Antonella Treglia

Hot carriers (HC) offer a potential route to surpass conventional efficiency limits in solar cells. Generated after light absorption, these high-energy carriers can significantly boost device performance if extracted before dissipating energy as heat. Although HC cooling has been studied in tin-based perovskites, interpreting these dynamics is challenging because high-energy, high-fluence pump pulses required to generate HCs have shallow penetration depths, confining excitation near the surface. In tin-based perovskites, surface properties differ from the bulk, and surface defects may strongly contribute to the optical response. Here, by using pump-push-probe ultrafast transient absorption spectroscopy, we photo-excite the semiconductor thin film with a low-energy pump, in a photon density range typical for solar cell operation, making relevant the bulk electronic properties, and selectively re-excite the photo-carriers only once they cool down, enabling direct tracking of HC relaxation in the relevant spectral region while minimizing surface contributions. We track the cooling process in FA0.85Cs0.15SnI3 (FA = Formamidinium) as a benchmark sample and extend the analysis to single-cation FA+, MA+ (MA = methylammonium), and Cs+ tin perovskites. Across all compositions, and at different doping densities, we find cooling timescales in the 150–600 fs time range. These results highlight the central role of phonon-mediated interactions and show that ultrafast cooling fundamentally limits hot carrier extraction in tin halide perovskites.

Two-dimensional van der Waals materials display rich nonequilibrium physics due to their reduced dimensionality and strong light–matter interactions. Layered metal phosphorus trichalcogenides (MPX3) feature bandgaps spanning the visible to ultraviolet range and exhibit a complex interplay between excitons, phonons, and defects. Here we use static and time-resolved photoluminescence and femtosecond broadband transient reflectivity to investigate the temperature-dependent ultrafast carrier dynamics in layered HgPSe3, a semiconductor with a quasi-direct bandgap near 2 eV. We identify distinct sub-bandgap photoluminescence features that become prominent at low temperature. Their temperature and fluence dependence strongly indicate radiative recombination involving localized states. The transient reflectivity map displays three features associated with interband optical transitions in the visible range, as corroborated by density functional theory band structure calculations. These features exhibit fluence-dependent decay dynamics, indicating a competition between carrier localization into sub-bandgap states, state-filling effects that limit the available electronic phase space for hot-carrier relaxation, and phonon-bottleneck-limited carrier cooling. Our results provide new insights into the ultrafast carrier dynamics of HgPSe3 and establish a framework for understanding the role of sub-bandgap states in MPX3 semiconductors, with potential implications for broadband optoelectronic and photodetection applications.

Mixed I-Br perovskites are promising top absorbers for tandem solar cells due to their tunable band gap and solution processability, yet photoinduced halide segregation—a critical but poorly understood degradation mechanism—hinders their reliability. Here, we demonstrate how the enhancement of the Br− binding energy in the perovskite inhibits I2Br− formation—the key intermediate species driving halide segregation. The incorporation of densely charged cations like Sr2+ in the perovskite strongly localizes halide anions, inhibiting their electron donation to photogenerated I2 and preventing triple halide ion formation, which eventually suppresses halide segregation. Additionally, this also reduces undercoordinated Pb at grain surfaces, suppressing nonradiative recombination. These synergistic effects yield high-performance solar cells with champion efficiencies of 20.18% at 1.77 eV and 19.02% at 1.84 eV, alongside enhanced stability. Our work establishes a design principle to mitigate halide segregation in mixed-halide perovskites, overcoming a major challenge in perovskite optoelectronics.

Vacuum-deposited lead halide perovskite thin films enable solvent-free fabrication, eliminating residual processing solvents that might compromise the long-term stability. Here, we investigate the stability of thermally evaporated mixed-cation compositions FA_0.8Cs_0.2PbI₃ and FA_0.8MA_0.2PbI₃ (FA⁺ = formamidinium and MA⁺ = methylammonium) under thermal and light stress. Although from a thermodynamic perspective the phase stability hierarchy is typically described as MA⁺ < FA⁺ < Cs⁺, with Cs-based perovskites expected to be the most stable, both compositions exhibit thermal robustness, retaining their structural, optical, and morphological properties after continuous heating at 85 °C for over 500 h. Under continuous illumination, however, distinct degradation pathways emerge: FA_0.8Cs_0.2PbI₃ shows the largest morphological and optical changes. This is attributed to chemical inhomogeneities caused by CsI-rich segregations during crystallization, which make point defects effective triggers for photodegradation. Film homogeneity improves by partially replacing iodide with bromide. Based on these results, we selected FA_0.8MA_0.2PbI₃ and FA_0.8Cs_0.2Pb­(I_0.8Br_0.2)₃ for device fabrication and evaluated their operational stability. The resulting perovskite solar cells maintain their performance after four months of outdoor operation and withstand 900 h under continuous sun-equivalent indoor illumination at room temperature. These results demonstrate how a high-quality crystallization process can reveal the potential of MA-containing perovskite formulations for long-lived perovskite photovoltaics.

Tin-halide perovskites are emerging as exceptional materials for near-infrared light-emitting diodes (NIR-LEDs). However, their extreme oxygen sensitivity remains a significant obstacle to practical applications. This work presents a facile yet effective strategy to overcome this limitation by designing self-encapsulated tin-halide perovskite films. Incorporating a rational molecule, 4,4′-diaminodiphenyl sulfone, into precursors, it forms isolated tin-iodide perovskite particles that are encapsulated in situ, achieving outstanding air stability. The resulting films show high crystallinity, reduced trap density, and mitigated p-doping density, boosting radiative charge recombination to reach an impressive photoluminescence quantum yield approaching 50%. Leveraging these advancements, the resulting NIR-LEDs demonstrate a record-breaking peak external quantum efficiency of 12.4%, accompanied by a substantial improvement in operational lifetime. Notably, for the first time, we demonstrated a functional tin-iodide perovskite-based device in ambient air. This work provides a robust pathway for realizing high-performance and stable tin-halide perovskite-based optoelectronic devices, addressing critical challenges for their widespread application.

Chiral metal halides are promising materials for nonlinear optics and spin-selective devices. Typically, chirality is introduced via large chiral organic cations, leading to low-dimensional structures and limitations in charge transport. Here, we design a family of chiral metal halides based on the relatively small ditopic R/S-3-aminoquinuclidine (3-AQ) cation, forming an (R/S-3AQ)-Pb₂Br₆ structure closely related to the 3D corner-sharing octahedral network of perovskites. The resulting material exhibits a direct bandgap, isotropic band structure, and fully 3D photoexcitation. Circular dichroism confirms a chiral anisotropy factor consistent with theoretical predictions. Moreover, the material displays a Rashba effect in the conduction band, which is attributed to spin-orbit coupling and the lack of inversion symmetry. Offering rich chemical tunability and efficient 3D charge transport, this new class of chiral semiconductors provides a promising platform for advancing nonlinear optoelectronic and spintronic devices.

Low-contact-loss and durable buried interface was engineering by a molecular hybridization strategy. The proton transfer from Me-4PACz to Histamine enables distinguished efficiency and operational stability of PSCs based on mix-halide perovskites.

The role of A-site cations (MA ⁺ , FA ⁺ , Cs ⁺ ) in the defect chemistry of metal halide semiconductor is well-studied in lead halide perovskites; here we investigate it in the less explored tin perovskites.

Tin‐halide perovskites (THP) exhibit complex carrier dynamics due to the interplay between electronic doping and carrier trapping, both of which affect device performance. Evaluating the impact of trap states is challenging because the timescales of photogenerated electron recombination with dopant holes and trapping often overlap. Here, Transient Absorption Spectroscopy (TAS) is used across a broad spectral and temporal range, spanning from visible to near‐infrared and from femtoseconds to microseconds, to probe both sub‐bandgap and band‐edge transitions, while manipulating defect and doping densities via chemical treatments. Focusing on tin triiodide perovskites, the rapid carrier recombination due to high electronic doping density is considered the main source of carrier loss. However, deep electron trap states originated by two distinct type of defects are identified: surface Sn(IV) defects and tin interstitials. Surface Sn(IV) defects play a key role in the loss of photo‐generated carriers, but their density can be mitigated by the addition of SnF₂, improving carrier lifetimes. Nevertheless, excessive SnF₂ promotes the stabilization of tin interstitial traps, highlighting a delicate balance in defect control. Moreover, near‐infrared TAS reveals sub‐bandgap transitions associated with shallow traps, which contribute to band‐edge repopulation within tens of picoseconds. This work disentangles the contributions of doping and trap‐mediated processes to the optoelectronic mechanisms in THP, offering insights into defect management for performance optimization.

Long-wavelength near-infrared light-emitting diodes (NIR LEDs) with peak emission wavelengths beyond 900 nm are of critical importance for various applications including night vision, biomedical imaging, sensing and optical communications. However, the low radiance and poor operational stability of state-of-the-art long-wavelength NIR LEDs based on soft materials remain the most critical factors limiting their practical applications. Here we develop NIR LEDs emitting beyond 900 nm with improved performance through the rational manipulation of p doping in all-inorganic tin perovskites (CsSnI₃) by retarding and controlling the crystallization process of perovskite precursors in tin-rich conditions. The resulting NIR LEDs exhibit a peak emission wavelength at 948 nm, high radiance of 226 W sr⁻¹ m⁻² and long operational half-lifetime of 39.5 h at a high constant current density of 100 mA cm⁻². Our demonstration of efficient and stable NIR LEDs operating at high current densities may also open up new opportunities towards electrically pumped lasers.