Jeremy Fields

Highly efficient charge transfer in nanocrystalline Si:H solar cells

We demonstrate that in nanostructured films of nanocrystalline silicon imbedded in a hydrogenated amorphous silicon matrix, carriers generated in the amorphous region are transported out of this region and therefore do not recombine in the amorphous phase. Electron paramagnetic resonance (EPR) and photoluminescence (PL) measurements show that the EPR and PL from the amorphous phase are rapidly quenched as the volume fraction of Si nanocrystals exceeds about 30 vol. %. We propose the use of similar structures to dramatically increase the open circuit voltages in solar cell devices.

Thermal activation of deep oxygen defect formation and hydrogen effusion in hydrogenated nanocrystalline silicon thin films

Deep oxygen related defects form in hydrogenated nanocrystalline silicon (nc-Si:H) as a consequence of thermal annealing, but their microscopic origins and formation mechanisms are not well understood. To gain insight to this behavior we intentionally drive-out hydrogen from nc-Si:H films by thermal annealing and monitor accompanying changes in the electronic and vibrational structure of the films with photoluminescence (PL) and Fourier transform infrared (FTIR) absorption spectroscopy. Hydrogen effusion (HE) data provide additional insight, because the annealing temperature range shown to induce a defect band, centered at ∼ 0.7 eV in PL studies, and that corresponding to the onset of thermally activated hydrogen desorption from grain boundaries, coincide. This coincidence suggests a probable link between the two processes. The activation energy obtained from correlated annealing-PL experiments, of ∼ 0.6 eV, for defect formation with thermal exposure, provides substantial insight regarding the mechanism.

Optical and electrical properties of crystalline silicon wire arrays

Silicon wire arrays have been synthesized through a two-step metal-assisted electrode-less etching from an n-type silicon wafer with (100) orientation. Field Emission Scanning Electron microscope (FESEM), Ultra violet-Visible-Near infrared (UV-VIS-NIR) spectrophotometer and Resonance-coupled photoconductivity decay (RCPCD) have been used to characterize the morphological, optical, and electrical properties of Si wires at varying etching times. The reflectivity of the wire arrays decreased with increasing etching time because of light scattering from the micro-roughness of the Silicon wire surfaces. The effective carrier lifetime decreased with increasing wire length due to the increased surface area. We also created smoother wire surfaces by thermal oxidation followed by HF dipping. From FESEM cross sectional images and reflectivity results, this treatment removes the micro-roughness, but the effective lifetime is lower than the as-grown wire arrays. A photoluminescence peak observed only in the smoother wires suggests that the lower effective lifetime is due to the diffusion of residual Ag atoms from the wire surfaces into the bulk during the thermal oxidation process.