Quantum Dot (QD) Sensitizer on the Surface of TiO2Film: Effect of Metal Salt Anions Dissolved in Chemical Bath on the Distribution Density of SILAR-Grown PbS QDs

Abstract

Recently, quantum dots (QDs) have attracted much attention for a variety of optoelectronic applications based on their strong light absorption and bright emission over a broad range of spectrum. One of the most recent and active areas is related with QD-sensitized solar cells that utilize inorganic QDs as a light harvester. QDs are considered as an ideal sensitizer because their absorption range from visible to IR spectrum can be tuned easily by changing the size or composition of QD itself. QDs have also demonstrated a very special characteristic by showing a possibility of two or more exciton generation per one photon absorbed, which is generally termed as multiple exciton generation (MEG). In most studies, QD sensitizers have been prepared by two different methods and incorporated into a variety of structures for photovoltaic devices; in-situ chemical bath-deposited QD sensitizers look suitable for mesoporous metal oxide-based photoelectrochemical cells, while ex-situ pre-synthesized colloidal QD sensitizers have been applied in very thin film-type solar cells composed of QD/polymer or QD alone between two electrodes. The successive ionic layer adsorption and reaction (SILAR) process is a representative chemical bath deposition technique for preparing target QDs on the surface of mesoporous metal oxide films by dipping the electrode alternatively in cationic or anionic chemical bath. The SILAR can be considered as a sort of ionic layer adsorption process to combine the target cation and anion successively at room temperature. However, there have been few investigations on the detailed experimental conditions of the SILAR process and their effects on the performance of asprepared QD sensitizers although it has been applied routinely so far in many studies. In this study, it has been observed clearly that the counter-anions (nitrate vs. acetate) of a metal cation (Pb) could play an important role in determining the distribution density of QDs adsorbed during the typical SILAR process for growing PbS QDs on the surface of TiO2 mesoporous films. In addition, it was successfully demonstrated that the phase image obtained with atomic force microscopy (AFM) could be used as a distinguishable map to show the distribution of QDs deposited on the surface of metal oxide at each growth stage, along with a high-resolution transmission electron microscopy (TEM) image. In the design of efficient QD sensitizers over the surface of mesoporous metal oxide, it is very important to get a knowledge of the adsorption pattern and distribution density of QDs and thus determine the experimental conditions for SILAR process for both effective light absorption and efficient charge separation at the interface of metal oxide and QD sensitizers.