Dynamics of semiconducting nanocrystal uptake into mesoporous TiO2 thick films by electrophoretic deposition

Abstract

Electrophoretic deposition (EPD) is a simple technique for the uptake of nanoparticles into mesoporous films, for example to graft semiconducting nanocrystals (quantum dots, QDs) on mesoporous oxide thick films acting as photoanodes in third generation solar cells. Here we study the uptake of colloidal QDs into mesoporous TiO₂ films using EPD. We examined PbS@CdS core@shell QDs, which are optically active in the near infrared (NIR) region of the solar spectrum and exhibit improved long-term stability toward oxidation compared to their pure PbS counterpart, as demonstrated by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. We applied Rutherford backscattering spectrometry (RBS) to obtain the Pb depth profile into the TiO₂ matrix. EPD duration in the range from 5 to 120 min and applied voltages from 50 to 200 V were considered. The applied electric field induces the fast anchoring of QDs to the oxide surface. Consequently, QD concentration in the solution contained in the mesoporous film drastically decreases, inducing a Fick-like diffusion of QDs. We modelled the entire process as a QD diffusion related to the formation of a QD concentration gradient, and a depth-independent QD anchoring, and were able to determine the electric field-induced diffusion coefficient D for QDs and the characteristic time for QD grafting, in very good agreement with the experiment. D increases from (1.5 ± 0.4) × 10⁻⁵ μm² s⁻¹ at 50 V to (1.1 ± 0.3) × 10⁻³ μm² s⁻¹ at 200 V. The dynamics of EPD may also be applied to other different colloidal QDs and quantum rod materials for the sensitization of mesoporous films. These results quantitatively describe the process of QD uptake during EPD, and can be used to tune the optical and optoelectronic properties of composite systems, which determine, for instance, the photoconversion efficiency in QD solar cells (QDSCs).