Journal Article

Bright triplet excitons in caesium lead halide perovskites

Bernstein, Noam, Kovalenko, Maksym V., Lyons, John L., Shabaev, Andrew, Efros, Alexander L., Michopoulos, John G., Mahrt, Rainer F., Sercel, Peter C., Nedelcu, Georgian, Becker, Michael A., Mehl, Michael J., Rainò, Gabriele, Lambrakos, Samuel G., Stöferle, Thilo, Norris, David J., Vaxenburg, Roman

2018

Overview

The lowest-energy exciton state in caesium lead halide perovskite nanocrystals is shown to be a bright triplet state, contrary to expectations that lowest-energy excitons should always be dark. A bright future for semiconductors Lead halide perovskite semiconductor nanocrystals are attracting considerable interest as materials for solar cells and light-emitting diodes because of their excellent photophysical properties. But what makes them so special? Excitons are the electronic excitations that are ultimately responsible for the emissive properties of nanostructured semiconductors, and prevailing wisdom is that the lowest-energy excitonic state will be long-lived and hence poorly emitting (or 'dark'). Michael Becker et al . now show that caesium lead halide perovskites disobey this rule: the lowest-energy excitons are instead unusually 'bright', emitting much faster than any other semiconductor nanocrystal. Furthermore, they identify the structural and electronic factors responsible for this anomalous behaviour, providing vital clues for the identification of other semiconducting materials that might behave similarly. Nanostructured semiconductors emit light from electronic states known as excitons 1 . For organic materials, Hund’s rules 2 state that the lowest-energy exciton is a poorly emitting triplet state. For inorganic semiconductors, similar rules 3 predict an analogue of this triplet state known as the ‘dark exciton’ 4 . Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX 3 , with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin–orbit coupling in the conduction band of a perovskite is combined with the Rashba effect 5 , 6 , 7 , 8 , 9 , 10 . We then apply our model to CsPbX 3 nanocrystals 11 , and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster 12 than any other semiconductor nanocrystal at room 13 , 14 , 15 , 16 and cryogenic 4 temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting 17 , lasers 18 and displays 19 , these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 119/118 / 140/125 / 639/301/1034/1038 / 639/624/399/1017 / 639/925/357/354 / Asymmetry / Atomic properties / Cesium / Conduction / Conduction bands / Cryogenic temperature / Crystals / Electron states / Emissions / Energy / Excitation / Excitons / Fine structure / Fluorescence / Group theory / Humanities and Social Sciences / Lasers / Lead compounds / letter / Light / Low temperature / multidisciplinary / Nanocrystals / Optoelectronic devices / Organic materials / Perovskite / Perovskites / Photon emission / Photons / Quantum dots / Science / Semiconductors / solar (photovoltaic), solar (fuels), solid state lighting, bio-inspired, electrodes - solar, defects, charge transport, materials and chemistry by design, optics, synthesis (novel materials), synthesis (scalable processing) / Spectrum analysis / Spin-orbit interactions / Theory / Triplet state / Ultrastructure