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1. Establishing charge-transfer excitons in 2D perovskite heterostructures

   https://doi.org/10.1038/s41467-020-16415-1  

   Zhang, Jia ; Zhu, Xixiang ; Wang, Miaosheng ; Hu, Bin ( May 2020 , Nature Communications)

   Charge-transfer excitons (CTEs) immensely enrich property-tuning capabilities of semiconducting materials. However, such concept has been remaining as unexplored topic within halide perovskite structures. Here, we report that CTEs can be effectively formed in heterostructured 2D perovskites prepared by mixing PEA₂PbI₄:PEA₂SnI₄, functioning as host and guest components. Remarkably, a broad emission can be demonstrated with quick formation of 3 ps but prolonged lifetime of ~0.5 μs. This broad PL presents the hypothesis of CTEs, verified by the exclusion of lattice distortion and doping effects through demonstrating double-layered PEA₂PbI₄/PEA₂SnI₄heterostructure when shearing-away PEA₂SnI₄film onto the surface of PEA₂PbI₄film by using hand-finger pressing method. The below-bandgap photocurrent indicates that CTEs are vital states formed at PEA₂PbI₄:PEA₂SnI₄interfaces in 2D perovskite heterostructures. Electroluminescence shows that CTEs can be directly formed with electrically injected carriers in perovskite LEDs. Clearly, the CTEs presents a new mechanism to advance the multifunctionalities in 2D perovskites.

    

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2. Tuning cesium–guanidinium in formamidinium tin triiodide perovskites with an ethylenediammonium additive for efficient and stable lead-free perovskite solar cells

   https://doi.org/10.1039/d0ma00520g  

   Tosado, Gabriella A. ; Zheng, Erjin ; Yu, Qiuming ( December 2020 , Materials Advances)

   null (Ed.)

   Achieving efficient and stable tin-based perovskite solar cells remains challenging. In this work, we incorporate the ethylenediammonium diiodide (EDAI 2 ) additive into a cesium–guanidinium doped formamidinium tin triiodide perovskite with the composition of (CsGA) x FA 1−2x SnI 3 + y % EDAI 2 . This new perovskite utilizes the strong hydrogen bonding of the guanidinium cation and the lattice strain relaxation of the small cesium cation as well as the hollowing and passivation effects of the EDAI 2 additive. The EDAI 2 additive not only yields pinhole-free cubic phase perovskite films but also decreases both shallow and deep trap states in the perovskite films. These effects are pronounced with the increase of substitution of the pair of GA + and Cs + . The new perovskites are deployed in inverted planar solar cells. A maximum power conversion efficiency (PCE) of 5.01% is achieved with the (CsGA) 0.15 FA 0.70 SnI 3 + 0% EDAI 2 device but the device degrades after storage in a nitrogen-filled glove box for 30 days. Both performance and stability are improved with the addition of EDAI 2 . A maximum PCE of 5.72% is achieved with the (CsGA) 0.15 FA 0.70 SnI 3 + 1.0% EDAI 2 device. The (CsGA) 0.15 FA 0.70 SnI 3 + 1.5% EDAI 2 devices exhibit a maximum PCE of 5.69% and the performance is further increased to 6.39% after storage in a nitrogen-filled glove box for 4 days; 70% of the initial PCE is retained after 45 days. This study demonstrates the benefit of tuning cation sizes and introducing divalent cations to integrate stabilizing factors into pure Sn perovskites, creating new routes for efficient and stable lead-free perovskite solar cells. 

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3. Room temperature synthesis of lead-free FASnI 3 perovskite nanocrystals with improved stability by SnF 2 additive

   https://doi.org/10.1063/5.0125100  

   Chen, Zeying ; Dhakal, Tara P. ( March 2023 , Applied Physics Reviews)

   Tin halide perovskites are among the candidates for replacing lead-based ones for less toxicity and comparable optical properties. However, stability remains a challenge due to the easier oxidation of Sn 2+ than Pb 2+ . Here, for the first time, we applied the ligand-assisted reprecipitation method to synthesize CH(NH 2 ) 2 SnI 3 (FASnI 3 ) orthorhombic perovskite nanocrystals with an average diameter of 7.7 nm and a photoluminescence emission at 825 [Formula: see text] 2 nm (1.5 eV). The influence of synthesis parameters, including precursor solvent, precipitation media, temperature, and time on optical properties of nanocrystals, was studied. By incorporating SnF 2 , the stability of the nanocrystals was improved, and the oxidation from FASnI 3 to FA 2 SnI 6 was significantly delayed, which was quantitively demonstrated and confirmed by observing the characteristic diffraction peaks of the perovskite phase using x-ray diffraction at various exposure time to air. The addition of SnF 2 is optimized to be 6%. The FASnI 3 nanocrystals stayed stable for at least 265 days under N 2 storage at room temperature and relative humidity of 20%. 

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4. Photoelectric Synaptic Plasticity Realized by 2D Perovskite

   https://doi.org/10.1002/adfm.201902538  

   Sun, Yilin ; Qian, Liu ; Xie, Dan ; Lin, Yuxuan ; Sun, Mengxing ; Li, Weiwei ; Ding, Liming ; Ren, Tianling ; Palacios, Tomás ( May 2019 , Advanced Functional Materials)

   Recently, several light‐stimulated artificial synaptic devices have been proposed to mimic photonic synaptic plasticity for neuromorphic computing. Here, the photoelectric synaptic plasticity based on 2D lead‐free perovskite ((PEA)₂SnI₄) is demonstrated. The devices show a photocurrent activation in response to a light stimulus in a neuron‐like way and exhibit several essential synaptic functions such as short‐term plasticity (STP) and long‐term plasticity (LTP) as well as their transmission based on spike frequency control. The strength of synaptic connectivity can be effectively modulated by the duration, irradiance, and wavelength of light spikes. The ternary structure of (PEA)₂SnI₄causes it to possess varied photoelectric properties by composition control, which enhances the complexity and freedoms required by neuromorphic computing. The physical mechanisms of the memory effect are attributed to two distinct lifetimes of photogenerated carrier trapping/detrapping processes modulated by controlling the proportion of Sn vacancies. This work demonstrates the great potential of (PEA)₂SnI₄as a platform to develop future multifunctional artificial neuromorphic systems.

    

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5. Kinetic versus thermodynamic metalation enables synthesis of isostructural homo- and heterometallic trinuclear clusters

   https://doi.org/10.1039/d0cc02346a  

   Hyun, Sung-Min ; Upadhyay, Apoorva ; Das, Anuvab ; Burns, Corey P. ; Sung, Siyoung ; Beaty, Jeremy D. ; Bhuvanesh, Nattamai ; Nippe, Michael ; Powers, David C. ( June 2020 , Chemical Communications)

   null (Ed.)

   Temperature-dependent metalation of the new hexadentate ligand (tris(5-(pyridin-2-yl)-1 H -pyrrol-2-yl)methane; H 3 TPM) enables the selective synthesis of both mononuclear ( i.e. Na(THF) 4 [Fe(TPM)], kinetic product) and trinuclear ( i.e. Fe 3 (TPM) 2 , thermodynamic product) complexes. Exposure of Na(THF) 4 [Fe(TPM)] to FeCl 2 or ZnCl 2 triggers cluster expansion to generate homo- or heterometallic trinuclear complexes, respectively. The developed approach enables systematic variation of ion content in isostructural metal clusters via programmed assembly. 

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