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Solar cell (grain boundaries in p-doped silicon)
Shown below is the example of simultaneous surface potential and scanning impedance imaging on polycrystalline p-doped silicon. This sample was prepared by polishing antireflection layer and n-doped layer off the solar cell. Grain structure was visualized by selective etching in 0.1M NaOH at 80C. The images were acquired from three grain junction region.
Surface topography (a) exhibits small topographic variations between the grains due to the difference in etching rate. Surface potential on grounded surface (b) exhibits positive potential regions associated with the grain boundaries consistent with positively charged interfaces in the p-doped material. Surface potential of forward (c) and reverse (d) interface show that potential drops indeed develop at the grain boundaries, i.e. grain boundary conductivity is lower than that of the surrounding regions. Finally, SIM phase image (e) shows that there is significant phase changes in the vicinity of the grain boundaries. SIM amplitude image (f) shows both amplitude decrease in the grains and at the grain boundaries. Additional information can be obtained from the potential and phase profiles across the interface shown below.
Surface potential profiles show that potential is virtually uniform in the higher-potential grain and exhibit wide tail in the lower biased grain. This behavior can be attributed to the minority carrier dynamics under illumination. Note that SIM phase profile (c) exhibits wide asymmetric feature at the grain boundary (as opposed to typical step in phase profile) consistent with SSPM data. Finally, amplitude profile (d) exhibits both ohmic decrease of amplitude in the grains and at the interface.