Proceedings of MATSUS23 & Sustainable Technology Forum València (STECH23) (MATSUS23)
DOI: https://doi.org/10.29363/nanoge.matsus.2023.042
Publication date: 22nd December 2022
Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies of single-junction devices now exceeding 25%.
On issue that still limits the implementation of silicon-perovskite tandem cells in particular, is the peculiar mechanisms underlying detrimental halide segregation in mixed iodide-bromide lead perovskites with desirable electronic band gaps near 1.75eV.[1,2,3,4] We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities.[2] However, remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains leads to enhanced radiative recombination.[3] Performance losses in photovoltaic devices may therefore potentially be mitigated by deployment of careful light management strategies. We further demonstrate[3] how a combination of simultaneous in-situ photoluminescence and X-ray diffraction measurements is able to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br0.5I0.5)3 and FA0.83Cs0.17Pb(Br0.4I0.6)3 films. While MAPb(Br0.5I0.5)3 exhibits rearrangement of halide ions only in localized volumes of perovskite, FA0.83Cs0.17Pb(Br0.4I0.6)3 lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation.[4] We further explore the influence of a hole-transport layer, necessary for a full device. We show that top coating FA0.83Cs0.17Pb(Br0.4I0.6)3 perovskite films with a poly(triaryl)amine (PTAA) hole-extraction layer surprisingly leads to suppression of halide segregation because photogenerated charge carriers are rapidly trapped at interfacial defects that do not drive halide segregation.[4]
We also discuss the charge-carrier dynamics in layered, 2D perovskites that have been found to improve the stability of metal halide perovskite thin films and devices.[5] We show that the 2D perovskite PEA2PbI4 exhibits an excellent long-range mobility of 8.0 cm2 (V s)–1, ten times greater than the long-range mobility determined for a comparable 3D material FA0.9Cs0.1PbI3. These values shows that the polycrystalline 2D thin films already have single-crystal-like qualities. We further demonstrate that these materials exhibit unexpectedly high densities of sustained populations of free charge carriers.
[1] A. J. Knight and L. M. Herz, Energy Environmental Science 13, 2024 (2020).
[2] S. G. Motti, J. B. Patel, R. D. J. Oliver, H. J. Snaith, M. B. Johnston, L. M. Herz, Nature Communications 12, 6955 (2021).
[3] A. J. Knight, J. Borchert, R. D. J. Oliver, J. B. Patel, P. G. Radaelli, H. J. Snaith, M. B. Johnston, and L. M. Herz, ACS Energy Letters 6, 799 (2021).
[4] Impact of hole-transport layer and interface passivation on halide segregation in mixed-halide perovskites, V. J.-Y. Lim, A. J. Knight, R. D. J. Oliver, H. J. Snaith, M. B. Johnston, and L. M. Herz, Advanced Functional Materials 32, 2204825 (2022).
[5] Excellent long-range charge-carrier mobility in 2D perovskites, M. Kober-Czerny, S. G. Motti, P. Holzhey, B. Wenger, J. Lim, L. M. Herz, and H. J. Snaith, Advanced Functional Materials 32, 2203064 (2022).
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