silicon and lead-sulfide

We fabricated three-dimensional silicon nanopillar array (SiNP)-based photovoltaic (PV) devices using PbS quantum dots (QDs) as the hole-transporting layers. The core-shell structured device, which is based on high aspect ratio SiNPs standing on roughed silicon substrates, displays a higher PV performance with a power conversion efficiency (PCE) of 6.53% compared with that of the planar device (2.11%). The enhanced PCE is ascribed to the increased light absorption and the improved charge carrier collections in SiNP-modified silicon surfaces. We also show that, for the core-shell structured device, the thickness of the shell layer plays a critical role in enhancing the PV performance and around five monolayers of QDs are preferred for efficient hole-transporting. Wafer-scale PV devices with a radial PbS/SiNP heterojunction can be fabricated by solution phase techniques at low temperatures, suggesting a facile route to fabricate unique three-dimensional nanostructured devices.

Topics: Lead; Nanoparticles; Quantum Dots; Silicon; Solar Energy; Sulfides

The basic characteristics of nanowire growth driven by screw dislocations were investigated by synthesizing hierarchical lead sulfide (PbS) nanowire "pine trees" using chemical vapor deposition of PbCl(2) and S precursors and systematically observing the effects of various growth parameters, such as hydrogen flow, temperature, pressure, and the growth substrates employed. Statistical surveys showed that the growth rate of the dislocation-driven trunk is about 6 mum/min and that of the vapor-liquid-solid (VLS) driven branch nanowire is about 1.2 mum/min under the typical reaction conditions at 600 degrees C, 900 Torr, and a hydrogen flow rate of 1.5 sccm. The onset of hydrogen flow plus the presence of fresh silicon have been identified as the critical ingredients for generating PbS nanowire trees reproducibly. To explain the experimental findings in the context of classical crystal growth theory, the former is suggested to create a spike in supersaturation of the actual sulfur precursor H(2)S and initiate dislocations with screw components that then propagate anisotropically to form the PbS nanowire trunks. Maintaining suitable hydrogen flow provides a favorable low supersaturation that promotes dislocation-driven trunk nanowire growth and enables the simultaneous VLS nanowire growth of branches. Furthermore, thermodynamic consideration and experiments showed that silicon fortuitously controls the supersaturation by reversibly reacting with H(2)S to form SiS(2) and that SiS(2) can also be a viable precursor for PbS nanowire growth. The key requirements of screw dislocation-driven nanowire growth are summarized. This study provides some general guidelines for further nanowire growth driven by screw dislocations.

Topics: Hydrogen; Lead; Materials Testing; Nanowires; Particle Size; Pressure; Silicon; Sulfides; Surface Properties; Temperature