Organic-inorganic lead bromide perovskites nanoparticlessynthesis, stability and photophysical properties

Resumen

Abstract The organic-inorganic lead halide perovskites have emerged as novel semiconductor materials for photovoltaic and related optoelectronic applications, due to their exceptional optical and electronic properties. Their preparation into mesoporous films gave rise to nanoparticles with new properties, such as photoluminescence, which attracted great interest owing to their potential in optoelectronic applications. The lack of a suitable synthetic procedure to obtain colloidal perovskite nanoparticles (0D materials), with sizes as small as several nanometers, motivated our interest in their preparation. Therefore, this thesis was focused on the synthesis of highly photoluminescent colloidal organic-inorganic lead bromide perovskite nanoparticles by means of a non-template approach based on the use of medium alkyl chain ammonium ligands, and the study of their conductive properties. We reported for the first time the preparation of green photoluminescent colloidal methylammonium lead bromide perovskite nanoparticles with a size of ca. 6 nm, a photoluminescence quantum yield of ca. 20 % and a 3D perovskite inorganic framework, by using medium alkyl ammonium salts and mild reaction conditions. The nanoparticles proved to be dispersible in aprotic and moderate polar organic solvents and highly stable in solid state (up to three months). Moreover, the toluene colloid was used in the preparation of photo- and electro-luminescent thin films. An improvement in the electroluminescence was observed in the device based on the perovskite nanoparticles film compared to the bulk device. Subsequently, the effect of the components molar ratio and the length of the alkyl chain ammonium, as well as of the non-coordinating solvent, on the synthesis, stability and unique optical properties of colloidal nanoparticles was studied. Thus, the photoluminescence quantum yield of methylammonium lead bromide nanoparticles was increased, to up to 83 % as colloid and 72 % as thin film, through the fine tuning of the molar ratio between the alkyl ammonium salts and lead bromide. These nanoparticles preserved their emissive properties in solid state and exhibited high photostability under continuous ultraviolet irradiation of the colloid. In addition, these non-template synthetic approach for the synthesis of colloidal perovskite also enabled the preparation of blue photoluminescent perovskites with a 2D inorganic framework, which showed a photoluminescence quantum yield of ca. 21% and proved to be easily processed. Later on, we demonstrated the efficient surface passivation of the methylammonium lead bromide nanoparticles by using 2-adamantylammonium bromide as the capping ligand, as demonstrated by their exceptional photoluminescence quantum yield of 100%. However, a considerable aggregation in solution and a long-lived photoluminescence were observed. Then, surface engineering by using 2-adamantylamine and different alkyl carboxylic acids was studied to decrease their aggregation tendency in solution with the final aim of prepare conductive nanoparticle solid films. Remarkably, this binary ligand systems enabled the preparation of highly dispersible and photoluminescent methylammonium lead bromide colloids (photoluminescence quantum yield ca. 100 %), reduced the tendency of aggregation of nanoparticles in solution and allowed the assembly of nanoparticles into densely packed solid films with thicknesses up to several hundreds of nanometers. The nanoparticles retained the pre-engineered confined morphologies in the film as well as their optical properties. Moreover, the conductive atomic force microscopy measurements demonstrated the presence of high conductive domains that can be attributed to conductive regions due to an effective electrical connectivity between the nanoparticles. Finally, we demonstrated the possibility of preparing colloidal ligand-free methylammonium lead bromide perovskite nanoparticles. The strategy consisted in the addition of a lipophilic potassium salt to the perovskite precursor solution and an excess of methylammonium bromide. The potassium cations eventually acted as the confinement agent and provided colloidal stability in the moderately polar solvent ethyl acetate. The assembly of methylammonium lead bromide nanoparticles on substrates led to densely packed nanoparticle solid films, which showed transport properties comparable to that of a polycrystalline bulk thin film.