The performance of semiconductor devices is fundamentally governed by charge-carrier dynamics within the active materials1,2,3,4,5,6. Although advances have been made towards understanding these dynamics under steady-state conditions, the importance of non-equilibrium phenomena and their effect on device performances remains elusive7,8. In fact, the ballistic propagation of carriers is generally considered to not contribute to the mechanism of photovoltaics (PVs) and light-emitting diodes, as scattering rapidly disrupts such processes after carrier generation via photon absorption or electric injection9. Here we characterize the spatiotemporal dynamics of carriers immediately after photon absorption in methylammonium lead iodide perovskite films using femtosecond transient absorption microscopy (fs-TAM) with a 10 fs temporal resolution and 10 nm spatial precision. We found that non-equilibrium carriers propagate ballistically over 150 nm within 20 fs of photon absorption. Our results suggest that in a typical perovskite PV device operating under standard conditions, a large fraction of carriers can reach the charge collection layers ballistically. The ballistic transport distance appears to be limited by energetic disorder within the materials, probably due to disorder-induced scattering. This provides a direct route towards optimization of the ballistic transport distance via improvements in materials and by minimizing the energetic disorder. Our observations reveal an unexplored regime of carrier transport in perovskites, which could have important consequences for device performance.
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The data underlying all figures in the main text and supplementary information are publicly available at https://doi.org/10.17863/CAM.44851.
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We thank the Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability for funding. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 758826). J.S. acknowledges financial support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A3A03009054). L.N. acknowledges support from the Jardine Foundation. C.S. acknowledges financial support from the Royal Commission for the Exhibition of 1851. A.S. acknowledges support from the UKRI Global Challenge Research Fund project, SUNRISE (EP/P032591/1), UKIERI project for the Physics of Sustainability (University of Cambridge) and Indo-UK joint project-APEX Phase-II.
The authors declare no competing interests.
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Sung, J., Schnedermann, C., Ni, L. et al. Long-range ballistic propagation of carriers in methylammonium lead iodide perovskite thin films. Nat. Phys. (2019). https://doi.org/10.1038/s41567-019-0730-2