Lead halide perovskite nanocrystals: enhancing commercial viability for light emitting applications

Abstract

Lead halide perovskite nanocrystals (NCs) have exhibited exceptional optoelectronic properties, which are extremely attractive for light-emitting applications. However, inadequate surface passivation and elaborate high-temperature preparation hinder their commercial viability. This project sought to address these problems by investigating and exploiting a promising alternative ligand, octylphosphonic acid (OPA). Through an initial ligand exchange approach, the mechanism of OPA binding to the surface of CsPbBr3 nanocrystals was revealed. Deprotonation of OPA resulted in an anionic phosphonate moiety, which bound to under-coordinated Pb2+ sites. The remaining two free functional groups on the bound ligand promoted the assembly an inter-ligand hydrogen-bonded network. These extensive ligand connections provided a novel mechanism for the passivation of perovskite nanocrystals. This OPA passivation significantly enhanced the photoluminescence quantum yield (PLQY), and the retention of PLQY through antisolvent purification. An antisolvent purification protocol was devised to enable the fabrication of high-efficiency light-emitting diodes (LEDs) from OPA-passivated CsPbBr3 nanocrystals. A second purification cycle with acetonitrile was crucial, improving the external quantum efficiency (EQE) of OPA-modified NCs from 2.0 % to 7.7 %. Without OPA, the EQE of twice-purified NCs was only 3.6 %; the efficiency enhancement provided by OPA was attributed to a combination of more robust surface passivation and the replacement of long oleyl ligands for less-resistive octyls. Following these promising results, a new protocol was devised which could prepare OPA-capped CsPbBr3 nanocrystals directly at room temperature for the first time. An understanding of the influence of OPA on the thermodynamics of nucleation and the kinetics of growth facilitated precise control over the nanocrystal diameter. The photoluminescence emission wavelength was tuned from 501 and 517 nm by adjusting the nanocrystal diameter between 6.6 and 13 nm. Vitally, PLQY was maintained above 80 % for all sizes, and the bandwidth remained very narrow (16 to 19 nm). Judicious size tuneability had not previously been achieved for perovskite nanocrystals without using a high temperature synthesis. The developed method attained excellent optoelectronic properties in a more efficient, less complicated, and less costly manner than the common high temperature, hot-injection approaches. Furthermore, the versatility of the synthetic framework designed offers an excellent foundation to tailor the nanocrystal properties for specific technologies. Ultimately, this work presented a clear enhancement of the commercial prospects of perovskite nanocrystals for both optically and electrically-driven applications. Currently, the stability of perovskite nanocrystals under electric field remains far below the necessary standards for commercial electrically-driven applications. In contrast, the flexible, cost-effective production of highly emissive nanocrystals exhibited herein could contribute towards their imminent inclusion in consumer electronics, as down-converters or colour filters for wide colour gamut displays.

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Identifiers

ORCID for Alasdair Brown: orcid.org/0000-0002-0714-3851

ORCID for Suan-Hui Pu: orcid.org/0000-0002-3335-8880

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