Highly mobile hot holes in Cs2AgBiBr6 double perovskite

Unraveling Defect-Mediated Enhancement of Transient Photoconductivity and Slower Carrier's Mobility Decay in Cu-Doped Cs2AgBiBr6 Nanocrystals Using Ultrafast Pump-Probe Spectroscopy

Lead-free double perovskite nanocrystals (A2B'(III)B″(I)X6 NCs) address the instability and toxicity concerns of lead-based counterparts, but their device performance is limited by subpar absorption and unexplored carrier dynamics. Impurity ion doping offers a route to tune electrical conductivity and charge carrier transport. Herein, we synthesized Cu-doped Cs2AgBiBr6 (CABB) nanocrystals using a hot-injection approach and investigated the charge carrier's dynamics through ultrafast pump-probe spectroscopy. Copper introduction into the CABB lattice enhanced absorption in the near-infrared region and introduced sub-band gap defect states in CABB NCs. The transient absorption study revealed a faster bleach decay with increased copper doping, as a result of charge transfer from the conduction band to copper defect states. Also, an optical pump terahertz probe study displays higher photoconductivity and mobility in Cu-doped CABB NCs. Slower mobility decay in Cu-doped systems was attributed to the charge carrier's entrapment at the defect state. These findings suggest that copper-doped CABB is a superior contender for optoelectronic applications over conventional CABB.

High-throughput screening of the transport behavior of tetragonal perovskites

Halide perovskites have attracted attention due to their low cost and excellent optoelectronic properties. Although their optical properties gained widespread consensus, there was still divergence in understanding carrier transport behavior. In this study, the mobility of tetragonal perovskites was investigated by empirical models, including longitudinal acoustic phonon (LAP) and polar optical phonon (POP) models. The results revealed that the mobility predicted from the LAP model was much higher than that from the POP model. A longitudinal optical phonon (LOP) was considered as the decisive scattering source for charge carriers in perovskites. Furthermore, the mobility was extremely sensitive to z-axis strain, and 8 types of perovskites with high carrier mobility were screened. Using the experimental lattice constants, the predicted mobility of CsSnI3 was μe,z = 1428 and μh,z = 2310 cm2 V-1 s-1, respectively. The tetragonal CsSnI3 has high mobility and moderate bandgaps, suggesting potential applications in high-efficiency solar cells.

Porphyrin-fused graphene nanoribbons

Graphene nanoribbons (GNRs), nanometre-wide strips of graphene, are promising materials for fabricating electronic devices. Many GNRs have been reported, yet no scalable strategies are known for synthesizing GNRs with metal atoms and heteroaromatic units at precisely defined positions in the conjugated backbone, which would be valuable for tuning their optical, electronic and magnetic properties. Here we report the solution-phase synthesis of a porphyrin-fused graphene nanoribbon (PGNR). This PGNR has metalloporphyrins fused into a twisted fjord-edged GNR backbone; it consists of long chains (>100 nm), with a narrow optical bandgap (~1.0 eV) and high local charge mobility (>400 cm2 V-1 s-1 by terahertz spectroscopy). We use this PGNR to fabricate ambipolar field-effect transistors with appealing switching behaviour, and single-electron transistors displaying multiple Coulomb diamonds. These results open an avenue to π-extended nanostructures with engineerable electrical and magnetic properties by transposing the coordination chemistry of porphyrins into graphene nanoribbons.

Phase-engineering compact and flexible CsPbBr3 microcrystal films for robust X-ray detection

All-inorganic CsPbBr3 perovskites have gained significant attention due to their potential in direct X-ray detection. The fabrication of stable, pinhole-free thick films remains challenging, hindering their integration in durable, large-area high-resolution devices. In this study, we propose a facile strategy using a non-conductive polymer to create a flexible, compact thick film under ambient conditions. Furthermore, we investigate the effect of introducing the 2D CsPb2Br5 phase into CsPbBr3 perovskite crystals on their photophysical properties and charge transport. Upon X-ray exposure, the devices consisting of the dual phase exhibit improved stability and more effective operation at higher voltages. Rietveld refinement shows that, due to the presence of the second phase, local distortions and Pb-vacancies are introduced within the CsPbBr3 lattice. This in turn presumably increases the ion migration energy barrier, resulting in a very low dark current and hence, enhanced stability. This feature might benefit local charge extraction and, ultimately, the X-ray image resolution. These findings also suggest that introducing a second phase in the perovskite structure can be advantageous for efficient photon-to-charge carrier conversion, as applied in medical imaging.

Synthesis and Optimization of Cs2B'B″X6 Double Perovskite for Efficient and Sustainable Solar Cells

Hybrid perovskite materials with high light absorption coefficients, long diffusion lengths, and high mobility have attracted much attention, but their commercial development has been seriously hindered by two major problems: instability and lead toxicity. This has led to lead-free halide double perovskite becoming a prominent competitor in the photovoltaic field. For lead-free double perovskites, Pb2+ can be heterovalent, substituted by non-toxic metal cations as a double perovskite structure, which promotes the flexibility of the composition. However, the four component elements and low solubility in the solvent result in synthesis difficulties and phase impurity problems. And material phase purity and film quality are closely related to the number of defects, which can limit the photoelectric performance of solar cells. Therefore, based on this point, we summarize the synthesis methods of Cs2B'B″X6 double perovskite crystals and thin films. Moreover, in the application of solar cells, the existing research mainly focuses on the formation process of thin films, band gap adjustment, and surface engineering to improve the quality of films and optimize the performance of devices. Finally, we propose that Cs2B'B″X6 lead-free perovskites offer a promising pathway toward developing highly efficient and stable perovskite solar cells.

Ultrafast glimpses of the excitation energy-dependent exciton dynamics and charge carrier mobility in Cs2SnI6 nanocrystals

Advancements in photovoltaic research suggest that tin-based perovskites are potential alternatives to traditional lead-based structures. Cs2SnI6, specifically, stands out as a notable candidate, exhibiting impressive performance. However, its complete potential remains untapped primarily owing to the limited understanding of its photophysics. In light of this, this study aims to bridge this knowledge gap. To commence our study, we first executed theoretical investigations to locate the energetically diverse excitons within the Brillouin zone. Building on this knowledge, we then utilized transient absorption spectroscopy to investigate their temporal evolution. Herein, we observed the formation of high-energy excitons even when the incident photon energy was below the necessary threshold, which is quite distinctive and intriguing. Of particular interest is the generation of ultraviolet (UV) domain exciton using visible photons, which implies that Cs2SnI6 has the potential for efficient solar light harvesting. Tracking the kinetics revealed that this unique finding arises due to the intertwined formation and decay pathways undertaken by the different excitons, aided by intervalley scattering and phonon absorption processes. In addition, we found that the decay of the UV exciton was unusually slow. Transient mobility investigations were undertaken to probe the carrier transport behavior that further established hot carriers (HCs) in Cs2SnI6 to be highly mobile and susceptible to polaron formation. Overall, our findings demonstrate that Cs2SnI6 is a strong candidate for HC-based photovoltaics because it possesses all the prerequisites desired for such applications.

A Thiophene Backbone Enables Two-Dimensional Poly(arylene vinylene)s with High Charge Carrier Mobility

Linear conjugated polymers have attracted significant attention in organic electronics in recent decades. However, despite intrachain π-delocalization, interchain hopping is their transport bottleneck. In contrast, two-dimensional (2D) conjugated polymers, as represented by 2D π-conjugated covalent organic frameworks (2D c-COFs), can provide multiple conjugated strands to enhance the delocalization of charge carriers in space. Herein, we demonstrate the first example of thiophene-based 2D poly(arylene vinylene)s (PAVs, 2DPAV-BDT-BT and 2DPAV-BDT-BP, BDT=benzodithiophene, BT=bithiophene, BP=biphenyl) via Knoevenagel polycondensation. Compared with 2DPAV-BDT-BP, the fully thiophene-based 2DPAV-BDT-BT exhibits enhanced planarity and π-delocalization with a small band gap (1.62 eV) and large electronic band dispersion, as revealed by the optical absorption and density functional calculations. Remarkably, temperature-dependent terahertz spectroscopy discloses a unique band-like transport and outstanding room-temperature charge mobility for 2DPAV-BDT-BT (65 cm2  V-1  s-1 ), which far exceeds that of the linear PAVs, 2DPAV-BDT-BP, and the reported 2D c-COFs in the powder form. This work highlights the great potential of thiophene-based 2D PAVs as candidates for high-performance opto-electronics.

Lattice Dynamics and Electron-Phonon Coupling in Double Perovskite Cs2NaFeCl6

Phonon-phonon and electron/exciton-phonon coupling play a vitally important role in thermal, electronic, as well as optical properties of metal halide perovskites. In this work, we evaluate phonon anharmonicity and coupling between electronic and vibrational excitations in novel double perovskite Cs₂NaFeCl₆ single crystals. By employing comprehensive Raman measurements combined with first-principles theoretical calculations, we identify four Raman-active vibrational modes. Polarization properties of these modes imply Fm3̅m symmetry of the lattice, indicative for on average an ordered distribution of Fe and Na atoms in the lattice. We further show that temperature dependence of the Raman modes, such as changes in the phonon line width and their energies, suggests high phonon anharmonicity, typical for double perovskite materials. Resonant multiphonon Raman scattering reveals the presence of high-lying band states that mediate strong electron-phonon coupling and give rise to intense nA _1g overtones up to the fifth order. Strong electron-phonon coupling in Cs₂NaFeCl₆ is also concluded based on the Urbach tail analysis of the absorption coefficient and the calculated Fröhlich coupling constant. Our results, therefore, suggest significant impacts of phonon-phonon and electron-phonon interactions on electronic properties of Cs₂NaFeCl₆, important for potential applications of this novel material.

Elucidating Polaron Dynamics in Cs2AgBiBr6 Double Perovskite

Lead-free Cs₂AgBiBr₆ double perovskites have recently emerged as a possible alternative to lead-based halide perovskites for photovoltaic and optoelectronic applications. Significant research efforts have been devoted toward device engineering to enhance the performance of Cs₂AgBiBr₆ double-perovskite-based photovoltaic and optoelectronic devices; however, less attention has been paid to address their intrinsic photophysical properties. In this work, we have shown that the small polaron formation under photoexcitation and polaron localization limits the carrier dynamics in Cs₂AgBiBr₆ double halide perovskites. Furthermore, temperature-dependent ac conductivity measurement reveals that single polaron hopping is the dominant conduction mechanism. Ultrafast transient absorption spectroscopy reveals that a deformed lattice under photoexcitation leads to the formation of small polarons which act as self-trapped states (STSs) and lead to the ultrafast trapping of charge carriers. Our findings provide an in-depth understanding of intrinsic limitations of Cs₂AgBiBr₆ perovskites, which can be applicable for other bismuth-based semiconductors.

Stable Mott Polaron State Limits the Charge Density in Lead Halide Perovskites

Large polarons are known to form in lead halide perovskites (LHPs). Photoinduced isolated polarons at low densities have been well-researched, but many-body interactions at elevated polaron densities, exceeding the Mott criterion (i.e., Mott polaron density), have remained elusive. Here, employing ultrafast terahertz spectroscopy, we identify a stable Mott polaron state in LHPs at which the polaron wavefunctions start to overlap. The Mott polaron density is determined to be ∼10¹⁸ cm^-3, in good agreement with theoretical calculations based on the Feynman polaron model. The electronic phase transition across the Mott density is found to be universal in LHPs and independent of the constituent ions. Exceeding the Mott polaron density, excess photoinjected charge carriers annihilate quickly within tens to hundreds of picoseconds, before reaching the stable and long-lived Mott state. These results have considerable implications for LHP-based devices and for understanding exotic phenomena reported in LHPs.

Probing the charge transfer mechanisms in type-II Cs2AgBiBr6-CdSe composite system: ultrafast insights

Lead-free halide-based double perovskites (DPs) have established themselves as the emerging nontoxic alternatives for photovoltaic (PV) applications thus substituting the long-standing lead halide perovskites. Among the prospective lead-free DPs, Cs₂AgBiBr₆has gained immense popularity owing to the fascinating properties demonstrated by them including low carrier effective mass and microsecond lifetime for electron-hole recombination. Nevertheless, the large, indirect bandgap remains the prime hurdle that restrains commercialization of the Cs₂AgBiBr₆DPs based PV devices. A rational solution could be designing its heterostructure with another suitable material that could mitigate the inadequacies of Cs₂AgBiBr₆DPs. With this line of thought, herein we synthesized a composite of Cs₂AgBiBr₆DPs with CdSe NCs and then performed transient absorption (TA) spectroscopic measurements to introspect its photophysical aspects. Executing excitation energy-dependent studies clearly reveal the carrier transfer efficiency to be strongly pump-dependent. Upon exciting with 350 nm pump, in compliance with the energy band alignment and tendency of both the constituents to be photoexcited across their bandgap, there is a bidirectional transfer of hot electrons anticipated in the composite system. Nevertheless, the TA outcomes indicate the transfer of hot electrons from CdSe to Cs₂AgBiBr₆to be more favorable out of the bidirectional pathways. Employing further lower pump energies (480 nm) when only CdSe NCs are capable of being excited, the transfer efficiency of the electrons from CdSe to Cs₂AgBiBr₆is noticed to be fairly low. Besides this, when the pump wavelength is tuned to 530 nm i.e. quite close to the CdSe band edge, no electron transfer is noticeable despite the anticipation from thermodynamic feasibility. Thus, as reflected by the TA kinetics, electron transfer is discerned to be more efficient from the hot states rather than the band edges. Most advantageously, charge separation is successfully achieved in this never explored composite architecture which eases the carrier extraction and minimizes the otherwise prevalent fast recombination processes.

Photoinduced large polaron transport and dynamics in organic-inorganic hybrid lead halide perovskite with terahertz probes

Organic-inorganic hybrid metal halide perovskites (MHPs) have attracted tremendous attention for optoelectronic applications. The long photocarrier lifetime and moderate carrier mobility have been proposed as results of the large polaron formation in MHPs. However, it is challenging to measure the effective mass and carrier scattering parameters of the photogenerated large polarons in the ultrafast carrier recombination dynamics. Here, we show, in a one-step spectroscopic method, that the optical-pump and terahertz-electromagnetic probe (OPTP) technique allows us to access the nature of interplay of photoexcited unbound charge carriers and optical phonons in polycrystalline CH₃NH₃PbI₃ (MAPbI₃) of about 10 μm grain size. Firstly, we demonstrate a direct spectral evidence of the large polarons in polycrystalline MAPbI₃. Using the Drude-Smith-Lorentz model along with the Frӧhlich-type electron-phonon (e-ph) coupling, we determine the effective mass and scattering parameters of photogenerated polaronic carriers. We discover that the resulting moderate polaronic carrier mobility is mainly influenced by the enhanced carrier scattering, rather than the polaron mass enhancement. While, the formation of large polarons in MAPbI₃ polycrystalline grains results in a long charge carrier lifetime at room temperature. Our results provide crucial information about the photo-physics of MAPbI₃ and are indispensable for optoelectronic device development with better performance.

Probing Carrier Dynamics in sp3-Functionalized Single-Walled Carbon Nanotubes with Time-Resolved Terahertz Spectroscopy

The controlled introduction of covalent sp³ defects into semiconducting single-walled carbon nanotubes (SWCNTs) gives rise to exciton localization and red-shifted near-infrared luminescence. The single-photon emission characteristics of these functionalized SWCNTs make them interesting candidates for electrically driven quantum light sources. However, the impact of sp³ defects on the carrier dynamics and charge transport in carbon nanotubes remains an open question. Here, we use ultrafast, time-resolved optical-pump terahertz-probe spectroscopy as a direct and quantitative technique to investigate the microscopic and temperature-dependent charge transport properties of pristine and functionalized (6,5) SWCNTs in dispersions and thin films. We find that sp³ functionalization increases charge carrier scattering, thus reducing the intra-nanotube carrier mobility. In combination with electrical measurements of SWCNT network field-effect transistors, these data enable us to distinguish between contributions of intra-nanotube band transport, sp³ defect scattering and inter-nanotube carrier hopping to the overall charge transport properties of nanotube networks.

The Role of Interspecific Hybridisation in Adaptation and Speciation: Insights From Studies in Senecio

Hybridisation is well documented in many species, especially plants. Although hybrid populations might be short-lived and do not evolve into new lineages, hybridisaiton could lead to evolutionary novelty, promoting adaptation and speciation. The genus Senecio (Asteraceae) has been actively used to unravel the role of hybridisation in adaptation and speciation. In this article, we first briefly describe the process of hybridisation and the state of hybridisation research over the years. We then discuss various roles of hybridisation in plant adaptation and speciation illustrated with examples from different Senecio species, but also mention other groups of organisms whenever necessary. In particular, we focus on the genomic and transcriptomic consequences of hybridisation, as well as the ecological and physiological aspects from the hybrids' point of view. Overall, this article aims to showcase the roles of hybridisation in speciation and adaptation, and the research potential of Senecio, which is part of the ecologically and economically important family, Asteraceae.