Prof Laura Herz FRS

Abstract:

Halide perovskites show great optoelectronic performance, but their favorable properties are paired with unusually strong anharmonicity. It was proposed that this combination derives from the ns2 electron configuration of octahedral cations and associated pseudo-Jahn–Teller effect. We show that such cations are not a prerequisite for the strong anharmonicity and low-energy lattice dynamics encountered in these materials. We combine X-ray diffraction, infrared and Raman spectroscopies, and molecular dynamics to contrast the lattice dynamics of CsSrBr3 with those of CsPbBr3, two compounds that are structurally similar but with the former lacking ns2 cations with the propensity to form electron lone pairs. We exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden in the cubic phase, as a fingerprint of anharmonicity and reveal that low-frequency tilting occurs irrespective of octahedral cation electron configuration. This highlights the role of structure in perovskite lattice dynamics, providing design rules for the emerging class of soft perovskite semiconductors.

Abstract:

The efficiency and longevity of metal-halide perovskite solar cells are typically dictated by nonradiative defect-mediated charge recombination. In this work, we demonstrate a vapor-based amino-silane passivation that reduces photovoltage deficits to around 100 millivolts (>90% of the thermodynamic limit) in perovskite solar cells of bandgaps between 1.6 and 1.8 electron volts, which is crucial for tandem applications. A primary-, secondary-, or tertiary-amino–silane alone negatively or barely affected perovskite crystallinity and charge transport, but amino-silanes that incorporate primary and secondary amines yield up to a 60-fold increase in photoluminescence quantum yield and preserve long-range conduction. Amino-silane–treated devices retained 95% power conversion efficiency for more than 1500 hours under full-spectrum sunlight at 85°C and open-circuit conditions in ambient air with a relative humidity of 50 to 60%.

Abstract:

Silver‐bismuth‐based perovskite‐inspired materials (PIMs) are increasingly being explored as non‐toxic materials in photovoltaic applications. However, many of these materials exhibit an ultrafast localization of photogenerated charge carriers that is detrimental for charge‐carrier extraction. In this work, such localization processes are explored for thermally evaporated thin films of compositions lying along the (AgI)x(BiI3)y series, namely BiI3, AgBi2I7, AgBiI4, Ag2BiI5, Ag3BiI6, and AgI, to investigate the impact of changing Ag+/Bi3+ content. A persistent presence of ultrafast charge‐carrier localization in all mixed compositions and BiI3, together with unusually broad photoluminescence spectra, reveal that eliminating silver will not suppress the emergence of a localized state. A weak change in electronic bandgap and charge‐carrier mobility reveals the resilience of the electronic band structure upon modifications in the Ag+/Bi3+ composition of the mixed‐metal rudorffites. Instead, chemical composition impacts the charge‐carrier dynamics indirectly via structural alterations: Ag‐deficient compositions demonstrate stronger charge‐carrier localization most likely because a higher density of vacant sites in the cationic sublattice imparts enhanced lattice softness. Unraveling such delicate interplay between chemical composition, crystal structure, and charge‐carrier dynamics in (AgI)x(BiI3)y rudorffites provides crucial insights for developing a material‐by‐design approach in the quest for highly efficient Bi‐based PIMs.