A Brief Review of Transparent Conducting Oxides (TCO): The Influence of Different Deposition Techniques on the Efficiency of Solar Cells.

Global-warming-induced climate changes and socioeconomic issues increasingly stimulate reviews of renewable energy. Among energy-generation devices, solar cells are often considered as renewable sources of energy. Lately, transparent conducting oxides (TCOs) are playing a significant role as back/front contact electrodes in silicon heterojunction solar cells (SHJ SCs). In particular, the optimized Sn-doped In2O3 (ITO) has served as a capable TCO material to improve the efficiency of SHJ SCs, due to excellent physicochemical properties such as high transmittance, electrical conductivity, mobility, bandgap, and a low refractive index. The doped-ITO thin films had promising characteristics and helped in promoting the efficiency of SHJ SCs. Further, SHJ technology, together with an interdigitated back contact structure, achieved an outstanding efficiency of 26.7%. The present article discusses the deposition of TCO films by various techniques, parameters affecting TCO properties, characteristics of doped and undoped TCO materials, and their influence on SHJ SC efficiency, based on a review of ongoing research and development activities.

Influence of Ultra-Thin Ge3N4 Passivation Layer on Structural, Interfacial, and Electrical Properties of HfO2/Ge Metal-Oxide-Semiconductor Devices.

We report the effects of the nitride passivation layer on the structural, electrical, and interfacial properties of Ge metal-oxide-semiconductor (MOS) devices with a hafnium oxide (HfO2) gate dielectric layer deposited on p-type 〈100〉 Ge substrates. X-ray photoelectron spectroscopy analysis confirmed the chemical states and formation of HfO2/Ge3N4 on Ge. The interfacial quality and thickness of the layers grown on Ge were confirmed by high-resolution transmission electron microscopy. In addition, the effects of post-deposition annealing (PDA) on the HfO2/Ge3N4/Ge and HfO2/Ge samples at 400 °C in an (FG+O2) ambient atmosphere for 30 min were studied. After PDA, the HfO2/Ge3N4/Ge MOS device showed a higher dielectric constant (k) of ~21.48 and accumulation capacitance of 1.2 nF, smaller equivalent oxide thickness (EOT) of 1.2 nm, and lower interface trap density (Dit) of 4.9×1011 cm-2 eV-1 and oxide charges (Qeff) of 7.8×1012 cm-2 than the non-annealed sample. The I-V analysis showed that the gate leakage current density of the HfO2/Ge3N4/Ge sample (0.3-1 nA cm-2 at Vg = 1 V) was half of that of the HfO2/Ge sample. Moreover, the barrier heights of the samples were extracted from the Fowler-Nordheim plots. These results indicated that nitride passivation is crucial to improving the structural, interfacial, and electrical properties of Ge-based MOS devices.

Ultrafast transient grating spectroscopy in silicon quantum dots.

Transient grating spectroscopy detects directly the relaxation of the excited carriers rather than time-resolved photoluminescence and thus it is particularly desired for the indirect semiconductors such as silicon quantum dots. We investigate ultrafast carrier dynamics in silicon quantum dots embedded in silicon oxide matrix using femtosecond transient grating spectroscopy. Two ultrafast decay components are observed with decay time of 800 fs and 4 ps at various detection wavelengths, which are attributed to the transverse optical and transverse acoustic phonon assisted relaxation. Photoexcited electrons and holes are effectively trapped into the localized states on the surface of the silicon quantum dots where electrons and holes have a slow recombination in the time scale of microseconds.

Silicon quantum dot/crystalline silicon solar cells.

Silicon (Si) quantum dot (QD) materials have been proposed for 'all-silicon' tandem solar cells. In this study, solar cells consisting of phosphorus-doped Si QDs in a SiO(2) matrix deposited on p-type crystalline Si substrates (c-Si) were fabricated. The Si QDs were formed by alternate deposition of SiO(2) and silicon-rich SiO(x) with magnetron co-sputtering, followed by high-temperature annealing. Current tunnelling through the QD layer was observed from the solar cells with a dot spacing of 2 nm or less. To get the required current densities through the devices, the dot spacing in the SiO(2) matrix had to be 2 nm or less. The open-circuit voltage was found to increase proportionally with reductions in QD size, which may relate to a bandgap widening effect in Si QDs or an improved heterojunction field allowing a greater split of the Fermi levels in the Si substrate. Successful fabrication of (n-type) Si QD/(p-type) c-Si photovoltaic devices is an encouraging step towards the realization of all-silicon tandem solar cells based on Si QD materials.

Fabrication of multilayered Ge nanocrystals by magnetron sputtering and annealing.

Multilayered Ge nanocrystals embedded in Si and Ge oxide films have been fabricated on Si substrate by a (SiO(2)+Ge)/(SiO(2)+GeO(2)) superlattice approach, using an rf magnetron sputtering technique with a Ge+SiO(2) composite target and subsequent thermal annealing in N(2) ambient at 750 °C for 5 min. X-ray diffraction (XRD) measurements indicated the formation of Ge nanocrystals with an average size estimated to be 9.8 nm. Raman scattering spectra showed a peak of the Ge-Ge vibrational mode shifted downwards to 298.8 cm(-1), which was caused by quantum confinement of phonons in the Ge nanocrystals. X-ray photoemission spectroscopy (XPS) analysis demonstrated that the Ge chemical state is mainly Ge(0) in the (SiO(2)+Ge) layer and Ge(4+) in the (SiO(2)+GeO(2)) layer in the superlattice structure. Transmission electron microscopy (TEM) revealed that Ge nanocrystals were confined in (SiO(2)+Ge) layers, and had good crystallinity. This superlattice approach significantly improved both the size uniformity of Ge nanocrystals and their uniformity of spacing on the 'Z' growth direction compared with the conventional Ge-ncs fabrication method using a single and thick SiO(2) matrix film.