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See detailSynthesis, theoretical and experimental characterisation of thin film Cu2Sn1-xGexS3 ternary alloys (x = 0 to 1): Homogeneous intermixing of Sn and Ge
Robert, Erika UL; Gunder, René; De Wild, Jessica UL et al

in Acta Materialia (2018), 151

Cu2Sn1-xGexS3 is a p-type semiconductor alloy currently investigated for use as an absorber layer in thin film solar cells. The aim of this study is to investigate the properties of this alloy in thin ... [more ▼]

Cu2Sn1-xGexS3 is a p-type semiconductor alloy currently investigated for use as an absorber layer in thin film solar cells. The aim of this study is to investigate the properties of this alloy in thin film form in order to establish relationships between group IV composition and structural, vibrational and opto-electronic properties. Seven single phase Cu2Sn1-xGexS3 films are prepared from x ¼ 0 to 1, showing a uniform distribution of Ge and Sn laterally and in depth. The films all show a monoclinic crystal structure. The lattice parameters are extracted using Le Bail refinement and show a linear decrease with increasing Ge content. Using density-functional theory with hybrid functionals, we calculate the Raman active phonon frequencies of Cu2SnS3 and Cu2GeS3. For the alloyed compounds, we use a virtual atom approximation. The shift of the main Raman peak from x ¼ 0 to x ¼ 1 can be explained as being half due to the change in atomic masses and half being due to the different bond strength. The bandgaps of the alloys are extracted from photoluminescence measurements and increase linearly from about 0.90 to 1.56 eV with increasing Ge. The net acceptor density of all films is around 1018 cm 3. These analyses have established that the alloy forms a solid solution over the entire composition range meaning that intentional band gap grading should be possible for future absorber layers. The linear variation of the unit cell parameters and the band gap with group IV content allows composition determination by scattering or optical measurements. Further research is required to reduce the doping density by two orders of magnitude in order to improve the current collection within a solar cell device structure. [less ▲]

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See detailFormation of nanometer-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films and their effect on solar cell efficiency
Schwarz, Torsten; Cojocaru-Miredin, Oana; Mousel, Marina et al

in ACTA MATERIALIA (2017), 132

Atom probe tomography and transmission electron microscopy are used to study the formation of nano sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin -films. For a Cu -rich precursor, which was deposited at 320 ... [more ▼]

Atom probe tomography and transmission electron microscopy are used to study the formation of nano sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin -films. For a Cu -rich precursor, which was deposited at 320 degrees C under Cu- and Zn-rich growth conditions, Cu2-xSe grains at the surface are detected. During annealing the precursor at 500 degrees C in a SnSe + Se atmosphere most of the Cu2-xSe is transformed to Cu2ZnSnSe4 via the consumption of excessive ZnSe and incorporation of Sn. However, atom probe tomography studies also reveal the formation of various nanometer-sized Cu-Sn-Se particles close to the CdS/Cu(2)ZnSnSe4 interface. One of those particles has a composition close to the Cu2SnSe3 compound. This phase has a smaller band gap than Cu2ZnSnSe4 and is proposed to lead to a significant drop in the open -circuit voltage and could be the main cause for a detrimental p-n junction and the zero efficiency of the final device. Possible effects of the other phases on solar cell performance and formation mechanisms are discussed as well. (C)2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. [less ▲]

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See detailMagnetic SANS study of a sintered Nd–Fe–B magnet: Estimation of defect size
Perigo, Elio Alberto UL; Gilbert, Elliot; Michels, Andreas UL

in Acta Materialia (2015), 87

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See detailUnraveling the nature of room temperature grain growth in nanocrystalline materials
Ames, M.; Markmann, J.; Karos, R. et al

in Acta Materialia (2008), 56

We report on the observation of real-time-resolved room temperature grain growth in nanocrystalline metals. We find that neither the time evolution of size can be modeled by standard growth theories nor ... [more ▼]

We report on the observation of real-time-resolved room temperature grain growth in nanocrystalline metals. We find that neither the time evolution of size can be modeled by standard growth theories nor are there any other systems aware to us that manifest a similar growth behaviour. We detect a transition from an initially self-similar slow growth to abnormal grain growth. Its onset seems to be associated with the simultaneous decrease of microstrain with increasing grain size. Abnormal grain growth is considered as a generic feature of nanocrystallinity but is a transient state since we observed in the late stage of coarsening, using orientational imaging microscopy, a monomodal grain size distribution. We empirically find a nonlinear-response-type of growth law which is in agreement with the observed coarsening kinetics. [less ▲]

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See detailThe influence of spin-misalignment scattering on the SANS data evaluation of martensitic age-hardening steels
Bischof, M.; Staron, P.; Michels, Andreas UL et al

in Acta Materialia (2007), 55(8), 2637-2646

Small-angle neutron scattering has proved to be a valuable technique for probing precipitates in steels. The investigated sample is thereby exposed to a large magnetic field and is assumed to consist of a ... [more ▼]

Small-angle neutron scattering has proved to be a valuable technique for probing precipitates in steels. The investigated sample is thereby exposed to a large magnetic field and is assumed to consist of a single domain with all the magnetic moments perfectly aligned with the external field. In this situation the dominating magnetic scattering contrast is expected to stem from the differences between the magnetization values of matrix and particles. However, the present work shows that strong additional scattering can be present which is due to spin misalignment. The effects of this spin-misalignment scattering are discussed as being exemplary for differently heat-treated martensitic steel samples. It is revealed that the amount of spin-misalignment scattering is very sensitive to the applied heat treatment and is most pronounced in the as-quenched condition. In particular, when considering nuclear scattering curves the influence of spin-misalignment scattering may be very large and must not be neglected. [less ▲]

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See detailModelling the influence of grain-size-dependent solute drag on the kinetics of grain growth in nanocrystalline materials
Michels, Andreas UL; Krill, C. E.; Ehrhardt, H. et al

in Acta Materialia (1999), 47(7), 2143-2152

The large relative change in total grain-boundary area that accompanies grain growth in a nanocrystalline material has a potentially strong influence on the kinetics of grain growth whenever grain ... [more ▼]

The large relative change in total grain-boundary area that accompanies grain growth in a nanocrystalline material has a potentially strong influence on the kinetics of grain growth whenever grain-boundary migration is controlled by solute (impurity) drag. As the grain-boundary area decreases, the concentration of solute or impurity atoms segregated to the boundaries is expected to increase rapidly, introducing a grain-size dependence to the retarding force on boundary migration. We have modified the Burke equation—which assumes the drag force to be independent of the average grain size—to take into account a linear dependence of grain-boundary pinning on grain size. The form of the resulting grain-growth curve is surprisingly similar to Burke's solution; in fact, a constant rescaling of the boundary mobility parameter is sufficient to map one solution approximately onto the other. The activation energies for grain-boundary motion calculated from the temperature dependence of the mobility parameter are therefore identical for both models. This fact provides an explanation for the success of Burke's solution in fitting grain-growth data obtained in systems, such as nanocrystalline materials, for which the assumption of grain-size-independent solute drag is incorrect. [less ▲]

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