A series of novel lanthanide metal-organic frameworks were synthesized using a ligand featuring three carboxylate groups stationed on a triazinyl central motif. The readily accessible multiple Lewis basic triazinyl N atoms allow for complexation of incoming metal ions. Such interactions have been established quantitatively.
A lanthanide metal-organic framework (MOF) compound of the formulation [Eu2(CO3)(ox)2(H2O)2]·4H2O (1, ox = oxalate) was prepared by hydrothermal synthesis with its structure determined crystallographically. Temperature-dependent but humidity-independent high proton conduction was observed with a maximum of 2.08 × 10(-3) S cm(-1) achieved at 150 °C, well above the normal boiling point of water. Results from detailed structural analyses, comparative measurements of conductivities using regular and deuterated samples, anisotropic conductivity measurements using a single-crystal sample, and variable-temperature photoluminescence studies collectively establish that the protons furnished by the Eu(III)-bound and activated aqua ligands are the charge carriers and that the transport of proton is mediated along the crystallographic a-axis by ordered hydrogen-bonded arrays involving both aqua ligands and adjacent oxalate groups in the channels of the open framework. Proton conduction was enhanced with the increase of temperature from room temperature to about 150 °C, which can be rationalized in terms of thermal activation of the aqua ligands and the facilitated transport between aqua and adjacent oxalate ligands. A complete thermal loss of the aqua ligands occurred at about 160 °C, resulting in the disintegration of the hydrogen-bonded pathway for proton transport and a precipitous drop in conductivity. However, the structural integrity of the MOF was maintained up to 350 °C, and upon rehydration, the original structure with the hydrogen-bonded arrays was restored, and so was its high proton-conduction ability.
Isostructural lanthanide metal-organic frameworks (MOFs) are synthesized through the spontaneous self-assembly of H3BTPCA (1,1',1″-(benzene-1,3,5-triyl)tripiperidine-4-carboxylic acid) ligands and lanthanide ions (we term these MOFs Ln-BTPCA, Ln = La(3+), Tb(3+), Sm(3+), etc.). Prompted by the observation that the different lanthanide ions have identical coordination environment in these MOFs, we explored and succeeded in the preparation of mixed-lanthanide analogues of the single-lanthanide MOFs by way of in situ doping using a mixture of lanthanide salts. With careful adjustment of the relative concentration of the lanthanide ions, the color of the luminescence can be modulated, and white light-emission can indeed be achieved. The mechanisms possibly responsible for the observed photophysical properties of these mixed-lanthanide MOFs are also discussed.
High-efficiency hole transport layer
free perovskite solar cells (HTL-free PSCs) with economical and simplified
device structure can greatly facilitate the commercialization of PSCs.
However, eliminating the key HTL in PSCs results usually in a severe
efficiency loss and poor carrier transfer due to the energy-level
mismatching at the indium tin oxide (ITO)/perovskite interface. In
this study, we solve this issue by introducing an organic monomolecular
layer (ML) to raise the effective work function of ITO with the assistance
of an interface dipole created by Sn–N bonds. The energy-level
alignment at the ITO/perovskite interface is optimized with a barrier-free
contact, which favors efficient charge transfer and suppressed nonradiative
carrier recombination. The HTL-free PSCs based on the ML-modified
ITO yield an efficiency of 19.4%, much higher than those of HTL-free
PSCs on bare ITO (10.26%), comparable to state-of-the-art PSCs with
a HTL. This study provides an in-depth understanding of the mechanism
of interfacial energy-level alignment and facilitates the design of
advanced interfacial materials for simplified and efficient PSC devices.
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