Can luminescence imaging replace lock-in thermography on solar cells and wafers?

Since 15 years lock-in thermography (LIT) is used to investigate solar cells and modules. While LIT was used at the beginning only for shunt detection, meanwhile special LIT-techniques have been developed to image the minority carrier lifetime, also on wafers, the monochromatic cell efficiency, the local series resistance, the ideality factor, or the physical properties of breakdown sites. A general limitation of LIT is its limited spatial resolution due to thermal blurring. LIT investigation of breakdown sites takes only a fracture of a second, but investigation under forward bias usually takes some minutes. In recent years photoluminescence (PL) and electroluminescence (EL) imaging have become very popular to characterize solar wafers, cells and modules. Just as LIT, luminescence imaging allows to image the local lifetime and local series resistances (with PL even quantitatively), stronger local shunts, breakdown sites, and other faults like cracks. A big advantage of luminescence imaging is that it does not suffer from thermal blurring. Moreover, imaging times in the range of seconds are possible, and the systems are cheaper than LIT systems. The question is: Do we need LIT imaging anymore at all? It will be demonstrated that some important kinds of characterization can be done only by LIT, such as the quantitative measurement of breakdown and leakage currents (especially space charge recombination currents, which are not detectable by EL/PL), ideality factor imaging, the detection of leakage currents below grid lines and bus bars, as well as the quantitative measurement of certain breakdown parameters. Thus, LIT and luminescence imaging are complementing each other. For a complete characterization of solar cells both techniques have to be used in parallel.