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.