Charge-Carrier Dynamics in 2D Hybrid Metal–Halide Perovskites

Abstract

Hybrid metal-halide perovskites are promising new materials for use in solar cells; however, their chemical stability in the presence of moisture remains a significant drawback. Quasi two-dimensional (2D) perovskites that incorporate hydrophobic organic interlayers offer improved resistance to degradation by moisture, currently still at the cost of overall cell efficiency. To elucidate the factors affecting the optoelectronic properties of these materials, we have investigated the charge transport properties and crystallographic orientation of mixed methylammonium (MA)-phenylethylammonium (PEA) lead iodide thin films as a function of the MA-to-PEA ratio and, thus, the thickness of the "encapsulated" MA lead-halide layers. We find that monomolecular charge-carrier recombination rates first decrease with increasing PEA fraction, most likely as a result of trap passivation, but then increase significantly as excitonic effects begin to dominate for thin confined layers. Bimolecular and Auger recombination rate constants are found to be sensitive to changes in electronic confinement, which alters the density of states for electronic transitions. We demonstrate that effective charge-carrier mobilities remain remarkably high (near 10 cmVs) for intermediate PEA content and are enhanced for preferential orientation of the conducting lead iodide layers along the probing electric field. The trade-off between trap reduction, electronic confinement, and layer orientation leads to calculated charge-carrier diffusion lengths reaching a maximum of 2.5 μm for intermediate PEA content (50%).

Structure of Organometal Halide Perovskite Films as Determined with Grazing‐Incidence X‐Ray Scattering Methods

“…[13,14,39,66] On the long-term, GIXS might help to unravel hitherto unresolved questions on film formation, device stability, or the influence of crystal shape Reproduced with permission. [59] Copyright 2016, American Chemical Society. b) Schematic of the heterojunction approach comprising 3D MAPI with a thinner layer of 2D perovskite on top.…”

“…Thus, a range of low dimensional perovskites can easily be produced as depicted in Figure 4a. [59] Associated with the confinement in 2D perovskites, the optoelectronic properties of the material are also affected: [59][60][61] Figure 4a shows the absorbance (black) and photoluminescence spectra (red), which indicate a pronounced blue-shift for lower dimensional varieties due to quantum confinement, with absorption and emission behavior becoming more excitonic in nature. In the spectra of the mixed dimensional species, additional contributions from the lower dimensional variants are observed.…”

Two-dimensional organic-inorganic hybrid perovskite: from material properties to device applications

“…Fortunately, 2D perovskite is highly anisotropic because of its inherent organic-inorganic stacking structure, which signifies that the charge transport ability could be improved by out-of-plane crystal growth. Vertical growth of inorganic layer is related to the species and proportions of organic cations as well as preparation methods [6,[14][15][16]. The 2D perovskite film is isotropy when the proportion of large-group ammonium halides is small.…”

“…However, with a larger proportion of insulating layers (smaller n), the electronic properties of 2D perovskite make it imperfect as solarcell absorber for the increased exciton binding energy as well as the decreased charge separation and transport abilities. The charge transport ability can be improved by engineering the vertical orientation of inorganic layers [6,[14][15][16].…”

Coherent Spin and Quasiparticle Dynamics in Solution‐Processed Layered 2D Lead Halide Perovskites

“…Early photophysics studies in 2D perovskites were mainly limited to their linear properties;10, 15, 16 while studies on their transient photophysics are few and far between 17, 18, 19. In particular, transient optical spectroscopy takes on a leading role in understanding these unusual phenomena and in unlocking the latent potential of these novel materials 20.…”