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

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

Electrical Resistivity of Assembled Transparent Inorganic Oxide Nanoparticle Thin Layers: Influence of Silica, Insulating Impurities, and Surfactant Layer Thickness

The electrical properties of transparent, conductive layers prepared from nanoparticle dispersions of doped oxides are highly sensitive to impurities. Production of cost-effective thin conducting films for consumer electronics often employs wet processing such as spin and/or dip coating of surfactant-stabilized nanoparticle dispersions. This inherently results in entrainment of organic and inorganic impurities into the conducting layer leading to largely varying electrical conductivity. Therefore, this study provides a systematic investigation on the effect of insulating surfactants, small organic molecules and silica in terms of pressure dependent electrical resistivity as a result of different core/shell structures (layer thickness). Application of high temperature flame synthesis gives access to antimony-doped tin oxide (ATO) nanoparticles with high purity. This well-defined starting material was then subjected to representative film preparation processes using organic additives. In addition ATO nanoparticles were prepared with a homogeneous inorganic silica layer (silica layer thickness from 0.7 to 2 nm). Testing both organic and inorganic shell materials for the electronic transport through the nanoparticle composite allowed a systematic study on the influence of surface adsorbates (e.g., organic, insulating materials on the conducting nanoparticle’s surface) in comparison to well-known insulators such as silica. Insulating impurities or shells revealed a dominant influence of a tunneling effect on the overall layer resistance. Mechanical relaxation phenomena were found for 2 nm insulating shells for both large polymer surfactants and (inorganic) SiO₂ shells

Effective Ligand Passivation of Cu₂O Nanoparticles through Solid-State Treatment with Mercaptopropionic Acid

In colloidal nanoparticle (NPs) devices, trap state densities at their surface exert a profound impact on the rate of charge carrier recombination and, consequently, on the deterioration of the device performance. Here, we report on the successful application of a ligand exchange strategy to effectively passivate the surface of cuprite (Cu₂O) NPs. Cu₂O NPs were prepared by means of a novel synthetic route based on flame spray pyrolysis. FTIR, XRD, XPS, and HRTEM measurements corroborate the formation of cubic cuprite Cu₂O nanocrystals, excluding the possible presence of undesired CuO or Cu phases. Most importantly, steady-state emission and transient absorption assays document that surface passivation results in substantial changes in the intensity of emissive excitonic statescentered at copper and oxygen vacanciesand in the lifetime of excitons near the band edge. To shed light onto ultrafast processes in Cu₂O nanocrystals additional pump probe experiments on the femtosecond and nanosecond time scales were carried out. Two discernible species were observed: on one hand, an ultrafast component (∼ps) that relates to the excitons; on the other hand, a long-lived component (∼μs) that originates from the defects/trap states