Volker Probst

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.