Synthesis and Properties of Stable Amino Metal Halide Molecular Clusters in the Solid State.

Nanosized molecular clusters (MCs) composed of PbBr2 and neutral ligand butylamine (BTYA) with unique optical properties in solution and solid states have been synthesized using ligand-assisted reprecipitation and spin-coating, separately. The studies of their optical properties using ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) show the first electronic absorption and PL band of the MCs at 401 and 411 nm, respectively, for the solution and solid state samples that exhibit good stability under ambient conditions. Low-temperature PL spectra below 30 K show vibronic peaks indicative of a single size or a very narrow size distribution of the MCs. On the basis of Raman, X-ray diffraction, and transmission electron microscopy measurements, a layered structural model is proposed for the MCs with a BTYA ligand capping on the surface of the corner-shared tilted [PbBr6]4- octahedral framework. The stable and retained structure of MCs in the solid state is promising for photonics applications.

Dependence of stability and electronic and optical properties of perovskite quantum dots on capping ligand chain length.

Methylammonium lead bromide (MAPbBr3) perovskite quantum dots (PQDs) passivated with capping ligands with different chain length, including butylamine-valeric acid (BUTY-VA), octylamine-caprylic acid (OCTY-CA), and dodecylamine-lauric acid (DODE-LA), are investigated to determine an optimized capping layer thickness for maximizing both electronic and antimoisture properties of perovskite materials in optoelectronic devices. The photoluminescence quantum yield (PLQY) is observed to be chain length dependent, where the PLQY of BUTY-VA, OCTY-CA, and DODE-LA MAPbBr3 PQDs is 82% ± 4%, 68% ± 7%, and 18% ± 2%, respectively. Electrochemical impedance spectroscopy (EIS) measurements of each PQD film reveal that there is a slight increase in conductivity from reducing the capping ligand chain length from 8 carbon atoms (OCTY-CA) to 4 carbon atoms (BUTY-VA). Using the Butler-Volmer equation, the charge transfer factor ß for BUTY-VA and OCTY-CA MAPbBr3 PQD films in a tetrabutylammonium hexafluorophosphate-dichloromethane electrolyte solution was calculated to be 0.36 and 0.31, respectively. From an Arrhenius analysis, the activation energy (Ea) for charge transport between the PQD film and the electrolyte was calculated to be 77 and 90 meV for BUTY-VA and OCTY-CA MAPbBr3 PQD films, respectively. Moreover, passivating PQDs with capping ligands with 12 carbon atoms (DODE-LA) almost completely insulates the PQDs and diminishes charge transport. This is also observed in transient photocurrent density measurements. The results suggest that the inter-PQD distance in this solid film is too long for effective tunneling to occur. However, using BUTY-VA capping ligands to improve electronic properties of PQD solid film comes with a cost of stability.