Local efficiency analysis of solar cells based on lock-in thermography

By evaluating four lock-in thermography images of a solar cell taken at four different biases and an independently measured series resistance image, images of all local two-diode parameters are obtained. Assuming the local validity of the two-diode model, this information enables the construction of local and global dark and illuminated characteristics and of realistic images of local solar cell parameters like efficiency, fill factor, and open circuit voltage with a good spatial resolution. Within this procedure, an injection-dependent lifetime may be regarded by assuming an ideality factor larger than unity for the diffusion current. The possibilities and limitations of this approach are discussed and selected results on a typical industrial multicrystalline cell are introduced. The proposed procedure is a valuable tool for judging which local defects are especially harmful for degrading the fill factor or the open circuit voltage, respectively, and extrapolating the properties of a cell where certain types of defects are excluded. A general limitation of this approach is that it assumes an individual but constant series resistance to each pixel, which neglects the distributed character of the series resistance.

Graphical AbstractCalculated dependence of the efficiency on illumination intensity for regions A, B, and C, and for the whole cell.Figure optionsDownload as PowerPoint slideHighlights
► A local solar cell efficiency analysis is provided, based on lock-in thermography.
► Input images are up to four LIT images and a series resistance image resp. value.
► The local two-diode parameters are calculated for each image point.
► From these, all local cell efficiency parameters are calculated.
► Local dark and illuminated characteristics are obtained for any position or region.