Development
of water-stable metal–organic frameworks (MOFs)
for promising visible-light-driven photocatalytic water splitting
is highly desirable but still challenging. Here we report a novel
p-type nickel-based MOF single crystal (Ni-TBAPy-SC) and its exfoliated
nanobelts (Ni-TBAPy-NB) that can bear a wide range of pH environment
in aqueous solution. Both experimental and theoretical results indicate
a feasible electron transfer from the H4TBAPy ligand (light-harvesting
center) to the Ni–O cluster node (catalytic center), on which
water splitting to produce hydrogen can be efficiently driven free
of cocatalyst. Compared to the single crystal, the exfoliated two-dimensional
(2D) nanobelts show more efficient charge separation due to its shortened
charge transfer distance and remarkably enhanced active surface areas,
resulting in 164 times of promoted water reduction activity. The optimal
H2 evolution rate on the nanobelt reaches 98 μmol
h–1 (ca. 5 mmol h–1 g–1) showing benchmarked apparent quantum efficiency (AQE) of 8.0% at
420 nm among water-stable MOFs photocatalysts.
Layered two-dimensional (2D) lead halide perovskites are a class of quantum well (QW) materials, holding dramatic potentials for optical and optoelectronic applications. However, the thermally activated exciton dissociation into free carriers in 2D perovskites, a key property that determines their optoelectronic performance, was predicted to be weak due to large exciton binding energy (E b , about 100−400 meV). Herein, in contrast to the theoretical prediction, we discover an ultrafast (<1.4 ps) and highly efficient (>80%) internal exciton dissociation in (PEA) 2 (MA) n−1 Pb n I 3n+1 (PEA = C 6 H 5 C 2 H 4 NH 3 + , MA = CH 3 NH 3 + , n = 2−4) 2D perovskites despite the large E b . We demonstrate that the exciton dissociation activity in 2D perovskites is significantly promoted because of the formation of exciton− polarons with considerably reduced exciton binding energy (down to a few tens of millielectronvolts) by the polaronic screening effect. This ultrafast and high-yield exciton dissociation limits the photoluminescence of 2D perovskites but on the other hand well explains their exceptional performance in photovoltaic devices. The finding should represent a common exciton property in the 2D hybrid perovskite family and provide a guideline for their rational applications in light emitting and photovoltaics.
Layered two-dimensional (2D) hybrid
perovskites are naturally formed
multiple quantum well (QW) materials with promising applications in
quantum and optoelectronic devices. In principle, the transport of
excitons in 2D perovskites is limited by their short lifetime and
small mobility to a distance within a few hundred nanometers. Herein,
we report an observation of long-distance carrier transport over 2
to 5 μm in 2D perovskites with various well thicknesses. Such
a long transport distance is enabled by trap-induced exciton dissociation
into long-lived and nonluminescent electron–hole separated
state, followed by a trap-mediated charge transport process. This
unique property makes 2D perovskites comparable with 3D perovskites
and other traditional semiconductor QWs in terms of a carrier transport
property and highlights their potential application as an efficient
energy/charge-delivery material.
Organic–inorganic
hybrid halide perovskites (ABX3), especially layered 2D
perovskites, have been recognized as promising
semiconductors due to their tunable crystal structure and unique optoelectronic
properties. A-site cations, as spacers, allow various metal halide
assemblies, but the stacking pattern and the influence of their collective
behavior on the properties of the resultant materials remain ambiguous.
Here, the cation-stacking effects in the 2D perovskite single crystals,
with a focus on the electron–phonon interaction, are investigated.
We reveal the different photoluminescence from the surface region
and the interior of the crystal, which is due to the residual strain
induced by A-site cation stacking. We also examine the cation-stacking
effects on the electron–phonon interaction, which is further
employed to tailor the optoelectronic properties of the resultant
2D crystals. By reducing the microstrain, we reduce the electron–phonon
coupling to improve the mobility and their stability against electric
field in the corresponding crystals. Our study suggests a way to manipulate
the optoelectronic properties in 2D perovskite materials by rational
design of cation stacking.
Wurtzite ZnO has many potential applications in optoelectronic devices, and the hydrogenated ZnO exhibits excellent photoelectronic properties compared to undoped ZnO; however, the structure of H-related defects is still unclear. In this article, the effects of hydrogen-plasma treatment and subsequent annealing on the electrical and optical properties of ZnO films were investigated by a combination of Hall measurement, Raman scattering, and photoluminescence. It is found that two types of hydrogen-related defects, namely, the interstitial hydrogen located at the bond-centered (H(BC)) and the hydrogen trapped at a O vacancy (H(O)), are responsible for the n-type background conductivity of ZnO films. Besides introducing two hydrogen-related donor states, the incorporated hydrogen passivates defects at grain boundaries. With increasing annealing temperatures, the unstable H(BC) atoms gradually diffuse out of the ZnO films and part of them are converted into H(O), which gives rise to two anomalous Raman peaks at 275 and 510 cm(-1). These results help to clarify the relationship between the hydrogen-related defects in ZnO described in various studies and the free carriers that are produced by the introduction of hydrogen.
We studied the effects of hydrogen plasma treatment on the electrical and optical properties of ZnO films deposited by radio frequency magnetron sputtering. It is found that the ZnO:H film is highly transparent with the average transmittance of 92% in the visible range. Both carrier concentration and mobility are increased after hydrogen plasma treatment, correspondingly, the resistivity of the ZnO:H films achieves the order of 10−3 Ω cm. We suggest that the incorporated hydrogen not only passivates most of the defects and/or acceptors present, but also introduces shallow donor states such as the VO-H complex and the interstitial hydrogen Hi. Moreover, the annealing data indicate that Hi is unstable in ZnO, while the VO-H complex remains stable on the whole at 400 °C, and the latter diffuses out when the annealing temperature increases to 500 °C. These results make ZnO:H more attractive for future applications as transparent conducting electrodes.
By combining the slow growth of the perovskite film and the introduction of a ZnO interlayer, highly efficient and stable perovskite solar cells with an efficiency of 16.8% were obtained.
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