Growth and Dispersion Control of SnO₂ Nanocrystals Employing an Amino Acid Ester Hydrochloride in Solution Synthesis:...

Tin oxide (SnO₂) is a technologically important semiconductor with versatile applications. In particular, attention is being paid to nanostructured SnO₂ materials for use as a part of the constituents in perovskite solar cells (PSCs), an emerging renewable energy technology. This is mainly because SnO₂ has high electron mobility, making it favorable for use in the electron transport layer (ETL) in these devices, in which SnO₂ thin films play a role in extracting electrons from the adjacent light-absorber, i.e., lead halide perovskite compounds. Investigation of SnO₂ solution synthesis under diverse reaction conditions is crucial in order to lay the foundation for the cost-effective production of PSCs. This research focuses on the facile catalyst-free synthesis of single-nanometer-scale SnO₂ nanocrystals employing an aromatic organic ligand (as the structure-directing agent) and Sn(IV) salt in an aqueous solution. Most notably, the use of an aromatic amino acid ester hydrochloride salt-i.e., phenylalanine methyl ester hydrochloride (denoted as L hereafter)-allowed us to obtain an aqueous precursor solution containing a higher concentration of ligand L, in addition to facilitating the growth of SnO₂ nanoparticles as small as 3 nm with a narrow size distribution, which were analyzed by means of high-resolution transmission electron microscopy (HR-TEM). Moreover, the nanoparticles were proved to be crystallized and uniformly dispersed in the reaction mixture. The environmentally benign, ethanol-based SnO₂ nanofluids stabilized with the capping agent L for the Sn(IV) ions were also successfully obtained and spin-coated to produce a SnO₂ nanoparticle film to serve as an ETL for PSCs. Several SnO₂ ETLs that were created by varying the temperature of nanoparticle synthesis were examined to gain insight into the performance of PSCs. It is thought that reaction conditions that utilize high concentrations of ligand L to control the growth and dispersion of SnO₂ nanoparticles could serve as useful criteria for designing SnO₂ ETLs, since hydrochloride salt L can offer significant potential as a functional compound by controlling the microstructures of individual SnO₂ nanoparticles and the self-assembly process to form nanostructured SnO₂ thin films.