Carbon dots are cost-effective, environmental friendly, and biocompatible nanoparticles with many potential applications in optoelectronics and biophotonics. Their dual fluorescence bands were ...observed and could be attributed to core and surface state emission. We also conduct temperature-dependent fluorescence measurements from cryogenic to room temperatures. The dual emission bands exhibit similar temperature dependence. The strong electron–electron interactions and weak electron–phonon interactions could account for the very broad photoluminescence (PL) band even at 77 K. Our experimental results also suggest that carbon dots exhibit similar temperature behavior as metallic quantum dots (nanoclusters) but are different from inorganic semiconductor quantum dots. Here, for the first time, we present the temperature-dependent spectroscopic results to shed some light on the presently unclear fluorescence mechanism.
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IJS, KILJ, NUK, PNG, UL, UM
Two-dimensional (2D) crystals are promising materials for developing future nano-enabled technologies
. The cleavage of weak, interlayer van der Waals bonds in layered bulk crystals enables the ...production of high-quality 2D, atomically thin monolayers
. Nonetheless, as earth-abundant compounds, metal oxides are rarely accessible as pure and fully stoichiometric monolayers owing to their ion-stabilized 'lamellar' bulk structure
. Here, we report the discovery of a layered planar hexagonal phase of oxides from elements across the transition metals, post-transition metals, lanthanides and metalloids, derived from strictly controlled oxidation at the metal-gas interface. The highly crystalline monolayers, without the support of ionic dopants or vacancies, can easily be mechanically exfoliated by stamping them onto substrates. Monolayer and few-layered hexagonal TiO
are characterized as examples, showing p-type semiconducting properties with hole mobilities of up to 950 cm
V
s
at room temperature. The strategy can be readily extended to a variety of elements, possibly expanding the exploration of metal oxides in the 2D quantum regime.
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GEOZS, IJS, IMTLJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZAGLJ
In this work, we demonstrated the successful construction of metal-free zero- dimensional/two-dimensional carbon nanodot （CND）-hybridized protonatedg=C3N4 （pCN） （CND/pCN） heterojunction ...photocatalysts b; means of electrostatic attraction. We experimentally found that CNDs with an average diameter of 4.4 nm were uniformly distributed on the surface of pCN using electron microscopy analysis. The CND/pCN-3 sample with a CND content of 3 wt.% showed thehighest catalytic activity in the CO2 photoreduction process under visible and simulated solar light. This process results in the evolution of CH4 and CO. Thetotal amounts of CH4 and CO generated by the CND/pCN-3 photocatalyst after 10 h of visible-light activity were found to be 29.23 and 58.82 molgcatalyst-1, respectively. These values were 3.6 and 2.28 times higher, respectively, than thearn＊ounts generated when using pCN alone. The corresponding apparent quantum efficiency （AQE） was calculated to be 0.076%. Furthermore, the CND/pCN-3 sample demonstrated high stability and durability after four consecutive photoreaction cycles, with no significant decrease in the catalytic activity.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
A hole‐transport‐material‐free planar solar cell of cesium lead mixed halide perovskite (CsPbIBr2) is deposited by dual source thermal evaporation for the first time, achieving an efficiency of 4.7%. ...The addition of iodine into the bromide lowers the bandgap resulting in wider solar spectrum absorption. Compared to the hybrid halide perovskites, CsPbIBr2 demonstrates better thermal stability.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The hot-phonon bottleneck effect in lead-halide perovskites (APbX
) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. ...Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA
/MA
/Cs
, X=I
/Br
) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI
. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials.
Abstract
Ultralow trap densities, exceptional optical and electronic properties have been reported for lead halide perovskites single crystals; however, ambiguities in basic properties, such as the ...band gap, and the electronic defect densities in the bulk and at the surface prevail. Here, we synthesize single crystals of methylammonium lead bromide (CH
3
NH
3
PbBr
3
), characterise the optical absorption and photoluminescence and show that the optical properties of single crystals are almost identical to those of polycrystalline thin films. We observe significantly longer lifetimes and show that carrier diffusion plays a substantial role in the photoluminescence decay. Contrary to many reports, we determine that the trap density in CH
3
NH
3
PbBr
3
perovskite single crystals is 10
15
cm
−3
,
only one order of magnitude lower than in the thin films. Our enhanced understanding of optical properties and recombination processes elucidates ambiguities in earlier reports, and highlights the discrepancies in the estimation of trap densities from electronic and optical methods.
Photon recycling and carrier diffusion are the two plausible processes that primarily affect the carrier dynamics in halide perovskites, and therefore the evaluation of the performance of their ...photovoltaic and photonic devices. However, it is still challenging to isolate their individual contributions because both processes result in a similar emission redshift. Herein, it is confirmed that photon recycling is the dominant effect responsible for the observed redshifted emission. By applying one‐ and two‐photon confocal emission microscopy on Ruddlesden–Popper type 2D perovskites, of which interplane carrier diffusion is strictly suppressed, the substantial PL redshift (72 meV) is well reproduced by the photon transport model. A comparison of 3D bulk CH3NH3PbBr3 single crystal to 2D perovskite by depth‐resolved two‐photon PL spectra reveals the contribution of carrier diffusion on energy transport at a distance beyond diffusion length is constantly negligible, though the carrier diffusion indeed exists in the 3D crystal. The investigation resolves the fundamental confusion and debate surrounding the issue and provides significant insights into carrier kinetics in perovskites, which is important for future developments in solar cells and other optoelectronic devices.
Photon recycling and carrier diffusion are the two plausible processes that primarily affect the carrier dynamics in halide perovskites, and therefore the evaluation of the performance of their photovoltaic and photonic devices. However, it is still challenging to isolate their individual contributions. Herein, it is confirmed that photon recycling is the dominant effect responsible for the observed redshifted emission.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Potassium (K+) doping has been recently discovered as an effective route to suppress hysteresis and improve the performance stability of perovskite solar cells. However, the mechanism of these K+ ...doping effects is still under debate, and rationalization of the improved performance in these perovskites is needed. Herein, the photoluminescence (PL) properties and device performance of mixed‐cation mixed‐halide perovskite are dynamically monitored with and without K+ doping under bias light illumination via a confocal fluorescence microscope, together with ultrafast transient absorption as well as time‐dependent and time‐resolved PL measurements. It is demonstrated that illumination is essential to trigger the passivation effect of K+ by forming KBr‐like compounds, leading to the elimination of interface trapping defects and suppression of mobile ion migration, thus resulting in improved power conversion efficiency and negligible current–voltage hysteresis of solar cells. This work provides novel insight into the hysteresis suppression upon K+ doping and highlights the significance of light illumination when using this protocol.
The passivation effect of K+ doping in mixed‐cation mixed‐halide perovskite is triggered by light illumination to form stable KBr‐like compounds and reduce the interface trapping defect density to facilitate a high‐efficiency and hysteresis‐free perovskite solar cell.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Organic–inorganic perovskite solar cells have attracted huge research interest due to rapid improvement in device performance showing great potential to be the next generation flexible solar cells. ...Unique defect properties in perovskite have been considered as the possible mechanism for the superior performance, and closely relevant to the effects of hysteresis and light soaking. To date, the quantitative correlation and in-depth understanding of defects in organic–inorganic perovskite are still lacking although extensive investigation have been undertaken. Here we study defect trapping states and carrier recombination dynamics in organic–inorganic halide perovskites. At low excitation the photoluminescence (PL) intensity exhibits a super-linear increase with increasing excitation, due to the slow depopulation rate of the defect states. The steady state and time-resolved photoluminescence (PL) carried out in this work reveal that the carrier recombination dynamics is ultimately correlated with both the defect density and the relaxation rate of the carriers in defects. A model is established for the relationship between the properties of the defect trapping state and steady state PL intensity. Two key parameters, (i) the ratio of the trap-state density to the depopulation rate of trapped states and (ii) ratio of the maximum density of covalence band electrons to the trapping rate, can be extracted from the model based on the excitation dependent steady state PL. This work demonstrates that the properties of defect trapping states are closely related to the fabrication technique, and suggests that the organic–inorganic halide perovskite is partly defect-tolerant.
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