Franz Karg

Rapid CIS-process for high efficiency PV-modules: development towards large area processing

Abstract The guideline for our CIGS-thin film process development is the scalability to large areas for low cost and high throughput module fabrication. Our technology consequently applies established large area sputter coating processes for the Cu-, In-, Ga, Se-precursors as well as for the back- and front-electrode. The characteristics of our absorber formation are an advanced two-step stacked elemental layer process including Cu(Ga)–In–Se precursor deposition, rapid thermal processing (RTP) to CIGS in a sulfur-containing ambient and a controlled sodium doping technique. Within our laboratory 12-cell mini-module baseline (substrate size: 10 cm×10 cm) peak and average conversion efficiencies of 14.7% and 13.2%± 1.5%, respectively, have been achieved. By varying the sulfur content from run to run in the gas atmosphere of the absorber formation process the total S/(Se+S) ratio in the obtained CIGS films has been changed between 0% and 18%. Although TEM-EDX- and SIMS-analyses on the CIGS-absorbers reveal an increased concentration of sulfur (and also gallium) towards the molybdenum back electrode, open circuit voltage and minority carrier lifetime monotonously increase with the average S/(Se+S) ratio determined by XRF. Our scaling up efforts towards pilot module processing necessitate a transfer of our laboratory RTP-technology to a fast heating system for large area substrates (60 cm×90 cm) at a high throughput. The essential task of heating the single-side-coated glass panel homogeneously is successfully demonstrated in a prototype heating chamber proving that a temperature deviation of ±10°C is not exceeded even at high heating rates.

Advanced Stacked Elemental Layer Process for Cu(InGa)Se 2 Thin Film Photovoltaic Devices

Targeting large area and low cost processing of highly efficient thin film solar modules an advanced stacked elemental layer process for Cu(InGa)Se 2 (CIGS) thin films is presented. Key process steps are i) barrier coating of the soda lime glass substrate combined with the addition of a sodium compound to the elemental Cu/In/Ga/Se-precursor stack and ii) rapid thermal processing (RTP) to form the CIGS compound. By this strategy exact impurity control is achieved and the advantageous influence of sodium on device performance and on CIGS film formation is demonstrated unambiguously by means of electrical characterisation, XRD, SEM, TEM and SIMS. Sodium enriched and sodium free precursor stacks were heated to intermediate states (300°C–500°C) of the RTPreaction process. The experiment clearly reveals that on the reaction pathway to the chalcopyrite semiconductor increased amounts of copper-selenide are formed, if sodium is added to the precursor films. TEM-electron diffraction unambiguously identifies the CuSe-phase which is localised at the surface of the forming CIGS-film. These experimental findings propose a sodium assisted quasi liquid growth model for the CIS formation taking into account that sodium promotes the existence of CuSe at higher temperatures and its effect as a flux agent. The model contributes to a better understanding of the observed superior crystal qualitiy for sodium enriched in contrast to sodium free CIGS films. Application of these experimental findings in the technique of the optimized and controlled sodium incorporation significantly improves process reproducibility, CIGS film homogenity over larger substrate areas and shifts the average efficiency of cells and modules to a significantly higher level. This is demonstrated by a 12-cell integrated series connected minimodule with an aperture area of 51 cm 2 and a confirmed efficiency of 11.75 %.

CIS module pilot processing applying concurrent rapid selenization and sulfurization of large area thin film precursors

The status of our pilot process for Cu(In,Ga)(S,Se)2 (CIGSSe) thin films on 60×90 cm2 glass substrates is described. In a newly developed large area rapid thermal processing (RTP) furnace the CIGSSe layer is formed from sputtered metallic precursors coated by an evaporated Se film. We present device characteristics of pilot line modules and discuss material issues critical for up scaling of our lab process. We demonstrate that thin silicon nitride layers effectively impede the Na diffusion from the float glass. The accurate sodium dose required for high efficiency devices is deposited on the Mo layer. The Na content in the reacted CIGSSe film is affected by oxygen in the Mo. The loss of sodium can be eliminated choosing appropriate Mo sputtering conditions. Cu(Ga) and In can be sputtered in a multi (>200) layer sequence on a lab scale. In the pilot process, only a few alternating layers are deposited. The structural properties of both types of precursors and reacted CIGSSe films are investigated. Absorbers processed in the large area, pilot line RTP show good crystal quality, grain sizes and uniformity of composition. Maps of the photoluminescence decay rate are shown. Average lifetimes of 30 ns are obtained. Cell efficiencies up to 13.5% are obtained using pilot line precursor and large area RTP. The module process currently is being optimized. Best circuit efficiencies on 30×30 cm2 substrates are at 11% to date.

XPS, TEM and NRA investigations of Zn(Se, OH)/Zn(OH)2 films on Cu(In, Ga)(S, Se)2 substrates for highly efficient solar cells

Abstract Structural and compositional properties of Zn(Se,OH)/Zn(OH) 2 buffer layers deposited by chemical bath deposition(CBD) on Cu(In,Ga)(S,Se) 2 (CIGSS) absorbers are investigated. Due to the aqueous nature of the CBD process, oxygen and hydrogen were incorporated into the ‘ZnSe’ buffer layer mainly in the form of Zn(OH) 2 as is shown by X-ray photoelectron spectroscopy and nuclear reaction analysis (NRA) measurements leading to the nomenclature ‘Zn(Se,OH)’. Prior to the deposition of Zn(Se,OH), a zinc treatment of the absorber was performed. During that treatment a layer mainly consisting of Zn(OH) 2 grew to a thickness of several nanometer. The whole buffer layer therefore consists of a Zn(Se,OH)/Zn(OH) 2 structure on CIGSS. Part of the Zn(OH) 2 in both layers (i.e. the Zn(Se,OH) and the Zn(OH) 2 layer) might be converted into ZnO during measurements or storage. Scanning electron microscopy pictures showed that a complete coverage of the absorber with the buffer layer was achieved. Transmission electron microscopy revealed the different regions of the buffer layer: An amorphous area (possibly Zn(OH) 2 ) and a partly nanocrystalline area, where lattice planes of ZnSe could be identified. Solar cell efficiencies of ZnO/Zn(Se,OH)/Zn(OH) 2 /CIGSS devices exceed 14% (total area).

Development and manufacturing of CIS thin film solar modules

Thin film modules based on CIS-technology with power outputs ranging between 5 and 40 W and corresponding circuit aperture area efficiencies between 9.6% and 11% have been introduced recently by Siemens Solar. Current status of production yield and performance is presented demonstrating significantly higher performance than alternative thin film technologies. Further developments have resulted in new champion efficiencies of 12.1% for a large commercial size modules and 14.7% for a small laboratory module.

Replacement of the CBD–CdS buffer and the sputtered i-ZnO layer by an ILGAR-ZnO WEL: optimization of the WEL deposition

Abstract As shown earlier the window extension layer (WEL) concept for thin film solar cells based on chalcopyrites results in device performances exceeding those of corresponding chemical bath deposited cadmium sulfide (CBD–CdS) buffered reference cells. The WEL concept is extended and it will be demonstrated, that now a single WEL successfully replaces both, the conventional buffer and the intrinsic part of the window bi-layer usually deposited by sputtering. Thus, one part of the window is deposited directly onto the absorber by a soft process called ion layer gas reaction (ILGAR). The optimization of ILGAR-ZnO WELs on Cu(In,Ga)(S,Se)2 absorbers with respect to the efficiencies of the completed solar cells is presented. This effort results in ‘total area’ efficiencies of 14.5% (best cell) which are comparable to those of devices with CBD–CdS buffer (14.7%—best cell) without any antireflecting coating.

Surface microstructure of CIS thin films produced by rapid thermal processing

Abstract The surfaces of polycrystalline CuInSe 2 thin films produced by rapid thermal processing (RTP) have been analyzed by scanning tunnelling microscopy and spectroscopy in ambient air. Deviating from standard measurement techniques the tunnelling microscope is driven by an AC sample voltage for surface morphology mapping in the constant current mode. Additionally, a Fermi energy mapping of the semiconductor surface is performed by mapping significant features of the I–V tunnelling characteristic. The polarity of the tunnelling current proves to be a reliable measure of the conductivity type of the material (n- or p-type); the observation of leakage currents at small bias voltages allows the identification of gap states around the Fermi level or metallic phases. Current-voltage curves taken at positions of different conduction type verigy the spectroscopic information in the maps. Typical areas imaged are (1.5 μm) 2 . Intra- and inter-granular nonuniformities of the conduction type are observed. Although the bulk material of all samples investigated is p-conductive, abrupt changes of the conductivity type of the surfaces from p- to n-type are observed as a function of the overall copper-to-indium ratio. The dominant current flow direction in slightly Cu-rich thin film bulk material is associated with p-type conduction, whereas In-rich samples exhibit largely n-type conductivity at the surface. Surfaces of copper-rich bulk materials show Fermi level pinning. The spectroscopic results do not depend on material and geometry of the tunnelling tip.

A Comparative Study of the Electronic Stability of Hydrogenated Amorphous Silicon and Silicon-Germanium Alloy Material

The electronic stability of a-Si:H and a-Si1-xGex:H films of different preparations has been investigated by keV-electron irradiation. Employing an electron dose of about 60 J/cm2, a metastable defect density near its saturation value was created. It was found that a-Si:H films exhibiting a high stability, if bulk sensitive measurement techniques (CPM, conductivity measurements) are applied, still have quite a large number of stable surface defects (detected by PDS) whose concentration is raised by irradiation. a-Si1-xGex:H alloy films are obviously more stable than pure a-Si:H films, whereby the relative stability increases with the Ge content x. Alloy films containing a small amount of germanium (≈5%) could be of special significance for practical application. The electronic properties of this material are almost identical to those of a-Si:H but the electronic stability is increased.

ILGAR technology. VIII. Sulfidic buffer layers for Cu(InGa)(S,Se)/sub 2/ solar cells prepared by ion layer gas reaction (ILGAR)

The ion layer gas reaction (ILGAR) is a novel deposition technique for chalcogenide compounds. This sequential and cyclic method, which involves a solid-gas reaction, is used to deposit sulfidic buffer layers on Cu(InGa)(S,Se)/sub 2/ (CIGSSe) absorbers. Chemical pretreatments of the absorber in several metal salt baths are investigated, which improve the device performance of the solar cell significantly. In the case of a Zn-bath a temperature above 100/spl deg/C forces the modification of the absorber surface resulting in a drastic increase of the open circuit voltage with increasing bath temperature. As a first result the combination of Cd-pretreatment and ILGAR-CdS buffer in a solar cell (0.5 cm/sup 2/) yields an efficiency of 14.2 % (total area) comparable with the quality of the corresponding standard device (CBD-CdS buffer, /spl eta/=14.1%); the Cd-free combination of Zn-pretreatment and ILGAR-ZnS buffer also results in a device with an efficiency of 14.2 % (total area).

Defect localization in CuInSe2 solar modules by thermal infrared microscopy

In this paper the IR Microscopy Thermosensoric Defect Localization method ((mu) -TDL) is presented. This technique is based on a novel IR microscopy lens which permits to take IR images with a spatial resolution of better than 10 micrometer, which is close to the theoretical limit. The (mu) -TDL method is demonstrated on defective CuInSe2 solar modules consisting of several solar cells serially interconnected and having solar efficiencies considerably below the average. By using the accurate localization of the defects by the (mu) -TDL method further investigations were performed and the origin for the defect was found. The (mu) -TDL method is also applicable to solar cells and modules consisting of other materials, such as amorphous Si or CdTe. The (mu) TDL method is suitable for the solar module development as well as for non- destructive production control.