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Slow carrier relaxation in tin-based perovskite nanocrystals

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An Addendum to this article was published on 01 October 2021

Abstract

The conversion efficiency of solar energy in semiconductors is fundamentally limited by ultrafast hot-carrier relaxation processes, and slowing down these processes is critical for improved energy harvesting. Here we report formamidinium tin iodide (FASnI3) nanocrystals where quantum confinement effects yield an evolution from a continuous band structure to separate energy states with decreasing nanocrystal size, as observed by transient absorption spectroscopy. The appearance of separate energy levels slows down the relaxation of hot carriers by two orders of magnitude at low injected carrier densities (<1 carrier pair per nanoparticle). The observed build up time of the ground-state bleach at the band edge is two orders of magnitude slower in FASnI3 nanocrystals than in lead halide perovskite bulk and nanocrystals, which we attribute to a phonon bottleneck effect. Our results highlight the promise of lead-free perovskite nanocrystals for high-efficiency photovoltaic applications operating above the Shockley–Queisser limit.

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Fig. 1: Characterization of FASnI3 nanocrystals.
Fig. 2: Optical measurements of FASnI3 nanocrystals with different sizes.
Fig. 3: Transient absorption spectroscopy of FASnI3 nanocrystals.
Fig. 4: Transient absorption spectroscopy of FASnI3 nanocrystals of different sizes.
Fig. 5: Diagrams of electronic states of FASnI3.
Fig. 6: Photodegradation and A-site doping of FASnI3 nanocrystals.

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Data availability

The data underlying this paper are available at https://doi.org/10.17863/CAM.72660.

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Acknowledgements

L.D. and Z.D. thank the Cambridge Trust and the China Scholarship Council for funding. F.A. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 670405). Z.Z. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Actions grant (grant no. 842271–TRITON project). The TEM infrastructure was supported by the EPSRC through the Sir Henry Royce Institute—Cambridge Equipment (EP/P024947/1). The calculations were performed at the U.K. National Supercomputing Service, ARCHER. Access was obtained via the UKCP consortium and funded by EPSRC under grant no. EP/P022596/1. This work was supported by the EPSRC (EP/M005143/1).

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Authors and Affiliations

  1. Cavendish Laboratory, University of Cambridge, Cambridge, UK

    Linjie Dai, Florian Auras, Heather Goodwin, Zhilong Zhang, Felix Deschler & Neil C. Greenham

  2. Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, UK

    Zeyu Deng, John C. Walmsley & Paul D. Bristowe

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  1. Linjie Dai
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Contributions

L.D. synthesized the FASnI3 nanocrystals, performed the TEM and XRD characterizations, and performed all of the optical measurements (absorption, photoluminescence, photoluminescence quantum efficiency, transient absorption and TCSPC) under the supervision of N.C.G. Z.D. conducted the density functional theory simulations under the supervision of P.D.B. F.A. analysed the data on superlattices. J.C.W. assisted with TEM measurements for air-sensitive samples. H.G. and Z.Z. assisted in interpreting results. L.D. analysed the results and drafted the manuscript, which was revised by F.D. and N.C.G.

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Correspondence to Neil C. Greenham.

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Extended data

Extended Data Fig. 1 TEM images of FASnI3 nanocrystals synthesized at 25 °C.

FASnI3 nanocrystals synthesized at 25 °C with a size distribution of 7.3 ± 0.8 nm (scale bar, 100 nm).

Extended Data Fig. 2 TEM images of FASnI3 nanocrystals synthesized at 125 °C.

FASnI3 nanocrystals synthesized at 125 °C with a size distribution of 12.1 ± 1.1 nm (scale bar, 100 nm).

Extended Data Fig. 3 TEM images and electron diffraction patterns of FASnI3 nanocrystals.

(a, c) HR-TEM images of FASnI3 nanocrystals (125 °C synthesis) with (b, d) corresponding Fourier transform images (electron diffraction patterns) (scale bar, 10 nm).

Extended Data Fig. 4 TEM images with wide field of view.

(a, b) FASnI3 nanocrystals (25 °C synthesis) with a large field of view (scale bar, 500 nm). (c-f) FASnI3 nanocrystals (~9.7 nm, FA 130 °C, SnI2 25 °C) with a large field of view (holey carbon films, (c, d) scale bar, 500 nm, (e, f) scale bar, 1 μm). See Additional Fig. 4 for individual images.

Extended Data Fig. 5 XRD sample preparations.

(a) The colloidal nanocrystals in hexane were drop-casted onto a glass substrate and dried for ten minutes in a glovebox without further processing. (b) A schematic diagram of XRD measurements for FASnI3 nanocrystal superlattices. (c) The drop-casted film was scraped by a blade so the superlattice structure was broken. (d) A schematic diagram of XRD measurements for scratched-off FASnI3 nanocrystal powder.

Extended Data Fig. 6 XRD of FASnI3 nanocrystal superlattices.

(a) XRD patterns of FASnI3 nanocrystal superlattices prepared by dropcasting nanocrystal solutions on glass substrates. The temperatures refer to the nanocrystal synthesis. (b) Magnified view of the 2θ ∼ 14° reflection of the superlattice constructed from the 110 °C nanocrystals. The x-axis is transformed to the scattering vector q = 4π sin(θ)/λ. Due to the periodicity of the superlattice, the Bragg reflection is split into a series of satellite peaks (indicated by blue lines). Inset: Plot of the satellite positions qn vs. the satellite peak n. The mean centre-to-centre distance between the nanocrystals in the superlattice Λ is given by Λ = 2π n/q.

Extended Data Fig. 7 TA bleach kinetics.

Early-time kinetics of the two bleaches of FASnI3 nanocrystals under (a) 1.97-eV pump and (b) 3.1-eV pump. Kinetics of two bleaches of under (c) 1.97-eV pump and (d) 3.1-eV pump.

Extended Data Fig. 8 TA kinetics of individual sweeps.

Kinetics from individual TA sweeps at the beginning and end of the overall measurement. There is no systematic difference between the kinetics of the first sweep and the last sweep, indicating that there is no change in the photophysics of the FASnI3 nanocrystals due to degradation during the TA measurements.

Extended Data Fig. 9 TA bleach kinetics of FASnI3 nanocrystals of different sizes.

Kinetics (−1 to 1500 ps, unnormalized) of the two bleaches of FASnI3 nanocrystals of different sizes. (a) ~12.1 nm, (b) ~10.9 nm, (c) ~9.7 nm, (d) ~8.5 nm.

Extended Data Fig. 10 TA map and fitted kinetics of FASnI3 films.

(a) TA map of FASnI3 film under 400-nm laser. (b) The kinetics of the high-energy bleach of FASnI3 film and the fitting curve of the decay. The fitting curve gives a decay of 0.463 ± 0.026 ps. For bulk FASnI3 films that have a continuous electronic structure, photoexcitations undergo a rapid relaxation process by emitting phonons, the process of which obeys the conservation of the crystal momentum and energy. A second bleach above the bandgap was also detected by TA spectroscopy, indicating the existence of the second transition in bulk FASnI3. This second transition (or second band) in FASnI3 could result from the density of states of bulk FASnI3 calculated by DFT simulation. Although a second transition was detected in bulk FASnI3, this high-energy state (or second band) in bulk material is barely capable of preserving hot carriers because of the continuity of the conduction band and the valence band, leading to a relaxation time of 0.46 ps directly observed from the bleach decay.

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Supplementary notes, Figs. 4 and 11–17, and Tables 1 and 2.

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Dai, L., Deng, Z., Auras, F. et al. Slow carrier relaxation in tin-based perovskite nanocrystals. Nat. Photon. 15, 696–702 (2021). https://doi.org/10.1038/s41566-021-00847-2

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