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Efficient vertical charge transport in polycrystalline halide perovskites revealed by four-dimensional tracking of charge carriers

Abstract

Fast diffusion of charge carriers is crucial for efficient charge collection in perovskite solar cells. While lateral transient photoluminescence microscopies have been popularly used to characterize charge diffusion in perovskites, there exists a discrepancy between low diffusion coefficients measured and near-unity charge collection efficiencies achieved in practical solar cells. Here, we reveal hidden microscopic dynamics in halide perovskites through four-dimensional (directions x, y and z and time t) tracking of charge carriers by characterizing out-of-plane diffusion of charge carriers. By combining this approach with confocal microscopy, we discover a strong local heterogeneity of vertical charge diffusivities in a three-dimensional perovskite film, arising from the difference between intragrain and intergrain diffusion. We visualize that most charge carriers are efficiently transported through the direct intragrain pathways or via indirect detours through nearby areas with fast diffusion. The observed anisotropy and heterogeneity of charge carrier diffusion in perovskites rationalize their high performance as shown in real devices. Our work also foresees that further control of polycrystal growth will enable solar cells with micrometres-thick perovskites to achieve both long optical path length and efficient charge collection simultaneously.

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Fig. 1: Self-filtering effect on the transient PL spectrum of semiconductors.
Fig. 2: Characterization of the out-of-plane charge diffusion across the FAPbI3 perovskite film.
Fig. 3: Local heterogeneity of the vertical diffusivity in FAPbI3 perovskite.
Fig. 4: Four-dimensional (x, y, z, t) tracking of free charge carriers in FAPbI3 perovskite.
Fig. 5: Anisotropic diffusion in a vertically stacked BA2PbI4 2D perovskite.

Data availability

The data underlying this paper are available at https://doi.org/10.17863/CAM.88680.

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Acknowledgements

This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) (EP/S030638/1). S.D.S. acknowledges the Royal Society and Tata Group (UF150033), the EPSRC (EP/R023980/1 and EP/M006360/1) and the European Research Council under the European Union’s Horizon 2020 research and innovation programme (HYPERION, grant agreement no. 756962; SOLARX, grant agreement no. 758826). S.F. acknowledges an EPSRC Doctoral Prize Fellowship and is grateful for support from the Winton Programme for the Physics of Sustainability. K.M.Y., M.N.T.K. and J.H.N. acknowledge the National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Science, ICT and Future Planning (MSIP); NRF-2020R1A2C3009115). J.-Y.H. and Y.-R.W. acknowledge the Ministry of Science and Technology (MOST) of Taiwan under grant nos 109-2221-E-002-196-MY2 and MOST 111-2923-E-002-009. T.C.-J.Y. acknowledges the support of a Marie Skłodowska-Curie Individual Fellowship from the European Union’s Horizon 2020 research and innovation programme (PeTSoC, grant agreement no. 891205). J.-Y.H. acknowledges support from the Simons Foundation (grant no. 601946).

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Contributions

C.C. and N.C.G. conceived the idea. C.C. performed the confocal TCSPC with the help of S.K. and S.D.S.; C.C. developed the analysis method and performed the diffusion and ray-optics models. S.F. set up and performed the broadband time-resolved PL (intensified charge-coupled device) characterization. K.M.Y. prepared the spin-coated 3D perovskite films and took their SEM images under the supervision of J.H.N.; Y.-W.J. optimized the 2D perovskite films and took their X-ray diffraction patterns under the supervision of M.C.; C.C. measured the UV–visible transmission. K.M.Y. and M.N.T.K. fabricated and characterized the device under the supervision of J.H.N.; J.-Y.H. performed the drift-diffusion model for full devices under the supervision of Y.-R.W.; T.C.-J.Y. prepared the vapour-deposited 3D perovskite films under the supervision of S.D.S.; and S.D.S. and N.C.G. supervised the work. All authors discussed the data and contributed to the manuscript.

Corresponding author

Correspondence to Neil C. Greenham.

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Competing interests

N.C.G. is a cofounder of a company commercializing perovskite emitters. S.D.S. is a cofounder of Swift Solar. The remaining authors declare no competing interests.

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Cho, C., Feldmann, S., Yeom, K.M. et al. Efficient vertical charge transport in polycrystalline halide perovskites revealed by four-dimensional tracking of charge carriers. Nat. Mater. 21, 1388–1395 (2022). https://doi.org/10.1038/s41563-022-01395-y

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