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In this contribution, we focus on improving the fundamental understanding of the carrier lifetime degradation and regeneration observed in block-cast multicrystalline silicon (mc-Si) wafers under illumination at elevated temperature. We observe a pronounced degradation in lifetime at 1 sun light intensity and 75̊C after rapid thermal annealing (RTA) in a belt-firing furnace at a set peak temperature of 900̊C. However, almost no lifetime instability is detected in mc-Si wafers which are fired at a peak temperature of only 650̊C, clearly showing that the firing step is triggering the degradation effect. Lifetime spectroscopy reveals that the light-induced recombination centre is a deep-level centre with an asymmetric electron-to-hole capture cross section ratio of 20±7. After completion of the degradation, the lifetime is observed to recover and finally reaches even higher carrier lifetimes compared to the initial state. While the lifetime degradation is found to be homogeneous, the regeneration shows an inhomogeneous behaviour, which starts locally and spreads later laterally throughout the sample. Furthermore, the regeneration process is extremely slow with time constants of several hundred hours. We demonstrate, however, that by increasing the regeneration temperature, it is possible to significantly speed up the regeneration process so that it might become compatible with industrial solar cell production. To explain the observed lifetime evolution, we propose a defect model, where metal precipitates in the mc-Si bulk dissolve during the RTA treatment and the mobile metal atoms bind to a homogeneously distributed impurity. Restructuring and subsequent dissociation of this defect complex is assumed to cause the lifetime degradation, whereas a subsequent diffusion of the mobile species to the sample surfaces and crystallographic defects explains the regeneration.

AbstractWe present n-type epitaxially grown wafers deposited in a reactor that allows a process transfer to inline high-throughput reactors. Those wafers exhibit an effective lifetime of up to 1720 μs locally for a phosphorous concentration of 2·1015 cm-3 and a wafer thickness of about 100 μm. In these wafers the most detrimental defects are stacking faults with polycrystalline silicon inclusions. Comparing two samples with stacking faults densities differing by one order of magnitude revealed a difference in average effective minority carrier lifetime of also one order of magnitude (reduction from more than 1500 μs down to 111 μs for the defective sample).A solar cell fabricated from a 200 μm thick epitaxial wafer of low stacking fault density and a phosphorous concentration of 3·1016 cm-3 reaches an independently confirmed efficiency of 20%, an open circuit voltage of up to 658 mV, a short circuit current density of up to 39.6 mA/cm2 and a fill factor of up to 76.9%. Differences between this cell and FZ references can be attributed to a reduced bulk lifetime caused by the high doping concentration and most probably additional recombination due to polycrystalline silicon inclusions in stacking faults, although their amount is comparably low. A second solar cell made of an epitaxial wafer with a high stacking fault density exhibits an efficiency reduction of 0.5% absolute compared to the cell made of the high quality epitaxial wafer. This result underlines the importance of minimizing the stacking fault density in epitaxial wafers, in particular the density of those with polysilicon inclusions.

AbstractThe German Energiewende is resulting in high grid load changes caused by renewable energies. Therefore flexibility of power plants is getting more and more important. Future CCS power plants are usually equipped with more components than conventional power plants, resulting in a more complex and inert reaction on changes in power output. Additionally, due to the change in price structures and higher fixed and operational costs for CCS power plants, it is harder for them to be economically efficient. This study will show different options to increase the flexibility of CCS power plants and evaluate their benefits.

AbstractNext-generation, crystalline-silicon solar cells might use different metallization concepts compared to current state-of-the-art cell designs. One specific design uses a thin aluminum back layer created with a physical vapor deposition process. In the solar industry, there is no reliable, cost-effective method of directly connecting metallic ribbons to such an aluminum layer to create cell strings. In the semiconductor industry ultrasonically bonding aluminum ribbon to the aluminum metallization of power semiconductors or high-volume production is a well-established process. This paper describes adapting ultrasonic ribbon bonding and the equipment used in the semiconductor industry to interconnect crystalline-silicon solar cells.

Different concepts of solar assisted heat pump systems with ground heat exchanger are simulated according to IEA SHC Task44/HPP Annex38 reference conditions. Two aspects of the concepts are investigated using TRNSYS simulations. First, the solar impact on system efficiency is assessed by the seasonal performance factor. Second, the solar impact on the possible shortening of the ground heat exchanger is evaluated by the minimum temperature at the ground heat exchanger inlet. The simulation results reveal diverging optimums for the concepts. The direct use of solar energy clearly achieves the best effect on the efficiency improvement. A simple domestic hot water system reaches a seasonal performance factor of 4.5 and solar combi-systems seasonal performance factors up to 6. In contrast, the use of solar energy on the cold side of the heat pump achieves the best effects on the shortening of the ground heat exchanger of up to 20%. Two highly sensitive influences are investigated with the developed transient system model. First, the minimum allowed heat source temperature is varied. Here 1 K equals a variation of 0.25 in the seasonal performance or of around 10% ground heat exchanger length. Second, the ground heat exchanger model is simulated without and with a pre-pipe that improves the transient model behavior. The influence of this pre-pipe on the SPF is small for conventionally designed ground heat exchangers, but of around 2 K for the minimum inlet temperature. Therefore, the dynamic model quality reveals potential to reduce the size of the ground heat exchanger corresponding to investment costs.

AbstractA tool for analysis of CO2 value chains has been developed in the ECCO project (EU’s Seventh Framework Programme). The tool supports networks of multiple CO2 sources and sinks, dynamics, and variations in macro-economic conditions. The performance of individual chain units, actors, and the system as a whole is evaluated. The tool has an object-oriented architecture, allowing flexibility and incorporation of a range of different techno-economic modules to model the various chain units.

13th International Conference on Greenhouse Gas Control Technologies, GHGT-13 / edited by Tim Dixon, Lyesse Laloui, Sian Twinning 13. International Conference on Greenhouse Gas Control Technologies, GHGT-13, Lausanne, Switzerland, 14 Nov 2016 - 18 Nov 2016; Amsterdam [u.a.] : Elsevier, Energy Procedia 114, 650-665 (2017). doi:10.1016/j.egypro.2017.03.1209 Published by Elsevier, Amsterdam [u.a.]