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NanoSPV - SPM Technique for Measuring Minority Charge Carrier Diffusion Lengths with High Spatial Resolution

Presentation held at International Conference on Nanoscience and Technology, ICN+T 2014, 21.07. - 25.07.2014, Vail, Colorado
: Stumpf, Florian

Präsentation urn:nbn:de:0011-n-3222320 (3.5 MByte PDF)
MD5 Fingerprint: c37293b72a1676a9652e1d066bd9eadc
Erstellt am: 22.1.2015

2014, 32 Folien
International Conference on Nanoscience and Technology (INC+T) <2014, Vail/Colo.>
Bundesministerium für Bildung und Forschung BMBF
Rastersondenmikroskopieverfahren zur Bestimmung der Ladungsträgerdiffusionslänge
Vortrag, Elektronische Publikation
Fraunhofer IISB ()
diffusion length; KPFM; surface photovoltage; lifetime

Polycrystalline silicon has become the dominating substrate material in the PV industry due to its relatively low production costs compared to monocrystalline substrates. Another route to reduce material and production costs relies on the fabrication of nano- and microcrystalline silicon thin film solar cells. Both technologies share the problems of measuring, controlling and achieving satisfying minority charge carrier diffusion lengths which are influenced by crystal defects, grain boundaries and electrically active impurities. The commonly used tools do not offer sufficient lateral resolution to explicitly allocate the cause of short diffusion lengths to grain boundaries or bulk material. In this work we present a measuring approach that fulfils those requirements, without additional s ample preparation, under room conditions and with very high spatial resolution. We introduce a SPM technique that combines Kelvin Probe Force Microscopy (KPFM) and Surface Photo Voltage (SPV) measurements in one setup. This allows us to significantly scale down the lateral resolution of standard SPV tools, which are usually limited by the diameter of the Kelvin probe, to the nanometre scale enabled by the KPFM tip (NanoSPV). For the SPV mode, four lasers (wavelengths between 650 nm and 980 nm) were added to a standard AFM setup to provide the option of measuring the SPV of illuminated samples at different wavelengths. These measurements allow calculating the diffusion length of the minority charge carriers. For the proof of concept we have produced homogeneously Fe implanted p-doped mono- Si reference samples and measured the diffusion lengths / lifetimes with established SPV and µ-PCD tools. The exact agreement with the theoretically expected values enables us to use these samples as calibration samples. Thus we can critically compare the diffusion lengths with our NanoSPV setup and quantitatively verify this approach. For more complex samples physical effects occur on the nanometre scale (e.g. grain boundary recombination). These have to be detected and characterized in detail for improved understanding of the dominant recombination parameters of such samples which in turn would allow improved optimization of cell processing. We have measured polycrystalline Si wafers with the established tools and observed areas where this information is clearly lost by the averaging ov er a large area. We will show measurements of these specific areas with the NanoSPV method. As those structures can lead to very complex charge carrier distributions numerical simulations are necessary to fully understand the results measured by the NanoSPV system and to determine the diffusion lengths.