Semiconductors group

Abstract:

Lead-free perovskites have emerged as environmentally benign alternatives to lead halide counterparts for photovoltaic and optoelectronic applications. Among them, the double perovskite Cs₂AgInCl₆ family, with proper composition engineering, exhibits remarkable white-light emission characteristics enabled by strong electron-phonon coupling and the formation of self-trapped excitons (STEs). Despite these advantages, the fundamental photophysics and structural dynamics governing their excited-state behavior remain poorly understood. Here, we report a long-lived metastable phase in the Cs₂AgInCl₆ double perovskite family and unravel this process and the concomitant electronic and structural evolution using a suite of tools including transient optical spectroscopy, time-resolved x-ray diffraction (TR-XRD), time-resolved x-ray absorption spectroscopy (TR-XAS), and inelastic x-ray scattering (IXS). We show that the photoinduced, transient metastable phase is associated with B-site [silver (Ag)-indium (In)] disorder, which induces a markedly reduced optical bandgap. Supported by TR-XRD and first-principles calculations, the Ag-In disorder drives the formation of Ag-rich and In-rich domains with millisecond lifetimes, with lifetimes increasing at lower temperatures. TR-XAS further reveals that the photogenerated STEs oxidize Ag⁺ to Ag²⁺, which facilitates this highly temporally asymmetric order-disorder transition. Our findings demonstrate a previously unidentified mechanism, mediated by hole-localized STE formation, that enables prolongation of transient light-induced states to the multimillisecond regime in halide double perovskites, opening possibilities to harvesting the functional properties of metastable phases of these material systems.

Abstract:

Effective charge-carrier transport is a key requirement of next-generation thin-film materials developed for solar cells. Perovskite-inspired materials (PIMs), including metal chalcohalides, show great promise as lead-free solar absorbers. However, intrinsic charge-carrier localization processes have frequently been reported to severely limit their transport properties. Recent research has thus focused on developing a rational understanding of this localization process and identifying strategies to eliminate it. Mixed-metal chalcohalides (A₂BCh₂X₃) may offer promising solutions, combining enhanced chemical stability with promising optoelectronic properties. Here, we demonstrate how charge-carrier localization can be overcome through judicious chemical substitution in this family of materials. Upon changing the M(II) cation on the A-site, the lattice symmetry shifts from the lower-symmetry monoclinic P2₁/c phase in Pb₂SbS₂I₃ to the higher-symmetry orthorhombic Cmcm phase in Sn₂SbS₂I₃. Crucially, a rapid localization of charge carriers within the first few picoseconds of their generation is observed only for Pb₂SbS₂I₃, whereas Sn₂SbS₂I₃ maintains a longer-lived nanosecond photoconductivity. We attribute this observation to the higher electronic dimensionality of the Cmcm Sn₂SbS₂I₃ structure, whose more symmetric lattice suppresses the charge-carrier localization dominating in the lower-dimensional P2₁/c Pb-analogue. These findings establish a direct link between structural and optoelectronic properties in metal chalcohalides, demonstrating how facile chemical tuning can be harnessed to overcome charge-carrier localization in PIM absorbers for solar energy harvesting.

Abstract:

Abstract Coupling CO₂ reduction with selective oxidation of biomass-derived molecules enables simultaneous carbon utilisation and value-added chemical synthesis, offering a sustainable pathway when driven by solar energy. Here, we report a bias-free photoelectrochemical platform that co-produces formate from CO₂ and 5-formyl-2-furoic acid, a key precursor to bio-based plastics. The system integrates a Co₃O₄/WO₃/g-C₃N₄ heterojunction photoanode on carbon paper, which selectively oxidises 5-hydroxymethyl-2-furoic acid to 5-formyl-2-furoic acid with a Faradaic efficiency of 95%, and a tin nanoparticle-decorated carbon paper cathode, which reduces CO₂ to formate with a Faradaic efficiency of 93% under solar irradiation and without external bias. The WO₃/g-C₃N₄ heterojunction promotes efficient Z-scheme charge transfer, enabling spatially separated oxidation and reduction reactions using earth-abundant materials and scalable electrode architectures. Life cycle assessment demonstrates low greenhouse gas emissions and low water consumption potential of the CO₂-to-formate pathway, highlighting the environmental sustainability of the process. By directly coupling CO₂ reduction with the upgrading of biomass-derived furanics into polymer-relevant intermediates, this work establishes a practically meaningful and scalable co-production strategy for solar-driven carbon utilisation and sustainable plastic precursor synthesis.

Abstract:

Bulk formamidinium lead triiodide (FAPbI3) films host spontaneously formed quantum-confined (QC) domains, but their structural origin remains unclear. Using controlled material degradation in humid air as a dynamic lattice perturbation, we track the evolution of QC features in thin-film absorption of FAPbI3. With aging, above-bandgap QC features redshift and diminish, indicating weakened electronic confinement. Concurrently, X-ray diffraction reveals that breakdown of α-phase connectivity coincides with the loss of short-range higher-order hexagonal (nH, n > 2) polytypes as the material converts to the 2H δ-phase. Such polytypic nanodomains may generate peaked absorption features by forming higher-energy barriers confining charge carriers within α-FAPbI3 or by introducing distinct electronic states associated with mixed octahedral connectivity. Progressive degradation dismantles this framework, causing the disappearance of the QC features. Our results identify the structural motifs underpinning QC effects and propose that controlling higher-order (n > 2) hexagonal polytypes offers a route to tuning quantum confinement in FAPbI3 films.

Abstract:

Two-dimensional perovskites have emerged as promising materials for optoelectronic applications owing to their excellent environmental stability and tunable quantum confinement. Such 2D perovskites can incorporate a particularly versatile range of organic cations of different size, chemical nature, and optoelectronic character. However, understanding and controlling thin-film transport for this vast family of materials remains a key challenge to their successful application in devices. Here, we systematically investigate odd-even effects in thin films of Ruddlesden-Popper-type (RP) lead-iodide 2D perovskites based on nonconjugated alkylammonium spacer cations with chain lengths ranging from three to eight carbon atoms. A pronounced odd-even dependence on the carbon number is observed in both optical and transport properties, including absorption coefficients, photoluminescence energies and lifetimes, and excitation diffusion dynamics. Notably, the coefficients for charge-carrier diffusion out of the film plane─extracted via a dynamic photon reabsorption approach─display an opposite odd-even trend to the in-plane charge-carrier mobility obtained from optical pump-terahertz probe measurements, causing a pronounced odd-even modulation of the thin-film mobility anisotropy. Grazing-incidence wide-angle X-ray scattering measurements reveal that this behavior is related to cation-controlled nanostructural orientation: even-numbered alkyl spacer cations induce lead-iodide planes lying highly oriented within the film plane, while odd-numbered ones cause more disordered stacking. Furthermore, the observed 1/d²-dependence on interplane distance d in ordered films demonstrates that Förster resonance energy transfer underpins diffusion of excitations between lead-iodide layers. Our findings establish a direct structure-transport correlation in 2D perovskite films and provide valuable guidelines for the design of optoelectronic devices.