Fast Charge Transfer and High Stability via Hybridization of Hygroscopic Cu-BTC Metal-Organic Framework Nanocrystals with a Light-Absorbing Layer for Perovskite Solar Cells.

Perovskite solar cells (PSCs) have great potential as an efficient solar energy harvesting system due to their outstanding optoelectronic properties, but the charge accumulation and recombination, as well as the moisture-induced degradation of the light-absorbing perovskite layers, remain great bottlenecks in practical applications for future technology. As a solution to this challenge, here we report a strategy to realize moisture-stable PSCs allowing fast charge transfer that, in turn, leads to high power conversion efficiency (PCE). Hybridization of hygroscopic copper(II) benzene-1,3,5-tricarboxylate metal-organic frameworks (Cu-BTC MOFs) with a light-absorbing perovskite layer for PSCs, where a moderate level of moisture attracted by Cu-BTC MOFs during the synthesis step, leads to enhanced perovskite crystallization. Besides, the perovskite-MOF hybrid facilitates the transfer of photoexcited electrons from the perovskite to TiO2 by providing additional channels for electron extraction. This enables a high PCE of 20.5% in a triple-cation perovskite-MOF device with negligible hysteresis compared to reference devices. Moreover, the perovskite-MOF hybrid exhibits high stability in ambient air under dark conditions over a long period (up to 22 months), while the unmodified counterpart quickly decomposes into PbI2. Consequently, this work provides a promising clue to realizeing fast charge transfer and high stability for high-performance PSCs.

In Situ Mapping and Local Negative Uptake Behavior of Adsorbates in Individual Pores of Metal-Organic Frameworks.

Herein, we report the adsorbate behavior in individual local pores of MIL-101, which is a metal-organic framework (MOF) with two heterogeneous mesopores and different metal sites, by combining adsorbate isotherms and in situ crystallography profiles. The in situ mapping shows that the substrate-adsorbate interaction affects the initial adsorption and pore condensation steps. The monolayer adsorption gradient changes greatly depending on the framework metal-adsorbate attraction force. Also, broad inflection points are found in adsorption isotherms, and the initial shape depends on the different metals. Besides, the capillary condensation at a pore draws adsorbates from other local pores. This leads to the local negative uptake behavior in individual pore isotherms. At higher pressure, they move to a larger space, whereas in a relatively low-pressure range the attraction force between the MOF framework and guest molecule influences the amount of rearranged guest molecules. Furthermore, the origin of the characteristic adsorption behavior based on the metals constituting the MOFs and the relative strength of substrate-adsorbate and adsorbate-adsorbate interactions are elucidated through the combined study of electron densities in pores, electron paramagnetic resonance spectroscopy spectra, and density functional theory and Monte Carlo simulations to uncover the previously veiled information on adsorption behavior.

Encapsulation of redox polysulphides via chemical interaction with nitrogen atoms in the organic linkers of metal-organic framework nanocrystals.

Lithium polysulphides generated during discharge in the cathode of a lithium-sulphur redox cell are important, but their dissolution into the electrolyte from the cathode during each redox cycle leads to a shortened cycle life. Herein, we use in situ spectroelectrochemical measurements to demonstrate that sp(2) nitrogen atoms in the organic linkers of nanocrystalline metal-organic framework-867 (nMOF-867) are able to encapsulate lithium polysulphides inside the microcages of nMOF-867, thus helping to prevent their dissolution into the electrolyte during discharge/charge cycles. This encapsulation mechanism of lithiated/delithiated polysulphides was further confirmed by observations of shifted FTIR spectra for the C = N and C-N bonds, the XPS spectra for the Li-N bonds from nMOF-867, and a visualization method, demonstrating that nMOF-867 prevents lithium polysulphides from being dissolved in the electrolyte. Indeed, a cathode fabricated using nMOF-867 exhibited excellent capacity retention over a long cycle life of 500 discharge/charge cycles, with a capacity loss of approximately 0.027% per cycle from a discharge capacity of 788 mAh/g at a high current rate of 835 mA/g.