Area-selective electrodeposition of micro islands for CuInSe2-based photovoltaics

For mass fabrication of highly-efficient photovoltaic modules based on Cu(In,Ga)Se 2 (CIGSe) absorber layers the availability and cost of the critical raw materials In and Ga present a potential bottleneck. The micro-concentrator solar cell concept provides a solution by using micro lenses to concentrate incoming sun light on an array of micro-sized CIGSe solar cells. The challenge is to fabricate CIGSe micro islands in exactly the desired positions using only the required material. Here, we analyze the area-selective electrodeposition of CuInSe 2 into holes in an insulating SiO 2 template layer as a material-efficient fabrication approach. We observe that the deposition process shows a strong dependence on the hole size, with a faster deposition around the hole perimeter. Based on a model developed for electrochemical reactions at ultra-micro electrodes, we develop numerical simulations for the electrochemical deposition process. The simulations consider the changing micro-electrode geometry throughout the deposition process, and provide a reasonable fit to the experimental data. Finally, it is shown that CuInSe 2 micro solar cells fabricated by electrodeposition reach efficiencies of 4.8% under 1 sun, providing a proof-of-concept demonstration meriting further development

Coupling of hydraulic and mechanical behaviour in unsaturated compacted clay

An experimental study was carried out to investigate the shear strength of a compacted non-active clay (Speswhite kaolin). A total of 33 statically compacted specimens were prepared, at vertical stresses of 300, 600 and 1200 kPa and water contents ranging from 0·24 to 0·34. This made it possible to explore a broad range of initial compaction-induced conditions and hence to examine whether differences in water retention characteristics and shear strength are due to differences in initial compaction-induced state or are to be attributed to fundamentally different microfabrics. A shearbox was modified to allow the direct measurement of negative pore water pressure using Trento high-suction tensiometers. Tests were carried out at constant water content by monitoring suction changes. In these tests, it was possible to investigate the transition from unsaturated to saturated states. In the first stage of the test, the specimen was compressed at constant water content. The specimen was therefore wetted, in the sense that its degree of saturation increased. Compression data were then used to investigate the influence of void ratio on the main wetting curve. Shear data were used to put forward a critical state framework for unsaturated compacted clay. This is described by three equations linking shear stress, normal total stress, suction, void ratio and water ratio

Mechanical damage on Bird-of-Paradise (Strelitzia reginae) postharvest

The marketing of strelitzia (Strelitzia reginae) has been increasing due to its beauty, exotic colors and high post-harvest longevity. It has potential to increase also in international exports, however this has been limited by the scarcity of information on the flower post-harvest management and packaging. Therefore, this study aimed to further the technology of strelitzia postharvest to maintain high quality of the product and to select a packaging system suitable for export, ensuring high quality to the final consumer. The damages of the stem and the petal color on the buds of Strelitzia reginae were evaluated and it was observed that the stalks harvested with closed buds had extended longevity, while the flower stalks harvested with the first opened button had the highest flower opening rate. As for mechanical damage, the sleeve net protection most affected the visual appearance of the flowers. The cardboard box packaging showed the lowest percentage of damage in the stems

Photoelectrochemical Screening of Solar Cell Absorber Layers: Electron Transfer Kinetics and Surface Stabilization

edox electrolyte contacts offer a simple way of testing the photocurrent generation/collection efficiency in partially completed thin-film solar cells without the need to complete the entire fabrication process. However, the development of a reliable quantitative method can be complicated by the instability of the semiconductor/electrolyte interface. In the case of Cu(In,Ga)Se2 (CIGSe) solar cells, these problems can be overcome by using samples that have undergone the next processing step in solar cell fabrication, which involves chemical bath deposition of a thin (ca. 50 nm) CdS buffer layer. The choice of redox system is also critical. The frequently used Eu3+/2+ redox couple is not suitable for reliable performance predictions since it suffers from very slow electron transfer kinetics. This leads to the buildup of photogenerated electrons near the interface, resulting in electron–hole recombination. This effect, which can be seen in the transient photocurrent response, has been quantified using intensity-modulated photocurrent spectroscopy (IMPS). The study has demonstrated that the more oxidizing Fe(CN)63–/4– redox system can be used when a CdS buffer layer is deposited on the CIGSe absorber. The wide bandgap CdS acts as a barrier to hole injection, preventing decomposition of the CIGSe and formation of surface recombination centers. The IMPS response of this system shows that there is no recombination; i.e., electron scavenging is very rapid. It is shown that measurements of the external quantum efficiency made using the Fe(CN)63–/4– redox couple with CdS-coated CIGSe layers can provide reliable predictions of the short-circuit currents of the complete solar cells. Similar results have been obtained using CdS-coated GaAs layers, suggesting that the new approach may be widely applicable

Cu2ZnSnSe4 device obtained by formate chemistry for metallic precursor layer fabrication

Cu2ZnSnSe4 films were prepared by selenization of metallic precursors obtained by a new wet process involving metal formates. Cu(HCOO)2 and Zn(HCOO)2 were used as copper and zinc sources respectively, while tin was introduced as a methoxide.The elemental analysis of the resulting absorber layers revealed a very low carbon content (less than 0.2wt%), which is believed to be a feature that chemical methods need to have in order to stand out as valuable alternatives to high-vacuum processes in this field. Solar cells with efficiencies of up to 2.39% with Voc of 207mV, a Jsc of 31.2mA/cm2 and a fill factor of 37.1% were achieved for these copper-poor CZTSe absorbers