Abstract
AbstractSilver‐bismuth based semiconductors represent a promising new class of materials for optoelectronic applications because of their high stability, all‐inorganic composition, and advantageous optoelectronic properties. In this study, charge‐carrier dynamics and transport properties are investigated across five compositions along the AgBiI4–CuI solid solution line (stoichiometry Cu4x(AgBi)1−xI4). The presence of a close‐packed iodide sublattice is found to provide a good backbone for general semiconducting properties across all of these materials, whose optoelectronic properties are found to improve markedly with increasing copper content, which enhances photoluminescence intensity and charge‐carrier transport. Photoluminescence and photoexcitation‐energy‐dependent terahertz photoconductivity measurements reveal that this enhanced charge‐carrier transport derives from reduced cation disorder and improved electronic connectivity owing to the presence of Cu+. Further, increased Cu+ content enhances the band curvature around the valence band maximum, resulting in lower charge‐carrier effective masses, reduced exciton binding energies, and higher mobilities. Finally, ultrafast charge‐carrier localization is observed upon pulsed photoexcitation across all compositions investigated, lowering the charge‐carrier mobility and leading to Langevin‐like bimolecular recombination. This process is concluded to be intrinsically linked to the presence of silver and bismuth, and strategies to tailor or mitigate the effect are proposed and discussed.
Highlights
K1 is the trap-mediated recombination rate, k2 is the radiative recombination rate constant, and the constant is defined such that 1 = k1 A n0. We note that this type of ordinary differential equation (ODE) has been previously applied to describe the recombination of charge carriers in conventional metal-halide perovskites,[74,75] where photoexcited charge carriers take the form of large polarons,[21,43] as opposed to the small polarons that we identify with the long-time optical-pump terahertz-probe (OPTP) signal
To gain insights into charge-carrier transport in Cu4x(AgBi)1 −xI4 semiconductors and how it may be affected by charge-carrier localization, we present in Figure 6 the values of the effective charge-carrier mobilities extracted from the fits to the early-time OPTP dynamics
We find that such charge-carrier localization leads to a modification of band-to-band bimolecular recombination mechanisms, typically described by a radiative-balance approach for lead halide perovskites, but for small polarons in Cu4x(AgBi)1−xI4 more akin to Langevin-type recombination, similar to processes in organic semiconductors
Summary
In order to probe the viability of these proposed concepts, and to understand how the addition of copper alters the electronic band structure in these silver-bismuth materials, we carried out UV–visible absorption measurements on spin-coated thin films, deposited on z-cut quartz substrates. To gain insights into charge-carrier transport in Cu4x(AgBi)1 −xI4 semiconductors and how it may be affected by charge-carrier localization, we present in Figure 6 the values of the effective charge-carrier mobilities extracted from the fits to the early-time OPTP dynamics These values combine the “true” mobilities of the large/small polaron states (μdeloc and μloc, respectively) with the photon-to-charge branching ratio φ, which may vary as the pump excess energy is altered. Cu+ addition can tailor charge-carrier dynamics and transport, both of which are crucial for determining suitability across a variety of optoelectronic applications We find that such charge-carrier localization leads to a modification of band-to-band bimolecular recombination mechanisms, typically described by a radiative-balance approach for lead halide perovskites, but for small polarons in Cu4x(AgBi)1−xI4 more akin to Langevin-type recombination, similar to processes in organic semiconductors. Given that these materials have to date received significantly less attention and research focus compared to their more prominent lead halide perovskites counterparts, such research is likely to form a highly promising future avenue toward lead-free materials for photovoltaic applications
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