Applications of imaging techniques to Si, Cu(In,Ga)Se2, and CdTe and correlation to solar cell parameters

Characterization by imaging has been performed on various sets of Si, Cu(In, Ga)Se₂ (CIGS), and CdTe solar cell samples. The imaging techniques include photoluminescence (PL) imaging, electroluminescence (EL) imaging, reverse-bias EL imaging (ReBEL), illuminated lock-in thermography (ILIT), and forward- and reverse-bias dark lock-in thermography (DLIT). PL imaging of Si has shown that the image intensity correlates to minority-carrier lifetime. PL imaging of CIGS shows brightness variations after the deposition of the CIGS that persist through the CdS deposition and subsequent processing steps to finish the devices. The PL and EL intensities on both Si and CIGS finished cells correlate to efficiency and open-circuit voltage. Also, for all materials, PL and EL imaging show dark areas due to carrier recombination induced by defects. These same areas often appear in ILIT, DLIT, and ReBEL as heated areas or breakdown sites where currents flow through weak diodes, shunts, and defects. For Si cells, we have correlated the cells´ fill factors to the amount of shunting detected by DLIT. For CIGS cells, we have identified these detrimental weak diodes and shunts by imaging and show an example of a defect analyzed in more detail by scanning electron microscopy techniques using top view and cross-sectional imaging.