Fast, contactless and spatially resolved measurement of sheet resistance by an infrared method

The principle of free carrier absorption in combination with a CCD camera sensitive in the infrared is used to establish a measurement method for the emitter sheet resistance of silicon solar cells. This combination allows for extremely fast measurements. For example a 100 x 100 mm wafer may be measured with a spatial resolution of 350 mu m within less than 10 s. An additional advantage of the optical measurement of emitter sheet resistance is the ability to measure inhomogeneities with an extremely good spatial resolution without errors due to size and orientation of the probe, which occur regularly in electrical measurements as, e.g., four-point probing if a spatial resolution of the order of or below the size of the probe is required. In the set-up presented spatial resolutions as low as 50 mu m were realized, but even higher resolutions would be attainable with an appropriate lens system. The measurement requires a solar cell precursor after emitter diffusion, but before metalization. Additionally a reference without emitter, but with identical surface properties is needed. The high spatial resolution facilitates a detailed investigation of selective emitters and a comparison of the results obtained by four-point probing. It is shown that the method developed here allows a considerably better quantitative evaluation of these structures than electrical measurements. Additionally the technique is used to test and optimize the homogeneity of a diffusion furnace