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I am interested in using Kelvin probes to determine information about the grain and grain boundary characteristics (i.e. effective work function due to traps, trap energy and density) of our thin film samples. As I am not familiar with the technology, could you provide some information regarding the application of Kelvin Probes for polycrystalline thin film characterisation.

Our in house research has give us the opportunity to work with Polycrystalline Si thin film for a number of years. The key point for us was understanding that the long term decay states at the dielectric interfaces played an important role in determining Voc and thus the fill factor and energy extraction. A surprising aside was to discover how the cells responded (detrimentally) to temperature change and for this point we have been very careful in interpretation claimed specifications determined at 20°C as in strong sunlight the surface temperature of the cell can be considerably higher and cell efficiency actually starts to degrade.

From the beginning of our research we were aware of SPV mapping on semiconductors using pulsed light through a semitransparent electrode. The objective of this experimentation was to determine chemical contamination of crystalline Si wafer via changes in de-trapping lifetime and involved high speed data acquisition (>1MHz). However such system simply cannot measure slow states (this is clearly the domain of the Kelvin Probe), e.g. at a speed of 1000 measurements per minute (referred to as DC-SPV), and it is within these slow states that much of the effect of Voc is determined. These SPV transients are easy to record and product surface potential changes of between 250 and 450 mV (depending on semiconductor and cell structure) make them easy to measure.

Our recent experiences have been with a series of solar cells and dye sensitive films. Our work has resulted in the commercial release of our Surface Photovoltage Spectroscopy equipment based upon a motorised Linear Variable Filter (high intensity, 400-700 nm, broad pass band) and a motorised monochromator (low intensity, 400-800 nm, narrow pass band). We currently use the SPS equipment in both DC-SPV and AC-SPV modes.

We have used a 50 micron diameter tip in our SKP5050 system to seek obvious macroscopic (optical) grain boundaries in oxide and nitride coated mc-Si and have recorded approx 70 mV surface potential changes across the grain boundaries. Further SEM/X-ray Crystallographic studies of micro-crystalline metal alloy surface indicate a complex grain boundary structure and although we have not yet found a way to resolve these surface potential changes laterally, the Kelvin Probe, electrically, via charge trapping and de-trapping is another matter. There is a high probability that charge trapping and release can be observed, however it would involve a separate technique to establish a relationship between electrical and physical/chemical characteristics.

To summarise, with reference to your specific queries: we have evidence that the effective work function is affected by grain boundaries and this is backed by the order of magnitude surface potential changes one would anticipate from Miller index variations. However dielectric or oxide coatings may tend to mask these changes and the spatial (lateral) resolution may not be adequate. Changes in effective work function due to light stimulated trapping and detrapping are another matter and we do recommend this as a strong possibility for future study, however other techniques/data will be required to correlate with the WF/SP changes observed.

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KP Technology
Burn Street, Wick, Caithness, KW1 5EH, Scotland

Telephone: +44 (0)1955 602 777
Fax: +44 (0)1955 602 555
Email: sales@kelvinprobe.com

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