Search

We report on the generation of a passive carrier-envelope phase (CEP) stable 1.7-cycle pulse in the mid-infrared by adiabatic difference frequency generation. With sole material-based compression, we achieve a sub-2 cycle 16 fs pulse at a center wavelength of 2.7 μm and measured a CEP stability of

Photo and charge carrier induced ion migration is a major challenge when utilizing metal halide perovskite semiconductors for optoelectronic applications. For mixed iodide bromide perovskites, the compositional instability due to light or electrical bias induced phase segregation restricts the exploitation of the entire bandgap range. Previous experimental and theoretical work suggests that excited states or charge carriers trigger the process, but the exact mechanism is still under debate. To identify the mechanism and cause of light induced phase segregation phenomena, the full compositional range of methylammonium lead bromide iodide samples are investigated, MAPb BrxI1 x 3 with x 0 1, by simultaneous in situ X ray diffraction XRD and photoluminescence PL spectroscopy during illumination. The quantitative comparison of composition dependent in situ XRD and PL shows that at excitation densities of 1 sun, only the initial stage of photo segregation is rationalized with the previously established thermodynamic models. However, a progression of the phase segregation is observed that is rationalized by considering long lived accumulative photo induced material alterations. It is suggested that additional photo induced defects, possibly halide vacancies and interstitials, need to be considered to fully rationalize light induced phase segregation and anticipate the findings to provide crucial insight for the development of more sophisticated models

In this communication, we present the phase diagram of CsPb BrxI1 amp; 8722;x 3 0 amp; 8804; x amp; 8804; 1, 300 585 K obtained by high throughput in situ GIWAXS measurements of a combinatorial thin film library. We find that all compositions convert to the cubic perovskite phase at high temperature and that the presence of bromide in the films stabilizes the metastable perovskite phases upon cool down. In accordance with recent predictions from DFT calculations, the transition temperatures monotonically decrease with increasing bromide content

To optimize the opto-electronic properties of compound semiconductors, a detailed understanding and control of compositional gradients forming during their synthesis is crucial. A common fabrication process for $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ (CIGS) thin-film solar cells uses annealing at high temperatures, which---contrary to what could be expected from simple Fickian diffusion---results in the formation of steep and stable $\mathrm{Ga}$ gradients, deviating from the optimal $\mathrm{Ga}$ profile for high-efficiency CIGS absorbers. Here, we show that the formation of elastic stresses inside the material during the interdiffusion can have a profound effect on the final $\mathrm{Ga}$ distribution, resulting in persistent $\mathrm{Ga}$ gradients inside CIGS absorber layers. A comparison of numerical diffusion and stress-formation calculations with real-time synchroton-based energy-dispersive x-ray diffraction data acquired in-situ during selenization of CIGS thin films demonstrates that the model can reproduce the stagnation of $\mathrm{In}$-$\mathrm{Ga}$ interdiffusion. We discuss that a detailed understanding of the interplay between stress and diffusion processes in thin films may open alternative fabrication strategies for generating desired stable compositional gradients to improve the opto-electronic properties of compound semiconductors, such as chalcopyrite, kesterite, and perovskite solar cell absorbers for solar cells.

The presence of in-gap states, especially deep ones, that act as recombination centers in solar-cell absorbers influence the phototransport properties of the films, and thus, strongly affect the performance of the corresponding solar cells. The type and density of such defects is influenced by the deposition process. In particular, for $\mathrm{Cu}({\mathrm{In}}_{1\text{\ensuremath{-}}x}{\mathrm{Ga}}_{x}){\mathrm{Se}}_{2}$, it is well established that the presence of a small amount of alkali metal atoms (e.g., $\mathrm{Na}$) improves the performance of the solar cell. Furthermore, in a coevaporation process, a $\mathrm{Cu}$-rich [[$\mathrm{Cu}$]/([$\mathrm{In}$] + [$\mathrm{Ga}$]) g 1] intermediate step decreases the density of extended structural defects, such as stacking faults. Here, we apply temperature-dependent and intensity-dependent photoconductivity measurements, along with Shockley-Read-Hall model calculations, to study the phototransport properties of $\mathrm{Cu}({\mathrm{In}}_{1\text{\ensuremath{-}}x}{\mathrm{Ga}}_{x}){\mathrm{Se}}_{2}$ films with and without a $\mathrm{Cu}$-rich process step, as well as with and without $\mathrm{Na}$. Our experimental and theoretical results indicate a correlation between the presence of planar defects and the formation of a shallow recombination level. Our results further suggest that $\mathrm{Na}$ eliminates deep recombination centers, and thus, increases the lifetime of the photogenerated charge carriers.

The properties of electronic defects and their relation to structural defects are of high relevance for $\mathrm{CuInS}{\mathrm{e}}_{2}$ photovoltaic absorbers. Here, we use Raman scattering and steady-state photoluminescence to study the intrinsic optoelectronic properties of near-stoichiometric $\mathrm{CuInS}{\mathrm{e}}_{2}$ samples with a lateral composition gradient around the Cu saturation point. Apart from a well-known shallow defect band at 0.97 eV, we also observe a deep defect band at 0.8 eV, which is not discernable in photoluminescence spectra at lower temperatures. The preparation of a laterally graded sample with a very precise relative composition range by in situ process control allows for a measurement of a significant decrease of the photoluminescence emission yield at the Cu-poor/Cu-rich transition on a very narrow composition scale. Possible assignments of the bands to structural point defects are discussed.

Significant structural evolution occurs during the deposition of ${\mathrm{CuInSe}}_{2}$ solar materials when the Cu content increases. We use in situ heating in a scanning transmission electron microscope to directly observe how grain boundaries migrate during heating, causing nondefected grains to consume highly defected grains. Cu substitutes for In in the near grain boundary regions, turning them into a Cu-Se phase topotactic with the ${\mathrm{CuInSe}}_{2}$ grain interiors. Together with density functional theory and molecular dynamics calculations, we reveal how this Cu-Se phase makes the grain boundaries highly mobile.

The selenization of metallic Cu-Zn-Sn-Ge precursors is a promising route for the fabrication of low-cost and efficient kesterite thin film solar cells. Nowadays, efficiencies of kesterite solar cells are still below 13 %. For Cu(In,Ga)Se2 solar cells, the formation of compositional gradients along the depth of the absorber layer has been demonstrated to be a key requirement for producing thin film solar cells with conversion efficiencies above the 22 % level. No clear understanding has been reached so far about how to produce these gradients in an efficient manner for kesterite compounds, but among the possible candidates, Ge arises as one of the most promising ones. In the present work, we evaluate the potential of incorporating Ge in Cu2ZnSnSe4 to produce compositional gradients in kesterites. Synchrotron-based in-situ energy-dispersive X-ray diffraction and X-ray fluorescence has been used to study the selenization of Cu-Zn-Sn-Ge metallic precursors. We propose a reaction mechanism for the incorporation of Ge atoms into the kesterite lattice after the formation of Cu2ZnSnSe4. Electron microscopy reveals that the annealing process leads to Cu2Zn(Sn,Ge)Se4 absorber layers with an increase of Ge content towards the back contact with independence of the original location of Ge in the precursor layer. The effect of the Ge gradient on the optoelectronic properties of the absorber layer has been evaluated with room temperature cathodoluminescence. The implications of the results for the development of kesterite solar cells are discussed, with the aim of encouraging new synthesis routes for compositionally graded absorbers.

Sequential Cu(In,Ga)Se 2 fabrication with a thermally activated reaction of Cu-In-Ga metal precursor layers in chalcogen atmosphere is an industrially attractive route for preparation of Cu(In,Ga)Se 2 absorber based thin film solar cells. Recent results show that controlling the selenium supply during rapid thermal processing has a huge impact on absorber growth. Especially a two stage process applying a first annealing step with or without Se at temperatures up to 400 °C was shown to have a positive effect on the elemental in-depth distribution. However, during this annealing, lateral phase separation, dewetting and coarsening may occur in the metal phase, leading to lateral non-uniformity of the absorber. In this study we show how the dewetting can be strongly decreased by adjusting the precursor architecture, applying faster heating rates and NaF addition on top of a precursor. In contrast, NaF deposited underneath the precursor increases the dewetting effect. Further we show that lateral phase separation during annealing increases with temperature and leads to phase domain sizes of several micrometers at 580 °C.

We study the Cu(In,Ga)Se2 absorber fabrication by fast atmospheric pressure selenization of metal precursor films utilizing elemental selenium vapor from independent sources. We find that a high Se supply during selenization leads to the generally observed Ga accumulation at the back contact, while a reduced Se supply leads to a more homogeneous Ga distribution within the absorber. By optimizing the precursor as well as the Ga in‐depth distribution in the CIGSe layer, a conversion efficiency of 15.5% was achieved.

Significant structural evolution occurs during the deposition of CuInSe2 solar materials when the Cu content increases. We use in situ heating in a scanning transmission electron microscope to directly observe how grain boundaries migrate during heating, causing nondefected grains to consume highly defected grains. Cu substitutes for In in the near grain boundary regions, turning them into a Cu-Se phase topotactic with the CuInSe2 grain interiors. Together with density functional theory and molecular dynamics calculations, we reveal how this Cu-Se phase makes the grain boundaries highly mobile.

To explain the persistence of stable Ga gradients inside thin-film Cu(In,Ga)Se 2 absorber layers, we propose to consider the inter-diffusion of In and Ga inside those layers and the subsequent formation of stresses. To substantiate this proposition we developed a theoretical model and performed numerical calculations of the inter-diffusion process and subsequent stress formation, showing that inter-diffusion will come to a halt before total intermixing of Ga and In occurred if the energy released by the diffusion process does not suffice to perform the elastic work required to deform the lattice and if no additional energy is released, e.g. by mechanisms of stress reduction through plastic deformation.

Earth-abundant copper barium thioselenostannate, $\mathrm{C}{\mathrm{u}}_{2}\mathrm{BaSn}{(\mathrm{S},\mathrm{Se})}_{4}$, absorbers have recently demonstrated promising optoelectronic and defect resistance properties for solar harvesting applications. The highest photovoltaic device efficiencies have been achieved in vacuum-based co-sputter deposited films, yet there is a tendency for a multilayer formation consisting of large, plateletlike surface grains and a smaller grain-sized underlayer (often accompanied by voids). In this work we use a combination of in situ and ex situ x-ray diffraction and scanning electron microscopy to unravel the coupling of phase evolution to film morphology. We find that Cu surface migration and associated Cu-rich phases play a defining role in determining the overall film structure.

High-efficiency Cu(In,Ga)Se2 (CIGS) thin-film solar cells are based on polycrystalline CIGS absorber layers, which contain grain boundaries, stacking faults, and dislocations. While planar defects in CIGS layers have been investigated extensively, little is still known about the impact of dislocations on optoelectronic properties of CIGS absorbers. Herein, evidence for an optoelectronic inactivity of dislocations in these thin films is given, in contrast to the situation at grain boundaries. This unique behavior is explained by the extensive elemental redistribution detected around dislocation cores, which is connected with the dislocation strain field, probably leading to a shift of defect states toward the band edges.

In this study, Cu2ZnSnS4 (CZTS) absorber layers were grown by sequential deposition of metallic Cu, Zn, and Sn layers by DC magnetron sputtering on Mo coated glass followed by annealing in sulfur atmosphere in a rapid thermal processing (RTP) system for 3 min at 500 °C, 550 °C and 600 °C. Two different metal stacking sequences were investigated: Mo/Zn/Sn/Cu and Mo/Cu/Sn/Zn/Cu – both with a Cu-poor and Zn-rich composition. For the Mo/Cu/Sn/Zn/Cu stack, the initial Mo/Cu/Sn layer was pre-annealed to yield homogeneous Cu Sn alloys on which Zn and the final Cu was deposited afterwards. Compositional inhomogeneities were studied by measuring X-ray fluorescence (XRF) before and after KCN etching of the sulfurized samples as well as by glow discharge optical emission spectroscopy (GDOES). Additionally, crystallographic properties were analyzed by X-ray diffraction (XRD) and Raman spectroscopy. For the Mo/Zn/Sn/Cu precursor, we find that the initial stacking order is partially preserved with a Cu-rich surface and a Zn-rich bottom. In contrast, the modified stacking Mo/Cu/Sn/Zn/Cu leads to an improved compositional homogeneity. While a Zn-rich composition is still found at the bottom part of the film, after sulfurization for 3 min at 600 °C the surface and bulk part show homogeneous element distributions close to that of CZTS.

The traceability of measured quantities is an essential condition when linking process control parameters to guaranteed physical properties of a product. Using Raman spectroscopy as an analytical tool for monitoring the production of Cu(In1–xGax)Se2 thin-film solar cells, proper calibration with regard to chemical composition and lateral dimensions is a key prerequisite. This study shows how the multiple requirements of calibration in Raman microscopy might be addressed. The surface elemental composition as well as the integral elemental composition of the samples is traced back by reference-free X-ray fluorescence analysis. Reference Raman spectra are then generated for the relevant Cu(In1–xGax)Se2 related compounds. The lateral dimensions are calibrated with the help of a novel dimensional standard whose regular structures have been traced back to the International System of Units by metrological scanning force microscopy. On this basis, an approach for the quantitative determination of surface coverage values from lateral Raman mappings is developed together with a complete uncertainty budget. Raman and X-ray spectrometry have here been proven as complementary nondestructive methods combining surface sensitivity and in-depth information on elemental and species distribution for the reliable quality control of Cu(In1-xGax)Se2 absorbers and Cu(In1-xGax)3Se5 surface layer formation.

A treatment of CuInS2 (CIS) based on rapid thermal processing (RTP) selenization is developed, aiming at tuning the absorber's band gap grading using the [Se]/([S] + [Se]) composition. X-ray photoelectron spectroscopy and X-ray fluorescence analysis measurements of RTP-treated CIS samples (with the used set of RTP-parameter ranges) show a greater treatment effect at the surface of the sample compared to the bulk. A tuning of the [Cu] : [In] : ([S] + [Se]) surface composition from a Cu-poor 1 : 3 : 5 to a 1 : 1 : 2 stoichiometry is also observed in RTP-treated CIS absorbers with lower to higher surface Se contents, respectively. Ultraviolet photoelectron spectroscopy measurements show a shift in valence band maximum toward the Fermi level, EF, in higher surface Se content samples [from (−0.88 ± 0.1) to (−0.51 ± 0.1) eV], as expected for a reduction of the (surface) band gap produced by exchanging S with Se. Ultraviolet-visible spectrophotometry reveals a reduction in the optical (bulk) band gap of samples with greater Se incorporation [from (1.47 ± 0.05) to (1.08 ± 0.05) eV], allowing for a working window for optimization purposes.

Buried wurtzite structures composed by stacking faults of the 111 planes in zinc blende and 112 planes in chalcopyrite structures can result in barriers for charge carrier transport. A precise understanding of stacking fault annihilation mechanisms is therefore crucial for the development of effective deposition processes. During co evaporation of Cu In,Ga Se2 a photovoltaic absorber material showing record efficiencies of up to 22.9 for thin film solar cells a reduction of stacking faults occurs at the transition from a Cu poor to a Cu rich film composition, parallel to grain growth, which is suggesting that the two phenomena are coupled. Here, we show by in situ synchrotron X ray diffraction during annealing of Cu poor CuInSe2 thin films that stacking faults can be strongly reduced through annealing, without passing through a Cu rich film composition. We simulate the evolution of the X ray diffraction stacking fault signal with a simple numerical model of grain growth driven by stacking fault energy and grain boundary curvature. The results support the hypothesis that the stacking fault reduction can be explained by grain growth. The model is used to make predictions on annealing times and temperatures required for stacking fault reduction and could be adapted for polycrystalline thin films with similar morphology

The bandgap of CuInSe2 thin film photovoltaic absorbers depends on the Cu content, although the nature of this dependence is still a matter of debate. While theoretical results predicted a widening or stable bandgap with decreasing Cu content, the few experimental data available point to a narrowing of the bandgap. Here, we apply photoreflectance spectroscopy at room temperature to near-stoichiometric polycrystalline CuInSe2/CdS heterojunctions with a lateral Cu gradient to analyze the electronic transitions in the vicinity of the fundamental absorption edge of CuInSe2 absorber as a function of Cu deficiency. The results indicate that the lowest bandgap transition at 1.02 eV notably decreases by 20–30 meV for slightly Cu deficient samples, strengthening the case for an association of a lower Cu content with a narrower bandgap. In contrast, the higher energy transition at 1.25 eV does not show a redshift, which requires further theoretical explanation.The bandgap of CuInSe2 thin film photovoltaic absorbers depends on the Cu content, although the nature of this dependence is still a matter of debate. While theoretical results predicted a widening or stable bandgap with decreasing Cu content, the few experimental data available point to a narrowing of the bandgap. Here, we apply photoreflectance spectroscopy at room temperature to near-stoichiometric polycrystalline CuInSe2/CdS heterojunctions with a lateral Cu gradient to analyze the electronic transitions in the vicinity of the fundamental absorption edge of CuInSe2 absorber as a function of Cu deficiency. The results indicate that the lowest bandgap transition at 1.02 eV notably decreases by 20–30 meV for slightly Cu deficient samples, strengthening the case for an association of a lower Cu content with a narrower bandgap. In contrast, the higher energy transition at 1.25 eV does not show a redshift, which requires further theoretical explanation.

To gain an in-depth understanding of the mechanisms and driving forces responsible for the microstructural changes during the Cu-Se deposition of the three-stage Cu(In,Ga)Se 2 co-evaporation process and its effect on the absorber quality, we have performed a series of in-situ studies as well as complementing ex-situ studies in the recent past. Our studies provide insights into several correlating changes of the structural and physical properties of the absorber layer during Cu-Se deposition or subsequent annealing, such as defect annihilation, stress relaxation, texture changes, and grain growth. In this contribution, we combine these various observations and discuss how they can be explained by a model of abnormal grain growth, driven by differences in the defect densities of neighboring grains.

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.

In this study, kesterite Cu2ZnSnS4 (CZTS) absorber layers were fabricated by DC magnetron sputtering deposition of metallic Cu-Zn-Sn precursors, followed by an annealing treatment in sulfur vapor atmosphere at 600 °C for 3 min using rapid thermal processing (RTP). Three types of stacked metallic films were prepared and included pre-annealing of Cu-Sn stacks in order to induce preferential Cu-Sn alloying. The chemical composition of the sulfurized films was obtained by X-ray fluorescence (XRF) before and after etching the samples in KCN solution. All CZTS thin films are found to be Cu-poor and Zn-rich. Structural characterizations were performed by X-ray diffraction (XRD) and Raman spectroscopy to investigate the impact of pre-annealing on the structural properties of the precursors and final CZTS films. Glow discharge optical emission spectroscopy (GDOES) shows that pre-annealing of the precursors can improve depth homogeneity of the CZTS films. Photoluminescence spectra and the optical band gap energy values are compatible with literature. Selected samples were processed to solar cells and characterized., Turkish Council of Higher Education (YOK)

Ga segregation at the backside of Cu(In,Ga)Se2 solar cell absorbers is a commonly observed phenomenon for a large variety of sequential fabrication processes. Here, we investigate the correlation between Se incorporation, phase formation and Ga segregation during fast selenisation of Cu–In–Ga precursor films in elemental selenium vapour. Se incorporation and phase formation are analysed by real-time synchrotron-based X-ray diffraction and fluorescence analysis. Correlations between phase formation and depth distributions are gained by interrupting the process at several points and by subsequent ex situ cross-sectional electron microscopy and Raman spectroscopy. The presented results reveal that the main share of Se incorporation takes place within a few seconds during formation of In–Se at the top part of the film, accompanied by outdiffusion of In out of a ternary Cu–In–Ga phase. Surprisingly, CuInSe2 starts to form at the surface on top of the In–Se layer, leading to an intermediate double graded Cu depth distribution. The remaining Ga-rich metal phase at the back is finally selenised by indiffusion of Se. On the basis of a proposed growth model, we discuss possible strategies and limitations for the avoidance of Ga segregation during fast selenisation of metallic precursors. Solar cells made from samples selenised with a total annealing time of 6.5 min reached conversion efficiencies of up to 14.2 % (total area, without anti-reflective coating). The evolution of the Cu(In,Ga)Se2 diffraction signals reveals that the minimum process time for high-quality Cu(In,Ga)Se2 absorbers is limited by cation ordering rather than Se incorporation. Copyright © 2014 John Wiley & Sons, Ltd.

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.

We use synchrotron-based energy-dispersive X-ray diffraction to study in real-time the phase transformations during the incorporation of thermally evaporated In into a thin Cu film at 770 K. Most of the phase transitions are in agreement with the Cu–In phase diagram. We measure a linear increase of the relative lattice expansion of the α -Cu(In) and δ -Cu 7 In 3 phases as In is incorporated into them. A radical change in the preferred orientation of the grains is observed at an In concentration of 37 at.% which we explain with the appearance of a liquid phase. In contrast to the phase diagram, we do not observe any crystalline phases for In concentrations higher than 52 at.%.

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.

Thin film solar cells based on co-evaporated Cu(In,Ga)Se2 absorber films present the highest efficiencies among current polycrystalline thin-film technologies. Thanks to the development of a novel experimental setup for in situ growth studies, it was possible to follow the formation of the crystalline phases during such deposition processes for the first time. This synchrotron-based energy-dispersive X-ray diffraction and fluorescence setup is suited for real-time studies of thin film vapor deposition processes. Focusing on the growth of CuInSe2 and CuGaSe2 fabricated by three-stage processing, we find that the phase transitions in the Cu-In-Se system follow the reported pseudo-binary In2Se3-Cu2Se phase diagram. This requires a transformation of the Se sublattice during the incorporation of Cu-Se into the In2Se3 precursor film from the first process stage. In the Cu-Ga-Se system, besides an increase in the lattice spacings, we observe no transformation of the Se sublattice. Furthermore, the structural defects of the Ga-Se precursor film are preserved until the CuGaSe2 stoichiometry is reached. By means of model calculations of the fluorescence signals, we confirm in both systems the segregation of Cu2Se at the surface near a concentration of 25 at.% Cu shortly after the recrystallization of the films. The modeling also reveals that Cu2Se penetrates into the CuInSe2 film, whereas it remains at the surface of the CuGaSe2 film.

Optical and electrical properties of complex semiconducting alloys like Cu(In,Ga)Se2 (CIGS) are strongly influenced by the reaction pathways occurring during their deposition process. This makes it desirable to observe and control these properties in real-time during the deposition. Here we show for the first time the evolution of the band gap and the sub-band-gap defect absorption of CIGS thin film as well as surface roughness during a three-stage co-evaporation process by means of an optical analysis technique, based on white light reflectometry (WLR). By simultaneously recording structural information with in-situ energy dispersive X-ray diffraction and X-ray fluorescence we can directly correlate the evolution of opto-electronic material parameters with the structural properties of the film during growth. We find that the surface roughness and the sub-gap light absorption can be correlated with the phase evolution during the transformation from (In,Ga)2Se3 to Cu(In,Ga)Se2 by the incorporation of Cu into the film. Sub-bandgap light absorption is found to be influenced by the Cu-saturated growth phase and is lowered close to the points of stoichiometry, allowing for an advanced process design.

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.

In view of the complexity of compound semiconductor based thin-film solar cells, which are comprised of a multitude of layers, interfaces, surfaces, elements, impurities, etc., it is crucial to characterize and understand the structural, chemical, and electronic properties of these components. Hence, this paper gives a review of our recent progress in the characterization of compound semiconductor thin films using synchrotron-based characterization methods. It is demonstrated how different analytical techniques are extraordinarily powerful to reveal the material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of these properties. Light will be shed on structural phase transitions, reactive thin-film formation mechanisms, surface off-stoichiometries, and the electronic structure of chalcopyrite and kesterite compound semiconductors. Copyright © 2012 John Wiley & Sons, Ltd.

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

Rapid thermal sulfurization of metallic precursors has proven to be a successful method for the preparation of Cu(In,Ga)S 2 based solar cells. However, during the sulfurization, several problems can be encountered. Due to the difference in reaction rates between ternary sulfides, the process can result in absorbers with a layered CuInS 2 /CuGaS 2 structure or slow and incomplete sulfurization that leads to samples where an unreacted Cu–Ga metallic phase remains at the back of the sample. The formation kinetics of single phase Cu(In,Ga)S 2 is a complex process which depends on several parameters. In this work, we focus on the influence of precursor stacking and investigate the growth of Cu(In,Ga)S 2 thin films using scanning electron microscopy and X-ray diffraction. It is observed that precursor alloying occurs prior to sulfurization and that the Cu(In,Ga)S 2 compound is formed by the interdiffusion of the ternary CuInS 2 and CuGaS 2 phases. Correlation between the structural properties of the precursors/absorbers and the obtained solar cells is made.

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.

Starting from a small area cell published in 1993, CuInS2 technology has been continuously improved with respect to performance and manufacturability. Major milestones include successful preparation by rapid thermal processing, a monolithically integrated module test structure on a 5×5 cm2 substrate, implementation of an industrial pilot line, incorporation of gallium for higher open circuit voltages and better performance and demonstration of Cd-free devices. Phase formation, reaction pathways and interdiffusion mechanisms have been investigated and modelled as have been electronic and device properties such as current transport. This review summarizes the most significant aspects of development and our current understanding of the technology.

A Cu(In,Ga)S 2 -based solar cell with a confirmed efficiency of 12.6% together with an open circuit voltage of 879 mV, prepared from sputtered metals subsequently sulfurized using rapid thermal processing in sulfur vapor, is reported. The performance of the new cell is superior to those obtained previously with multi-source evaporated absorbers. We show that by carefully adjusting the temperature profile, good absorber properties could be transferred from a long process to a rapid thermal process. The improved efficiency is due to an appropriate degree of gallium diffusion toward the surface, which could be achieved despite the short sulfurization time. Absorber and solar cell characteristics are presented.

The understanding of the interplay between structural and electronic properties of photovoltaic materials as well as a deeper insight into growth pathways and phase formation kinetics of the absorber layer in a thin film solar cell give a crucial contribution to the continuous improvement of the solar cell efficiency. Among the various experimental methods used for the investigation of the structure and microstructure of photovoltaic materials, neutron, and X-ray (photon) scattering are key techniques of choice. Both techniques are complementary, which is demonstrated in the present paper. Neutron powder diffraction is used to detect different kinds of intrinsic point defects in chalcopyrite type and kesterite type semiconductors. The calculated defect concentrations may lead to the expectation of a clustering of anti-site defects and vacancy to the electrical inactive defect pairs (2 VCu + InCu) and (InCu + CuIn). By the means of energy dispersive X-ray diffraction (EDXRD) phase formations and grain growth in thin films are studied in real time. The potential of EDXRD for in situ studies of reactions during the formation of chalcopyrite thin films is demonstrated.

Multistage evaporation processes are capable of producing state of the art Cu(In,Ga)Se2-thin-films for use in solar cells. The morphology of films grown by this process changes in a rapid manner as soon as their composition becomes copper rich beyond stoichiometry. For investigation of the structural changes, synchrotron radiation energy-dispersive X-ray diffractometry was used. Cu-rich and Cu-poor absorbers with an in-depth gallium gradient were grown by physical vapor deposition. These were transferred to a thermal processing reaction chamber which allows in-situ characterization. In the Cu-rich case changes in energy and shape of the measured diffraction signals caused by heating the samples could be attributed to the leveling of indium and gallium gradients within the layer. As a secondary method glow discharge optical emission spectroscopy was used to confirm this result. Cu-poor reference samples did not show a similarly significant intermixing of indium and gallium during annealing. We conclude, that the presence of excess copper in the layer at elevated temperatures (>470∘C) causes interdiffusion of indium and gallium.

CuInS2-based solar cells have a strong potential of achieving high efficiencies due to their ideal band gap of 1.5 eV. A further increase in the efficiency is expected from doping the absorber film with gallium due to enlargement of the band gap (Eg) and correspondingly the open-circuit voltage (VOC). We investigated Cu(In,Ga)S2 solar cells obtained from stacked metal layers sputtered from In and (Cu,Ga) targets followed by rapid thermal processing (RTP) in sulfur vapor. Depending on the actual RTP temperature profile, the films might exhibit CuInS2/CuGaS2 (top/bottom) segregation, which is rather detrimental for a large VOC. We found that only precursors sulfurized at sufficiently high temperatures exhibit the desired interdiffusion of the segregated CuInS2/CuGaS2 layers resulting in an increased VOC. Moreover, temperature dependent current–voltage profiling (suns-VOC-analysis) yielded strong indications for improved current collection and reduced losses for devices with proper interdiffusion of the CuInS2/CuGaS2 layers. A more fundamental question is related to the variation and formation of defect states in differently processed absorber films. The studied samples were thus further investigated by means of admittance spectroscopy, which allowed us to confirm the presence of three individual defect states in both absorber configurations. Two defects exhibit activation energies, which remain unchanged upon varying the RTP temperature whereas a third state exhibits significantly increased activation energy in devices showing interdiffusion of CuInS2/CuGaS2 layers. According to the characteristic shift of the conduction band edge upon Ga-doping we conclude that the latter defect level corresponds with the minority carriers in the p-type absorbers.

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.