CIGS

A simulation study of a Cu(In1 - xGax)Se2 (CIGS) thin film solar cell has been carried out with maximum efficiency of 24.27 % (Voc = 0.856 V, Jsc = 33.09 mA/cm(2) and FF = 85.73 %). This optimized efficiency is obtained by determining the optimum band gap of the absorber and varying the doping concentration of constituent layers. The Ga content denoted by x = Ga/(In + Ga) is selected as 0.35 which provides the optimum band gap of absorber layer as 1.21 eV. Theoretically, the effects of Ga fraction "x" on CIGS absorber band gap are investigated and to avoid the lattice mismatch effect, the efficiency measurements due to the CIGS band gaps >1.21 eV have not come to the consideration. A one-dimensional simulator ADEPT/F 2.1 has been used to analyze the fabricated device parameters and hence to calculate open circuit voltage, short circuit current, fill factor and efficiency.

Polycrystalline CuInS2 chalcopyrite thin films were formed on a Mo-coated glass substrate by annealing of spray deposited precursor films in a sulfur atmosphere. Structural and photoelectrochemical analyses of CuInS2 films obtained by annealing at 500 °C and 600 °C revealed that a well-defined crystalline film was obtained by the 600 °C annealing. Owing to these favorable properties, the solar cell with an Al:ZnO/CdS/CIS/Mo/glass structure based on the 600 °C annealed CuInS2 film showed higher conversion efficiency than that obtained on the cell derived from the 500 °C annealed CuInS2. Partial incorporation of Ga in the CuInS2 film with a Ga/In ratio of ca. 0.2 to form a Cu(In,Ga)S2 mixed crystal without any reduction of photoelectrochemical properties can be achieved by introduction of a Ga source in the sprayed solution. As a result, the solar cell based on the 600 °C annealed Cu(In,Ga)S2 film showed the best conversion efficiency (5.8%) of the present sprayed chalcopyrite films. By introduction of a CdS thin layer followed by loading Pt deposits, moreover, the 600 °C annealed Cu(In,Ga)S2 film worked as a photocathode for photoelectrochemical water splitting with applied bias potential of >0.65 V.

Con el fin de incrementar la seguridad jurídica de la contratación internacional, la ley
aplicable al contrato debe contar con un elevado nivel de previsibilidad. Para alcanzarlo,
los Estados han venido implementando normas materiales uniformes y normas
de conflicto uniformes. Sin embargo, las normas de conflicto internas en países como
Colombia, lamentablemente todavía tienen un rol principal. La jerarquía y aplicación
de estas disposiciones representa un enorme reto para el operador jurídico colombiano.
El presente artículo tiene como principal objetivo establecer los criterios que debe
seguir el juez colombiano para determinar la ley aplicable al contrato internacional de
compraventa.

Real time spectroscopic ellipsometry (RTSE) fromthe near-infrared to ultraviolet has been applied for analysis of
the deposition of polycrystalline thin films that formthe basis of two key photovoltaic heterojunction configurations,
superstrate SnO2/CdS/CdTe and substrate Mo/Cu(In1 − xGax)Se2/CdS. The focus of this work is to develop
capabilities for monitoring and controlling the key steps in the fabrication of these device structures. Analysis
of RTSE data collected during sputter deposition of CdS on a rough SnO2 transparent top contact provides the
time evolution of the CdS effective thickness, or film volume per unit substrate area. This thickness includes interface,
bulk, and surface roughness layer components and affects the CdS/CdTe heterojunction performance
and the quantum efficiency of the solar cell in the blue region of the solar spectrum. Similarly, analysis of RTSE
data collected during co-evaporation of Cu(In1 − xGax)Se2 (CIGS; x ~ 0.3) on a rough Mo back contact provides
the evolution of a second phase of Cu2 − xSe within the CIGS layer. During the last stage of CIGS deposition, the
In, Ga, and Se co-evaporants convert this Cu2 − xSe phase to CIGS, and RTSE identifies the endpoint, specifically
the time at which complete conversion occurs and single-phase, large-grain CIGS is obtained in this key stage.

In the scale up from small area solar cells to large area production modules, it is important to understand the effects of macroscopic nonuniformities in basic properties on the ultimate performance of the devices. In this study, we have spatially correlated non-uniformities in thin-absorber (~ 0.7 μm) CIGS [Cu(In1-xGax)Se2] based solar cells with the corresponding performance parameters of small area devices. Non-contacting spectroscopic ellipsometry (SE) mapping of constituent layers over a 10 cm × 10 cm area has been performed step by step during batch processing, providing CIGS composition, as well as bulk and surface roughness thickness maps for each layer of the device structure. After film stack preparation and mapping, an 18 × 9 array of 0.5-cm2-area solar cells was fabricated in order to establish a spatial correlation between the composition/ thicknesses and cell performance maps over the same area. This approach serves to characterize the inherent non-uniformities that occur during large-area, thin-layer deposition, and is uniquely suited for industrial application. In addition, given sufficient non-uniformity, correlations between layer properties, deduced by SE at different spatial points of a large area cell structure, and the solar cell performance at those points enables expeditious optimization. As an example of the application of this capability, we have demonstrated the feasibility of depositing efficient, very thin CIGS in a three stage process, designed to reduce materials cost and increase throughput of solar modules.

Here we use nanoscale resolved Kelvin Probe Force Microscopy (KPFM) to locally probe the open circuit voltage in CIGS thin film solar cells. Illumination-dependent KPFM shows that the grain boundaries and grain cores present variations in surface photovoltage, as a consequence of the local variation of the open circuit voltage. Additionally, room temperature sub-micron photoluminescence (PL) scans were used to map the recombination centers in the polycrystalline CIGS films.

A spectroscopic ellipsometry (SE) capability having
the potential to scan production-scale areas at high speed has been
developed and successfully applied tomap the alloy composition of
copper–indium–gallium–diselenide (CuIn1−xGaxSe2: CIGS) thin
films. This technique not only generates a compositional map but
simultaneously provides maps of the more typical SE-determined
properties as well, including bulk layer and surface roughness layer
thicknesses. As a result, the methodology is suitable for characterization
in online production-scale applications. In order to develop
the mapping capability, CIGS films having different molar
Ga contents x and fixed copper stoichiometry were deposited and
measured in situ by SE in order to extract the complex dielectric
functions (ε = ε1+iε2 ) of these films. For mathematical interpolation
between the available alloy contents, the (ε1 , ε2 ) spectra
were parameterized using an oscillator sum. Best-fitting equations
were obtained that relate each oscillator parameter to the Ga content
x, as determined by energy dispersive X-ray analysis. This
approach reduces the number of fitting parameters for (ε1 , ε2 )
from several to just one: the Ga content x. Because (ε1 , ε2) is now
represented by this single parameter, the chances of parameter
correlations during fitting are reduced, enabling production-scale
compositional mapping of chalcopyrite films by SE.

Cu(In,Ga)Se2 thin-film solar cells with Ga-graded absorber
layers and a [Cu]/([In] + [Ga]) ratio varying between 0.5
and 1.0 were prepared by coevaporation and investigated. Except for the sample with a final [Cu]/([In] + [Ga]) ratio of 1.0, the samples were Cu-poor at all times during the evaporation. The variation in copper was found to influence the material properties in several ways: 1) Changing the Cu content had a strong impact on In and Ga interdiffusion, resulting in decreased Ga gradients in samples with large Cu deficiency; 2) the Cu-poor Cu(In,Ga)3 Se5 phase was detected in absorbers with [Cu]/([In] + [Ga]) ratios of 0.65 and below; and 3) the grain size changed significantly with the Cu variation. We observe a trend of reduced solar cell efficiencies for [Cu]/([In] + [Ga]) ratios of 0.65 and below, with an efficiency of 13.4% for the sample with a [Cu]/([In] + [Ga]) ratio of only 0.5, i.e., far from stoichiometry. We tentatively attribute the efficiency loss to a high concentration of point defects caused by the Cu deficiency.

Through-the-glass and film side spectroscopic
ellipsometry (SE) are being developed as in situ, on-line,
and off-line mapping tools for large area thin film
photovoltaics. Given that such instrumentation allows one
to extract thicknesses, as well as parameterized optical
functions versus wavelength, there exists the possibility to
utilize this information further to predict the optical
quantum efficiency (QE) and optical losses, the latter
including the reflectance and inactive layer absorbances.
By spatially resolving this information, one can gain a
better understanding of the origin of performance
differences between small area cells and large area
modules. We have demonstrated these techniques for
thin film hydrogenated amorphous silicon (a-Si:H) and
Cu(In1-xGax)Se2 solar cell structures. For solar cells on
glass superstrates, film-side SE can be supplemented
with through-the-glass SE, which helps to increase the
sensitivity of the analysis to the critical transparent
conducting oxide and window layer properties. A
comparison of predicted and experimental QE can reveal
optical and electronic losses and light trapping gains.

In this work we study the CdS/Cu (In, Ga) Se 2 pn junction region in Cu (In, Ga) Se 2 thin-film solar cells using atom probe tomography. A Cu-, Ga-depleted, and Cd-doped region of about 1 nm thickness is detected at the Cu (In, Ga) Se 2 side of the CdS/Cu (In, Ga) Se 2 interface. Furthermore, Cd is also found to be enriched at Cu (In, Ga) Se 2 grain boundaries connected to the CdS layer.

Electrochemical deposition of indium (In) on a copper-covered molybdenum-coated glass substrate from several acidic InCl3 solutions was studied for fabrication of CuInS2-based solar cells. When In was deposited using a simple acidic InCl3 solution at −0.80 V (vs. Ag/AgCl), island-shaped growth was observed, whereas a homogeneous In film was obtained from InCl3 solution containing citric acid and sodium citrate at −0.98 V (vs. Ag/AgCl). Electrochemical and structural analyses revealed that the citric acid additive had a function for smoothing the surface of the In deposit. The mixing with sodium citrate induced appreciable inhibition of H2 evolution during the In deposition, leading to high current efficiency of >90%. The CuInS2 film derived from the homogeneous In had a uniform thickness with a smooth surface, while the CuInS2 film obtained from the island-shaped In deposit showed a large variation in thickness with recessed areas. The CuInS2 film derived from the homogeneous In was showed better photoelectrochemical response than that of the film fabricated from the island-shaped In. As expected from these differences, the solar cell with an Al:ZnO/CdS/CuInS2/Mo structure derived from the homogeneous In film showed the best conversion efficiency of 7.8% with relatively high reproducibility.

In the scale-up of Cu(In1-xGax)Se2 (CIGS) solar
cell processing for large area photovoltaics (PV) technology, the
challenge is to achieve optimum values of layer thicknesses as
well as CIGS Cu stoichiometry and alloy composition x within
narrow ranges and simultaneously over large areas. As a result,
contactless metrologies -- those that provide such information in
real time or in-line process step by step, with the capabilities of
large area mapping -- are of great interest in this technology. We
have demonstrated high-speed multichannel spectroscopic
ellipsometry (SE) in a number of modes for CIGS metrology
including (i) single spot real time SE monitoring of (In1-xGax)2Se3
(IGS) as the first stage in multi-source evaporation of three-stage
CIGS; (ii) control of Cu stoichiometry in the second and third
stages of the process; (iii) single spot in situ SE analysis of alloy
composition and grain size averaged through the thickness for
the final CIGS film; (iv) off-line mapping of CIGS thickness and
composition over large areas, as well as mapping after each
device fabrication step for correlation with local small area cell
performance; (v) ex situ single spot analysis of alloy composition
profiles in CIGS and of completed solar cell stacks to extract
thicknesses and properties of semiconductor and contact layers;
and (vi) predictive capability for quantum efficiency based on the
results of SE multilayer analysis. With the future development of
new instrumentation, the off-line and ex-situ capabilities in
multilayer analysis and mapping will be possible in-line for both
rigid and roll-to-roll flexible substrates.

Surface trap states in copper indium gallium selenide semiconductor nanocrystals (NCs), which serve as undesirable channels for nonradiative carrier recombination, remain a great challenge impeding the development of solar and optoelectronics devices based on these NCs. In order to design efficient passivation techniques to minimize these trap states, a precise knowledge about the charge carrier dynamics on the NCs surface is essential. However, selective mapping of surface traps requires capabilities beyond the reach of conventional laser spectroscopy and static electron microscopy; it can only be accessed by using a one-of-a-kind, second-generation four-dimensional scanning ultrafast electron microscope (4D S-UEM) with subpicosecond temporal and nanometer spatial resolutions. Here, we precisely map the collective surface charge carrier dynamics of copper indium gallium selenide NCs as a function of the surface trap states before and after surface passivation in real space and time using S-UEM. The time-resolved snapshots clearly demonstrate that the density of the trap states is significantly reduced after zinc sulfide (ZnS) shelling. Furthermore, the removal of trap states and elongation of carrier lifetime are confirmed by the increased photocurrent of the self-biased photodetector fabricated using the shelled NCs.

Through-the-glass and film side spectroscopic ellipsometry (SE) are being developed as in situ, on-line, and off-line mapping tools for large area thin film photovoltaics. Given that such instrumentation allows one to extract thicknesses, as well as parameterized optical functions versus wavelength, there exists the possibility to utilize this information further to predict the optical quantum efficiency (QE) and optical losses, the latter including the reflectance and inactive layer absorbances. By spatially resolving this information, one can gain a better understanding of the origin of performance differences between small area cells and large area modules. We have demonstrated these techniques for thin film hydrogenated amorphous silicon (a-Si:H) and Cu(In1-xGax)Se2 solar cell structures. For solar cells on glass superstrates, film-side SE can be supplemented with through-the-glass SE, which helps to increase the sensitivity of the analysis to the critical transparent conducting oxide and window layer properties. A comparison of predicted and experimental QE can reveal optical and electronic losses and light trapping gains.

Surface trap states in semiconductor copper indium gallium selenide nanocrystals (NCs) which serve as undesirable channels for non-radiative carrier recombination, remain a great challenge impeding the development of solar and optoelectronics devices based on these NCs. In order to design efficient passivation techniques to minimize these trap states, a precise knowledge about the charge carrier dynamics on the NCs surface is essential. However, selective mapping of surface traps requires capabilities beyond the reach of conventional laser spectroscopy and static electron microscopy; it can only be accessed by using a one-of-a-kind, second-generation four-dimensional scanning ultrafast electron microscope (4D S-UEM) with sub-picosecond temporal and nanometer spatial resolutions. Here, we precisely map the collective surface charge carrier dynamics of copper indium gallium selenide NCs as a function of the surface trap states before and after surface passivation in real space and time using S-UEM. The time-resolved snapshots clearly demonstrate that the density of the trap states is significantly reduced after zinc sulfide (ZnS) shelling. Furthermore, removal of trap states and elongation of carrier lifetime are confirmed by the increased photocurrent of the self-biased photodetector fabricated using the shelled NCs.