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The temperature dependence of Si-based thin-film single junction solar cells on the phase of the intrinsic absorber is investigated in order to find the optimal absorber at high operating temperatures. For comparison, hydrogenated amorphous, protocrystalline, and microcrystalline silicon solar cells are fabricated by plasma-enhanced chemical vapor deposition and hot-wired CVD techniques. Photo J-V characteristics are measured using a solar simulator at the ambient temperature range of 25-85°C. It is found that the cells with a higher open-circuit voltage usually show lower temperature-dependent behaviors; the protocrystalline silicon solar cells provide the lowest temperature coefficient of efficiency, while the microcrystalline silicon solar cells are highly sensitive to the temperature. Therefore, protocrystalline silicon solar cells are promising for use in high temperature regions.
Pt thin films of various thicknesses (30 nm ∼ 200 nm) were deposited on Si wafers with SiO2, Ti, TiO2, or IrO2 buffer layers at various temperatures (room temperature ∼200 °C) by a direct current magnetron sputtering process. The Pt films showed a strong (111)-preferred texture irrespective of the thickness, under-layer, and growth temperature. The authors previously reported [J-E. Lim, D-Y. Park, J.K. Jeong, G. Darlinski, H.J. Kim, and C.S. Hwang, Appl. Phys. Lett. 81, 3224 (2002)] that the films were composed of three kinds of grains with slightly different (111) lattice parameters (bulklike, 1.0% and 2.1% larger). This study details the microstructural variations of the Pt films according to the variations of experimental parameters. The different deposition conditions produced slightly different crystalline structures, but the three different (111) lattice parameters were always found. Epitaxial (200) Pt films on a (200) MgO substrate and a highly (111) textured Au thin film on a SiO2/Si did not show the same splitting in the lattice parameter. The grains with 1.0% and 2.1% larger (111) lattice parameter almost disappeared after postannealing at 1000 °C. However, surface chemical binding of the Pt film before and after annealing was unchanged. Therefore, it is believed that the lattice parameter splitting in the (111) textured Pt film originated from the interfacial grains with the distorted crystal structure due probably to growth stress.
A drastic change of the growth orientation in epitaxial Pt films grown by sputtering on MgO(001) was observed. At higher substrate temperatures, practically pure (001) epitaxial Pt films grow. On the other hand, epitaxial (111) Pt films grow at lower substrate temperatures. Interestingly, the gradual transition from (111) to (001) orientation occurs at lower temperatures when applied at a lower deposition rate. The degree of supersaturation in growth conditions is proposed as a key driver of the orientation change. When homogeneous nucleation occurs under a higher supersaturation, a large number of nuclei grow at the flat (001) terraces possessing (111) tetrahedral orientation. Under a lower supersaturation, a small number of nuclei aligned in (001) orientation to each other form dominantly at surface defects, resulting in a (001) epitaxial film with no grains. Our results suggest that one can selectively prepare either (001) or (111) epitaxial Pt films by properly adjusting substrate temperature and/or deposition rate.
Thickness dependence of leakage current behaviors was investigated in epitaxial (Ba0.5Sr0.5)TiO3 thin films with different thicknesses of 55 - 225 nm prepared on Pt(001)/MgO(001) substrates by a radio-frequency magnetron sputtering technique. Below a certain critical film thickness (≤ 55 nm), the Schottky emission is a ruling leakage conduction mechanism over a wide electric field range. In contrast, in thicker films (> 55 nm), the Schottky emission still operates at low electric fields, however at high electric fields the Fowler-Nordheim (F-N) tunneling dominates. The transition film thickness appears to be associated with overlapping of the depletion layers formed at the top and bottom electrode interfaces.
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