Unraveling Electron Dynamics in Perovskite Nanocrystals from Milli-to Femtoseconds

The fundamental understanding of carrier dynamics in- and between photovoltaic materials is of utmost importance for the advancement of material and device engineering. These dynamics includes both transfer of electron and holes through the material and between layers of different materials. The focus herein will be on charge transfer and relaxation of excited electrons in nanocrystals of the lead halide perovskite CsPbBr₃. CsPbBr₃ is material that has shown promise as absorber layer in devices and it is also relatively stable under x-ray illumination compared to its hybrid cousins. This allows for long measurements that often is a requirement for studies of electron dynamics. Furthermore, nanocrystals allows for efficient tuning of its electronic properties by changing their size and shape [1,2].<br/><br/>Reviewing the timescales in which charge transfer occurs in lead halide perovskites that are similar to our system, it is shown that it varies through several orders of magnitude. For an interface between methylammonium lead iodide and the 2D material WS₂, pump-probe measurements with 45 femtoseconds time resolution did not capture the charge transfer process, indicative that it occurred on a shorter timescale. However, a small fraction of the electrons remained in the perovskite layer for up to 2 ns [3] showing that multiple processes are competing and for nanowires of CsPbBr₃ it has been reported that the exciton radiative recombination is in the few nanosecond regime [4]<b>.</b><br/><br/>We have used time resolved x-ray photoelectron spectroscopy on CsPbBr₃ nanocrystals to study the longer time scale and specifically recombination processes and have measured a decay constant in the range of 0.5 millisecond for the CsPbBr₃ nanocrystals on a gold substrate which can be compared with 20 microseconds that we recorded for a spin-coated CsPbBr₃thin film on TiO₂.<br/><br/>To study the other faster end of the timescale we used two photon photoemission, were the decay of the valence excited state of CsPbBr₃ nanocrystals with two different ligands was followed. The technique allows for studies of hot electron cooling and polaron formation, processes that occur in the picosecond range [5]. Our measurement revealed several time dependencies, showing hot electron cooling in the 0.5 picosecond range independent of which ligand was attached to the nanocrystal and a slower component, that could describe trap states, with decay constants in the several picosecond range. However, this slow component is dependent on which ligand is attached to the nanocrystals.<br/><br/>For the truly ultrafast timescale inside the nanocrystals, on the few femtosecond and even attosecond scale, core-hole clock spectroscopy was used [6,7]. This chemically specific, resonant x-ray spectroscopy uses the core-hole lifetime as an internal clock and allows for studies of pure electronic charge transfer <i>i.e. </i>tunneling. Our results indicate that charge transfer occurs on the few femtosecond timescale when resonantly exciting both the cesium L₃-edge and the lead M₅-edge.<br/><br/>To summarize, we have measured charge transfer and electron dynamics in CsPbBr₃ on timescales spanning twelve orders of magnitude, from femtoseconds to milliseconds using different spectroscopic techniques. Providing a comprehensive review of the electron dynamics in this promising material system.<br/><br/><br/>References:<br/>[1] Akkerman <i>et al</i>. <i>Science, </i>377, 1406-1412, (2022)<br/>[2] Wahl <i>et al.</i> <i>Phys. Chem. Chem. Phys.</i>, 24, 10944-10951, (2022)<br/>[3] Bauer <i>et al. J. Phys. Chem. C</i>, 122, 28910-28917, (2018)<br/>[4] Fang <i>et al. J.</i> <i>Phys. Chem. Lett.</i> 9, 1655, (2018)<br/>[5] Evans <i>et al. J.</i> <i>Phys. Chem. C</i>, 122 13724-13730, (2018)<br/>[6] Johansson <i>et al.</i> <i>J. Phys. Chem. C </i>, 122, 12605-12614, (2018)<br/>[7] Johansson <i>et al.</i> <i>Phys. Rev. B</i>, 102, 035165, (2020)