Two-dimensional (2D) hybrid perovskites are generating
broad scientific
interest because of their potential for use in photovoltaics and microcavity
lasers. It has recently been demonstrated that mixtures of quantum
wells with different thicknesses can be assembled in films with heterogeneous
quantum well distributions. Large (small) quantum wells are concentrated
at the air side (substrate side) of the films, thereby promoting directional
energy and/or electron transfer. However, profiles of the quantum
well concentrations have not been directly measured throughout the
full thicknesses of the films. Similarly, the lateral motions of the
excitations in these systems are not well-characterized. In this work,
we perform focused ion beam milling tests to establish quantum well
concentrations as a function of depth in layered 2D perovskite films.
In addition, transient absorption microscopy is used to investigate
carrier diffusion and two-body recombination processes. Comparisons
of the layered films with phase-pure single crystals reveal that diffusion
is suppressed by grain boundaries in the films, which in turn promotes
two-body recombination. Similar behaviors were previously observed
in bulk perovskite films and single crystals. These studies suggest
that the morphology of the film, rather than the identity of the material,
is the primary factor that governs the two-body recombination dynamics.
Enhancement of the two-body recombination processes is desirable for
applications such as microcavity lasers.
Two-dimensional coherent photocurrent spectroscopies directly probe the electronic states and processes that are relevant to the performance of a photovoltaic device. In this Letter, we apply two-pulse nonlinear photocurrent spectroscopy to a photovoltaic device based on layered perovskite quantum wells. The method effectively decomposes the photovoltaic response into contributions from separate quantum wells and excited-state species (i.e., either single excitons or biexcitons). Our experiments show that the efficiency of photocurrent generation increases with the size of the quantum well. Overall, the results suggest that energy funneling processes in layered perovskites, which are most prominent in transient absorption spectroscopies, are largely irrelevant to the function of a photovoltaic cell.
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