Charge-Carrier Dynamics in Metal Halide Perovskite Heterostructures

Rebecca L. Milot

Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, UK CV4 7AL

Metal halide perovskites have attracted much attention for use in optoelectronic applications, particularly solar cells. However, the standard materials used in many devices (typically 3D materials such as formamidinium lead triiodide, FAPbI₃) 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 halide perovskites are incorporated into 3D perovskite thin films as either a mixture or a capping layer, thus forming complex heterostructures. A commonly used material is phenylethylammonium lead iodide (PEA₂PbI₄), which adopts a Ruddlesden-Popper where a layer of corner-sharing lead iodide octahedra is separated by a bilayer PEA cations. However, materials such as PEAPbI₄ typically have exciton binding energies of 100s of meV and can thus greatly alter optoelectronic properties due to a large population of excitons present at ambient temperatures [1].
For heterostructures, understanding charge transport can be challenging using just one spectroscopic technique because multiple materials, phases, and photoexcited species can coexist. To untangle the contributions from all of these different species, we have used a combination of visible transient absorption spectroscopy (TAS) and optical pump/THz probe spectroscopy (OPTP) [2]. With this combination of techniques, we have evaluated various lead-based 3D perovskite thin films which have been treated with PEA salts purported to preferentially form PEAPbI₄ layers at the surface of the films, thus forming RP/3D heterostructures [2]. However, TAS measurements show spectroscopic signatures of other Ruddlesden-Popper phases (PEA₂A_n−1B_nX_3n+1, where A is a smaller organic cation and n is the number of lead-iodide octahedra in a layer), as the various RP phases in PEAPbI₄ are well separated spectrally at visible frequencies (Figure 1A). TAS thus provides a sensitive method to distinguish these phases when they are not apparent in absorption or photoluminescence measurements. However, exciton and free- carrier effects can be more difficult to distinguish due to overlapping spectral features. 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 [3]. In addition to finding that the charge-carrier dynamics are sensitive to the film preparation method, we distinguish bulk and surface passivation effects (Figure 1B,C) and query charge transfer between RP and 3D species.

Figure 1: (A) Heatmaps showing the transient absorption spectra measured upon front excitation of a thin film of MAPbI₃ treated with a solution with a PEA concentration of 40 mM (denoted PEA-40). The spectra were obtained by exciting from the front (PEA-treated side) and with excitation at 410 nm at a fluence of ~200 µJ cm⁻². A logarithmic timescale is used with excitation occurring at time zero. The GSB features present in the films with thicker RP capping layers are indicated with dashed lines. Normalised OPTP transients for (b) MAPI and (c) PEA-40 with excitation at a fluence of 27 µJ cm⁻² at both 410 nm and 700 nm. Due to the short penetration depth, excitation at 410 nm targets carrier dynamics at the surface.

References
[1] D. Sirbu, F. Balogun, R. L. Milot, P. Docampo, Adv. Energy Mater. 11(24), 2003877 (2021)
[2] J. D. Hutchinson, E. Ruggeri, J. M. Woolley, G. Delport, S. D. Stranks, R. L. Milot, Adv. Funct. Mater. 33(50), 13 (2023)
[3] F. H. Balogun, N. P. Gallop, D. Sirbu, J. D. Hutchinson, N. Hill, J. M. Woolley, D. Walker, S. York, P. Docampo, R. L. Milot, Mater. Res. Express 11(2), 13 (2024)