Diffused junctions in multicrystalline silicon solar cells studied by complementary scanning probe microscopy and scanning electron microscopy techniques

Abstract

The junction location in textured n + -p multicrystalline Si solar cell devices is quantitatively located. A comprehensive comparison is presented between secondary ion mass spectrometry (SIMS) and three microscopy techniques: scanning capacitance microscopy (SCM), scanning Kelvin probe microscopy (SKPM), and secondary electron contrast (SE) in the scanning electron microscope. The comparison includes capabilities of junction allocation, applicability of the measurement to give a two-dimensional analysis of a textured device, apparent imaging of the depletion region, depletion edge and depletion width determination, sample structure requirements for the specific methods, ease of sample preparation, and data acquisition time for high-quality measurements. We find that SE not only allows the most straightforward sample preparation and provides the quickest measurement, but also seems to show the depletion edge and width with a high degree of accuracy. This is verified by a comparison to PC1D simulations of the band bending in the bulk of the device. These data also contribute to our understanding of the origin of SE doping contrast. SKPM provides reliable junction identification but poor capability in determining the depletion width. All three techniques are able to show how well the diffused n + emitter region tracks the surface topography, with SCM and SE yielding the clearest images.

Manufacturing metrology for c-Si module reliability and durability Part II: Cell manufacturing

“…SEM is often used to image structural defects associated with shunts like crystallographic defects [148], material defects [151], precipitates [148], and etch-pits [41,152,154]. For non-linear shunts caused by gaps in doping, SEM can be used to monitor PSG coverage uniformity [87] and SE contrasting can be used to confirm local doping inhomogeneities [85][86][87]. TEM has also been used for high resolution imaging of recombination active crystal defects as well as Si 3 N 4 inclusions and SiC precipitates [148].…”

“…It is imperative to determine whether or not the P diffusion process plays a direct role in PID as this would constitute yet another boundary condition for emitter optimization along with the traditional recombination and resistive losses. Linear shunt [148] Poor junction isolation [148] Dark lock-in thermography (DLIT) [168] Light beam-induced current (LBIC) [175] Scanning electron microscopy (SEM) Electron beam-induced current (EBIC) [81] Electroluminescence (EL) imaging [170] Photoluminescence (PL) imaging [173,174] Cracks prior to diffusion [148] SiC filaments [151] Non-linear shunt [148] Metal contacting base via gaps in emitter doping [148] DLIT [168] EL imaging [170] Recombination sites created at scratches on front surface [148] SEM EBIC [81] SE contrast [82,85,86] Transmission electron microscopy (TEM) Energy-dispersive x-ray analysis (EDX) Recombination active crystal defects [148] Si 3 N 4 inclusions [148] Junction prebreakdown [152] Type I: Surface contamination prior to diffusion [153] SEM EDX Reverse bias EL imaging DLIT [168] EBIC [81] Micro-x-ray fluorescence (µ-XRF) TEM…”

“…Being able to image the p-n junction allows for the detection of discontinuities, which can be missed with one-dimensional profiling techniques. The EBIC method [81] and secondary electron (SE) contrast imaging [82][83][84][85][86][87], both performed in a SEM, are well suited for solar cell emitter characterization. The final product for the end-user is the module (as part of a PV installation), and not individual solar cells, therefore additional understanding is required to determine what role if any the emitter formation process plays in module reliability.…”

Scanning Electron Microscopy Dopant Contrast Imaging of Phosphorus‐Diffused Silicon

“…The ability to spatially resolve dopant profiles is of key interest for characterizing photovoltaic solar cells employing features like localized doping or diffused nanotextures. Recent advances imaging of phosphorus emitters are less common, including homogeneous diffused emitters, [34][35][36] and localized selective emitters. [20,37] A more recent PV application of SEMDCI imaging has been characterizing nanotextured BSi.…”

“…SEMDCI has now been used widely to characterize industrial Si wafers with p‐type (boron) diffused emitters [ 18–22 ] and aluminum localized back surface contacts (LBSF). [ 23–33 ] However, reports of SEMDCI imaging of phosphorus emitters are less common, including homogeneous diffused emitters, [ 34–36 ] and localized selective emitters. [ 20,37 ] A more recent PV application of SEMDCI imaging has been characterizing nanotextured BSi.…”