We report on a two-dimensional investigation of the p-n junction in multicrystalline silicon solar cells using scanning Kelvin probe force microscopy (SKPFM). The junction location and depth were identified by SKPFM potential measurement and subsequent data analysis, where a procedure taking bias-voltage-induced changes in the potential and electric field was developed to avoid the effects of surface Fermi level pinning. Device simulation supported the junction identification procedure and showed a possible deviation of ∼40 nm in the junction identification. The two-dimensional electric-field images show that the shape of the junction follows the surface topography of the device, or, in other words, the junction depth is identical over the device.
Peculiar features of Deep Level Transient Spectroscopy (DLTS) measurements on resistive samples are pointed out. Depending on the value of the sample resistance in series with the diode capacitance, the DLTS signal can be strongly reduced and even its sign may be reversed, entailing a possible confusion between majority and minority carrier traps. Means of detecting these effects are discussed and a correction procedure is proposed, based on varying the circuit impedance by means of an additional resistance in series with the sample. Illustrative examples include ion implanted silicon and the case of a germanium bicrystal.
We report on a characterization study of laser edge isolation in multicrystalline silicon (mc-Si) solar cells using microscopic electrical, structural, and morphological tools of scanning capacitance microscopy (SCM), conductive atomic force microscopy (C-AFM), electron backscattering diffraction (EBSD), and scanning electron microscopy (SEM), as well as a macroscopic electrical characterization of lock-in thermography (LIT). SCM and C-AFM measurements revealed that the emitter was not completely removed by the laser ablation, and considerable amounts of emitter dopant were driven into the material. A portion of the ablated or molten material was redeposited or recrystallized on top of the laser groove, forming either single-or polycrystalline stripes. Si particles with either polycrystalline or amorphous structures were also formed on the grooves. LIT measurement on a shunted device exhibits a high temperature region centered on the groove line, indicating inadequate isolation. SEM observations show a significant different morphological/structural surface of the groove from that of the isolated devices. These techniques provide useful characterizations for failure analysis of the laser edge isolation.
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