Observations on the spectral characteristics of defect luminescence of silicon wafer solar cells

Recent studies of luminescence from silicon wafer solar cells under reverse bias show the potential for detection of defects in these solar cells. It was noticed that there is some distinction between breakdown voltages for various defect types, but there is no clear method to identify the particular chemical or structural origin of the defects being observed. The diode breakdown luminescence spectrum of silicon wafer solar cells was reported to peak in the visible range of the electromagnetic spectrum at around 700 nm, suggesting that photons with energies higher than the silicon bandgap are being emitted during diode breakdown. In the experiment presented here we use both Si and InGaAs cameras combined with a set of long-pass filters in the 900-1500 nm range to study the spectrum of defect luminescence. We have observed broadband reverse-bias breakdown luminescence spanning the wavelength range from 500 nm to 1500 nm. This suggests that diode breakdown luminescence does not have its peak in the visible part of the electromagnetic spectrum, as it clearly extends into the infrared wavelength range. We also demonstrate that sub-bandgap luminescence (which is typically associated with dislocations) and breakdown luminescence (which is associated with impurities) does not correspond one-to-one, and thus in principle can distinguish the types of defects present in the material. Further studies combining structural and chemical analysis need to be performed to confirm the accuracy of the technique, and to improve the model of diode breakdown luminescence.