Wolfgang Riedl

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 %.

Novel rapid-thermal-processing for CIS thin-film solar cells

Device quality CuInSe/sub 2/ thin films have been synthesized by rapid-thermal-processing (RTP) of elemental Cu-In-Se stacked layers. The authors demonstrate for the first time solar cells with an efficiency above 10% produced at short heating cycles and without using toxic gases. The microstructure of these films is analysed in detail by SEM, TEM and SIMS. Morphology, crystal quality and solar cell efficiency are influenced drastically by the Cu/In atomic ratio. The predominant CIS crystal defects observed in indium-rich films are twins whereas in copper-rich films dislocations, stacking faults and precipitates are also observed.<<ETX>>

Laser patterning of a-Si solar modules with transparent conducting zinc oxide back electrodes

A patterning process has been developed for a Nd:YAG picosecond pulse laser and for a frequency-doubled Nd:YLF nanosecond laser. Best results were achieved with a 10 ns laser pulse at a wavelength of 523 nm focused through the glass substrate. The authors obtained a whole-area efficiency of 9.8% for a 300*310 mm/sup 2/ pin-a-Si:H solar module. A tentative process model is given.<<ETX>>

The impact of controlled sodium incorporation on rapid thermal processed Cu(InGa)Se/sub 2/-thin films and devices

The alkali content of Cu(InGa)Se/sub 2/ thin films fabricated by RTP on a dense molybdenum backelectrode is below the detection limit of ESCA. Starting from this virtually sodium-free case, alkali impurities are successively added in order to study their impact on film morphology and device performance. Two novel techniques have been developed to add sodium to the chalcopyrite thin film: (1) By shifting the stress of the Mo-backelectrode from compressive to tensile its alkali permeability increases and in consequence so does the alkali content in the CIGS film. (2) Adding sodium compounds directly to the Cu-In-Ga-Se precursor film controls the final alkali content in CIGS independent from the substrate. Along with the addition of sodium a significant increase in performance of CIGS/CdS/ZnO cells (1.85 cm/sup 2/ active area) was found that peaked at 13.2%.

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.