Silicon vapor from a magnetron sputter source was deposited onto highly oriented pyrolytic graphite, resulting in the formation of nanoscale wires. The structures were analyzed by scanning tunneling microscopy. The wires are from 3 to 7 nm in diameter and at least 100 nm long. They tend to be assembled parallel in bundles. In order to understand the observed quasi-one-dimensional structures, diamondlike and fullerenelike wire models are constructed. Molecular-orbit calculations yield binding energies and band gaps of such structures, and lead us to propose a fullerene-type Si 24 -based atomic configuration for nanowires of silicon.

Thin films of tungsten oxide were fabricated by reactive RF magnetron sputtering for applications related to the direct solar splitting of water. To investigate band gap reduction by nitrogen doping, films were deposited with nitrogen introduced into the sputtering ambient at partial pressures in the range of 0-6 mTorr N 2 . For dilute doping (pN 2 < 2 mTorr), the films exhibit a small increase in band gap, and show a degradation of the carrier collection efficiency. Structurally, X-ray diffraction indicates a decrease in crystallinity with nitrogen incorporation. For higher levels of nitrogen doping (pN 2 > 3 mTorr), the diffraction pattern shows the evolution of a new phase that shows an increase in scattering power relative to the pure WO 3 . For these samples, a reduction of the optical band gap to <2.0 eV is measured. However, the photocurrent density under AM1.5G illumination showed a degradation from 2.68 mA/cm 2 for pure WO 3 to 0.67 mA/cm 2 for the nitrogen doped sample (6 mTorr). The poor photocurrent for the nitrogen doped sample is attributed a degradation of the electron transport properties as a result of a highly defective lattice.

Solar cells based on kesterite-type Cu 2 ZnSnS 4 (CZTS) were fabricated on molybdenum coated soda lime glass by evaporation using ZnS, Sn, Cu, and S sources. The coevaporation process was performed at a nominal substrate temperature of 5508C and at a sulfur partial pressure of 2-3 Â 10 À3 Pa leading to polycrystalline CZTS thin films with promising electronic properties. The CZTS absorber layers were grown copper-rich, requiring a KCN etch step to remove excess copper sulfide. The compositional ratios as determined by energy-dispersive X-ray spectroscopy (EDX) after the KCN etch are Cu/(Zn þ Sn): 1.0 and Zn/Sn: 1.0. A solar cell with an efficiency of 4.1% and an open-circuit voltage of 541 mV was obtained.

The electronic structure of the CdS/Cu2ZnSnS4 (CZTS) heterojunction was investigated by direct and inverse photoemission. The effects of a KCN etch of the CZTS absorber prior to CdS deposition on the band alignment at the respective interface were studied. We find a “cliff”-like conduction band offset at the CdS/CZTS interface independent of absorber pretreatment and a significant etch-induced enhancement of the energetic barrier for charge carrier recombination across the CdS/CZTS interface.

No abstract

No abstract

No abstract

No abstract