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 β 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 (E_a) 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.