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Niobium-doped TiO 2 films as highly transparent conducting oxides for electrical contacts were investigated. As-deposited films were amorphous and exhibited high resistivities ranging from 10 to 10 5 Ω cm. A slight oxygen deficiency in as-deposited films was essential to gain low resistivities (10 − 3 Ω cm) and low optical absorption coefficients ( α 550 nm 3 cm − 1 ) in the annealed films. Therefore, we controlled the oxygen stoichiometry during the film deposition by adjusting the magnetron discharge voltage, while the oxygen gas flow was kept constant. The Hall mobility of degenerately doped films (electron concentration > 10 20 cm − 3 ) increased with decreasing substrate temperature owing to metal-like phonon scattering in these samples.

CdS and Zn(O,S) grown by chemical bath deposition (CBD) are well established buffer materials for Cu(In,Ga)Se2 (CIGS) solar cells. As recently reported, a non-contiguous coverage of CBD buffers on CIGS grains with {112} surfaces can be detected, which was explained in terms of low surface energies of the {112} facets, leading to deteriorated wetting of the chemical solution on the CIGS surface. In the present contribution, we report on the effect of air annealing of CIGS thin films prior to the CBD of CdS and Zn(O,S) layers. In contrast to the growth on the as-grown CIGS layers, these buffer lay- ers grow densely on the annealed CIGS layer, even on grains with {112} surfaces. We explain the different growth behavior by increased surface energies of CIGS grains due to the annealing step, i.e., due to oxidation of the CIGS surface. Reference solar cells were processed and completed by i-ZnO/ZnO:Al layers for CdS and by (Zn,Mg)O/ZnO:Al for Zn(O,S) buffers. For solar cells with both, CdS and Zn(O,S) buffers, air-annealed CIGS films with improved buffer coverage resulted in higher power-conversion efficiencies, as compared with the devices containing as-grown CIGS layers. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

The widespread application of solar water splitting for energy conversion depends on the progress of photoelectrodes that uphold stringent criteria from photoabsorber materials. After investigating almost all possible elemental and binary semiconductors, the search must be expanded to complex materials. Yet, high structural control of these materials will become more challenging with an increasing number of elements. Complex metal oxides offer unique advantages as photoabsorbers. However, practical fabrication conditions when using glass based transparent conductive substrates with low thermal stability impedes the use of common synthesis routes of high quality metal oxide thin film photoelectrodes. Nevertheless, rapid thermal processing RTP enables heating at higher temperatures than the thermal stabilities of the substrates, circumventing this bottleneck. Reported here is an approach to overcome phase purity challenges in complex metal oxides, showing the importance of attaining a single phase multinary compound by exploring large growth parameter spaces, achieved by employing a combinatorial approach to study CuBi2O4, a prime candidate photoabsorber. Pure CuBi2O4 photoelectrodes are synthesized after studying the relationship between the crystal structures, synthesis conditions, RTP, and properties over a range of thicknesses. Single phase photoelectrodes exhibit higher fill factors, photoconversion efficiencies, longer carrier lifetimes, and increased stability than nonpure photoelectrodes. These findings show the impact of combinatorial approaches alongside radiative heating techniques toward discovering highly efficient multinary photoabsorbers

Determining elemental distributions dependent on the thickness of a sample is of utmost importance for process optimization in different fields e.g. from quality control in the steel industry to controlling doping profiles in semiconductor labs. Glow discharge optical emission spectrometry GD OES is a widely used tool for fast measurements of depth profiles. In order to be able to draw profound conclusions from GD OES profiles, one has to optimize the measurement conditions for the given application as well as to ensure the suitability of the used emission lines. Furthermore a quantification algorithm has to be implemented to convert the measured properties intensity of the emission lines versus sputtering time to more useful parameters, e.g. the molar fractions versus sample depth depth profiles . In this contribution a typical optimization procedure of the sputtering parameters is adapted to the case of polycrystalline Cu In,Ga S,Se 2 thin films, which are used as absorber layers in solar cell devices, for the first time. All emission lines used are shown to be suitable for the quantification of the depth profiles and a quantification routine based on the assumption of constant emission yield is used. The accuracy of this quantification method is demonstrated on the basis of several examples. The bandgap energy profile of the compound semiconductor, as determined by the elemental distributions, is compared to optical measurements. The depth profiles of Na the main dopant in these compounds are correlated with measurements of the open circuit voltage of the corresponding devices, and the quantification of the sample depth is validated by comparison with profilometry and X ray fluorescence measurements

The present tutorial review provides a practical guide to the analysis of semiconductor devices using electron beam induced currents EBIC . The authors focus on cross sectional EBIC measurements that provide an experimental assay of the efficiency of charge carrier collection in a semiconductor diode. The tutorial covers the fundamental physics of the technique, specimen preparation, data acquisition and numerical simulation and analysis of the experimental data. A key focus is put on application cases from the field of thin film photovoltaics as well as specific pitfalls that may occur, such as effects occurring in high level injection and at grain boundaries of polycrystalline materials

Thin film solar cells based on Cu In,Ga Se2 absorber layers have reached conversion efficiencies of well above 20 . One key of this success is the incorporation of alkali metals such as Na and K into the surface and the volume of the Cu In,Ga Se2 thin film. The present work discusses the impact of Na and K on the grain boundary GB properties in Cu In,Ga Se2 thin films, i.e., on the barriers for charge carriers, amp; 934;b, and on the recombination velocities at the GBs, sGB. The authors first revise the physics connected with these two quantities as well as their impact on the device performance, and then provide values for the barrier heights and recombination velocities from the literature. The authors measured sGB values by means of cathodoluminescence analysis of Na K free CIGSe layers as well as on CIGSe layers on Mo sapphire substrates which were submitted to only NaF or only KF PDTs. Overall, passivating effects on GBs by neither Na nor K can be confirmed. The GB recombination velocities seem to remain on the same order of magnitude, in average about 103 104 cm s, irrespective of whether Cu In,Ga Se2 thin films are Na K free or Na K containing

Based on density functional theory calculations, the authors develop a general model for nucleation of III-V semiconductors on vicinal nonpolar (111)-oriented substrates. This model predicts, in particular, that the atomic structure of the step edges at the substrate surface is decisive for the formation and suppression of detrimental rotational twin defects. These predictions are in full agreement with the experimental analysis done on a series of samples with the technologically important material combination of GaP grown on As-modified Si(111). The authors thus derive a complete picture of the formation and suppression of rotational twins relevant for low-defect III-V-on-Si integration.

Cuprous oxide (Cu2O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu2O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu2O surface as well as the interface between Cu2O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu2O and provide guidance for future material development., While cuprous oxide is a promising solar-to-fuel conversion material, photoelectrochemical devices substantially underperform. Here, the authors use femtosecond time-resolved two-photon photoemission spectroscopy to correlate photoexcited electron energetics and dynamics with performance losses.

Inorganic cesium lead halide compounds have gained an increasing interest in the perovskite photovoltaics research community. These compounds are mixed into state of the art organic lead halide perovskite solar cells to provide for more thermal stability, and CsPbX 3 (x=I, Br, Cl) nanocubes are investigated as standalone emitter material in light emitting diodes. Eventually, reproducible, single-phase CsPbBr 3 thin films couldalso provide us with a more stable inorganic material for perovskite solar cells. In the present work, we report on microscopic structural and optoelectronic properties of Cs-Pb-Br thin films prepared by different synthesis methods and studied using various electron-microscopy techniques.

The highest conversion efficiencies for Cu(In,Ga)Se$_{\mathbf {2}}$ (CIGS) solar cells have reached levels of more than 22%. The present case study was performed on a CIGS device with stacking sequence ZnO:Al/(Zn,Mg)O/CdS/Cu(In,Ga)Se$_{\mathbf {2}}$/Mo/glass, of which the co-evaporated CIGS absorber underwent a RbF postdeposition treatment, exhibiting a high conversion efficiency of almost 21% without anti-reflection coating. Electron backscatter diffraction, energy-dispersive X-ray spectrometry (EDX), electron-beam-induced current (EBIC), and cathodoluminescence (CL) measurements were performed on cross-sectional specimens in order to get insight to microstructural, compositional, electrical, and optoelectronic properties. It was found that the average grain size of the CIGS layer was about $0.5 {\mu } \mathrm {m}$. The recombination velocities extracted from CL profiles across grain boundaries were about 5x10$^{\mathbf {3}}$ cm/s in average. From the EBIC results, the width of the space charge region, $w _{\mathbf {SCR}}$, and the minority-carrier diffusion length in the quasi-neutral region, $L _{\mathbf {D}}$, were determined. EDX elemental distribution maps did not show any substantial change in composition in the lateral direction; only the Ga/In gradients perpendicular to the substrate were detected, which can be correlated well with the peak shifts visible in the hyperspectral CL map. In contrast, $w _{\mathbf {SCR}}$ and $L _{\mathbf {D}}$ were found to be inhomogeneously distributed along the $p {-} n$junction, indicating lateral fluctuations in the net doping and the carrier lifetime. The present contribution discusses these fluctuations as possible origin for the limited open-circuit voltage of the CIGS solar cell.

The unprecedented speed at which the performance of solar cells based on solution-processed perovskite thin films has increased, in some ways, appears to violate conventional understanding of device optimization. The relatively poor coverage of the TiO2 electron transport layer by the absorber should cause shunting of the cell. This, however, is not the case. In this paper, it is attempted to explain this “discrepancy.” Insights into coverage, morphology, local elemental composition, and spatially resolved electronic structure of CH3NH3PbI(3-x)Clx perovskite absorbers wet-chemically deposited on planar compact TiO2 electron transport material (ETM) are revealed. Microscopy images indicate an incomplete coverage of the ETM. Depending on the degree of coverage, a variation in iodine oxidation and metallic lead formation is found. With the electronic structure of the absorber and the ETM established experimentally and taking literature on the commonly used hole transport material spiro-MeOTAD into account, it is revealed that excellent charge selectivity occurs at the interfaces between the absorber and both the hole and electron transport layers. It can also be surmised that, crucially, any direct interface between the TiO2 and spiro-MeOTAD would be characterized by a large recombination barrier preventing shunts; to some extent minimizing the negative effects of absorber pinholes.

Chemical and structural phase compositions of two single-crystalline samples prepared with different cooling rates from stoichiometric FeTe 0.6 Se 0.4 melts were studied. Both types of samples were investigated in a very comprehensive way using magnetic and electrical transport measurements combined with X-ray, neutron and electron backscatter diffraction. We show that slowly cooled samples are homogeneous on a microscopic scale with only a small excess of iron. Those slowly cooled samples do not exhibit bulk superconductivity down to 1.8 K. In contrast, fast-cooled samples are superconducting below about 14 K but are composed of several chemical phases: they consist of a matrix preserving the crystal structure of slow-cooled samples, and of core-shell structured dendritic inclusions (about 20–30 vol%). These have different crystal structures and chemical compositions and order magnetically at temperatures far above the superconducting transition temperature of the inhomogeneous samples. These structural and chemical inhomogeneities seem to play a vital role in the superconducting properties of this and similar iron-based systems as they lead to internal stress and act in a similar way as the application of the external pressure that reportedly increase the superconducting transition temperature in many iron pnictides and chalcogenides. We argue that a phase pure, homogeneous and stress-free FeTe 0.6 Se 0.4 is non-superconducting.

In a recent publication by Abou-Ras et al., various techniques for the analysis of elemental distribution in thin films were compared, using the example of a 2-µm thick Cu(In,Ga)Se2 thin film applied as an absorber material in a solar cell. The authors of this work found that similar relative Ga distributions perpendicular to the substrate across the Cu(In,Ga)Se2 thin film were determined by 18 different techniques, applied on samples from the same identical deposition run. Their spatial and depth resolutions, their measuring speeds, their availabilities, as well as their detection limits were discussed. The present work adds two further techniques to this comparison: laser-induced breakdown spectroscopy and grazing-incidence X-ray fluorescence analysis, The work was supported in part by National Research Foundation of Korea (NRF) grant funded by Korea government (MEST, No. 2013- 064113), by the Spanish MINECO within the Ramón y Cajal programme (RYC-2011- 08521), and by the European Metrology Research Program (EMRP) within the projects IND07 Thin Films and ENG53 ThinErgy

High band gap, high open circuit voltage solar cells with methylammonium lead tribromide MAPbBr3 perovskite absorbers are of interest for spectral splitting and photoelectrochemical applications, because of their good performance and ease of processing. The physical origin of high performance in these and similar perovskite based devices remains only partially understood. Using cross sectional electron beaminduced current EBIC measurements, we find an increase in carrier diffusion length in MAPbBr3 Cl based solar cells upon low intensity a few percent of 1 sun intensity blue laser illumination. Comparing dark and illuminated conditions, the minority carrier electron diffusion length increases about 3.5 times from Ln 100 50 nm to 360 22 nm. The EBIC cross section profile indicates a p amp; 8722;n structure between the n FTO TiO2 and p perovskite, rather than the p amp; 8722;i amp; 8722;n structure, reported for the iodide derivative. On the basis of the variation in space charge region width with varying bias, measured by EBIC and capacitance amp; 8722;voltage measurements, we estimate the net doping concentration in MAPbBr3 Cl to be 3 amp; 8722;6 1017 cm amp; 8722;3

The impact of synthesis conditions, post-preparation heating procedure, aging and influence of pressure on the superconducting properties of FeSe 0.4 Te 0.6 crystals is reported. Two FeSe 0.4 Te 0.6 single crystals were used in the study, prepared from stoichiometric melt but cooled down with very different cooling rates, and investigated using magnetic bulk and electrical-resistivity methods. The fast-cooled crystal contains large inclusions of F e 3 Se 2.1 Te 1.8 and exhibits bulk superconductivity in its as-prepared state, while the other is homogeneous and shows only traces of superconductivity. AC susceptibility measurements under hydrostatic pressure show that the superconducting transition temperature of the inhomogeneous crystal increases from 12.3 K at ambient pressure to Tsc = 17.9 K at 9 kbar. On the other hand, neither pressure nor mechanically-induced stress is sufficient to induce superconductivity in the homogeneous crystal. However, an additional heat treatment at 673 K followed by fast cooling down and/or long-term aging at ambient conditions leads to the appearance of bulk superconductivity also in the latter sample. This sample remains homogeneous on a scale down to few μm but shows an additional magnetic phase transition around 130 K suggesting that it must be inhomogeneous. For comparison also F e 3 Se 2.1 Te 1.8 polycrystals have been prepared and their magnetic properties have been studied. It appears that this phase is not superconducting by itself. It is concluded that nano-scale inhomogeneities that appear in the F e S e x Te 1 − x system due to a spinodal decomposition in the solid state are necessary for bulk superconductivity, possibly due to minor changes in the crystal structure and microstructure. Macroscopic inclusions quenched by fast cooling from high temperatures lead obviously to strain and hence variations in the lattice constants, an effect that is further supported by application of pressure/stress.

Cu(In,Ga)Se2 thin-film solar cells exhibit record power-conversion efficiencies of currently 22.6%. Such performance is impressive in view of the rather small average grain sizes of the Cu(In,Ga)Se2 absorber. This work gives insight to the chemistry at linear and planar defects in Cu(In,Ga)Se2 absorber at the nanometer scale by means of atom-probe tomography. Moreover, the tip sample is investigated by transmission electron microscopy prior to the atom-probe tomography experiments to determine the structure of the planar defects. These experimental results are compared with those from theoretical predictions, and consequences for the energy-band diagrams around these planar defects are proposed.

By extracting profiles perpendicular to the substrate from images obtained by electron-beam-induced current measurements on Cu(In,Ga)Se 2 (CIGS) solar cells, fluctuations in the width of the space-charge region as well as in the diffusion length in the quasi-neutral region were studied. Devices with various solution-grown buffer layers as well as different [Ga]/([Ga]+[In]) ratios in the CIGS absorber were investigated. Complementary analyses by means of energy-dispersive X-ray spectrometry, and cathodoluminescence provide additional insight into the local optoelectronic and compositional properties of investigated cells.

The continuous improvement of Cu(In,Ga)Se2 (CIGSe) solar cells relies considerably on advanced characterization of individual layers in the solar-cell stacks as well as of completed CIGSe devices. The present contribution provides an overview of corresponding efforts performed by various research groups at Helmholtz-Zentrum Berlin fur Materialien und Energie GmbH. In-situ growth monitoring of CIGSe absorber layers by means of energy-dispersive X-ray spectrometry and light scattering is described, as well as structural analyses by means of X-ray and neutron diffraction. In addition, the characterization of surfaces and interfaces by soft X-ray and electron spectroscopy, the microscopic analysis by means of correlative electron microscopy, and optoelectronic characterization by optical spectroscopy are highlighted. The present contribution shows which substantial efforts in a research network are necessary in order to obtain deeper insight into materials properties and potentially limiting factors for the device performance, as well as to be able to control these factors during the solar-cell production.

Cu(In,Ga)Se₂ (CIGS)-based solar cells reach high power-conversion efficiencies of above 22%. In this work, a three-stage co-evaporation method was used for their fabrication. During the growth stages, the stoichiometry of the absorbers changes from Cu-poor ([Cu]/([In] + [Ga]) 1) and finally becomes Cu-poor again when the growth process is completed. It is known that, according to the Cu-In-Ga-Se phase diagram, a Cu-rich growth leads to the presence of Cu₂–ₓSe (x = 0–0.25), which is assumed to assist in recrystallization, grain growth, and defect annihilation in the CIGS layer. So far, Cu₂–ₓSe precipitates with spatial extensions on the order of 10–100 nm have been detected only in Cu-rich CIGS layers. In the present work, we report Cu₂–ₓSe platelets with widths of only a few atomic planes at grain boundaries and as inclusions within grains in a polycrystalline, Cu-poor CIGS layer, as evidenced by high-resolution scanning transmission electron microscopy (STEM). The chemistry of the Cu–Se secondary phase was analyzed by electron energy-loss spectroscopy, and STEM image simulation confirmed the identification of the detected phase. These results represent additional experimental evidence for the proposed topotactical growth model for Cu–Se–assisted CIGS thin-film formation under Cu-rich conditions.

The interaction of point defects with extrinsic Frank loops in the photovoltaic absorber material $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ was studied by aberration-corrected scanning transmission electron microscopy in combination with electron energy-loss spectroscopy and calculations based on density-functional theory. We find that Cu accumulation occurs outside of the dislocation cores bounding the stacking fault due to strain-induced preferential formation of ${\mathrm{Cu}}_{\mathrm{In}}^{\ensuremath{-}2}$, which can be considered a harmful hole trap in $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$. In the core region of the cation-containing $\ensuremath{\alpha}$-core, Cu is found in excess. The calculations reveal that this is because Cu on In-sites is lowering the energy of this dislocation core. Within the Se-containing $\ensuremath{\beta}$-core, in contrast, only a small excess of Cu is observed, which is explained by the fact that ${\mathrm{Cu}}_{\mathrm{In}}$ and ${\mathrm{Cu}}_{\mathrm{i}}$ are the preferred defects inside this core, but their formation energies are positive. The decoration of both cores induces deep defect states, which enhance nonradiative recombination. Thus, the annihilation of Frank loops during the $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ growth is essential in order to obtain absorbers with high conversion efficiencies.

We show that monolayer graphene can be grown isothermally on polycrystalline copper foils via ultra-high vacuum chemical vapor deposition (UHV-CVD), using acetylene as a carbon precursor. The growth is self-limiting, yielding monolayer graphene with a quality comparable to that of graphene grown by atmospheric- or low-pressure chemical vapor deposition. Copper sublimation, a typical concern for UHV-CVD, is shown to be suppressed by growing graphene domains. Further, the roughness of the copper surface after growth is similar to that of copper foils after growth processes at higher pressures. A dependency of the growth kinetics on the surface orientation of the copper grains is observed and a growth model including all stages of growth is presented and discussed. Similar to observations at higher growth pressures, the graphene domains possess sigmoidal growth, however the overall growth behavior is more complicated with two subsequent growth modes. The role of hydrogen is investigated and shows that, contrary to reports for higher growth pressures, dissolved hydrogen in the copper foil plays an essential role for graphene growth whereas ambient hydrogen does not have a noticeable influence.

The gallium gradient in Cu(In,Ga)Se2 (CIGS) layers, which forms during the two industrially relevant deposition routes, the sequential and co-evaporation processes, plays a key role in the device performance of CIGS thin-film modules. In this contribution, we present a comprehensive study on the formation, nature, and consequences of gallium gradients in CIGS solar cells. The formation of gallium gradients is analyzed in real time during a rapid selenization process by in situ X-ray measurements. In addition, the gallium grading of a CIGS layer grown with an in-line co-evaporation process is analyzed by means of depth profiling with mass spectrometry. This gallium gradient of a real solar cell served as input data for device simulations. Depth-dependent occurrence of lateral inhomogeneities on the µm scale in CIGS deposited by the co-evaporation process was investigated by highly spatially resolved luminescence measurements on etched CIGS samples, which revealed a dependence of the optical bandgap, the quasi-Fermi level splitting, transition levels, and the vertical gallium gradient. Transmission electron microscopy analyses of CIGS cross-sections point to a difference in gallium content in the near surface region of neighboring grains. Migration barriers for a copper-vacancy-mediated indium and gallium diffusion in CuInSe2 and CuGaSe2 were calculated using density functional theory. The migration barrier for the InCu antisite in CuGaSe2 is significantly lower compared with the GaCu antisite in CuInSe2, which is in accordance with the experimentally observed Ga gradients in CIGS layers grown by co-evaporation and selenization processes. Copyright © 2014 John Wiley & Sons, Ltd.

Nanocrystal-based Cu2Zn(SnyGe1-y)(SxSe4-x) (CZTGeSSe) thin-film solar cell absorbers with tunable band gap have been prepared. Maximum solar-conversion total area efficiencies of up to 9.4% are achieved with a Ge content of 30 at.%. Improved performance compared with similarly processed films of Cu2ZnSn(SxSe4-x) (CZTSSe, 8.4% efficiency) is achieved through controlling Ge loss from the bulk of the absorber film during the high-temperature selenization treatment, although some Ge loss from the absorber surface is still observed following this step. Despite limitations imposed by elemental losses present at the absorber surface, we find that Ge alloying leads to enhanced performance due to increased minority charge carrier lifetimes as well as reduced voltage-dependent charge carrier collection. Copyright © 2013 John Wiley & Sons, Ltd.

We study by means of real time X-ray diffraction the effect of the Cu and Na content on the diffusion of Ga during the formation of Cu(In,Ga)Se 2 films for solar cell applications. We analyze the diffraction data recorded during the annealing of stacks of different compositional ratios. A model for the film formation is suggested, which relies on two distinct steps: accumulation of Ga near the Mo back contact and In–Ga-interdiffusion. The process of Ga-acumulation near the back contact is stronger for the films containing Na. The interdiffusion step starts at about 750 K and is strongest for films with low Na content. We observe that Cu–Se strongly enhances the interdiffusion when using a barrier to prevent Na diffusion from the glass substrate. Microstructural characterization of films with different copper content shows that the steepest Ga-depth-profiles are obtained for a [Cu]/([In]+[Ga]) ratio of about 1.

I Introduction 1. Introduction to thin-film photovoltaics II Device characterization 2. Fundamental electrical characterization of thin-film solar cells 3. Electroluminescence analysis of thin-film solar modules 4. Capacitance spectroscopy of thin-film solar cells III Materials characterization 5. Characterizing the light trapping properties of textured surfaces with scanning near-field optical microscopy 6. Ellipsometry 7. Photoluminescence analysis of Si and chalcopyrite-type thin films for solar cells 8. Steady state photocarrier grating method 9. Time-of-flight analysis 10. Electron Spin Resonance on Si thin films for solar cells 11. Scanning probe microscopy on thin films for solar cells 12. Electron microscopy on thin films for solar cells 13. X-ray and neutron diffraction of materials for thin film solar cells 14. Raman Spectroscopy on thin films for solar cells 15. Soft x-ray and electron spectroscopy: a unique "tool chest" to characterize the chemical and electronic properties of surfaces and interfaces 16. Elemental distribution profiling of thin films for solar cells 17. Hydrogen effusion experiments IV Materials and device modelling 18. Ab-initio modelling of semiconductors 19. One-dimensional electro-optical simulations of thin film solar cells 20. Two-dimensional electrical simulations of thin film solar cells

Microstrain distributions were acquired in functional thin films by high-resolution X-ray microdiffraction measurements, using polycrystalline CuInSe2 thin films as a model system. This technique not only provides spatial resolutions at the submicrometre scale but also allows for analysis of thin films buried within a complete solar-cell stack. The microstrain values within individual CuInSe2 grains were determined to be of the order of 10−4. These values confirmed corresponding microstrain distribution maps obtained on the same CuInSe2 layer by electron backscatter diffraction and Raman microspectroscopy.

Cu(In,Ga)Se $_2$ (CIGSe) absorber layers used in thin-film solar cells exhibit, when grown in a multistage process, compositional gradients of Ga and In. In this study, the correlations between the Ga gradient and the microstructure are studied by means of transmission electron microscopy (TEM) imaging combined with energy-dispersive X-ray spectroscopy (EDX), allowing the determination of structural defects and elemental distributions at identical sample positions. The occurrence of linear defects (dislocations) and planar defects (stacking faults and microtwins) of CIGSe layers was studied by means of TEM images. The Ga distributions obtained from EDX elemental distribution maps and structural parameters from the literature were used to calculate the lattice parameters c and a and the gradient dc/ dx perpendicular to the substrate. We found a correlation between the magnitude of dc/dx and the occurrence of dislocations within individual large grains. From the presented results, a threshold value of the Ga gradient of 12–13at.%/μm can be estimated for the formation of misfit dislocations.

Grain boundaries (GBs) in polycrystalline Cu(In,Ga)Se 2 thin fi lms exhibit only slightly enhanced recombination, as compared with the grain interiors, allowing for very high power-conversion effi ciencies of more than 20% in the corresponding solar-cell devices. This work highlights the specifi c compositional and electrical properties of Cu(In,Ga)Se 2 GBs by application of appropriate subnanometer characterisation techniques: inline electron holography, electron energy-loss spectroscopy, and atom-probe tomography. It is found that changes of composition at the GBs are confi ned to regions of only about 1 nm in width. Therefore, these compositional changes are not due to secondary phases but atomic or ionic redistribution within the atomic planes close to the GBs. For different GBs in the Cu(In,Ga)Se 2 thin fi lm investigated, different atomic or ionic redistributions are also found. This chemical fl exibility makes polycrystalline Cu(In,Ga)Se 2 thin fi lms particularly suitable for photovoltaic applications.

Compound semiconductors belong to the most important materials for optoelectronic applications. Many of them exhibit favorable optical properties, such as a direct energy band gap (in contrast to silicon) and high-absorption coefficients over a wide spectral range. Moreover, varying the composition of the compound or substituting some of its elements often allows for controlled band gap engineering and optimization for specific applications. Because many compound semiconductors enable efficient conversion of light into electricity and vice versa, they are commonly used materials for optoelectronic devices.

In this letter, we report externally confirmed total area efficiencies reaching up to 12.9% for CdS/Cu(In,Ga)S2 based solar cells. These are the highest externally confirmed efficiencies for such cells. The absorbers were prepared from sputtered metals subsequently sulfurized using rapid thermal processing in sulfur vapor. Structural, compositional, and electrical properties of one of these champion cells are presented. The correlation between the Ga distribution profile and solar cell properties is discussed. Copyright © 2012 John Wiley & Sons, Ltd.

This paper discusses the advantages of pulsed laser atom-probe tomography (APT) to analyze Cu(In,Ga)Se2-based solar cells. Electron backscatter diffraction (EBSD) was exploited for site-specific preparation of APT samples at selected Cu(In,Ga)Se2 grain boundaries. This approach is very helpful not only to determine the location of grain boundaries but also to classify them as well. We demonstrate that correlative transmission electron microscopy (TEM) analyses on atom-probe specimens enable the atom-probe datasets to be reconstructed with high accuracy. Moreover, EBSD and TEM can be very useful to obtain complementary information about the crystal structure in addition to the compositional analyses. The local chemical compositions at grain boundaries of a solar grade Cu(In,Ga)Se2 film are presented here. Na, K, and O impurities are found to be segregated at grain boundaries. These impurities most likely diffuse from the soda lime glass substrate into the absorber layer during cell fabrication and processing. Based on the experimental results, we propose that Na, K, and O play an important role in the electrical properties of grain boundaries in Cu(In,Ga)Se2 thin films for solar cells.

Microscopy and Microanalysis, 17 (5), ISSN:1431-9276, ISSN:1435-8115

We explore here the possibilities of using highly absorbing chalcopyrite semiconductors of the type Cu(In,Ga)Se2 in a quantum well solar cell structure. Thin alternating layers of 50 nm CuInSe2 and CuGaSe2 were grown epitaxially on a GaAs(100) substrate employing metalorganic vapor phase epitaxy. The optical properties of a resulting structure of three layers were investigated by photoluminescence and photoreflectance, indicating charge carrier confinement ∗To whom correspondence should be addressed †Helmholtz-Zentrum Berlin ‡Universidad Politecnica de Madrid ¶University of Illinois §University of Jyvaskyla ‖Current address: Universitat des Saarlandes, Uni Campus, Gebaude A5.1, 66123 Saarbrucken Germany

Despite many recent research efforts, the influence of grain boundaries (GBs) on device properties of CuIn1−xGaxSe2 solar cells is still not fully understood Here, we present a microscopic approach to characterizing GBs in polycrystalline CuIn1−xGaxSe2 films with x = 0.33. On samples from the same deposition process we applied methods giving complementary information, i.e., electron backscatter diffraction (EBSD), electron-beam induced current measurements (EBIC), conductive atomic force microscopy (c-AFM), variable-temperature Kelvin probe force microscopy (KPFM), and scanning capacitance microscopy (SCM). By combining EBIC with EBSD, we find a decrease in charge-carrier collection for non-∑3 GBs, while ∑ 3 GBs exhibit no variation with respect to grain interiors. In contrast, a higher conductance of GBs compared to grain interiors was found by c-AFM at low bias and under illumination. By KPFM, we directly measured the band bending at GBs, finding a variation from − 80 up to + 115 mV. Depletion and even inversion at GBs was confirmed by SCM. We comparatively discuss the apparent differences between the results obtained by various microscopic techniques.

Recrystallization processes during the sulfurization of CuInS 2 (CIS) thin films have been studied in-situ using energy dispersive X-ray diffraction (EDXRD) with synchrotron radiation. In order to observe the recrystallization isolated from other reactions occurring during film growth, Cu-poor, small grained CIS layers covered with CuS on top were heated in a vacuum chamber equipped with windows for synchrotron radiation in order to analyze the grain growth mechanism within the CIS layer. In-situ monitoring of the grain size based on diffraction line profile analysis of the CIS-112 reflection was utilized to interrupt the recrystallization process at different points. Ex-situ studies by electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDX) performed on samples of intermediate recrystallization states reveal that during the heat treatment Cu and In interdiffuse inside the layer indicating the importance of the mobility of these two elements during CuInS 2 grain growth.

The recrystallization of Cu–In–S thin films has been monitored in real time by means of synchrotron-based energy-dispersive X-ray diffraction. To trigger recrystallization, nanocrystalline Cu–In–S layers with [Cu]/[In] 1. The bilayer films were heated to 773 K and the evolution of the microstructure was monitoredin situ viadiffraction spectra. In the first step of the analysis, the diffraction data were used to identify solid-state phase transitions as a function of temperature. In a further step, single-line profile analysis of the 112 CuInS2reflection was used to study grain growth in this material system. The recrystallization was investigated under two sulfur pressure conditions and for different [Cu]/[In] ratios. The recrystallization is composed of three steps: consumption of the CuIn5S8phase, grain growth, and a transition from the Cu–Au-type to the chalcopyrite-type structure of CuInS2. Increasing the sulfur pressure during heating systematically reduces the temperature at which grain growth sets in. Various paths to control the recrystallization of Cu–In–S thin films are proposed.

In the present work, the effect of the copper incorporation into the In-Ga-Se precursor layer during the stage 2 of the three stage growth process of Cu(InGa)Se2 thin films is investigated. Break-off experiments at certain points during stage 2 were performed, corresponding to different [Cu]/([Cu]+[In]+[Ga]) ratios. The elemental distribution in the layers at the break-off points was investigated by energy-dispersive X-ray spectroscopy on cross-sections of the layers. Although stage 1 includes a sequential deposition of In-Se and Ga-Se layers, these layers appear completely mixed in the beginning of stage 2, i.e., compositional gradients are absent, after the deposition of copper and selenium at a substrate temperature of 525 °C during this stage. However, the deposition of copper and selenium at lower substrate temperatures and for short deposition durations, leading to [Cu]/([Cu]+[In]+[Ga]) ratios much smaller than 0.5, results in thin films in which the sequential order of the precursor layers remains clearly visible in the EDX profiles. Grazing incidence X-ray diffraction measurements were carried out to investigate the formation of different phases, e.g., that of ordered vacancy compounds, during the incorporation of copper into the In-Ga-Se precursor layers. The influence of the copper incorporation on the Cu(In,Ga)Se2 lattice parameter is discussed. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We study some physical properties of CuIn 1―x Ga x Se 2 thin-films fabricated by evaporation from elemental sources under various Selenium environments. Specifically, thin-films were fabricated under growth conditions such as Se deficiency, near stoichiometry and excess Se during coevaporation to investigate the impact of the Se environment on absorber film properties and ultimately the device performance. We determine the chemical activity of Se in the evaporation process has a strong influence on film macrostructure (prefered orientation) and microstructure, particularly at the grain and grain boundary level. It is shown that the optoelectronic properties at grain boundaries are affected by the Se environment used resulting in absorber thin-films with distictive defect distribution and defect density. Consequently, the performance of the solar cells fabricated from those films is also affected by the Se environment. These effects on solar cell performance and absorber properties are reported in a (i) structural analysis of the CuIn 1―x Ga x Se 2 /Mo/glass samples by X-ray and electron backscattering techniques; (ii) optolectronic radiative characteristics of the absorbers by cathode luminescence and photoluminescence studies and (iii) current―voltage, quantum efficiency and capacitance―voltage measurements for the solar cells made from the absorbers fabricated under the pre prescribed Se growth conditions.