Terahertz photonics

Abstract:

High-performance ultrafast terahertz devices such as modulators, photoconductive emitters, and detectors rely on an ultrafast change in conductivity. Semiconductors are suitable for this purpose since their charge-carrier density can be varied by external factors, such as incident photons or an applied electrical potential. High-performance ultrafast terahertz devices require semiconductors with high charge-carrier mobility but low charge-carrier lifetimes. The trade-off between charge-carrier mobility and the short charge-carrier lifetimes of standard semiconductors limits the signal-to-noise ratio of current terahertz devices. Hence, it is essential to develop high-performance materials to overcome these limitations and enable ultrafast terahertz technology. Low-temperature GaAs (LT-GaAs) has shown excellent performance in overcoming this limitation by providing a high charge-carrier mobility and, at the same time, a low charge-carrier lifetime. In this study, we have conducted terahertz time-domain spectroscopy (THz-TDS) and terahertz optical-pump terahertz-probe spectroscopy (OPTPS) on 2 µm-thick LT-GaAs thin films bonded with benzocyclobutene (BCB) to sapphire substrates. With these techniques, we demonstrate excellent sub-picosecond charge-carrier lifetimes across different infrared pump beam fluences. The measured samples showed a charge-carrier lifetime in LT-GaAs of 0.46 ps. We discuss observed saturation effects and how they can influence the shape of the photoconductivity spectrum. We explain the origin of this effect in the context of the experimental technique and charge-carrier dynamics in the material.

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.