SIMS depth profiling of CdTe-based solar cells grown on sapphire substrates

CdTe/CdS/TCO solar cell structures were grown on sapphire substrates especially to investigate the impurity distributions in them. The study was performed using quantitative secondary ion mass spectrometry (SIMS) depth profiling, from the CdTe back surface through to the TCO layer. The species investigated were F, Br, Na, Si, Sn, In, O, Cl, S and Te, and for the purpose of comparison, some of the structures were investigated in their as-grown state. It has been shown that while Si concentration profiles are similar to those for structures grown on glass, Na was more than one order of magnitude lower when sapphire was used as substrate instead of glass. This demonstrates that Na diffused from the glass substrate. Moreover, the overall Na concentration in structures with and without CdCl2 treatment showed that Na also originated from the CdCl2, which is in accordance with our recently reported studies of impurities in CdCl2. The concentration of Br was found to be at least 10 times higher in the etched structures compared to the un-etched ones, especially near the CdTe surface where there was a clearly resolved diffusion profile. In the CdTe active layer, the concentration of F was about 4 × 1015 cm− 3 with a relatively flat level but with surface segregation following heat treatment. Finally, the potential diffusion of Sn and In from the sputtered SnO2/ITO:F TCO layer into the CdTe active layer was investigated. It was found that there was no measurable diffusion of Sn from the SnO2 layer into CdTe. Moreover, the SnO2 layer played an important role in preventing the diffusion of In from the In-containing TCO layer.