solution with high NaBr concentration. The obtained Na-doped CsPb2Br5 single crystal has a tetragonal structure with a slight lattice contraction of 0.34% for Cs0.88Na0.12Pb2Br5. In comparison with the previously reported pure CsPb2Br5 single crystal, the Na-doped CsPb2Br5 single crystal shows interesting self-trapped exciton (STE) emission with emission peak at 689 nm, full width at half-maximum of 246 nm, and average photoluminescence lifetime of 429 ns. The STE emission feature of Na-doped CsPb2Br5 single crystal was further evidenced by the wavelength-dependent photoluminescence excitation spectra and intensity-dependent photoluminescence spectra measurements. By increasing the Na doping concentration, the photoluminescence quantum yields of Na-doped CsPb2Br5 single crystal can be greatly improved to a maximum value of 28.5% for Cs0.88Na0.12Pb2Br5. In addition, the methodology was also extended to fabricate Li-doped CsPb2Br5 single crystal with STE emission. The available Na-doped or Li-doped CsPb2Br5 single crystals are promising candidates for both fundamental research and the exploration of photonic applications. 

Shangjun Cheng

Papers

What Happens When Halide Perovskites Meet with Water?

Ultrafast imaging of coherent polariton propagation and interactions

Ultrafast imaging of polariton propagation and interactions

Centimeter-Sized Na-Doped CsPb2Br5 Single Crystals with Efficient Self-Trapped Exciton Emission