Atomic scale structure and its impact on the band gap energy for Cu2Zn Sn,Ge Se4 kesterite alloys

Kesterite based materials gain more and more relevance in the pursuit of affordable, efficient and flexible absorber materials for thin film photovoltaics. Alloying Cu(2)ZnSnSe(4)with Ge could allow controlled band gap engineering as already established for Cu(In,Ga)(S,Se)(2)based solar cells. This study investigates the local atomic arrangements of Cu2Zn(Sn,Ge)Se(4)alloys by means of low temperature Extended x-ray Absorbtion Fine Structure Spectroscopy. The element specific bond lengths are used together with x-ray diffraction data to derive the anion positions of the different local configurations.Ab initiotheoretical calculations are performed to predict the influence of structural parameters such as anion position and lattice constants on the band gap energy. Combining the results of the experimental and theoretical studies suggests that the overall influence of the structural changes on the band gap bowing due to alloying is significant yet smaller than the total non-linear change of the band gap energy. Consequently, it is concluded, that band gap bowing stems from both structural and electronic changes

Atomic Scale Structure of Ag,Cu 2 ZnSnSe4 and Cu2Zn Sn,Ge Se 4 Kesterite Thin Films

Kesterite based materials are being researched and developed as affordable, efficient, and mechanically flexible absorber materials for thin film photovoltaics. Both Ag,Cu 2ZnSnSe4 and Cu2Zn Sn,Ge Se4 based devices have shown great potential in overcoming some of the remaining challenges for further increasing the conversion efficiency of kesterite based solar cells. This study therefore investigates the long range crystallographic structure and the local atomic scale structure of technologically relevant thin films by means of grazing incidence X ray diffraction and low temperature X ray absorption spectroscopy. As expected, the unit cell dimensions change about an order of magnitude more than the element specific average bond lengths. In case of Cu2Zn Sn,Ge Se4, the thin film absorbers show a very similar behavior as Cu2Zn Sn,Ge Se4 powder samples previously studied. Small amounts of residual S in the thin films were taken into account in the analysis and the results imply a preferential formation of Sn S bonds instead of Ge S bonds. In Ag,Cu 2ZnSnSe4, the dependence of the Ag Se and Cu Se bond lengths on Ag Ag Cu might indicate an energetic advantage in the formation of certain local configuration

Point defects, compositional fluctuations, and secondary phases in non stoichiometric kesterites

The efficiency of kesterite based solar cells is limited by various non ideal recombination paths, amongst others by a high density of defect states and by the presence of binary or ternary secondary phases within the absorber layer. Pronounced compositional variations and secondary phase segregation are indeed typical features of non stoichiometric kesterite materials. Certainly kesterite based thin film solar cells with an off stoichiometric absorber layer composition, especially Cu poor Zn rich, achieved the highest efficiencies, but deviations from the stoichiometric composition lead to the formation of intrinsic point defects vacancies, anti sites, and interstitials in the kesterite type material. In addition, a non stoichiometric composition is usually associated with the formation of an undesirable side phase secondary phases . Thus the correlation between off stoichiometry and intrinsic point defects as well as the identification and quantification of secondary phases and compositional fluctuations in non stoichiometric kesterite materials is of great importance for the understanding and rational design of solar cell devices. This paper summarizes the latest achievements in the investigation of identification and quantification of intrinsic point defects, compositional fluctuations, and secondary phases in non stoichiometric kesterite type material

Recrystallization of amorphous nano-tracks and uniform layers generated by swift-ion-beam irradiation in lithium niobate.

The thermal annealing of amorphous tracks of nanometer-size diameter generated in lithium niobate (LiNbO3) by Bromine ions at 45 MeV, i.e., in the electronic stopping regime, has been investigated by RBS/C spectrometry in the temperature range from 250°C to 350°C. Relatively low fluences have been used (<1012 cm−2) to produce isolated tracks. However, the possible effect of track overlapping has been investigated by varying the fluence between 3×1011 cm−2 and 1012 cm−2. The annealing process follows a two-step kinetics. In a first stage (I) the track radius decreases linearly with the annealing time. It obeys an Arrhenius-type dependence on annealing temperature with activation energy around 1.5 eV. The second stage (II) operates after the track radius has decreased down to around 2.5 nm and shows a much lower radial velocity. The data for stage I appear consistent with a solid-phase epitaxial process that yields a constant recrystallization rate at the amorphous-crystalline boundary. HRTEM has been used to monitor the existence and the size of the annealed isolated tracks in the second stage. On the other hand, the thermal annealing of homogeneous (buried) amorphous layers has been investigated within the same temperature range, on samples irradiated with Fluorine at 20 MeV and fluences of ∼1014 cm−2. Optical techniques are very suitable for this case and have been used to monitor the recrystallization of the layers. The annealing process induces a displacement of the crystalline-amorphous boundary that is also linear with annealing time, and the recrystallization rates are consistent with those measured for tracks. The comparison of these data with those previously obtained for the heavily damaged (amorphous) layers produced by elastic nuclear collisions is summarily discussed

Ion beam induced effects at 15 K in alpha Al2O3 of different orientations

Ion beam induced effects in a Al2O3 of c , a and r orientation were studied by Rutherford backscattering spectrometry RBS in channelling configuration using 1.4 MeV He ions. 150 keV Ar, 150 keV K or 80 keV Na ions were step by step implanted at 15 K followed immediately by the RBS analysis without changing the sample environment. Defect annealing was observed during the RBS measurement, which is attributed to the electronic energy loss of the He ions. A similar effect occurs due to the electronic energy loss of the implanted ions, resulting in a reduced defect concentration between surface and profile maximum. The electronic energy loss of ions may change the charge state of defects thus enhancing their mobility and causing defect annealing. The results suggest that within the collision cascade of individual ions in perfect sapphire only point defects are produced, the concentration of which is well reproduced by SRIM calculations taking into account suggested values of the displacement energies of EAld 20 eV and EOd 50 eV for aluminum and oxygen, respectively. The lower efficiency for point defect production measured in c oriented material can be explained by the heavily reduced visibility of Al atoms sitting on vacant ocahedral sites, which are hidden in this direction. Point defect recombination is observed when the collision cascades start to overlap. Above a critical concentration point defects are altered into clusters which rapidly grow during further irradiation until a saturation is reache

Discrepancy between integral and local composition in off stoichiometric Cu2ZnSnSe4 kesterites A pitfall for classification

High efficiency kesterite based thin film solar cells typically feature Cu poor, Zn rich absorbers although secondary phases occur easily in non stoichiometric Cu2ZnSnSe4. We therefore applied high resolution X ray fluorescence analysis using a synchrotron nanobeam to study the local composition of a CZTSe cross section lamella cut from a sample with an integral composition of Zn Sn 1.37 and Cu Zn Sn 0.55. We find submicrometer sized ZnSe , SnSe SnSe2 , and even CuSe Cu2Se like secondary phases, while the local compositions of the kesterite are highly Zn rich yet barely Cu poor with 1.5 Zn Sn 2.2 and Cu Zn Sn 1.0. Consequently, great care must be taken when relating the integral composition to other material properties including the device performanc

Fine Structure in Swift Heavy Ion Tracks in Amorphous SiO2

International audienceWe report on the observation of a fine structure in ion tracks in amorphous SiO2 using small angle x-ray scattering measurements. Tracks were generated by high energy ion irradiation with Au and Xe between 27 MeV and 1.43 GeV. In agreement with molecular dynamics simulations, the tracks consist of a core characterized by a significant density deficit compared to unirradiated material, surrounded by a high density shell. The structure is consistent with a frozen-in pressure wave originating from the center of the ion track as a result of a thermal spike

Interplay of Performance Limiting Nanoscale Features in Cu2ZnSn S,Se 4 Solar Cells

Highly performing kesterite-based Cu2ZnSn(S,Se)(4)(CZTSSe) thin-film solar cells are typically produced under Cu-poor and Zn-rich synthesis conditions. However, these processing routes also facilitate the formation of secondary phases as well as deviations from stoichiometry, causing intrinsic point defects. Herein, the local composition of CZTSSe absorbers prepared with different nominal cation concentrations is investigated by applying energy dispersive X-ray spectroscopy and synchrotron X-ray fluorescence spectroscopy at the nanoscale to cross-sectional lamellae. The findings confirm the formation of ZnS(Se) secondary phases, whose presence, number, and dimension strongly increase with the reduction of the nominal Cu and increment of the nominal Zn content. Furthermore, the local compositions of the CZTSSe phase within the absorber reveal strong variations, leading to collateral and multiple off-stoichiometry types of the kesterite phase in the absorber, which cause different intrinsic point defects. Therefore, the off-stoichiometry type determined from the integral composition does not represent the complete true picture of this complex material system. Accordingly, the correlation of integral composition with electrical properties or conversion efficiency may be misleading. Overall, the approach provides new experimental insights into the nanoscale relationship among local compositional fluctuations, off-stoichiometry types, and secondary phases in these promising photovoltaic materials