Large-Scale Synthesis of PbS–TiO₂ Heterojunction Nanoparticles in a Single Step for Solar Cell Application

Abstract

The demand for low cost solar energy technology calls for manufacturing processes using economic liquid- or gas-phase synthesis of the corresponding materials. In this regard, manufacturing of quantum dot-sensitized solar cells is particularly complicated through multiple-step preparations. Material pairs such as TiO₂–PbS heterojunctions have shown high absorption of visible light and good electron transfer properties. However, traditional solution processing requires extensive surface functionalization or the use of surfactants to obtain well-defined films. Such surfactants, unfortunately, often lower electron hopping/tunneling in the system (surfactants are usually insulators) and therefore have to be removed or exchanged before completing device fabrication. Similarly, the so far presented processes to deposit PbS directly on TiO₂ are very time consuming. In this paper, we present a single-step, large-scale, operable process to synthesize PbS–TiO₂ heterojunction particles by aerosol synthesis using reducing flame spray pyrolysis. Nanopowders with different lead sulfide to titanium dioxide ratios were produced and characterized. Thermodynamic equilibrium calculations of the gaseous environment during the combustion process show that the process is robust with regard to usual process changes or fluctuations. We further showed how this approach allowed us to vary the structure and size of the PbS–TiO₂ heterojunction particles, as long as an excess of sulfur species (S/Pb = 2.5) was applied during processing