Bright triplet excitons in lead halide perovskites (1707.03071v1)

Abstract: Nanostructured semiconductors emit light from electronic states known as excitons[1]. According to Hund's rules[2], the lowest energy exciton in organic materials should be a poorly emitting triplet state. Analogously, the lowest exciton level in all known inorganic semiconductors is believed to be optically inactive. These 'dark' excitons (into which the system can relax) hinder light-emitting devices based on semiconductor nanostructures. While strategies to diminish their influence have been developed[3-5], no materials have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in quasi-cubic lead halide perovskites is optically active. We first use the effective-mass model and group theory to explore this possibility, which can occur when the strong spin-orbit coupling in the perovskite conduction band is combined with the Rashba effect [6-10]. We then apply our model to CsPbX3 (X=Cl,Br,I) nanocrystals[11], for which we measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright character of the lowest exciton immediately explains the anomalous photon-emission rates of these materials, which emit 20 and 1,000 times faster[12] than any other semiconductor nanocrystal at room[13-16] and cryogenic[17] temperatures, respectively. The bright exciton is further confirmed by detailed analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals[18], which are already used in lighting[19,20], lasers[21,22], and displays[23], these optically active excitons can lead to materials with brighter emission and enhanced absorption. More generally, our results provide criteria for identifying other semiconductors exhibiting bright excitons with potentially broad implications for optoelectronic devices.

• The paper identifies bright triplet excitons in lead halide perovskites, challenging the notion that the lowest exciton state in inorganic semiconductors is dark.
• It employs effective-mass modeling and group theory, with nanocrystalline experiments on CsPbX₃ validating rapid emission rates up to 1000× faster at cryogenic temperatures.
• The findings underscore the impact of Rashba spin–orbit coupling in tuning excitonic properties, offering new pathways for optimizing optoelectronic devices.

Bright Triplet Excitons in Lead Halide Perovskites

The research presented in "Bright Triplet Excitons in Lead Halide Perovskites" provides a compelling scrutiny of the optoelectronic properties of lead halide perovskites, specifically focusing on exciton dynamics. Traditional understanding suggests that in inorganic semiconductors, the lowest exciton state is dark due to spin and orbital interactions that prevent photon emission. However, this work outlines a contrary finding for lead halide perovskites, showcasing that their lowest exciton levels are indeed optically active.

The researchers employed a combination of effective-mass model and group theory analyses to explore how strong spin–orbit coupling in conjunction with the Rashba effect yields bright excitons within quasi-cubic perovskites. These theoretical insights were further substantiated through empirical evaluations at the nanocrystalline level, specifically in CsPbX$_3$ (where X = Cl, Br, and I) nanocrystals. The results demonstrated size- and composition-dependent fluorescence rates that greatly surpassed those of other semiconductors under similar conditions, achieving emission rates up to 1000 times faster at cryogenic temperatures.

Key Findings and Implications

1. Bright Exciton Characterization: The explicit identification of bright triplet excitons in lead halide perovskites challenges the established belief that the lowest energy exciton state in inorganic semiconductors is universally dark. This finding has profound implications for optoelectronic applications such as light-emitting diodes and laser technologies, where brighter emission and enhanced absorption are desirable.
2. Radiative Lifetimes: Through meticulous experiments, the authors report significantly reduced radiative lifetimes in CsPbX$_3$ nanocrystals, with measurements showing decay times around the order of sub-nanoseconds. This outcome is consistent with the theoretical predictions made using the effective-mass model combined with Rashba spin–orbit coupling considerations.
3. Impact of Rashba Effect: The paper highlights the importance of the Rashba effect in altering the fine structure of excitons, whereby the inversion asymmetry in perovskite nanocrystals leads to a significant lowering of the bright triplet exciton's energy. This observation underscores the potential of tailoring material properties through controlled asymmetry to optimize excitonic behaviors.
4. New Methodological Criteria: The research outlines methodological criteria for identifying semiconductors with bright excitons, suggesting that materials with no inversion symmetry alongside strong spin–orbit coupling and specific band symmetries could be promising candidates.

Future Directions

The implications of these findings open up several avenues for further research. The prospect of engineering other inorganic systems exhibiting similarly bright excitons is particularly enticing for future developments in solid-state lighting and display technologies. Additionally, the fine control over spin–orbit interactions and Rashba effects could evolve into an innovative toolkit for semiconductors, offering new pathways to enhancing photovoltaic and quantum dot devices. Furthermore, exploring the electrical injection efficiencies and the characteristics of related optoelectronic applications would be a logical next step to leverage the bright exciton states identified.

Conclusively, this paper provides a solid foundation and a novel perspective in the understanding of excitonic properties in lead halide perovskites, presenting substantial opportunities for the advancement of various optoelectronic functionalities.