Rebecca Milot1
University of Warwick1
Metal halide perovskites are promising materials for many optoelectronic applications, particularly solar cells. However, the standard materials used in many high efficiency devices (e.g. 3D materials such as methylammonium lead triiodide, MAPbI<sub>3</sub>) are unstable under ambient conditions due to their sensitivity to a number of environmental factors including humidity and light. To address stability issues, layered or quasi-2D materials are incorporated into 3D perovskite thin films as either a mixture or a capping layer. However, these materials can greatly alter the optoelectronic properties since the exciton binding energies in 2D perovskites can be as high as 100s of meV, meaning that a large population of excitons can be present at ambient temperatures.<sup>1</sup> <br/><br/>Since the beginning of the perovskite field about a decade ago, ultrafast spectroscopy techniques including optical pump/THz probe spectroscopy (OPTP) have provided vital information about the charge transport and charge-carrier dynamics in metal halide perovskites.<sup>2</sup> For these newly-developed mixed 2D/3D materials, however, understanding charge transport can be challenging using just one spectroscopic technique due to the presence of multiple materials and photoexcited species. To untangle the contributions from all of these different species, we have used a combination of visible transient absorption spectroscopy (TAS) and OPTP. While TAS can easily distinguish the excited states of various phases of 2D and 3D perovskites, OPTP is sensitive only to mobile free charge carriers and can be used to evaluate the charge-carrier mobility and separate excitonic effects when compared to TAS data. With this combination of techniques, we have evaluated various lead-based 3D perovskite thin films which have been treated with phenylethylammonium salts in order to preferentially form Ruddlesden-Popper phases at the surface of the films. In addition to finding that the charge-carrier dynamics are sensitive to the film preparation method, we distinguish between bulk and surface passivation effects and query charge transfer between 2D and 3D species.<br/><br/>1. Sirbu, D., Balogun, F. H., Milot, R. L. & Docampo, P. Layered Perovskites in Solar Cells: Structure, Optoelectronic Properties, and Device Design. <i>Adv. Energy Mater.</i> <b>11</b>, 2003877 (2021).<br/>2. Pereira dos Santos, T.<i> et al.</i> The 2021 Roadmap on Ultrafast Spectroscopic Probes of Condensed Matter. <i>J. Phys. - Condens. Mat.</i> <b>33</b>, 353001 (2021).